Crystal structure of TeOF2

L. Guilleta, A. Iderb, J.P. Lavala, B. Frita,*

aLaboratoire de MateÂriaux CeÂramiques et Traitements de Surface, ESA-CNRS no. 6015, Faculte des Sciences, Universite de Limoges,

123 Avenue A. Thomas 87060, Limoges Cedex, FrancebLaboratoire de CeÂramiques et Verres, DeÂpartement de Chimie, Faculte des Sciences et Techniques, Universite Hassan Ier, Settat, Maroc

Received 18 June 1998; accepted 17 July 1998

Abstract

TeOF2 crystallizes in the monoclinic system (space group P21) with unit cell parameters a�551.3(1)pm, b�828.9(1)pm, c�530.7(1)pm,

��96.22(2)8, and Z�4. Its crystal structure was solved and re®ned to RB�0.058 and Rwp�0.116 on the basis of a Rietveld analysis of its

X-ray powder pattern. Each tellurium atom is surrounded by six anions [4�2] (two oxygen, two ¯uorine and two other ¯uorine atoms at

much longer distances). Because of the strong stereochemical activity of their lone pairs E, the coordination polyhedra can be described as

distorted trigonal bipyramids TeO2F2E. These polyhedra associated by sharing O corners, form quasi-independent helical chains coiling

along b. Te±F weak bonds, connecting the chains, give a 3D character to the structure. Structural relationships with �-TeO2 have been found

and analysed. # 1999 Elsevier Science S.A. All rights reserved.

Keywords: Crystal structure; Tellurium oxide¯uoride; TeOF2; Structural relationships

1. Introduction

A phase diagram study of the TeO2±TeF4 system, under

equilibrium and non-equilibrium conditions, allowed iden-

ti®cation and characterization of a large glass-forming

domain (25±83 mol% TeF4), and two new crystalline TeIV

oxide¯uorides with the formulae: TeOF2 and Te2O3F2 [1].

The crystal structure of Te2O3F2 was determined by analysis

of X-ray diffraction single crystal data [2]. This paper

continues these studies and reports on the ab initio deter-

mination, from conventional powder diffraction data, of the

TeOF2 crystal structure.

2. Experimental ± determination of the structure

TeOF2 was prepared by heating an intimate equimolar

mixture of TeF4 and TeO2 in a sealed platinum tube for 24 h

at 1408C followed by water quenching. TeO2 was a com-

mercial high-purity product (Aldrich). TeF4 was prepared by

thermal decomposition of sodium penta¯uorotellurate (IV)

NaTeF5, according to the reaction: NaTeF5!NaF�TeF4 [3].

All the starting products and the ®nal compounds were

handled and stored in a glove box under a strictly dried and

deoxygenated argon atmosphere. TeOF2 so prepared is

obtained as a very hygroscopic white powder. The X-ray

diffraction pattern has been indexed in the monoclinic

system (the extinction conditions correspond to the P21/m

or P21 space groups, a�551.3(1)pm, b�828.9(1)pm,

c�530.7(1)pm, ��96.22(2)8, Z�4), with the help of the

automatic indexing programs TREOR [4] and ITO [5].

These parameters are close to those proposed previously

by Ider et al. [1] on the basis of the X-ray powder pattern

recorded with a D5000 diffractometer equipped with a

position sensitive detector.

TeOF2 decomposes peritectically at 1698C, and melting at

lower temperatures of TeF4-rich mixes leads to the forma-

tion of stable glasses, very dif®cult to recrystallize. So,

numerous attempts to grow single crystals suitable for X-

ray diffraction measurements were unsuccessful. We there-

fore decided to solve the structure by an ab initio method,

from powder diffraction data. The powder pattern was

recorded on a D5000 Siemens diffractometer equipped with

a back monochromator, under the experimental conditions

reported in Table 1. Because of the extremely hygroscopic

character of the powder, a special air-tight sample holder

was used, and the recording time was limited to the mini-

mum acceptable.

One hundred and eighty-nine structure factors were

extracted from the powder pattern by using Le Bail's method

[6] and the FULLPROF Rietveld program [7]. Two tellurium

Journal of Fluorine Chemistry 93 (1999) 33±38

*Corresponding author.

