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The crystal structure of the high-pressure (4-8 GPa) form of zirconium tetra­fluoride, [gamma]-ZrF4, is based on the association by corner- and edge-sharing of ZrF8 triangulated dodeca­hedra, forming a three-dimensional framework. It presents some analogies with high-temperature [alpha]-ZrF4 but clearly constitutes a new MX4 structure type. The main MX4 ionic structure types, and especially those deriving from the `anion-excess ReO3-type', are compared and it is shown that the TeF4 structure can also be included in this family.

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Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614014338/ov3051sup1.cif
Contains datablocks New_Global_Publ_Block, I

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Structure factor file (CIF format) https://doi.org/10.1107/S2053229614014338/ov3051Isup2.hkl
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CCDC reference: 1009055

Introduction top

Zirconium tetra­fluoride is the basis of a wide family of crystalline phases (fluoro­zirconates) long studied for their structural diversity and of vitreous phases (fluoride glasses) presenting exceptional optical properties (IR transparency). The determination of the glass structure of these important materials necessitates a good knowledge of the crystalline phases of close composition and/or recrystallizing from these fluoride glasses.

Zirconium tetra­fluoride is known in three crystalline and one amorphous form (Chrétien & Gaudreau, 1958, 1959; Gaudreau, 1965).

The stable variety, called β-ZrF4, is monoclinic and has been described either in the C2/c space group or in the nonstandard I2/a space group (Burbank & Bensey, 1956). It is isostructural with many MF4 phases, such as UF4 (Larson et al., 1964), ThF4, HfF4 (Gaudreau, 1966; Benner & Mueller, 1990), TbF4, PrF4 and CeF4 (Schmidt & Mueller, 1999; Legein et al., 2006), and transuranic tetra­fluorides, e.g. PuF4, NpF4, AMF4 etc (Zachariasen, 1949).

α-ZrF4 (Gaudreau, 1965) is a high-temperature metastable form, stabilized by quenching. It crystallizes in the tetra­gonal system (space group P42/m) and has been structurally characterized (Papiernik et al., 1982). Moreover, a solid solution Zr(O,F)4-x derived from its structure has been described (Papiernik et al., 1983). α-HfF4 (Gaudreau, 1966) is isotypic with α-ZrF4.

A third crystalline variety had been obtained previously (Chrétien & Gaudreau, 1958, 1959; Gaudreau, 1965) by low-temperature thermolysis (473–653 K) of NH4ZrF5 and by reaction of fluorine gas on massive ZrCl4 at 473 K. Poorly crystallized, it could not be indexed and irreversibly transformed to β-ZrF4 at 723 K. A very hygroscopic amorphous variety was also obtained in the same study by reaction of fluorine gas on ZrCl4 in thin layers at 423–473 K. It crystallizes as β-ZrF4 in 2 h at 773 K.

A high-pressure study of zirconium fluoride allowed the preparation of the γ crystalline phase, presenting a powder pattern similar to Gaudreau's low-temperature variety, but much better crystallized (Papiernik et al. 1986). Small single crystals were obtained and these are perfectly stable. The crystals have been used to determine the structure of this previously unreported γ-phase of ZrF4.

Experimental top

Synthesis and crystallization top

Pure γ-ZrF4 was obtained by Papiernik et al. (1986) by heating at 1173–1273 K, under high pressure (4–8 GPa), a powder sample of pure anhydrous β-ZrF4, in a platinum crucible covered with a fitted lid inserted between two mobile pistons. After a day of heating, the temperature and the pressure were rapidly lowered. Under these conditions, some single crystals were obtained but their small size and poor quality had prevented their use for structural determination and only an un-indexed powder pattern had been published. Re-examining the samples 27 years later, the present author, after several fruitless attempts, succeeded in selecting a single crystal suitable for a structural determination with a more modern apparatus (Nonius KappaCCD).

