9
Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation Yann Garcia,* Stewart J. Campbell, James S. Lord, § Yves Boland, ² Vadim Ksenofontov, # and Philipp Gu 1 tlich # Unite ´ de Chimie des Mate ´ riaux Inorganiques et Organiques, De ´ partement de Chimie, Faculte ´ des Sciences, UniVersite ´ Catholique de LouVain, Place Louis Pasteur 1, 1348 LouVain-la-NeuVe, Belgium, School of Physical, EnVironmental and Mathematical Sciences, The UniVersity of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia, ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQX, U.K., and Institut fu ¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg UniVersita ¨t Mainz, Staudingerweg 9, 55099 Mainz, Germany ReceiVed: March 27, 2007; In Final Form: June 22, 2007 The thermal spin transition that occurs in the polymeric chain compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 above room temperature has been investigated by zero-field muon spin relaxation (μSR) over the temperature range 8- 402 K. The depolarization curves are best described by a Lorentzian and a Gaussian line that represent fast and slow components, respectively. The spin transition is associated with a hysteresis loop of width ΔT ) 34 K(T 1/2 v ) 346 K and T 1/2 V ) 312 K) that has been delineated by the temperature variation of the initial asymmetry parameter, in good agreement with previously published magnetic measurements. Zero-field and applied field (20-2000 Oe) μSR measurements show the presence of diamagnetic muon species and paramagnetic muonium radical species (A ) 753 ( 77 MHz) over the entire temperature range. Fast dynamics have been revealed in the high-spin state of [Fe(NH 2 trz) 3 ](NO 3 ) 2 with the presence of a Gaussian relaxation mode that is mostly due to the dipolar interaction with static nuclear moments. This situation, where the muonium radicals are totally decoupled and not able to sense paramagnetic fluctuations, implies that the high-spin dynamics fall outside the muon time scale. Insights to the origin of the cooperative effects associated with the spin transition of [Fe(NH 2 trz) 3 ](NO 3 ) 2 through muon implantation are presented. 1. Introduction Spin transition (ST) coordination compounds represent an attractive class of materials in which the spin state can be reversibly addressed by various external stimuli including temperature, pressure, and electromagnetic radiation. Although the spin crossover (SCO) phenomenon has been observed for electronic configurations ranging from 3d 4 to 3d 7 , most of the synthetic efforts and physical investigations have been carried out for iron(II) (3d 6 ) mononuclear complexes. 1,2 Investigation of iron(II) polynuclear SCO complexes started more recently with representative examples from dinuclear, 3 trinuclear, 4 and tetranuclear compounds, 5 and also from some coordination polymers. 6 Iron(II) 1,2,4-triazole polymeric chain compounds have attracted particular attention, as their abrupt ST is generally associated with both hysteretic and thermochromic effects (some of them around the room-temperature region), thus providing a basis for their potential use in thermal display, memory devices and sensors. 7-9 [Fe(NH 2 trz) 3 ](NO 3 ) 2 (NH 2 trz ) 4-amino-1,2,4-triazole) is an important example of this family of smart materials because it displays a distinct thermochromic effect between pink in the low-spin (LS) state and white in the high-spin (HS) state; its complete and reversible abrupt ST is centered around 331 K and is associated with a cyclable hysteresis loop of width 35 K. 10,11 Such latter characteristics have been attributed to the presence of three rigid N1,N2-1,2,4-triazole linkages between the metal centers, 12-14 allowing an efficient transmission of elastic expansion associated with the spin state change of individual iron(II) ions along the chain 11 (Figure 1). The type of linkage, however, cannot solely explain the ST behavior of [Fe(NH 2 trz) 3 ](NO 3 ) 2 because other iron(II) triply bridged 1,2,4- triazole chain compounds present hysteresis effects of lower width ranging from 2 to 20 K. 6 Despite the few crystal structures available on some copper(II) 4R-1,2,4-triazole chain com- pounds, 13c,f,15,16 no single crystals of the analogous iron(II) complexes suitable for a X-ray diffraction analysis could be grown due to their poor crystallinity despite many attempts using different methods (evaporation, diffusion, ...). A structural modification from a linear conformation in the LS state to a zigzag one in the HS state was first proposed to account for the absence of long range ordering in X-ray absorption experiments (EXAFS, WAXS) in the HS state observed on these materials. 13a,b,17 EPR measurements of the diluted sample [Fe 0.9 Cu 0.1 (Htrz) 2 trz) 3 ](BF 4 ) 2 (Htrz ) 4H-1,2,4- triazole, trz ) 1,2,4-triazolato) showed systematic rhombic distortion in the HS state, 18 thus supporting this structural hypothesis that could have nicely accounted for the observed hysteresis of 40 K for the nondoped iron(II) ST derivative. 19 EXAFS experiments performed on 1D iron(II) 1,2,4-triazole compounds at the Fe-K edge have, however, demonstrated that chains remain perfectly linear in both spin states, ruling out the initially suggested HS zigzag conformation. Indeed, the disap- pearance of the 7 Å signal corresponding to the multiple * Corresponding author. E-mail: [email protected]. ² Universite ´ Catholique de Louvain. The University of New South Wales. § Rutherford Appleton Laboratory. # Johannes Gutenberg Universita ¨t Mainz. 11111 J. Phys. Chem. B 2007, 111, 11111-11119 10.1021/jp072399k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2007

Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation

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Page 1: Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation

Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric ChainCompound [Fe(NH2trz)3](NO3)2. Muon Spin Relaxation

Yann Garcia,*,† Stewart J. Campbell,‡ James S. Lord,§ Yves Boland,†Vadim Ksenofontov,# and Philipp Gu1 tlich#

