JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008, p. 765

Corresponding author: Karima Horchani-Naifer (E-mail: karima_horchani@yahoo.com; Tel.: +216-71-430470)

Crystal structure and infrared spectrum of thallium holmiumpolyphosphate, TlHo(PO3)4

Karima Horchani-Naifer, Jaouher Amami, Mokhtar Férid(Unité de Recherches de Matériaux de Terres Rares, Centre National de Recherches en Sciences des Matériaux, B.P. 95 Hammam-Lif, 2050, Tunisie)

Received 27 January 2008; revised 4 September 2008

Abstract: Crystals of thallium-holmium polyphosphate TlHo(PO3)4 were grown by flux method technique and characterized by single crys-tal X-ray diffraction. Structure of TlHo(PO3)4 was solved for the first time, and it crystallized in the monoclinic P21/n space group with thefollowing unit-cell dimensions: a=1.02225(3) nm, b=0.88536(2) nm, c=1.09541(4) nm, β=105.888(1)°, V=0.95354(5) nm3 and Z=4. Thecrystal structure was solved from 2174 independent reflections with final R1(F2)=0.0442 and Rw(F2)=0.0861 refined with 164 parameters.The atomic arrangement could be described as a long chain polyphosphate organization. Holmium atoms had eightfold coordination. Thestructure of TlHo(PO3)4 consisted of HoO8 polyhedra sharing oxygen atoms with phosphoric group PO4. Infrared spectrum was investigatedat room temperature in the frequencies range, 350–4000 cm–1, showing some characteristic vibration bands of infinite chain structure of PO4

tetrahedra linked by bridging oxygen.

Keywords: polyphosphate; holmium; crystal structure; infrared spectroscopy; rare earths

Rare earth polyphosphates MILn(PO3)4 (where MI=monovalent cation, and Ln=lanthanides) have been exten-sively studied due to their interesting optical properties[1–6].The common chemical features of these polyphosphates in-dicate that they are relatively stable under normal conditionsof temperature and humidity. Some polyphosphates of rareearth and thallium, TlCe(PO3)4

[7], TlNd(PO3)4[8],

TlLa(PO3)4[9] and TlGd(PO3)4

[10] are reported in the literature.The literature gives scarce information on the crystallo-graphic data and the spectroscopic properties of TlHo(PO3)4.This work enters within the framework of a systematic in-vestigation of the crystal structures and luminescent proper-ties of the double polyphosphate of the typeMILn(PO3)4

[9–11]. This paper dealt with the synthesis andsingle crystal structure analysis of TlHo(PO3)4 by X-raydiffraction, and the title compound was isotypic withTlNd(PO3)4

[8]. In addition, the titled compound was char-acterized by infrared spectroscopy.

1 Experimental

1.1 Synthesis

Single crystal of TlHo(PO3)4 was prepared by a fluxmethod. At room temperature, 5 g of Tl2CO3 and 0.4 g ofHo2O3 were slowly added to 12 ml of phosphoric acid

H3PO4 (85%). The mixture was then slowly heated to 573 Kduring 24 h and was kept at this temperature. After 7 d, col-orless, transparent and parallelepiped crystals were separatedfrom the excess phosphoric acid by washing the product inboiling water. Subsequently, a second washing with nitricacid is necessary to eliminate the remaining oxide Ho2O3.

1. 2 Structure analysis

A single crystal study was carried out using an EnrafNonius CAD-4 automated four-circle diffractometer withgraphite monochromatized Mo Kα (0.071073 nm) radiation;3904 reflections were measured for 2174 independent re-flections. Twenty-five reflections were located and centeredby searching reciprocal space and indexed to obtain the unitcell parameters optimized by least-squares method. Lorentzand polarizing effect corrections were carried out beforeproceeding to the refinement of the structure.

The structure resolution was performed with the WinGXprogram[12–14]. The positions of the holmium atoms wereobtained using the Patterson heavy atom method and suc-cessive Fourier analysis allowed the other atoms to be lo-cated. The recording conditions and crystallographic dataare reported in Table 1. The atomic coordinates, isotropicand anisotropic displacement parameters are shown in Ta-bles 2 and 3.