Refinement top

The Zr-atom position was determined by direct methods using SHELXS96 (Sheldrick, 2008) and the F atoms were determined on four sites from a Fourier difference map using SHELXL97 (Sheldrick, 2008). An extinction correction was applied to the data using suggested values. All atoms were refined with anisotropic displacement parameters. Crystal data, data collection and structure refinement details are summarized in Table 1. Selected bond lengths are reported in Table 2.

Results and discussion top

The structure of γ-ZrF4 is based on the association by corner- and edge-sharing of ZrF8 triangulated dodecahedra, forming a three-dimensional framework. The triangulated dodecahedron, also present in α-ZrF4, is, with the square Archimedean anti­prism, sometimes distorted to a bicapped trigonal prism, the most stable eight-coordination polyhedron. In γ-ZrF4 as in α-ZrF4, this polyhedron is slightly distorted by reference to the ideal one described by Wells (1975), but is close to the most stable one defined by Hoard & Silverton (1963). As shown in Fig. 1 and in the comparison reported in Table 3, the Zr—F bond lengths [2.034 (3)–2.200 (3) Å] are within the limits generally observed in the ambient-pressure structures of fluoro­zirconates.

Each fluoride anion is connected to two Zr2+ cations, directly giving the nominal composition: ZrF8/2. But, in contrast to the β-phase, in which the Archimedean square anti­prisms share only corners, the dodecahedra also share F1–F1 vertices. These vertices correspond in this γ-phase to the shortest F···F distances (2.338 Å cf. > 2.44 Å for the other F..F contacts) and to the shortest Zr···Zr distances (3.597 Å cf. 4.10 and 4.19 Å for connections through F-atom corners), in agreement with the values generally observed in fluoro­zirconates. The same situation was emphasized in α-ZrF4, in which the F–F vertices between two dodecahedra were also very short (2.302 Å cf. F···F = 2.661 Å). It is noted that the shortest contact distances between two F atoms are generally about 2.40 Å, but that some very short distances (2.25–2.30 Å) are occasionally observed.

In all the following descriptions, the plane net symbols and descriptions refer to O'Keeffe & Hyde (1980) using Schläffli notation modified by Cundy & Rollet (1961).

In the (001) plane, Zr2+ cations form 44 undulated square nets in which the ZrF8 dodecahedra (Fig. 2) are alternately connected by F2 and F3 corners. These planes are stacked along the [001] direction and connected through F4 corners or F1–F1 edges (Fig. 3). The three-dimensional framework results from the inter­section of (110) and (110) plane nets with the (001) 44 undulated square nets described above. Fig. 5(c) shows the (110) layer of ZrF8 dodecahedra and the corresponding [32.43][3.43] nonregular cationic plane net.

Comparison with MX\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~4\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~ structures top

The known MX4 structures, with X representing mainly halides, belong to a few structure types which can be classified by the nature and coordination of the cationic polyhedra and by their connectivity. A widely used classification of the MX4 halides by Wells (1975) distinguished:

– for low-size cations: tetra­hedral coordination in molecular structures (SnBr4 and SnI4);

– for M cations of medium size, often in o­cta­hedral coordination: chain (α-NbI4 and TcCl4) and layer structures (SnF4 and isotypic PbF4, TaOF3 and NbF4). More recently, several new MF4 structure types, based on various grouping of MF6 o­cta­hedra, have been described, viz. isolated triple columns for TiF4 (Bialowons et al., 1995), isolated layers of distorted o­cta­hedra sharing corners for VF4 (Becker & Mueller, 1990), MnF4 (Mueller & Serafin, 1987), RuF4 (Casteel et al., 1992), double columns of edge- and corner-sharing o­cta­hedra in α-CrF4 ( Kraemer & Mueller, 1995), quadruple columns of corner-sharing o­cta­hedra in β-CrF4 (Benkic et al., 2002), a three-dimensional framework in IrF4 (Bartlett & Tressaud, 1974) and isotypic PtF4 (Mueller & Serafin, 1992), in which Ir(Pt)F6 o­cta­hedra, sharing four F-atom corners, delimit tunnels, and two F- anions are terminal.