Unite de Chimie des Mate´riaux Inorganiques et Organiques, De´partement de Chimie, Faculte´ des Sciences,UniVersiteCatholique de LouVain, Place Louis Pasteur 1, 1348 LouVain-la-NeuVe, Belgium, School ofPhysical, EnVironmental and Mathematical Sciences, The UniVersity of New South Wales, Australian DefenceForce Academy, Canberra, ACT 2600, Australia, ISIS, Rutherford Appleton Laboratory, Chilton, Didcot,OX11 OQX, U.K., and Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes GutenbergUniVersitat Mainz, Staudingerweg 9, 55099 Mainz, Germany

ReceiVed: March 27, 2007; In Final Form: June 22, 2007

The thermal spin transition that occurs in the polymeric chain compound [Fe(NH2trz)3](NO3)2 above roomtemperature has been investigated by zero-field muon spin relaxation (µSR) over the temperature range∼8-402 K. The depolarization curves are best described by a Lorentzian and a Gaussian line that represent fastand slow components, respectively. The spin transition is associated with a hysteresis loop of width∆T ) 34K (T1/2

v ) 346 K andT1/2V ) 312 K) that has been delineated by the temperature variation of the initial

asymmetry parameter, in good agreement with previously published magnetic measurements. Zero-field andapplied field (20-2000 Oe)µSR measurements show the presence of diamagnetic muon species andparamagnetic muonium radical species (A ) 753( 77 MHz) over the entire temperature range. Fast dynamicshave been revealed in the high-spin state of [Fe(NH2trz)3](NO3)2 with the presence of a Gaussian relaxationmode that is mostly due to the dipolar interaction with static nuclear moments. This situation, where themuonium radicals are totally decoupled and not able to sense paramagnetic fluctuations, implies that thehigh-spin dynamics fall outside the muon time scale. Insights to the origin of the cooperative effects associatedwith the spin transition of [Fe(NH2trz)3](NO3)2 through muon implantation are presented.

1. Introduction

Spin transition (ST) coordination compounds represent anattractive class of materials in which the spin state can bereversibly addressed by various external stimuli includingtemperature, pressure, and electromagnetic radiation. Althoughthe spin crossover (SCO) phenomenon has been observed forelectronic configurations ranging from 3d4 to 3d7, most of thesynthetic efforts and physical investigations have been carriedout for iron(II) (3d6) mononuclear complexes.1,2 Investigationof iron(II) polynuclear SCO complexes started more recentlywith representative examples from dinuclear,3 trinuclear,4 andtetranuclear compounds,5 and also from some coordinationpolymers.6 Iron(II) 1,2,4-triazole polymeric chain compoundshave attracted particular attention, as their abrupt ST is generallyassociated with both hysteretic and thermochromic effects (someof them around the room-temperature region), thus providing abasis for their potential use in thermal display, memory devicesand sensors.7-9

[Fe(NH2trz)3](NO3)2 (NH2trz ) 4-amino-1,2,4-triazole) is animportant example of this family of smart materials because itdisplays a distinct thermochromic effect between pink in thelow-spin (LS) state and white in the high-spin (HS) state; itscomplete and reversible abrupt ST is centered around∼331 Kand is associated with a cyclable hysteresis loop of width∼35

K.10,11 Such latter characteristics have been attributed to thepresence of three rigidN1,N2-1,2,4-triazole linkages betweenthe metal centers,12-14 allowing an efficient transmission ofelastic expansion associated with the spin state change ofindividual iron(II) ions along the chain11 (Figure 1). The typeof linkage, however, cannot solely explain the ST behavior of[Fe(NH2trz)3](NO3)2 because other iron(II) triply bridged 1,2,4-triazole chain compounds present hysteresis effects of lowerwidth ranging from 2 to 20 K.6 Despite the few crystal structuresavailable on some copper(II) 4R-1,2,4-triazole chain com-pounds,13c,f,15,16 no single crystals of the analogous iron(II)complexes suitable for a X-ray diffraction analysis could begrown due to their poor crystallinity despite many attempts usingdifferent methods (evaporation, diffusion, ...).

A structural modification from a linear conformation in theLS state to a zigzag one in the HS state was first proposed toaccount for the absence of long range ordering in X-rayabsorption experiments (EXAFS, WAXS) in the HS stateobserved on these materials.13a,b,17EPR measurements of thediluted sample [Fe0.9Cu0.1(Htrz)2trz)3](BF4)2 (Htrz ) 4H-1,2,4-triazole, trz ) 1,2,4-triazolato) showed systematic rhombicdistortion in the HS state,18 thus supporting this structuralhypothesis that could have nicely accounted for the observedhysteresis of∼40 K for the nondoped iron(II) ST derivative.19

EXAFS experiments performed on 1D iron(II) 1,2,4-triazolecompounds at the Fe-K edge have, however, demonstrated thatchains remain perfectly linear in both spin states, ruling out theinitially suggested HS zigzag conformation. Indeed, the disap-pearance of the 7 Å signal corresponding to the multiple

* Corresponding author. E-mail: [email protected].† UniversiteCatholique de Louvain.‡ The University of New South Wales.§ Rutherford Appleton Laboratory.# Johannes Gutenberg Universita¨t Mainz.

11111J. Phys. Chem. B2007,111,11111-11119

10.1021/jp072399k CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 08/30/2007

Page 2: Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation

scattering path Fe-Fe-Fe at room temperature and above couldbe unambiguously attributed to the large Debye-Waller factorsat these temperatures and not to a misalignment of the chains.13e,f

Supramolecular interactions between linear chains involving thenitrate anion may be the source of cooperative processes duringthe SCO in [Fe(NH2trz)3](NO3)2 but, up to now, no experimentalevidence to this claim could be given. This contrasts with[Fe(Htrz)2(trz)](BF4), which displays a hysteretic ST of 40 Kwidth, for which direct hydrogen bonding between chainsthrough the non coordinated anion (B-F‚‚‚H) was deduced fromWAXS experiments.17 The planar geometry of the anion wasinitially thought to be relevant for maintaining importantcooperative effects in these materials. Iron(II) NH2trz chaincomplexes prepared with planar anions such as tosylate deriva-tives20,21or naphtalene sulfonate derivatives22 nevertheless didnot reveal a ST associated with a wide hysteresis loop. [Fe-(NH2trz)3](NO3)2 thus remains an interesting compound forinvestigation to elucidate the origin of its intriguing ST behavior.