0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

P I I : S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 2 7 1 - 1

atoms were ®rst located by direct methods by using the

TREF option of the SHELXS86 program [8], on the basis of

the P21 space group. After least-squares re®nement of their

coordinates with the SHELX 93 program, successive dif-

ference Fourier syntheses allowed location of the ¯uorine

and oxygen atoms on six crystallographic positions. At this

stage we returned to the Rietveld method with the aid of the

FULLPROF program. In the ®nal re®nement some dif®cul-

ties had to be overcome. They essentially correspond to the

presence in the pattern:

� of weak extra lines due to traces of crystalline Te2O3F2;

they have been refined simultaneously on the basis of the

crystal structure proposed by Ider et al. [2];

� of some diffuse scattering, limiting the accuracy of the

background determination, and probably related to the

presence of some amount of glass.

Despite these dif®culties it was possible to get a good

®nal re®nement (RB�0.058, Rwp�0.116) with reasonable

Table 1

Conditions for data recording and Rietveld refinement for TeOF2

Space group P21

Z 4

Lattice parameters (pm) a�551.3(1); b�828.9(1);

c�530.7(1); ��96.22(2)8Volume (pm3) 241.1�106

Density (calculated) dcalc.�5.005 mg mÿ3

Wavelength (pm) CuKa (154.0598)

Angular limits (2�) 15±1208Counting step (2�) 0.048Counting time (s) 75

Number of reflections observed 2626

Zero point (2�) ÿ0.0647

Temperature (8C) 22

Rietveld program FULLPROF [7]

Profile function pseudo-Voigt

Extraction of structure factors Le Bail's method [6]

Halfwidth parameters U�0.03283; V�ÿ0.0212;

W�0.01068

Structure solution Direct methods�SHELXS86 [8]

Number of refined parameters 47

Reliability factors (%) RB�5.8; Rp�8.8; Rwp�11.6

Table 2

Final refined atomic coordinates and isotropic thermal factors for the

TeOF2 structure

Atom x y z B (10ÿ4 pm2)

Te(1) 0.3545 (5) 0.1464a 0.5114 (5) 0.51 (5)

Te(2) 0.0418 (5) 0.4081 (5) 0.8750 (5) 0.63 (5)

O(1) 0.046 (5) 0.121 (4) 0.301 (6) 2.6 (2)

O(2) 0.222 (6) 0.348 (4) 0.597 (6) 2.6 (2)

F(1) 0.837 (4) 0.571 (3) 0.187 (5) 2.6 (2)

F(2) 0.717 (5) 0.316 (3) 0.650 (4) 2.6 (2)

F(3) 0.457 (4) 0.257 (3) 0.221 (4) 2.6 (2)

F(4) 0.688 (4) 0.061 (2) 0.007 (4) 2.6 (2)

e.s.d's. are given in parentheses.aFixed origin.

Table 3

Main interatomic distances (pm), angles (8), symmetry operations and bond valences in TeOF2

Te(1) F(1) F(2)1 F(2)2 F(3) O(1) O(2) Symmetry

F(1) 210 (3) 384 (4) 336 (4) 399 (4) 275 (4) 260 (4) 1ÿx, yÿ0.5, 1ÿz

F(2)1 113.4 248 (3) 495 (4) 260 (4) 427 (4) 272 (4)

F(2)2 83.1 134.7 288 (3) 386 (4) 285 (4) 462 (4) 1ÿx, y�0.5, 1ÿz

F(3) 164.1 70.9 104.9 193 (3) 260 (4) 260 (4)

O(1) 85.7 148.8 69.2 84.5 194 (3) 257 (4)

O(2) 80.9 75.4 149.8 85.7 84.1 189 (3)

Te(2) F(1)1 F(1)2 F(2) F(4) O(1) O(2)

F(1)1 250 (3) 501 (4) 355 (4) 291 (4) 276 (4) 437 (4) 1ÿx, y, z�1

F(1)2 136.2 290 (3) 323 (4) 422 (4) 472 (4) 260 (4) 1ÿx, y�0.5, 1ÿz

F(2) 98.6 77.6 218 (3) 411 (4) 285 (4) 284 (4) xÿ1, y, z

F(4) 79.9 118.2 158.7 200 (3) 242 (4) 274 (4) 1ÿx, y�0.5, 1ÿz

O(1) 74.5 146.3 85.1 74.0 203 (3) 278 (4) ÿx, y�0.5, 1ÿz

O(2) 162.2 61.4 87.3 88.5 89.5 193 (3)

Te(1) Te(2) �

F(1) 0.658 0.226 0.961

0.077

F(2) 0.234 0.535 0.849

0.080

F(3) 1.050 1.050

F(4) 0.871 0.871

O(1) 1.090 0.868 1.958

O(2) 1.235 1.128 2.363

� 4.347 3.704

e.s.d.'s are given in parentheses, notations are those indicated in Figs. 2±4.

