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Structural characterization of NaMgH 2 F and NaMgH 3 A. Bouamrane a , J.P. Laval b, *, J.-P. Soulie a , J.P. Bastide a a Laboratoire de Thermodynamique Applique ´e, Institut National des Sciences Applique ´es de Lyon, Ba ˆt 401, 20 Av. Albert Einstein, 69621 Villeurbanne Cedex, France b Science des Proce ´de ´s Ce ´ramiques et de Traitements de surface, UMR-CNRS 6638, 123 Av. A. Thomas, 87060 Limoges Cedex, France (Refereed) Received 30 April 1999; accepted 2 June 1999 Abstract The direct reaction of hydrogen on a mixture of Na 1 Mg or NaF 1 Mg allowed for synthesis of NaMgH 3 and NaMgH 2 F, respectively. Both phases were indexed with an orthorhombic unit cell with dimensions a 5 546.34, b 5 770.30, c 5 541.08 pm for NaMgH 3 and a 5 547.59, b 5 769.68, c 5 540.31 pm for NaMgH 2 F. The crystal structures were refined by X-ray Rietveld refinement. They derive from the perovskite structure type by a Pmm 3 Pnma distortion. In the hydridofluoride, H 2 and F 2 anions are equally distributed in the two anionic sites in a disordered way. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; A. Hydrides; C. X-ray diffraction; D. Crystal structure 1. Introduction This work is a part of a general investigation of complex hydrides of light metals, synthesis routes, structural characterization, and physical properties, focusing on thermody- namic, electric, and magnetic properties. The KMgH 32n F n series with n 5 0, 1, 2, 3 has been studied [1– 4] from a thermodynamic and structural point of view. Recently [5], standard enthalpies of formation of the NaMgH 32n F n series with n 5 0, 1, 2 were calculated by * Corresponding author. Fax: 133-5-55-45-72-70. E-mail address: [email protected] (J.P. Laval). Pergamon Materials Research Bulletin 35 (2000) 545–549 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0025-5408(00)00249-X

Structural characterization of NaMgH2F and NaMgH3

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Page 1: Structural characterization of NaMgH2F and NaMgH3

Structural characterization of NaMgH2F and NaMgH3A. Bouamranea, J.P. Lavalb,*, J.-P. Souliea, J.P. Bastidea

aLaboratoire de Thermodynamique Applique´e, Institut National des Sciences Applique´es de Lyon, Baˆt 401,20 Av. Albert Einstein, 69621 Villeurbanne Cedex, France

bScience des Proce´des Ceramiques et de Traitements de surface, UMR-CNRS 6638, 123 Av. A. Thomas,87060 Limoges Cedex, France

(Refereed)Received 30 April 1999; accepted 2 June 1999

Abstract

The direct reaction of hydrogen on a mixture of Na1 Mg or NaF1 Mg allowed for synthesisof NaMgH3 and NaMgH2F, respectively. Both phases were indexed with an orthorhombic unit cellwith dimensionsa 5 546.34,b 5 770.30,c 5 541.08 pm for NaMgH3 and a 5 547.59,b 5769.68, c 5 540.31 pm for NaMgH2F. The crystal structures were refined by X-ray Rietveldrefinement. They derive from the perovskite structure type by aPmm3 Pnmadistortion. In thehydridofluoride, H2 and F2 anions are equally distributed in the two anionic sites in a disorderedway. © 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Inorganic compounds; A. Hydrides; C. X-ray diffraction; D. Crystal structure

1. Introduction

This work is a part of a general investigation of complex hydrides of light metals,synthesis routes, structural characterization, and physical properties, focusing on thermody-namic, electric, and magnetic properties. The KMgH32nFn series with n5 0, 1, 2, 3 has beenstudied [1–4] from a thermodynamic and structural point of view. Recently [5], standardenthalpies of formation of the NaMgH32nFn series with n5 0, 1, 2 were calculated by

* Corresponding author. Fax:133-5-55-45-72-70.E-mail address:[email protected] (J.P. Laval).

Pergamon Materials Research Bulletin 35 (2000) 545–549

0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.PII: S0025-5408(00)00249-X

Page 2: Structural characterization of NaMgH2F and NaMgH3

reaction calorimetry. In the present paper, the Mg–H2–NaH andMg–H2–NaF systems areinvestigated and the crystal structure of the new phases is determined.