Although the ST of iron(II) coordination compounds has beenextensively investigated using a wide range of physical tech-niques, the use of muon spin relaxation (µSR) in the study ofthese switchable materials has developed more recently. Pre-liminary experiments of Nagamine and co-workers23 on[Fe(phen)2(NCS)2] have been followed by comprehensiveµSRstudies of a few other iron(II) compounds, whose SCO occursbelow room temperature.24-27 In these coordination compounds,the implanted spin-polarized positive muonsµ+ act as probesof the local magnetization,28-30 providing information aboutmagnetic fluctuations and spin dynamics around a broad timewindow of ∼10-9 to 10-5 s. TheµSR technique could thusnicely complement other physical methods that allow differentcharacteristic measurement time windows such as, for instance,bulk magnetic susceptibility measurements and57Fe Mossbauerspectroscopy.28 Our initial µSR investigations have enabled usto track two types of complete ST being either gradual24 orabrupt.26 This was achieved by following the initial asymmetryparameter,a0, and the relaxation rate constants in zero field(ZF), as a function of temperature. The longitudinal-field (LF)dependence ofa0 at selected temperatures allowed us to proposea comprehensive mechanism associated to this evolution involv-ing diamagnetic and paramagnetic muonic species.24,26

Our µSR investigations were performed on selected iron(II)spin crossover compounds for which the muon location couldbe assessed.24,26Here we consider the zero-field (ZF) and appliedfield µSR measurements carried out on [Fe(NH2trz)3](NO3)2

over the temperature range 8-402 K and field range of 20-2000 Oe. This study gives us the unique opportunity toinvestigate theµSR behavior of this compound within thebistability region that is situated slightly above room temper-ature. It also gives us the possibility to probe its lattice dynamics

through muon implantation and to obtain useful insights to itssupramolecular organization. The initial set of ZFµSR measure-ments on this bistable compound and their preliminary analyseshave recently been communicated.31

2. Experimental Section

[Fe(NH2trz)3](NO3)2 was synthesized as a pink powder usinga procedure described elsewhere10 starting with FeSO4‚7H2O,Ba(NO3)2, and NH2trz, all commercially available. TheµSRmeasurements were performed on the MuSR and DEVAspectrometers at the ISIS pulsed muon source, RutherfordAppleton Laboratory, U.K. The powdered sample was insertedinto an aluminum mount and covered with a Mylar film window.The holder was masked from the muon beam by a silver plate.The muons with energy∼3.2 MeV and implantation ranges ofthe order of 100 mg cm-2 thermalize within the sample on timescales much shorter than the spin relaxation times. The muonsenter the sample with their polarization antiparallel to the beamdirection, chosen as thez direction, and decay with a meanlifetime of 2.2 µs, emitting positrons preferentially in thedirection of the muon spin; the positrons are then detected byplastic scintillation detectors that surround the sample. Theasymmetry is described by the following function witha0 beingthe initial asymmetry,Gz(t) the depolarization function,NF andNB, the number of the decay positrons detected by the forwardand backward counters, respectively, andR an experimentalcalibration constant that is dependent on sample position anddetector efficiencies that was determined from 20 Oe transverse-field (TF) experiments:

TheµSR data were recorded in longitudinal geometry over thetemperature range 8-402 K using either a closed-cycle refrig-erator (on MuSR) or a He flow cryostat (on DEVA). Both MuSRand DEVA spectrometers allow fields as high as 2000 Oe tobe attained. TheµSR data were analyzed using the WiMDAsoftware.32

Mossbauer spectroscopy was carried out in transmissiongeometry using a conventional Wissel spectrometer equippedwith a 57Co(Rh) radioactive source (Amersham, U.K.) operatingat room temperature and a NaI scintillation detector. The pinkpowdered sample was inserted in a Plexiglas sample holder ofknown dimensions and sealed with Parafilm. The spectrum wasfitted to the sum of Lorentzians by a least-squares refinementusing Recoil 1.05 Mo¨ssbauer Analysis Software.33 The isomershifts are given with respect to center of anR-Fe calibrationsample at room temperature.

3. Results

3.1. Zero-Field µSR. Zero-field µSR spectra of [Fe(NH2-trz)3](NO3)2 have been recorded on the DEVA spectrometer,first on warming from 200 to 402 K, and second on cooling to8 K. The reproducibility of our ZF measurements could besuccessfully checked with measurements performed 10 monthsearlier on the MuSR spectrometer by warming and cooling thesame sample following the thermal sequence: 300 Kf 392 Kf 312 K. Two characteristic ZF relaxation spectra are shownin Figure 2. At 8 K, where the sample is in the LS state, theasymmetry parameter first slowly decays in the 4µs range, andthen is approximately constant. At 402 K, where the sample isin the HS state, a monotonic relaxation behavior is observed.The overall change in the relaxation behavior between these

Figure 1. A schematic side view of the polymeric chain in [Fe(NH2-trz)3](NO3)2 as deduced from X-ray absorption studies.12,13a

a(t) ) a0Gz(t) )NF(t) - RNB(t)

NF(t) + RNB(t)(1)

11112 J. Phys. Chem. B, Vol. 111, No. 38, 2007 Garcia et al.

Page 3: Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation

two regimes has been tracked by fitting the decay curves witha stretched exponential:

a0 represents the relaxing amplitude,λ is the decay constant,andabg stands for the background amplitude, which was allowedto vary during the fit procedure. The temperature dependence(8-402 K) of theâ parameter is shown in the inset to Figure2. A Lorentzian-type relaxation behavior (â ∼ 1) is observedon warming the sample from∼270 to∼310 K, above which atransition to a predominant Gaussian relaxation mode (â ∼ 1.6)is observed from∼351 to ∼402 K. On cooling, the majorGaussian relaxation behavior is maintained down to∼312 K,with the predominant Lorentzian-type relaxation mode recoveredat lower temperatures. A hysteretic behavior is thus observedwith the “transition temperatures” differing on warming andcooling asT1/2

v ) 345 K andT1/2V ) 310 K. This initial analysis

therefore allowed us to clearly delineate the hysteresis regionand to identify two relaxation modes. Given the spread ofâvalues, fits were then carried out with a Kubo-Toyabefunction,34 but our ZF data could only be described aboveT g380 K with this function.