766 JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008

Table 1 Crystallographic data, recording conditions and re-finement results for TlHo(PO3)4

Diffractometer Nonius Kappa CCD

Empirical formula TlHo(PO3)4

Formula weight 685.18

Temperature 293(2) K

Wavelength 0.071073 nm

Crystal system Monoclinic

Space group P21/n

Unit cell dimensions a=1.02225(3) nm, b=0.88536(2) nm

c=1.09541(4) nm, β=105.888°(1)

Volume, Z 0.95354(5) nm3, 4

Density (calculated) 4.773 g/cm3

Absorption coefficient 25.853mm–1

F(000) 1216

Theta range for data collection 2.42° to 27.45°

Limiting indices –13≤h≤13, –11≤k≤9, –14≤l≤14

Reflections collected 3904

Independent reflections 2174[R(int)=0,0340]

Absorption correction Integration

Refinement method Full-matrix least-square on F2

Data/restraints/parameters 1815/0/164

Goodness-of-fit on F2 1.091

Final R indices [I>2σ(I)] R1=0.0331, wR2=0.0706

R indices (all data) R1=0.0442, wR2=0.0861

Extinction coefficient 0.00210(16)

Table 2 Atomic coordinates and equivalent isotropic displace-ment parameters for TlHo(PO3)4 (Ueq is defined asone third of the trace of the orthogonalized Uij tensor)

Atom x y z Ueq. /10–2 nm2

Ho –0.00170(4) 0.22826(5) 0.18183(4) 0.00644(15)

Tl 0.19759(5) –0.06014(5) 0.46070(4) 0.02554(17)

P1 0.3520(2) 0.0946(3) 0.2413(2) 0.0061(5)

P2 0.1727(2) 0.3898(3) 0.4753(2) 0.0059(5)

P3 0.2554(2) 0.0218(3) 0.7795(2) 0.0058(5)

P4 –0.0418(2) –0.1726(3) 0.1348(2) 0.0058(5)

O1 0.1025(7) 0.4620(7) 0.1746(6) 0.0103(14)

O2 0.2336(7) 0.1813(8) 0.2549(6) 0.0129(15)

O3 0.0229(7) –0.2914(7) 0.2460(6) 0.0114(14)

O4 0.1703(7) –0.4526(7) 0.4036(6) 0.0114(14)

O5 0.0630(7) 0.2899(7) 0.4016(6) 0.0091(14)

O6 –0.1865(6) 0.1705(8) 0.0155(6) 0.0106(14)

O7 –0.3635(7) 0.0486(7) 0.0996(6) 0.0087(14)

O8 –0.1489(7) 0.0901(7) 0.2797(6) 0.0084(13)

O9 –0.1675(6) 0.4116(7) 0.1962(6) 0.0091(13)

O10 0.3103(7) –0.3442(7) 0.6588(6) 0.0099(14)

O11 0.0654(7) 0.2466(7) –0.0098(6) 0.0106(14)

O12 0.0351(7) –0.0295(7) 0.1627(6) 0.0093(13)

Fig.1 shows a projection on the ac plane of the structureof TlHo(PO3)4. The atomic arrangement can be described asa long chain polyphosphate organization. These chains arejoined to each other by HoO8 dodecahedra. The projectionof PO4 tetrahedra are shown in Fig.2.

The main geometrical features of PO4 tetrahedra are re-ported in Table 4. As can be seen, the P–O distances may bedivided into the linking or bridging P–O(Lij) distances thatrange from 0.1596 nm to 0.1611 nm and the exterior P–O(Eij)distances which vary from 0.1474 nm to 0.1485 nm. TheO–P–O angles are understood to be between 98.3° and 121°,which are in good agreement with those usually met inpolyphosphate anions. Eight oxygen atoms are involved inthe co-ordination of the holmium atom (Fig.3) with Ho-O

Fig.1 Projection of the structure of TlHo(PO3)4 along the a axis,showing arrangement of the HoO8 polyhedra and PO4 tetrahedra

Table 3 Anisotropic displacement parameters for TlHo(PO3)4

Atom U11 U22 U33 U23 U13 U12

Ho 0.0057(2) 0.0061(2) 0.0072(2) –0.00073(15) 0.00117(17) 0.00038(15)

Tl 0.0262(3) 0.0313(3) 0.0175(3) –0.00277(17) 0.00323(19) 0.00159(18)

P1 0.0049(11) 0.0065(11) 0.0059(11) 0.0017(9) –0.0005(9) 0.0009(9)

P2 0.0044(11) 0.0082(11) 0.0047(11) 0.0007(9) 0.0005(9) 0.0000(9)

P3 0.0054(11) 0.0056(11) 0.0070(11) 0.0000(9) 0.0027(9) 0.0004(9)

P4 0.0050(11) 0.0041(11) 0.0080(12) 0.0011(9) 0.0012(9) 0.0015(8)

O1 0.011(3) 0.014(3) –0.003(3) –0.003(3) 0.005(3) –0.002(3)