34 L. Guillet et al. / Journal of Fluorine Chemistry 93 (1999) 33±38

Fig

.1

.O

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nce

(bel

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sfo

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Ver

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ower

bar

s).

L. Guillet et al. / Journal of Fluorine Chemistry 93 (1999) 33±38 35

isotropic thermal coef®cients (all anions were constrained to

have the same thermal coef®cient) and bond lengths.

Atomic coordinates and thermal parameters are given in

Table 2. Selected bond distances, angles, and bond valences

calculated by using Brown's method [9] are listed in

Table 3. The bond valence values clearly show that the

O2ÿ and Fÿ anions are perfectly ordered on the anionic sites.

The experimental, calculated and difference powder pat-

terns are shown in Fig. 1.

3. Description of the structure

Each tellurium atom is surrounded by six anions (four

¯uorine and two oxygen atoms) forming a highly distorted

octahedron. In fact, only ®ve of them (three ¯uorine and two

oxygen atoms) contribute signi®cantly to the bond valence

sum (see Table 3). The coordination polyhedra then corre-

spond to distorted square pyramids which can be described

as TeX5E octahedra (Te(1)O(1)O(2)F(1)F(3)F(2)1E and

Te(2)O(1)O(2)F(2)F(4)F(1)1E), with one corner occupied

by the lone pair E of the central tellurium atom. If we

consider only the shortest bond distances (i.e. Te±

F�220 pm), each coordination polyhedron can be reason-

ably described as a distorted trigonal bipyramid TeO2F2E

with one equatorial corner occupied by the lone pair E. This

kind of TeX4 polyhedron is quite common in the stereo-

chemistry of TeIV and is observed in crystal structures like

the � and � forms of TeO2 [10±12], H2Te2O3F4 [13],

KTeOF3 [14] and Te2O3F2 [2]. In both Te(1)O2F2E and

Te(2)O2F2E polyhedra, O atoms logically occupy the equa-

torial positions. By sharing alternately O(1) and O(2) cor-

ners these, TeO2F2E polyhedra constitute quasi-independent

helical chains, parallel to the b axis and separated by the

lone pairs E (Figs. 2 and 3(a) and (b)). The inner part of

these chains is formed by the bridging O atoms, and the

outer part by the non-bridging F atoms. Such a description

emphasizes the pseudo-molecular character of the structure,

character which could justify (as for TeF4 structure, con-

stituted of independent chains of corner-sharing TeF5E

polyhedra), the low melting temperature (1698C) and the

hygroscopic character of TeOF2 [1].

The Te(1)±F(2)1 (248 pm) and Te(2)±F(1)1 (250 pm)

weak bonds connect the TeOF2 chains, respectively, along

the a and b axes, giving a partial 3D-character to this

structure. If these weak bonds are taken into account, the

structure can actually be described as a 3D-network of

TeO2F3E polyhedra sharing alternately O and F corners.

As shown in Fig. 4, two kinds of tunnels can therefore be

found:

� Small ones, built around the strong Te(1)±O(2)±Te(2) or

Te(1)±O(1)±Te(2) bridges.

� Wide ones, built around the weak Te(1)±F(2)±Te(2) or

Te(1)±F(1)±Te(2) bridges, and enclosing the non-brid-

ging F(3) and F(4) atoms.

4. Comparison with the �-TeO2 crystal structure

The �-form of TeO2 (paratellurite) crystallizes with

tetragonal symmetry (space group: P41212 or P43212,

a�480.8 pm, c�761.2 pm, Z�4) [11]. Its crystal structure,

in the left-handed form as re®ned by Champarnaud [15], is

shown in Fig. 5. It exhibits great similarities to the TeOF2

structure:

� The basic building units TeO4E (Fig. 2(c)) are nearly

identical to those of TeO2F2E (Fig. 2(a) and (b)).