The final comprehensive fits to the ZFµSR spectra (∼8-402 K) were obtained using a Lorentzian and a Gaussianfunction:

whereaf andas represent the amplitudes of the asymmetry offast and slow components, respectively, andλf andσs are thedecay constants associated with the fast and slow components,respectively.abg represents the background amplitude that wasfixed at 3.58(8)% over the entire temperature range of investiga-tion, after having been determined by fitting the depolarizationcurve at 402 K, a temperature that is well above the spin statecrossover region (310-350 K). As shown in Figure 3, twodifferent relaxation type behaviors can be found within thebistability domain of [Fe(NH2trz)3](NO3)2 depending on whetherthis region is approached on warming (at 321 K, LS state) oron cooling (at 322 K, HS state). The thermal history of thissample is thus of particular importance and its bistability domainclearly evidenced by ZF-µSR.

As shown in Figure 4a,31 the initial asymmetry parameter,a0, increases in an essentially linear manner from∼13% at 8 Kto ∼17% at 331 K, with a sharp increase toa0 ∼ 21% (whichcorresponds on the DEVA spectrometer to 100% muon polar-ization), forT g 361 K. It is noted that the same slope for∆a0/∆T is obtained above the transition (∼361-402 K) as belowthe transition (∼8-331 K). On cooling,a0 slowly decreases tothe value of∼20.5% at 320 K, before sharply decreasing to∼16%, thus delineating a well-defined hysteresis loop. A lineardecrease ofa0 is observed forT e 300 K, again matching theslope ∆a0/∆T obtained in the heating mode. Two transitiontemperatures for half of the HS/LS state population differingon warming and cooling can therefore be evaluated, withT1/2

v

Figure 2. Zero-field µSR relaxation curves for [Fe(NH2trz)3](NO3)2

in the LS region at 8 K (b) and in the HS region at 402 K (O). Thelines represent fits to the data as explained in the text. The inset showsthe hysteresis loop delineated by the temperature dependence of theâparameter on warming (2) and on cooling (3) (see text).

a(t) ) a0e-(λt)â

+ abg (2)

a(t) ) afe-λft + ase

(-σs2t2) + abg (3)

Figure 3. Zero-field µSR relaxation curves for [Fe(NH2trz)3](NO3)2

in which the hysteretic behavior is revealed by comparison of the curvesobtained at 321 K on heating (b) and at 322 K on cooling (O). Thelines represent fits to the data as explained in the text.

Figure 4. (a) Dependence of the initial asymmetry parameter,a0, inthe heating (2) and cooling (3) modes for [Fe(NH2trz)3](NO3)2. Thesolid lines act as guides to the eye.31 The inset shows a comparisonbetween the temperature dependences ofµeff (b, right scale, adaptedfrom ref 10) anda0 on heating (2) and on cooling (3) (left scale) overthe temperature range∼80-380 K. (b) Temperature dependence ofthe fast relaxation rate constant,λf, in the heating (2) and cooling (3)modes for [Fe(NH2trz)3](NO3)2. The inset shows the temperaturedependence of the slow relaxation rate constant,σs, over the temperaturerange∼8-402 K. The dotted line at 300 K demonstrates the shift ofthe hysteresis loop when comparing (a) and (b).

Dynamics and Organization of [Fe(NH2trz)3](NO3)2 J. Phys. Chem. B, Vol. 111, No. 38, 200711113

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∼ 346 K andT1/2V ∼ 312 K. Figure 4b shows the temperature

dependence of the fast relaxation decay constant,λf, on bothwarming and cooling the sample from∼8-402 K. λf firstdecreases linearly fromλf ∼ 4.3 µs-1 at 8 K to λf ∼ 1.6 µs-1

at 300 K, before decreasing toλf ∼ 0 aboveT1/2v ∼ 313 K on

further warming. On cooling from 402 K,λf remains close tozero before increasing atT1/2

V ∼ 275 K, whereas below∼200K, the λf data are consistent with theλf values obtained onwarming. As is evident in Figure 4b, comparison of theλf dataon warming and cooling reveals clear hysteretic behavior, inaccord with the hysteresis identified from the analysis by thestretched exponential (inset to Figure 2) and the temperaturedependence ofa0 (Figure 4a). Interestingly, this hysteresis loopis slightly shifted downward and thus exactly centered on theroom-temperature region (dotted line in Figure 4). The tem-perature dependence of the slow relaxation rate constant,σs,did not, however, reveal any hysteretic effect (inset to Figure4b). Indeed,σs first decreases linearly from∼8 to ∼280 K,above whichσs remains approximately constant aroundσs ∼0.24 µs-1.

3.2. Applied Field µSR. Figure 5 shows the ZF and 20 Oetransverse field (TF) relaxation curves obtained at 8 and 402K, in the LS and HS state, respectively. The TFµSR spectrawere fitted best using the following asymmetry function:

whereω represents the precession frequency andæ the phase.The precession frequency in the LS and HS state is found equal

to ω ) 137 MHz T-1, this behavior being characteristic ofdiamagnetic muons (ω ) 135.5 MHz T-1).30 Comparison ofthese curves with the ZF relaxation spectra reveal approximatelythe same initial asymmetry (∼21% at 402 K,∼10% at 8 K)compared to the ZF values (see Figure 2), and similar relaxationrates (σs ∼ 0.06(1)µs-1 at 402 K;σs ∼ 0.09(1)µs-1 at 8 K)compared to the ZF values (σs ∼ 0.23(1)µs-1 at 402 K;σs ∼0.48(1)µs-1 at 8 K).