O2 0.009(4) 0.017(3) 0.011(4) –0.006(3) –0.001(3) –0.002(3)

O3 0.014(4) 0.007(3) 0.012(4) 0.007(3) 0.003(3) 0.007(3)

O4 0.015(4) 0.008(3) 0.010(3) 0.002(3) 0.000(3) 0.000(3)

O5 0.010(3) 0.007(3) 0.010(3) –0.001(3) 0.001(3) –0.004(3)

O6 0.005(3) 0.022(4) 0.006(3) 0.002(3) 0.002(3) –0.006(3)

O7 0.006(3) 0.010(3) 0.012(3) 0.007(3) 0.005(3) 0.003(2)

O8 0.007(3) 0.011(3) 0.009(3) –0.004(3) 0.005(3) –0.005(3)

O9 0.006(3) 0.014(3) 0.008(3) 0.000(3) 0.002(3) 0.004(3)

O10 0.008(3) 0.009(3) 0.014(4) 0.004(3) 0.007(3) 0.001(3)

O11 0.006(3) 0.014(3) 0.011(3) 0.003(3) 0.001(3) 0.003(3)

O12 0.011(3) 0.002(3) 0.017(4) 0.000(3) 0.005(3) 0.001(2)

Karima Horchani-Naifer et al., Crystal structure and infrared spectrum of thallium holmium polyphosphate, TlHo(PO3)4 767

Fig.2 Projection of the polyphosphate chain unit with 8 PO4 tetrahedra

Table 4 Main bond distances (10–1 nm) and angles (°) in TlHo(PO3)4

HoO8 polyhedron

Ho–O6 2.294(6) Ho–O5 2.379(6)

Ho–O12 2.332(6) Ho–O9 2.383(6)

Ho–O1 2.339(6) Ho–O11 2.386(7)

Ho–O2 2.355(7) Ho–O8 2.407(6)P1 tetrahedron

P1–O2 1.474(7) O2–P1–O1 118.3(4)

P1–O1 1.487(7) O2–P1–O4 109.8(4)

P1–O4 1.596(7) O1–P1–O4 110.2(4)

P1–O3 1.605(7) O2–P1–O3 108.7(4)

O1–P1–O3 109.6(4)

O4–P1–O3 98.3(4)

P2 tetrahedron

P2–O5 1.481(7) O5–P2–O6 118.4(4)

P2–O6 1.485(7) O5–P2–O4 110.3(4)

P2–O4 1.598(7) O6–P2–O4 110.0(4)

P2–O7 1.602(7) O5–P2–O7 110.5(4)

O6–P2–O7 107.5(4)

O4–P2–O7 98.3(4)

P3 tetrahedron

P3–O9 1.481(7) O9–P3–O8 116.6(4)

P3–O8 1.483(7) O9–P3–O7 107.5(4)

P3–O7 1.601(7) O8–P3–O7 111.1(4)

P3–O10 1.601(7) O9–P3–O10 108.7(4)

O8–P3–O10 109.7(4)

O7–P3–O10 102.3(4)

P4 tetrahedron

P4–O11 1.477(7) O11–P4–O12 121.0(4)

P4–O12 1.478(7) O11–P4–O3 109.9(4)

P4–O3 1.609(7) O12–P4–O3 108.6(4)

P4–O10 1.611(7) O11–P4–O10 105.7(4)

O12–P4–O10 110.9(4)

O3–P4–O10 98.4(4)

distances ranging from 0.2294 nm to 0.2407 nm. The HoO8

dodecahedron shares all its oxygen atoms with the cornersof neighbouring PO4 tetrahedra, although it is considerablydistorted, they are isolated from each other and they do notshare any common oxygen atoms. Fig.4 shows the pro-jection of HoO8 dodecahedron on the ab plane. Sevenoxygen atoms constitute the coordination of the thallium atom,

Fig.3 Coordination of the holmium atom

Fig.4 Projection of HoO8 polyhedra arrangement on the bc plane

768 JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008

Table 5 Observed infrared frequencies (cm–1) and band as-signment for TlHo(PO3)4

*

IR Assignment

1298 s νas (PO2)

1250 s

1166 m

1150 s

1126 m

1116 m

1080 vs

1060 s

1028 vs

982 m

940 s

874 s

808 s

782 vs

738 w

726 m

714 m

616 vs

598 vs

554 m

534 m

518 m

498 s

464 vs

438 s

418 vs

402 vs

388 s

376 s

νs (PO2)

νas (POP)

νs (POP)

δ(PO2)

δ(POP)

* vs=very strong; s=strong; m=medium; w=weak; vw=very weak

Fig.5 Infrared spectrum of TlHo(PO3)4

forming a TlO8 polyhedra with Tl–O distance ranging from0.2841 to 0.3315 nm.