2. Experimental

2.1. Preparation

Because of the extremely high moisture sensitivity of the hydrides and hydridofluorides,all handlings were performed in a glove box under perfectly dried and deoxygenated argonatmosphere. The hydridofluoride NaMgH2F was obtained by reaction of an intimate mixtureof Mg and NaF (1:1 molar proportion) at 480°C for 24 h under 10 bar of hydrogen pressure.The hydride NaMgH3 was prepared under the same conditions by reaction of an equimolarmixture of Mg and NaH. The reaction products were recovered in a glove box and werewhite-gray powders.

2.2. Characterization

The hydrogen content in the synthesized phases was determined by the intermediate of aToepler pump [6]. A precise quantity of hydride or hydridofluoride was pyrolyzed inaccordance with the reaction MH2(s)3 M(s) 1 H2.

The released hydrogen was transferred from the pyrolysis reactor to a volume-calibratedchamber (at the same temperature) by the intermediate of a Toe¨pler pump. When thehydrogen volume was determined, the temperature and pressure were measured. The numberof moles of hydrogen was then deduced by considering hydrogen a perfect gas in ourexperimental conditions. The purity of the hydride was given by comparing the experimentalamount to the theoretical content of the initial mixture. The fluorine contained in thehydridofluoride was electrochemically determined using a F2 specific electrode.

The X-ray diffraction pattern of NaMgH2F revealed the presence of a pure phase, whichcan be indexed, in the orthorhombic system. Cell parameters were similar to those oforthorhombic perovskites such as CrNdO3 (space groupPnma). The pattern ofNaMgH3shows the presence of a similar phase. Main reflections correspond approximately toNaMgH2.72, the only Na–Mg hydride described in the ICSD database [42-1143] (space groupCmmm), but with slightly larger cell parameters (Table 1). However, many small peaksindicate a better correspondence with a CrNdO3- or NaMgH2F-type phase (space group

Table 1Cell parameters (pm) of NaMgH2F, NaMgH3, and the reference samples NaMgH2.72 and NaMgF3

NaMgHsF NaMgH3 NaMgH2.72

(ICSD 42-1143)NaMgF3

(ICSD 13-0303)

a 547.59 546.34 546.6 550.3b 769.68 770.30 384.5 767.6c 540.31 541.08 540.9 536.3

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Pnma). This situation implies a doubling of one cell parameter (384.5 pm3 770.3 pm). Thisdifference can be explained by

Y A disordering of the apparently nonstoichiometric NaMgH2.72phase, whose structure is notyet solved, compared with the stoichiometric NaMgH3 newly prepared phase.

Y An X-ray pattern of NaMgH2.72 which did not take the weak peaks into account andtherefore indexed on the basis of a sublattice.

However, NaMgH3 could not be obtained in a pure form. Small quantities of unreacted NaHand of MgO resulting from oxidation of Mg were always present, in spite of the precautionstaken. In order to fully characterize the hydride and the hydridofluoride, we decided to refinethe crystal structure of both phases.

3. Results and discussion

Despite several attempts, no single crystals of NaMgH3 and NaMgH2F could be prepared.Thus, the structure of both phases was determined by a Rietveld analysis of their X-ray

Fig. 1. Observed (1 1 1), calculated (——), and difference (bottom row) X-ray powder patterns for NaMgH2Fand NaMgH3. Vertical bars indicate the reflection positions for the nominal phases and for NaH and MgOimpurities (only for NaMgH3).

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Page 4: Structural characterization of NaMgH2F and NaMgH3

powder patterns. The X-ray patterns were measured on a D5000 diffractometer equippedwith a back-monochromator and using Cu Ka radiation.

The recording limits were 10–120° (2u) for NaMgH2F, 15–130° (2u) for NaMgH3 witha step of 0.04° (2u). The recording time was 17 s/step. The structures were refined using thestructural parameters of CrNdO3 as a basis. For NaMgH3, MgO and NaH impurities wererefined simultaneously to the nominal hydride. Both refinements quickly converge to rea-sonable RB, Rp and Rwp values in thePnmaspace group (RB 5 0.039 for NaMgH2F and0.023 for NaMgH3). For NaMgH2F, the distribution of H2 and F2 anions on the two anionicsites was refined. This shows that a full anionic disorder occurred in this hydridofluoride. TheF/H distribution in both anionic sites was 63–37% on the first site and 60–40% on the secondsite, which is in good agreement with the nominal formula (67–33%), within the limit ofaccuracy. Fig. 1 shows the comparison of the experimental and calculated X-ray pattern forboth phases and Table 2 summarizes the results of the structural study.