Longitudinal fieldµSR relaxation spectra were recorded infields of 50-2000 Oe at selected temperatures over the range8-402 K. Representative examples of LF-µSR spectra at 8 and402 K are shown in Figure 6. At 8 K, repolarization is reachedover 2000 Oe, which contrasts to the situation at 402 K wherea field of only 50 Oe is necessary to repolarize the muonsensemble. These spectra were successfully fitted to eq 3 exceptfor the data recorded in the high-temperature regime (380-402 K). A dynamic KT function that is suitable for the presenceof fluctuating nuclear spins was used but without success; thisis presumably due to the presence of a remaining electroniccontribution at these high temperatures.35 A Keren analyticalapproximation of the Kubo-Toyabe function that is more suitedfor intermediate spin dynamics was thus applied (eq 5) to fitthese spectra considering the muon spin polarizationP(t).36

Figure 5. Comparison between the ZF (O) and TF-20 Oe relaxationcurves (b) at 402 K (upper) and 8 K (lower) for [Fe(NH2trz)3](NO3)2.The lines represent fits to the data as explained in the text.

a(t) ) afe-λft cos(ωt + æf) + ase

(-σs2t2) cos(ωt +

æs) + abg (4)

Figure 6. (a) ZF (b) and selected LFµSR (O) relaxation curves for[Fe(NH2trz)3](NO3)2 at 8 K. (b) ZF (b) and LF-50 Oe (O) for [Fe-(NH2trz)3](NO3)2 at 402 K. The lines represent fits to the data asexplained in the text.

P(t) ) exp(-Γ(t)t)

Γ(t)t )2∆2

{[ωL2 + ν2]νt + [ωL

2 - ν2][1 - e-νt cos(ωLt)] - 2νωLe-νt sin(ωLt)}

(ωL2 + ν2)2

(5)

11114 J. Phys. Chem. B, Vol. 111, No. 38, 2007 Garcia et al.

Page 5: Dynamics and Supramolecular Organization of the 1D Spin Transition Polymeric Chain Compound [Fe(NH 2 trz) 3 ](NO 3 ) 2 . Muon Spin Relaxation

This expression depends on the width of the internal field

distribution at the muon site,∆ ) γµx⟨∆h2⟩, whereγµ is the

muon gyromagnetic ratio (2π × 13.55 kHz/G) andx⟨∆h2⟩ isthe mean square amplitude of field fluctuations sensed bymuons. Equation 5 also depends on the internal field fluctuationfrequency,ν, and on the nuclear Larmor frequency in LF,ωL

) γµB. The use of this expression is, however, only valid when

ν g γµx⟨∆h2⟩. Becauseν ∼ 0.4 µs-1 at 402 K, the ap-

proximation is valid whenx⟨∆h2⟩ e 5 Oe, a situation that was

met because we findx⟨∆h2⟩ ) 2.6 Oe in the whole HStemperature domain.

The final fits were obtained with a modified Keren functiontaking into account a fractionp of muons that feel electronicdynamics with 0< p < 1 (eq 6).37

The fit to the data at 402 K and 2000 Oe givesλ ) 0.226µs-1

and p ∼ 0.2, which corresponds to 20% of muons sensingelectronic spins.

An Arrhenius plot of the fluctuation rate ofν for [Fe(NH2-trz)3](NO3)2 in the HS state on cooling from 402 K is presentedin Figure 7. It decreases linearly from∼0.4 µs-1 at 402 K to0.3 µs-1 at 341 K. The fit to the data provided an activationenergy of∼5 kJ mol-1, which is comparable to the reorientationof the cyclopentadiene ring of ferrocene (EA ∼ 5.4 kJ mol-1).38

The set of LF-µSR measurements of the initial asymmetryparameter shown in Figure 8 was fitted to the isotropicrepolarization function given by eqs 7 and 8.

where

adia and apara represent the diamagnetic and paramagneticcontributions, respectively, in an applied field28 and γe ) 28GHz T-1. The hyperfine coupling constantA was not fixedduring the fit and assumed to be temperature independent.Analysis of the recorded repolarization data over the temperature

range∼8-311 K, resulted in a value ofA ) 753 ( 77 MHz.The shape of the repolarization curves at 8 K, obtained onconnecting the data points in Figure 8, indicates a slight increasefor low fields (H e 50 Oe) whereas a tendency toward saturationin a0 is observed at higher fields,H g 1000 Oe. The samebehavior is observed at higher temperatures with, however,constant values for low fields (H e 50 Oe). This behavior ofthe initial asymmetrya0(H) provides evidence for the formationof a paramagnetic state such as muonium Mu‚ ) µ+e-, ananalogue of the hydrogen atom. At 402 K,a0 remains at aconstant value forH e 200 Oe, before increasing slightly athigher fields. The presence of a fraction of paramagneticmuonium is not clearly revealed because of its very highrelaxation rate. At very high fields,H g 1000 Oe, anyparamagnetic muonium present would be re-polarized when theLarmor frequency of the electron exceeds the hyperfine couplingof an electron and a muon. Clear hysteretic behavior of thetransition curve is evidenced at 311 K in Figure 8 with adifferent repolarization curve obtained depending on the thermalhistory of the sample.