– for higher-size cations:

layer (ThI4) structure (Zalkin et al., 1964), composed of isolated sheets of ThI8 square anti­prisms, each anti­prism sharing one edge and two faces. Each I atom is bonded to two Th atoms, giving a 8:2 structure.

three-dimensional structures, stable for cations of high size (Zr, Hf, lanthanides and actinides) and anions of small size (Cl and F) and composed by the association of square anti­prisms or dodecahedra forming several structure types:

– in the U(Th)Cl4 type (Mooney, 1949), each U(Th)Cl8 dodecahedron shares one edge with each of four other dodecahedra, forming helical arrays around 41 axes;

– in β-ZrF4, both types of non-equivalent Zr4+ ions are surrounded by eight F atoms forming slightly distorted square anti­prisms, each sharing vertices with eight other square anti­prisms (8:2 structure). A detailed comparison with the ThZr2F12 (Taoudi et al., 1996a) and PrZr2F11 (Laval & Abaouz, 1992a) structures, suggested by the closeness of their unit cells, has been carried out. Considering β-ZrF4 as a double-fluoride ZrZr2F12, the three structures are based on the same stacking of Th, Pr and Zr rhombic plane nets alternating with sheets of ZrF8 and/or ZrF7 polyhedra with the same Zr cationic net deriving from a 32.4.3.4 plane net (Taoudi et al., 1996a).

– in α-ZrF4 (Papiernik et al., 1982), successive (001) layers of ZrF8 corner-sharing dodecahedra, in which Zr2+ cations form a 32.4.3.4 plane net, are connected alternately by corner- and edge-sharing. Each F- anion is bonded to two Zr2+ cations, hence the 8:2 formula. ThZrF8 structure derives from the α-ZrF4 type by a cationic ordering between the larger Th4+ cation (r = 1.09 Å) and the smaller Zr4+ cation (r = 0.84 Å). The ZrF8 dodecahedra are replaced by corner-shared ZrF7 monocapped trigonal prisms and ThF9 tricapped trigonal prisms in a manner described by Taoudi et al. (1996b).

γ-ZrF4 is a new member of the class of three-dimensional MX4 structures. Considering the ionic radius of Zr4+ (r = 0.84 Å), which is smaller than that of U4+ (r = 1.00 Å), Th4+ (r = 1.09 Å) and tetra­valent lanthanides or actinides cations, it could seem possible that some fluoro­zirconates, under pressure, increase their coordination to 9, similar to some fluoro­uranates(IV) and many fluoro­thorates. In fact, the MF4 stoichiometry, in contrast to that of the MF3 tysonite type, likely does not allow the existence of stable phases based only on MF9 polyhedra; only mixed phases are allowed, as ThZr2F12 and ThZrF8 contain MF9 polyhedra. Moreover, the LnTh2F11 series (Abaouz et al., 1997) is structurally related to the tysonite (LaF3) type. In this MF4 type, columns of face-shared MF11 Edshammar's polyhedra are connected by corner-sharing instead of the edge-sharing seen in LaF3. Ln4+ and Th4+ cations are disordered on the M site. The cationic coordination is likely locally reduced around Ln4+ cations, changing the composition from MF4 to MF3.67. Theoretically, ThF4 should present this kind of structure with only ThF11 polyhedra but, until now, the only known ThF4 structure is a very stable one with the β-ZrF4 type and eightfold coordination about the Th4+ cation. The specific influence of high pressure on the structure of ZrF4 can be better approached by a comparison of the α- and γ-ZrF4 structures, respectively, with the `anion-excess ReO3-type'.