1. 3 Infrared spectrum

At room temperature, infrared spectrum of the title com-pound was recorded on a Perkin Elmer (FTIR 2000) spec-trometer in the range of 350–4000 cm–1. Sample in powderform was pressed into disk using pellet of KBr. Fig.5shows IR spectrum of TlHo(PO3)4. All the bands were as-signed by the comparison with other condensed phos-phates[15]. The values for the corresponding frequenciesbands are given in Table 5. The bands due to the symmetricand antisymmetric stretching frequencies of PO2 are ob-served in the regions 1250 cm–1 to 1028 cm–1. The bandsobserved in the regions 940–874 cm–1 and 714–808 cm–1 areassigned to the antisymmetric and symmetric P-O-Pstretching modes. The bands due to δ(PO2) and δ(POP) de-formations are assigned in frequencies range of 402–554cm–1. All these bands are characteristic of a structure typebased on an infinite chain of PO4 tetrahedra bound bybridging oxygen.

2 Conclusion

Synthesis and crystal structure were described for thal-lium holmium polyphosphate TlHo(PO3)4. The structurewas determined by a single crystal X-ray analysis, and itwas shown that this compound crystallized in a monoclinicsystem P21/n. The main geometrical feature of this structurewas the existence of two infinite (PO3)α chains, with a pe-riod of eight PO4 tetrahedra that formed two-dimensionalzigzag. In this structure, holmium atoms were in eightfoldcoordination. The (PO3)α chains were joined to each otherby HoO8 dodecahedra, giving a three-dimensional frame-work structure. The energies of the vibrational modes of thecrystal were assigned on the basis of the characteristic vibra-tions of the P–O–P bridge and PO2 groups.

Acknowledgements: This work was supported by the Min-istry of Higher Education, Scientific Research and Tech-nology of Tunisia.

References:

[1] Blasse G, Grabmaier B C, Luminescent Materials, Springer-Verlag, 1994.

[2] Weber M J. Handbook of Laser Wavelengths, CRC Press,1999.

[3] Otsuka K, Yamada T. Transversely pumped LNP laser per-formance. Appl. Phys. Lett., 1975, 26: 310.

[4] Koizumi H. An efficient laser material, lithium neodymiumphosphate LiNdP4O12. Acta Cryst., 1976, B32: 266.

[5] Miyazawa S, Koizumi H, Kubodera K, Iwasaki H. Epitaxialgrowth of KNdP4O12 laser waveguides. J. Cryst. Growth, 1979,47: 351.

[6] Hong H Y P. Crystal structure of potassium neodymium

Karima Horchani-Naifer et al., Crystal structure and infrared spectrum of thallium holmium polyphosphate, TlHo(PO3)4 769

metaphosphate, KNdP4O12, a new acentric laser material. Ma-ter. Res. Bull., 1975, 10:1105.

[7] Rzaigui M, Trabelsi M, Kbir Ariguib N. Phase equilibrium re-lations in the pseudo binary system TlPO3-Ce(PO3)3. J. SolidState Chem., 1983, 50: 86.

[8] Palkina K K, Saifuddinov V Z, Kuznetsov V G, Chudinova NN. The crystal structure of TlNd(PO3)4. Dokl. Akad. NaukSSSR, 1977, 237(4): 837.

[9] Férid M, Trabelsi M, Ariguib N K, Etude du systeme TlPO3-LaP3O9. Thermochim. Acta, 1984, 81: 175.

[10] Amami J, Férid M, Trabelsi-Ayedi M. Crystal structure andspectroscopic studies of NaGd(PO3)4. Mater. Res. Bull., 2005,

40: 1144.[11] Amami J, Horchani K, Merle D, Férid M. Caractérisation et

étude structurale de NaHo(PO3)4. J. Phys. IV, 2004, 122: 111.[12] Win G X-version 1.63, Farrugia L J. WinGX suite for small-

molecule single-crystal crystallography. J. Appl. Cryst., 1999,32: 837.

[13] Sheldrick G M. SHELXS 97 Program for Crystal StructureDetermination, University of Göttingen, Germany 1997.

[14] Sheldrick G M. SHELXL-97 Program for the Refinement ofCrystal Structures, University of Göttingen, Germany, 1997.

[15] Corbridge D E C, Lowe E J. Infrared spectra of some inor-ganic phosphorus compounds. J. Chem. Soc., 1954, B15: 493.