3.3. Mo1ssbauer Spectroscopy.A careful inspection of themagnetic properties recorded at room temperature for [Fe(NH2-trz)3](NO3)2 (inset to Figure 4)10 shows a slight increase of theslope∆µeff/∆T that could indicate the presence of a few HSspecies. A57Fe Mossbauer spectrum was thus recorded at 300K to probe the spin state population. It was found to be bestfitted with two quadrupole doublets of nonequivalent intensities(Figure 9). The first signal with isomer shiftδ ) 0.44(1)mm‚s-1, quadrupole splitting∆EQ ) 0.17(2) mm‚s-1, and widthat half-maximumΓ ) 0.26(2) mm‚s-1 corresponds to LS iron-(II) ions. The splitting of the lines is attributed to the latticecontribution to the quadrupole splitting, indicating a distortedenvironment, as expected within the chain for the LS iron(II)octahedra. A second signal withδ ) 1.15(6) mm‚s-1, ∆EQ )2.9(1) mm‚s-1, andΓ ) 0.7(1) mm‚s-1, which corresponds toa population of around 6%, corresponds to the presence of HSiron(II) ions with a larger quadrupole splitting taking intoaccount both the lattice and valence contributions to the electricfield gradient.39 It is interesting to note that a previouslypublished Mo¨ssbauer spectrum of [Fe(NH2trz)3](NO3)2 at roomtemperature showed an identical isomer shift (δLS ) 0.44(1)mm‚s-1) to the predominant resonance in the spectrum but withΓ ) 0.42(2) mm‚s-1.10 The broadening of the lines noted inthis earlier study10 was attributed to the presence of an

Figure 7. Arrhenius plot of the fluctuation rate of the internal field,ν, for [Fe(NH2trz)3](NO3)2. The dotted line represents a fit leading toEA ∼ 5 kJ/mol as discussed in the text (note the point at 320 K (O),where LS ions appear on cooling from 402 K (see Figure 4a), has beenneglected in this fit).

a(t) ) arel exp(-Γ(t)t)[pe-λt + (1 - p)] + abg (6)

a0(B) ) adia + apara(1 + xx + 1) (7)

x ) (γe - γµ)BA

(8)

Figure 8. Dependence of the initial asymmetry parameter,a0, of[Fe(NH2trz)3](NO3)2 on magnetic field,H, up to 2000 Oe at thetemperatures indicated [b, T ) 8 K; O, T ) 200 K; 2, T ) 311 K onwarming from 8 K;3, T ) 311 K on cooling from 402 K;9, T ) 402K]. The dotted lines act as guides to the eye.

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unresolved quadrupole splitting that is now clearly detected inour spectrum (Figure 9). Thus, on this basis, HS species shouldbe considered to be present at room temperature, although onlyto small extent.

4. Discussion

The thermally induced hysteretic transition in the spin statefrom LS (S ) 0) to HS (S ) 2) in [Fe(NH2trz)3](NO3)2 hasbeen delineated by the variation of the ZF initial asymmetryand theâ parameter of eq 1. The transition temperatures derivedfrom thea0 variation (T1/2

v ∼ 346 K andT1/2V ∼ 312 K; Figure

4a) agree well with the transition temperatures derived frommagnetic measurements (T1/2

v ∼ 342 K andT1/2V ∼ 310 K),10

as shown in the inset to Figure 4a. This agreement betweenthese two techniques of wide disparity in characteristic measur-ing time implies that no LST HS relaxation effects40 shouldbe considered for the Fe spins. The transition temperaturesderived from the temperature dependence of the fast relaxationconstant (T1/2

v ∼ 313 K andT1/2V ∼ 275 K; Figure 4b) are found

to be lower than expected from the ST behavior ofa0 and themagnetic measurements. This precursor effect thus does notstrictly correspond to the thermal ST itself becauseλf ∼ 0 at342 K (where 50% of HS ions are present as evidenced byµeff

in the inset to Figure 4a) but may indicate the presence of asmall fraction of spins having, on average, a lower transitiontemperature. This is consistent with the slight increase ofµeff

centered above∼ 300 K as was noted above. Such a decreaseof the fast relaxation constant before the LSf HS transitiontakes place was also noticed for theµSR study of themononuclear compound [Fe(ptz)6](ClO4)2.24b

Several muonic species could be identified on the basis ofZF and applied fieldµSR measurements. Indeed, the linearvariation of the initial asymmetry parameter over the temperatureranges∼8-330 K on warming and∼402-320 K on coolingfrom ∼13% to ∼21% (Figure 4a) can be attributed to thepresence of diamagnetic muons as a result of thermalization.As discussed above, comparison of the TF (20 Oe) and ZFmeasurements demonstrates that diamagnetic muons are actuallypresent over the entire temperature range∼8-402 K. Interest-ingly, diamagnetic muons species were also identified in [Fe-(phen)2(NCS)2] (as polymorph I), but their variation, as reflectedby the slow component of the initial asymmetry, was found toincrease by only 1% between 10 K (LS state region) and 280K (HS state region).24 The present behavior for [Fe(NH2trz)3]-

(NO3)2 may be attributed to the fact that the slow and fastcomponents could not be separated, and as a result, only anaverage estimation of the evolution of diamagnetic muons hasbeen obtained. Diamagnetic muon species are expected to belocated in regions of large electron density30b and, as such,location on negative charge groups such as the lone pair of theamino function of the NH2trz ligand is possible (Figure 1).However, this position is not considered likely as the doublypositively charged metallic chains are expected to repel positivemuons; this could in turn result in the vicinity of noncoordinatednitrate anions, providing a suitable negative trap for the muons.The presence of “free” interstitial diamagnetic muon species inthe lattice can also not be excluded fully. Given that therelaxation rate constants for [Fe(NH2trz)3](NO3)2 recorded inthe ST regime from 270 to 402 K (σs ∼ 0.24µs-1 andλf

max ∼1.5µs-1) (and also for [Fe(ptz)6](ClO4)2; λs

max ∼ 0.15µs-1 andλf

max ∼ 2 µs-1 (ref 24b)) are significantly lower than therelaxation constant (λf ∼ 4.5µs-1) obtained in the HS state for[Fe(phen)2(NCS)2], which does not contain any noncoordinatedspecies;26 this indicates that the diamagnetic muon species arelocated far from the HS spins (Table 1).