Many phases of stoichiometry comprised between MX3 and MX4 have been structurally related to the cubic ReO3 type through mechanisms involving accomodation of anionic excess either in disordered solid solutions or in partially or fully ordered superstructures. In cubic (Zr,M)(O,F)3+x solid solutions, Tofield, Poulain and later Papiernik (Poulain et al., 1973; Tofield et al., 1979; Poulain & Tofield, 1981; Papiernik & Frit, 1984; Papiernik et al., 1986) had proposed a simple structural mechanism of statistical replacement of some X (O or F) corners of MX6 o­cta­hedra by XX pairs or X3 triangular faces, transforming corner-sharing o­cta­hedra into edge-sharing penta­gonal bipyramids or dodecahedra (Fig. 4a). This transformation implied only local adjustments of the structure and allowed extensive anionic excess insertion (`ReO3+x-type').

Later, many fluoro­zirconates, fluoro­hafnates, fluoro­stannates, fluoro­terbates(IV) etc, either fully or partly ordered, have also been gathered in a new structural family and denoted as `anion-excess ReO3+x superstructures'. The SmZrF7 structure (Poulain et al., 1973; Graudejus et al., 1994) is the first model of such an ordered superstructure. It associates ZrF6 o­cta­hedra and SMF8 dodecahedra and has been derived from the ReO3-type by a mechanism analogous to the Crystallographic Shear (CS) processes previously developed for anion-deficient `ReO3-x' phases in Mo(W)O2–Mo(W)O3 mixed-valence systems (Wells, 1975). Papiernik described a similar rather complex multistep CS process to derive the α-ZrF4 structure from the ReO3-type structure (Papiernik et al., 1982).

In further studies (Laval & Abaouz, 1992a,b; Laval et al., 1995; Taoudi et al., 1996b), it has been shown that this complex CS process could be replaced in a simpler way by a sum of two elementary processes whose addition allowed the description of many structure types:

(i) the same substitutional mechanism for anion insertion as described by Poulain and Papiernik in cubic (Zr,M)(O,F)3+x solid solutions and corresponding to the replacement of some F-atom corners in MF6 o­cta­hedra by F–F edges or by F–F–F triangular faces (Fig. 4a).

(ii) a new mechanism involving direct anionic bridging (Fig. 5a) between opposite Zr corners of a face of an ReO3 cubic lattice, transforming this square face into two triangular ones. After some cationic and anionic local regularization, the 44 cationic plane nets are transformed by this bridging in semi- or nonregular plane nets. The kind of plane nets, e.g. 32.4.3.4 or [32.43][3.43] nets, depends on the three-dimensional distribution of the anionic insertion, as shown in Figs. 5(b) and 5(c).

In this way, MF7 penta­gonal bipyramids, MF8 dodecahedra and/or MF9 tricapped trigonal prisms can replace MF6 o­cta­hedra. Rhombohedral α- and β-MZr3F15 (Caignol et al., 1988; Popov et al., 1991; Gervais et al., 1994; Laval et al., 1995), monoclinic LnMF7 (Poulain et al., 1973; Graudejus et al., 1994), β-BaZr2F10 structure types (Laval et al., 1988) are based on various distributions of excess anions by these two mechanisms, forming these ReO3+x-related superstructures.

α- and γ-ZrF4 structures can be easily related to the same structural family:

(i) α-ZrF4 derives from ReO3 by two different insertion mechanisms, easily recognized, respectively, in Figs. 4(b) and 5(b).

(ii) substitution of half the apical F-atom corners by F4–F5 edges along the [001] direction; this tranformation shortens the Zr···Zr distances from about 4.1 to 3.56 Å.

(iii) anionic bridging through F6 anions inside (001) planes, transforming the cationic 44 square plane net of the ReO3 type into a semiregular 32.4.3.4 net.

The structural equations corresponding to the two-step transformation ReO3 α-ZrF4 are:

MF6/2 - F + F4 + F5 MF7/2; MF7/2 + F6 MF8/2

This mechanism is much simpler than the original one described by Papiernik, which involved tranformation processes analogous to the crystallographic shear (CS) deriving from the ReO3 type by creation of anionic vacancies via shearing of blocks or columns of polyhedra along definite axes.