� These units constitute, by sharing corners, infinite helical

chains, coiling up around 41 or 43 axes, and very closely

related to the quasi-independent helical TeOF2 chains, as

shown by comparison of Fig. 3(b) with Fig. 5.

Fig. 2. Anionic polyhedra around Te(1) (a) and Te(2) (b) atoms in TeOF2

and, for comparison, around Te atoms in �-TeO2 (c). Arrows indicate the

direction toward which the lone pairs E point.

36 L. Guillet et al. / Journal of Fluorine Chemistry 93 (1999) 33±38

However, because of different connections of the TeX4E

basic units (by a tilt of these units the Teax:Oeq:Te bridges in

�-TeO2 become Teeq:Oeq:Te bridges in TeOF2) the helical

chains, which are interconnected in �-TeO2 and so form a

regular 3D-framework, are in the TeOF2 structure separated

by voluminous square tunnels. These tunnels, shown in

Fig. 3(b), contain one half of the non-bridging ¯uorine

atoms (the F(3) and F(4) atoms), enclosed within the wide

pseudo-chains built around the weak Te±F±Te bridges. It is

these ¯uorine atoms which formally account for the change

of stoichiometry, from TeX2 to TeX3.

It is worth pointing out here that there are strong relation-

ships between the Te2O3F2 structure and the �-form of TeO2

[2]. In both structures, bipolyhedral units (Te2O6E2 units in

the case of �-TeO2, Te2O4F2E2 in the case of Te2O3F2)

constitute, by sharing O corners, similar independent

twisted sheets. The rectangular holes of these sheets accom-

modate the lone pairs E of Te atoms in �-TeO2 and both the

lone pairs E and the non-bridging F atoms in Te2O3F2.

However, in the �-TeO2 structure, the lone pairs are directed

toward the centre of all the rectangular cavities (two lone

pairs E per cavity), which are therefore all identical,

whereas in the Te2O3F2 structure, only one half of the

cavities are occupied by the lone pairs E and by F atoms

(four lone pairs E and two F atoms per cavity), which are

therefore strongly distorted as compared with the unoccu-

pied ones.

5. Conclusions

The crystal structures of the two simple TeIV oxide¯uor-

ides, TeOF2 and Te2O3F2, are now known, and some

common features should be noted:

� Both compounds exhibit strong structural relationships

with the �- and �-TeO2 polymorphs, respectively.

� In both structures the lone pairs E of TeIV atoms are

stereochemically active.

� In both structures bond valence calculations indicate a

perfect O/F order. O atoms logically occupy the equa-

torial positions and are bridging atoms; F atoms on the

contrary occupy axial positions and are systematically

non-bridging. They are mainly located within the inter-

sheet (Te2O3F2) or inter-chain (TeOF2) space, so ensur-

Fig. 3. Projections of the TeOF2 structure normal to c (a) and b (b) axes,

visualising the independent helical chains parallel to b axis. Arrows

indicate the direction toward which the lone pairs E point.

Fig. 4. The 3D-network of TeO2F3E polyhedra sharing alternatively O(1),

O(2) and F(1), F(2) corners.

L. Guillet et al. / Journal of Fluorine Chemistry 93 (1999) 33±38 37

ing some cohesion via weak Te±F bonds between the

highly covalent sheets or chains. Such a long range O/F

order, assigning a different structural role to O2ÿ and Fÿ

anions, has been observed in numerous other oxide

fluorides [16±20].

Moreover, if we consider that TeIV is quasi-systematically

®vefold-coordinated in pure ¯uoride compounds as for

example TeF4 [21], KTeF5 [22] or BaTe2F10 [23] is threefold

(e.g. Tl2TeO3 [24] or BaTeO3 [25]) or fourfold-coordinated

(�- and �-TeO2 [10±12]) in pure oxide compounds, the [4],

[4�1] and [5] coordination numbers observed in these oxide

¯uorides are quite logical.

Acknowledgements

L. Guillet thanks the `̀ Conseil ReÂgional du Limousin''

for ®nancial support.

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Fig. 5. Projection of the �-TeO2 structure (left-handed form, space group: P43212), onto (1 1 0) plane.

38 L. Guillet et al. / Journal of Fluorine Chemistry 93 (1999) 33±38