The observation (Figure 4a) that on increasing the temperaturefrom 8 to 402 K,a0 increases from∼13% to 21% (correspond-ing to 100% muon polarization on the DEVA spectrometer),supports the presence of a large fraction of paramagnetic muons.As in the cases of [Fe(ptz)6](ClO4)2

24 and [Fe(phen)2(NCS)2],26

which contain aromatic coordinated ligands, muonium radicalspecies are expected to form on the 1,2,4-triazole ligands of[Fe(NH2trz)3](NO3)2. Indeed, the hyperfine coupling constantof [Fe(NH2trz)3](NO3)2 (753( 77 MHz) is consistent with thevalues found for [Fe(ptz)6](ClO4)2 (540 ( 15 MHz)24 and[Fe(phen)2(NCS)2] (∼500 MHz)26 (Table 1). Two locations canbe considered for the muon on the carbon or nitrogen (N1 orN2) atoms of the triazole, because it would preferably be locatedon double bonds.30 The bonding of muonium to these nitrogenatoms would require the muon to be close to the Fe ion, whichis unlikely for steric reasons. Also, a decrease of the crystalfield strength is expected, which should stabilize the HS state,but such an effect is not observed. Rather, a clear correlationbetween the magnetic data and the thermal variation of theµSRinitial asymmetry parameter in zero field was observed. Thefollowing configuration with an unpaired electron delocalizedon three atoms is thus proposed:

Muonium could also be present as the free interstitial speciesor located on the unsaturated bonds of the uncoordinated nitrateanions,45 although it is more likely that a muon in this locationforms a neutral diamagnetic species such as MuNO3.

As described above, the ZF-µSR spectra are best describedby a fast Lorentzian and a slow Gaussian function over the entiretemperature range (8-402 K) with these two components likelyto account for two possible muon sites. In the predominant LSregime (8-300 K), both paramagnetic muonium/radicals anddiamagnetic muons interact with local nuclear fields althoughonly the diamagnetic muons will give the Gaussian relaxationwith a measurable relaxation rate.28 Paramagnetic species willusually be depolarized completely in ZF or drop to one-sixthof the initial asymmetry instead of one-half (this former valuecorresponds to one-third of a KT tail times one-half of the low-

Figure 9. 57Fe Mossbauer spectrum of [Fe(NH2trz)3](NO3)2 recordedat room temperature. The subspectral fits are as described in the text.

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field muonium polarization). Indeed, because most of iron(II)ions are in theS ) 0 state in [Fe(NH2trz)3](NO3)2 at T e 300K, as deduced from Mo¨ssbauer spectroscopy, the relaxationspectra would be expected to be fitted with a major Gaussianfunction, appropriate for a distribution of static nuclear dipolarfields. However, the relaxation spectra were best fitted with asum of a Gaussian function (originating from the diamagneticmuons) and a Lorentzian function with a lower amplitude andslightly faster relaxation (originating from the paramagneticspecies, with relaxation due to fluctuations or occasional HSspins). This apparent contradiction was also found for [Fe(ptz)6]-(ClO4)2 and [Fe(phen)2(NCS)2], with both compounds exhibitinga complete ST in the LS state.24,26 In the case of [Fe(PM-X)2-(NCS)2] (X ) PEA, AZA), this behavior was explained by thepresence of a residual fraction of HS spins that correspondinglyreveal dynamic behavior.25 In the present case, the diffusion ofmuons as well as interactions with other charge carriers,although unlikely in the LS state,46 could also be considered toaccount for this behavior. Indeed, in the event of diffusion, manymuons would briefly encounter the few remaining HS Fe(II)ions, leading to slow Lorentzian relaxation, rather than themajority of muons sensing the LS spins with only a few muonsbeing next to the HS ions and strongly relaxed. The possibilityof muon trapping could also be considered. If the muons candiffuse, they may find defect locations (near the ends of Fe-NH2trz chains, for instance) and stop there. These locations mayalso be likely places for Fe spins to remain in the HS state whenthe bulk of the sample is in the LS state (and vice versa).

Once all iron(II) ions have warmed through the transitionand are entirely in the HS state (S) 2), on cooling through thetemperature range∼402-312 K (Figure 4a) the depolarizationcurves are best described by a major Gaussian relaxationcomponent and a minor Lorentzian function. The HS spins ofthe iron(II) ions couple very strongly to the electron spin in theradical and fluctuate so rapidly that the muon spin cannot followthem. The iron spins must interact magnetically with theirneighbors, or with phonons via crystal field interactions, to havesuch a high fluctuation rate. A Gaussian relaxation, mostly dueto the dipolar interaction with static nuclear moments, istherefore observed. This situation, where the muonium radicalsare totally decoupled, implies that the dynamics of the fast-fluctuating electronic spins fall outside the muon time scale.35

This hypothesis is consistent with the fact that a longitudinalapplied field as small as 50 Oe, is found to decouple the muonensemble (Figure 6b), as expected for relaxation due to staticdistribution of nuclear magnetic moments.35 Because theµSRrelaxation curves in the HS regime could not be fitted solely interms of a Gaussian relaxation, a fraction of muonium radicalsor muons experiencing the electronic species was also consid-ered by the use of a modified Keren function (eq 6). These“diamagnetic like” muonic species could presumably be im-

planted in the neighboring defects, on uncoordinated anions ora few iron(II) chain ends.

The overall relaxation behavior describes iron(II) spindynamics in [Fe(NH2trz)3](NO3)2, which are faster than thoseoccurring in the mononuclear compounds [Fe(ptz)6](ClO4)2 and[Fe(phen)2(NCS)2], for which the relaxation curves in the HSand LS states were described by Lorentzian lines.24,26 AGaussian relaxation behavior in the HS state was, however, alsoencountered for the ST compound [Fe(PM-PEA)2(NCS)2] (PM-PEA) N-(2′-pyridylmethylene)-4-(phenylethynyl)anilino)25 andfor the oxide LaCoO3 in the intermediate spin state (S ) 1),47

although in these two cases no muonium species were identified.Two reasons can be mentioned to account for the fast dynamicsin the coordination polymer [Fe(NH2trz)3](NO3)2. First, the SToccurs around∼320 K, which is significantly higher (∼170 K)than for [Fe(ptz)6](ClO4)2.24 Second, as a result of its polymericnature, the iron(II) ions in [Fe(NH2trz)3](NO3)2 are separatedby ∼3.5 Å13 and are thus much closer to each other than in thecase of mononuclear compounds (Table 1). This fast dynamicbehavior indicates that muonium radical species are very closeto iron(II) ions (e.g., on the triazole rings or on nearby interstitialsites to the chains). As a result the electron spin of the radicalis strongly coupled to the iron spin, and fluctuates rapidly,effectively decoupling the muon itself from the magnetic spinfluctuations.