(iv) γ-ZrF4 derives from the ReO3 structure type by a one-step process of anionic bridging which connects, via an F1–F1 common edge, the opposing Zr2+ cations of a square face of a 44 plane net of the ReO3 reference structure. Identified for the first time in `anion-excess ReO3+x superstructures', this double anionic bridging highly distorts the (110) and (110) homologous planes of dodecahedra. The corresponding Zr plane net, as represented in Fig. 5(c), is a [32.43][3.43] net, already identified in α- and β-MZr3F15 and also in ThZrF8. The structural equation corresponding to this one-step transformation ReO3 γ-ZrF4 is:

MF6/2 + 2F1 MF8/2

Therefore, the main effect of high pressure on zirconium tetra­fluoride seems to consist in favouring F-edge-bridging rather than an increase of the density or the coordination number of Zr. Indeed, the density of γ-ZrF4 is not significantly different (dx = 4.627 Mg m-3) than in α (dx = 4.62 Mg m-3) and β (dx = 4.65 Mg m-3) varieties, which is rather in favour of a greater structural distortion for this high-pressure variety.

TeF\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~4\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~: an anion-excess ReO\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~3+x\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\~ superstructure distorted by the stereochemical activity of the electronic lone pair of Te\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ÎV\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\^ top

In the classical structural descriptions, TeF4 (Fig. 6a) is an unique structure composed of TeF5 polyhedra connected via F-atom corners and forming parallel zigzag chains along the [010] axis of the orthorhombic P212121 structure (Edwards & Hewaidy, 1968; Kniep et al., 1984). The TeF5 polyhedron, a square pyramid, is characteristic of the environment of cations presenting a stereochemical activity of their electron lone pair, e. TeIV, SbIII, SnII and BiIII. In a series of structural determinations of fluoro­tellurates and oxyfluoro­tellurates(IV) (Boukharrata et al., 2008; Laval & Boukharrata, 2009; Laval et al., 2008), it has been shown that many of these structures derived from known aristotypes (rutile, fluorite, α-PbO2) in considering not only the short MX bond distances, but also much longer ones (2.5–3.5 Å) belonging to the coordination sphere of the cations, e.g. PbTeF6 derives from the ReO3 type, as shown by Ider et al. (1996).

The same criteria applied to TeF4 show that this last phase also derives from the family of `anion-excess ReO3+x superstructures'. Fig. 6(b) shows that the integration of long Te—F bonds transforms a TeF5 square pyramid into a TeF5+3 dodecahedron distorted by the repulsive effect of the E lone pair of Te4+ on the triangular opposite anionic face. Fig. 6(c) shows how the (001) plane of TeF4 represented on Fig. 6a is transformed by this new representation to a plane of TeF5+3 dodecahedra analogous to the (001) plane in γ-ZrF4 (Fig. 2). The stereochemical activity of the E lone pair does not destroy the highly undulated 44 cationic plane net, but the Te4+ cations are clearly off-center inside these distorted anionic dodecahedra.

However, TeF4 is not really isostructural with α- or γ-ZrF4, the dodecahedra being inter­connected only by corner-sharing, contrary to α- and γ-ZrF4 which present some edge-sharing. The transformation ReO3 TeF4 is a one-step process involving anionic bridging through F4 anions inside both (110) and (110) planes (Fig. 6d), transforming two cationic 44 square plane nets of ReO3 type to semi-regular 32.4.3.4 plane nets. The third, perpendicular, 44 net (Fig. 6c) formally remains a square plane net although it is highly undulated. The structural equation corresponding to this transformation is :

MF6/2 + F4 + F4 MF8/2.

Conclusion top

The crystal structure of the high-pressure variety of zirconium tetra­fluoride, γ-ZrF4, is based on the association by corner- and edge-sharing of ZrF8 triangulated dodecahedra, forming a three-dimensional framework. This constitutes a new MX4 structure type. As with α-ZrF4 and TeF4, it belongs to the `anion-excess ReO3+x-type' through an original process of anionic bridging via F1–F1 common edges between ZrF8 dodecahedra, the connection likely made easier by the high pressure.