Table 2Atomic coordinates, isotropic temperature factor B (pm2 3 1024) (esd’s are given in parentheses) forNaMgH2F, NaMgH3, and NaMgF3

NaMgH2F NaMgH3 NaMgF3 (25°C)

Na(x,1/4,z) x 0.0280(4) 0.0209(4) 0.0443z 20.0071(7) 0.006(1) 20.0107B 0.84(5) 1.40(4) 1.34

Mg(0,0,1/2) B 0.030(3) 0.36(3) 0.33F1(H1)(x,1/4,z) x 0.480(1) 0.503(7) 0.4716

z 0.086(1) 0.093(8) 0.0865B 0.46(37) 1 0.56tF1 0.184(5)tH1 0.316(5)

F2(H2)(x,y,z) x 0.298(1) 0.304(6) 0.2953y 0.041(1) 0.065(4) 0.0468z 0.709(1) 0.761(8) 0.7047B 1.3(3) 1 0.63tF2 0.40(1)tH2 0.60(1)

Table 3Comparison of the main average bond lengths (pm) in NaMgX3 (X 5 H, H 1 F, F) phases

NaMgH2F NaMgH3 NaMgF3

Mg–X1 198.2(2) 199(1) 197.8(2)Mg–X2 200.8(6) 224(5) 198.1(5)Mg–X2 194.7(6) 175(5) 198.5(5)Na–F1 304.4(8) 287(4) 318.5(6)Na–F1 252.3(8) 267(5) 240.2(6)Na–F1 313.6(9) 324(6) 320.2(4)Na–F1 229.3(9) 217(6) 216.1(4)Na–F2 267.1(6) 249(5) 258.4(5)Na–F2 269.7(6) 295(5) 284.7(5)Na–F2 231.9(6) 235(5) 212.5(5)Na–F2 328.2(6) 326(5) 337.2(5)

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The Mg–(F, H) and Na–(F, H) distances in NaMgH2F are very close to those of theisostructural NaMgF3 [7], as shown in Table 3.For example, in the MgX6 octahedron, theyare 198.2, 200.8, and 194.7 pm in NaMgH2F and 197.8, 198.1, and 198.5 pm in NaMgF3,respectively. However, in NaMgH3, if the Mg–H1 distance (199 pm) is similar to that of itshomologue, the Mg–H2 distances differ (175 and 224 pm). This may not result from asignificant distortion of the structure but rather from higher uncertainties of the refinementof the anionic sites caused by the very light scattering power of the H2 anion (the B thermalvibration factor of H was fixed for the refinement), and by the impurities present in themeasured sample. A better refinement should be obtained by a neutron diffraction experi-ment on a pure deuterated sample.

4. Conclusion

The direct reaction of hydrogen on metals or mixtures of a metal and a fluoride is aconvenient method to synthesize new hydrides and hydridofluorides. NaMgH2F andNaMgH3 were prepared by this method and structurally characterized. Both phases belongto the family of orthorhombic perovskites, and have the following properties:

Y NaMgH2F contains H2 and F2 anions of close size, fully disordered on the two anionicsites.

Y NaMgH3 seems different from known NaMgH2.72, in that NaMgH3 exhibits a betterlong-range ordering. Indeed, NaMgH2.72 is indexed on the basis of a sublattice of theorthorhombic perovskite type (b 5 b/2 (perovskite)). This discrepancy could resultfrom an ideal stoichiometry or a better crystallinity of the phase prepared by ourmethod.

References

[1] J.P. Bastide, A. Bouamrane, P. Claudy, J.M. Letoffe, J Less-Common Met 136 L1 (1987)[2] A. Bouamrane, J.P. Bastide, J Less-Common Met 152 L19 (1989)[3] J.P. Bastide, A. Bouamrane, P. Claudy, J.M. Letoffe, Eur J Solid State Inorg Chem 26 (1989) 575.[4] A. Bouamrane, J.P. Bastide, J.M. Letoffe, P. Claudy, P. Gonnard, Mater Res Bull 25 (4) (1990) 421.[5] A. Bouamrane, C. de Brauer, J.-P. Soulie, J.M. Letoffe, J.P. Bastide, Thermochim Acta 326 (1999) 37.[6] R. Grob, J. Casanovas, J. Mathieu, Analusis 9 (8) (1981) 367.[7] E. Chao, Am Mineral 46 (1961) 379.

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