The fluctuation of the magnetic field distribution arising fromnuclear spins in the HS state is likely to originate from molecularmotions or from nuclei having a short relaxation timeT1 causedby the iron spins, by a mechanism similar to that for the muonitself. These dynamics are thermally activated, but the deducedactivation energy (EA ∼ 5 kJ/mol; Figure 7) is too weak toaccount for a free rotation of the amino group of the 1,2,4-triazole or tumbling of the noncoordinated nitrate anion. Thoughthe amino group may not be able to rotate, the muonated (andcharged) NH2Mu+ group is more symmetrical and might havea lower barrier, even though its presence is not likely as stressedabove for repulsions reasons. Hydrogen bonding to NO3

- isvery likely, thus increasing the barrier in either case.

These molecular motions may be hindered as a result ofhydrogen bonding involving these species in the network, thusproviding a signature of interchain interactions for [Fe(NH2-trz)3](NO3)2. This supramolecular feature provides a 3D char-acter for this iron(II) spin crossover compound and, in combi-nation with Coulombic anion-cation interactions promoted bythe expected weak interchain distance,48,49may account for theobserved large hysteresis width (∼35 K) of its spin transition.This behavior contrasts with the ST behavior of other chainsof the [Fe(4R-1,2,4-triazole)3](anion)2 family6 whose structureis presumably of lower dimensionality.

TABLE 1: Information on Fe ‚‚‚Fe Distances, Transition Temperatures and RelevantµSR Parameters of a Series of Iron(II)Spin Crossover Complexes

[Fe(ptz)6](ClO4)2

(ref 24)[Fe(phen)2(NCS)2]

(ref 25)[Fe(NH2trz)3](NO3)2

(this work)[Fe(PM-PeA)2(NCS)2]

(ref 25)[Fe(PM-AzA)2(NCS)2]

(ref 25)

Fe‚‚‚Fe (Å) 10.8 (HS)41 8.31 (HS);8.13 (LS)42

∼3.513 8.49 (HS);8.73 (LS)43

8.60 (HS);8.39 (LS)44

T1/2 (magnetic, K) 150 177(1) 342v ; 310V 10 231v ; 194V 43 192v ; 186V 44

T1/2 (ao, K) ∼135 178(2) 346v ; 312VA (MHz) 540(15) ∼500 753(77)slow relaxation rate HS (µs-1) ∼0.06 0.25(3) 0.23(1) 0.047a

slow relaxation rate LS (µs-1) ∼0.15 0.25(3) 0.48(1) 0.047a

fast relaxation rate HS (µs-1) ∼2 4.53a ∼0 ∼0fast relaxation rate LS (µs-1) ∼0.7 4.53a 4.3 ∼1

a Fixed value.

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5. Conclusions

The hysteretic spin transition in [Fe(NH2trz)3](NO3)2 (T1/2v

) 346 K; T1/2V ) 312 K) has been followed byµSR measure-

ments for the first time in a linear chain compound, leading tonovel insight to the dynamical and structural properties of thistype of compound. The ZF relaxation curves over the temper-ature range∼8-402 K are found to be described well byGaussian and Lorentzian components. Some diamagnetic muonsexist over the entire temperature range, as evidenced by thelinear increase of the initial asymmetry parameter below andabove the spin transition (Figure 4a) and by low transverse fieldexperiments. On the basis of our earlierµSR studies on themononuclear complexes of [Fe(ptz)6](ClO4)2

24 and [Fe(phen)2-(NCS)2],26 diamagnetic-like and paramagnetic muonium sub-stituted radicals are expected to be present on the 1,2,4-triazolering of [Fe(NH2trz)3](NO3)2. Muons are expected to be locatedfar away from the iron spins, being either present on the nitrateanions (forming a diamagnetic neutral molecule) or close tothe Fe spins on 1,2,4-triazoles (as radicals), although locationof the muons on interstitial sites cannot be excluded fully. Fastdynamics implying an almost nuclear relaxation (likely tooriginate from molecular motions or from nuclei having a shortrelaxation time) have been revealed in the HS regime of [Fe-(NH2trz)3](NO3)2. Finally, supramolecular interactions between1D chains have been sensed through muon implantation andwere proposed to explain the large hysteresis effect associatedwith the spin transition of intriguing physical properties,particularly its thermochromic behavior.

Acknowledgment. We acknowledge support from theEuropean Union under Framework 5 for access to the MUSRand DEVA instruments at the ISIS Facility, Rutherford AppletonLaboratory, U.K, and support from the European Commissionfor granting Contract No. ERB-FMRX-CT98-0199EEC/TMR.We also thank the Fonds Special de Recherche of the Universite´Catholique de Louvain, the Fonds National de la RechercheScientifique (Cre´dit aux chercheurs), the IAP-VI (P6/17)program, the Deutsche Forschungsgemeinschaft (Priority Pro-gram 1137), the Fonds der Chemischen Industrie and theMaterialwissenschaftliches Forschungszentrum of the Universityof Mainz for financial help. S.J.C. acknowledges support fromthe Access to Major Research Facilities Program, AustralianNuclear Science and Technology Organization. The Fonds pourla Formation a` la Recherche dans l’Industrie et dans l’Agricultureis also thanked for a doctoral scholarship allocated to Y.B. Wethank J. Ladrie`re for providing access to a radioactive source.

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