A discussion about the main MX4 ionic structure types, and especially those deriving from the `anion-excess ReO3+x-type', has been developed.

Related literature top

For related literature, see: Abaouz et al. (1997); Bartlett & Tressaud (1974); Becker & Mueller (1990); Benkic et al. (2002); Benner & Mueller (1990); Bialowons et al. (1995); Boukharrata et al. (2008); Burbank & Bensey (1956); Caignol et al. (1988); Casteel, Wilkinson, Borrmann, Serfass & Bartlett (1992); Chrétien & Gaudreau (1958, 1959); Cundy & Rollet (1961); Edwards & Hewaidy (1968); Gaudreau (1965, 1966); Gervais et al. (1994); Graudejus et al. (1994); Hoard & Silverton (1963); Ider et al. (1996); Kniep et al. (1984); Kraemer & Mueller (1995); Larson et al. (1964); Laval & Abaouz (1992a, 1992b); Laval & Boukharrata (2009); Laval et al. (1988, 1995, 2008); Legein et al. (2006); Mooney (1949); Mueller & Serafin (1987, 1992); Papiernik & Frit (1984); Papiernik et al. (1982, 1983, 1986); Popov et al. (1991); Poulain & Tofield (1981); Poulain et al. (1973); Schmidt & Mueller (1999); Taoudi et al. (1996a, 1996b); Tofield et al. (1979); Wells (1975); Zachariasen (1949); Zalkin et al. (1964).

Computing details top

Data collection: COLLECT (Nonius, 2004); cell refinement: DIRAX/LSQ (Duisenberg & Schreurs, 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 2012); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. ZrF8 dodecahedra constants in γ-ZrF4 (see Table 3). θ(A) and θ(B) correspond to the average half value of the A—Zr—A and B—Zr—B angles, respectively. [Symmetry codes: (i) -x+2, y+1/2, -z+3/2; (ii) -x+2, -y, -z+2; (iii) -x+1, y+1/2, -z+3/2; (iv) x, -y+1/2, z-1/2.]
[Figure 2] Fig. 2. The (001) layer of ZrF8 dodecahedra in γ-ZrF4, showing the 44 undulated cationic square plane net derived from the ReO3-type.
[Figure 3] Fig. 3. The (010) projection of ZrF8 dodecahedra in γ-ZrF4.
[Figure 4] Fig. 4. (a) The basic substitutional mechanism for anion insertion in the ReO3 structure, transforming corner-sharing octahedra into edge-sharing pentagonal bipyramids. (b) Ordered anionic insertion in the (010) [identical to the homologous (100)] plane of the α-ZrF4 structure.
[Figure 5] Fig. 5. Tranformation of the ReO3-type phase to, respectively, α- and γ-ZrF4 structures through anionic bridging. (a) The basic bridging mechanism for anion insertion. (b) Ordered anionic bridging by F6 corners creating a 32.4.3.4 (001) cationic plane net in α-ZrF4. (c) F1–F1 edges creating a [32.43][3.43] (110) [identical to the homologous (110)] cationic plane net in γ-ZrF4.
[Figure 6] Fig. 6. Structural relationship of the TeF4 structure to the α- and γ-ZrF4 structures via anionic bridging. (a) A classical representation of the (001) TeF4 structure with zigzag chains of TeF5 polyhedra sharing corners. (b) The TeF5+3 dodecahedron in TeF4. The orientation of the electron lone pair of Te4+ is represented as an arrow. (c) The (001) plane in the TeF4 structure, showing a 44 cationic plane net, for comparison with the homologous (001) plane in γ-ZrF4 (Fig. 2). (d) The (110) plane in the TeF4 structure showing a distorted 32.4.3.4 cationic plane net and the anionic bridging via F4 corners, for comparison with the homologous plane in α-ZrF4 (Fig. 5b).
Zirconium tetrafluoride top
Crystal data top
ZrF4Z = 4
Mr = 167.22F(000) = 304
Monoclinic, P21/cDx = 4.627 Mg m3
Hall symbol: -P 2ybcMo Kα radiation, λ = 0.71070 Å
a = 5.554 (2) ŵ = 4.46 mm1
b = 5.639 (2) ÅT = 293 K
c = 7.973 (3) ÅPrismatic, colourless
β = 105.98 (5)°0.01 × 0.01 × 0.01 mm
V = 240.06 (15) Å3
Data collection top
Nonius KappaCCD
diffractometer
541 independent reflections
Radiation source: fine-focus sealed tube513 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.047
Detector resolution: 9 pixels mm-1θmax = 27.5°, θmin = 5.3°
CCD scansh = 77
Absorption correction: multi-scan
(SADABS; Bruker, 2004)
k = 77
Tmin = 0.920, Tmax = 0.976l = 1010
5384 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.026 w = 1/[σ2(Fo2) + (0.011P)2 + 3.0227P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.071(Δ/σ)max < 0.001
S = 1.29Δρmax = 1.34 e Å3
541 reflectionsΔρmin = 0.72 e Å3
47 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.008 (2)
0 constraints
Crystal data top
ZrF4V = 240.06 (15) Å3
Mr = 167.22Z = 4
Monoclinic, P21/cMo Kα radiation
a = 5.554 (2) ŵ = 4.46 mm1
b = 5.639 (2) ÅT = 293 K
c = 7.973 (3) Å0.01 × 0.01 × 0.01 mm
β = 105.98 (5)°
Data collection top
Nonius KappaCCD
diffractometer
541 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2004)
513 reflections with I > 2σ(I)
Tmin = 0.920, Tmax = 0.976Rint = 0.047
5384 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.02647 parameters
wR(F2) = 0.0710 restraints
S = 1.29Δρmax = 1.34 e Å3
541 reflectionsΔρmin = 0.72 e Å3
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Zr10.78760 (9)0.16627 (9)0.82954 (6)0.0083 (2)
F11.1417 (6)0.1578 (6)1.0205 (4)0.0141 (7)
F21.0054 (6)0.0420 (6)0.7090 (4)0.0148 (7)
F30.4827 (6)0.0438 (6)0.7347 (4)0.0149 (7)
F40.6865 (6)0.2614 (7)1.0476 (4)0.0145 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Zr10.0089 (3)0.0084 (3)0.0083 (3)0.00007 (19)0.00380 (19)0.00002 (18)
F10.0151 (16)0.0120 (16)0.0142 (15)0.0002 (13)0.0024 (13)0.0011 (13)
F20.0171 (16)0.0140 (18)0.0137 (16)0.0050 (13)0.0049 (13)0.0010 (13)
F30.0157 (16)0.0134 (18)0.0161 (16)0.0048 (13)0.0054 (13)0.0013 (13)
F40.0150 (16)0.0186 (17)0.0104 (15)0.0019 (14)0.0043 (13)0.0006 (13)
Geometric parameters (Å, º) top
Zr1—F32.034 (3)Zr1—F4iv2.200 (3)
Zr1—F42.040 (3)Zr1—Zr1ii3.597 (2)
Zr1—F2i2.078 (3)F1—Zr1ii2.160 (3)
Zr1—F22.099 (3)F2—Zr1v2.078 (3)
Zr1—F12.131 (4)F3—Zr1vi2.183 (3)
Zr1—F1ii2.160 (3)F4—Zr1vii2.200 (3)
Zr1—F3iii2.183 (3)
F3—Zr1—F494.21 (14)F1ii—Zr1—F3iii140.53 (13)
F3—Zr1—F2i147.57 (13)F3—Zr1—F4iv77.06 (14)
F4—Zr1—F2i102.20 (14)F4—Zr1—F4iv140.28 (13)
F3—Zr1—F291.70 (14)F2i—Zr1—F4iv72.42 (13)
F4—Zr1—F2149.95 (14)F2—Zr1—F4iv69.71 (13)
F2i—Zr1—F287.89 (5)F1—Zr1—F4iv131.31 (13)
F3—Zr1—F1139.71 (13)F1ii—Zr1—F4iv132.76 (14)
F4—Zr1—F179.25 (14)F3iii—Zr1—F4iv69.86 (13)
F2i—Zr1—F171.58 (13)F3—Zr1—Zr1ii106.44 (11)
F2—Zr1—F177.27 (13)F4—Zr1—Zr1ii76.72 (10)
F3—Zr1—F1ii73.67 (14)F2i—Zr1—Zr1ii104.48 (10)
F4—Zr1—F1ii78.54 (14)F2—Zr1—Zr1ii73.35 (9)
F2i—Zr1—F1ii136.74 (13)F1—Zr1—Zr1ii33.28 (9)
F2—Zr1—F1ii74.94 (13)F1ii—Zr1—Zr1ii32.77 (9)
F1—Zr1—F1ii66.05 (15)F3iii—Zr1—Zr1ii146.44 (9)
F3—Zr1—F3iii84.29 (4)F4iv—Zr1—Zr1ii143.00 (10)
F4—Zr1—F3iii70.75 (14)Zr1—F1—Zr1ii113.95 (15)
F2i—Zr1—F3iii75.23 (14)Zr1v—F2—Zr1157.34 (18)
F2—Zr1—F3iii139.23 (13)Zr1—F3—Zr1vi166.34 (19)
F1—Zr1—F3iii128.64 (13)Zr1—F4—Zr1vii150.07 (18)
Symmetry codes: (i) x+2, y+1/2, z+3/2; (ii) x+2, y, z+2; (iii) x+1, y+1/2, z+3/2; (iv) x, y+1/2, z1/2; (v) x+2, y1/2, z+3/2; (vi) x+1, y1/2, z+3/2; (vii) x, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaZrF4
Mr167.22
Crystal system, space groupMonoclinic, P21/c
Temperature (K)293
a, b, c (Å)5.554 (2), 5.639 (2), 7.973 (3)
β (°) 105.98 (5)
V3)240.06 (15)
Z4
Radiation typeMo Kα
µ (mm1)4.46
Crystal size (mm)0.01 × 0.01 × 0.01
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 2004)
Tmin, Tmax0.920, 0.976
No. of measured, independent and
observed [I > 2σ(I)] reflections
5384, 541, 513
Rint0.047
(sin θ/λ)max1)0.650
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.026, 0.071, 1.29
No. of reflections541
No. of parameters47
Δρmax, Δρmin (e Å3)1.34, 0.72

Computer programs: COLLECT (Nonius, 2004), DIRAX/LSQ (Duisenberg & Schreurs, 2000), EVALCCD (Duisenberg et al., 2003), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 2012), DIAMOND (Brandenburg, 1999), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

Selected bond lengths (Å) top
Zr1—F32.034 (3)Zr1—F1ii2.160 (3)
Zr1—F42.040 (3)Zr1—F3iii2.183 (3)
Zr1—F2i2.078 (3)Zr1—F4iv2.200 (3)
Zr1—F22.099 (3)Zr1—Zr1ii3.597 (2)
Zr1—F12.131 (4)
Symmetry codes: (i) x+2, y+1/2, z+3/2; (ii) x+2, y, z+2; (iii) x+1, y+1/2, z+3/2; (iv) x, y+1/2, z1/2.
Comparison of the geometric features of ZrF8 dodecahedra in γ- and α-ZrF4, and in ideal dodecahedra (the F–F edge lengths are calculated as multiples of the mean Zr—F distance in Å) top
γ-ZrF4α-ZrF4Ideal dodecahedron (rigid spheres model)Most favourable dodecahedron (Hoard & Silverton model)
a1.1861.1681.1991.17
m1.1681.1551.1991.17
g1.2501.2491.1991.24
b1.4251.4441.4991.49
θ(A)34.035.236.935.2
θ(B)74.474.969.573.5
MA/MB1.051.031.001.03
 

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