The channel structure of trithallium pentaantimonate(V), Tl3Sb5O14

The channel structure of trithallium pentaantimonate(V) Tl3Sb5O14 is characterized and compared to the K, Rb and Cs analogues.


Chemical context
During an extensive study of M[SbF 6 ] compounds (M = Li, NH 4 , Na, Tl), precursors in the form of MSbO 3 were synthesized. Whereas the chosen conditions (1273 K, 12 h) yielded the expected product for LiSbO 3 and NaSbO 3 , the Tl-poor title compound Tl 3 Sb 5 O 14 was inadvertently obtained in the case of Tl. TlSbO 3 was later successfully synthesized at 1073 K. In fact, prior syntheses of TlSbO 3 were performed at even lower temperatures (Bouchama & Tournoux, 1975).
The analogues K 3 Sb 5 O 14 (Hong, 1974), Rb 3 Sb 5 O 14 and Cs 3 Sb 5 O 14 (Hirschle et al., 2001) have been synthesized at 1373 K using more involved routes. The first structural characterization of K 3 Sb 5 O 14 was published by Aurivillius (1966). However, the author gives an incorrect Sohncke space-group symmetry of type Pba2, which was later corrected to Pbam by (Hong, 1974). Hong (1974) noted unusual enlargement of the atomic displacement parameters (ADP) of K in K 3 Sb 5 O 14 , which are located in distinct channels, suggesting ion conductivity. In fact, the author could partially substitute K for Rb, Ag and Tl in the respective nitrate salt melts. Accordingly, it is expected that the hitherto structurally uncharacterized Ag 3 Sb 5 O 14 likewise exists. In contrast, substitution with the smaller Na + ion in an NaNO 3 melt led to a collapse of the structure and formation of the Na-poor Na 2 Sb 4 O 11 . The instability of M 3 Sb 5 O 14 with small ions might explain the successful syntheses of MSbO 3 (M = Li, Na) at 1273 K.

Structural commentary
Tl 3 Sb 5 O 14 crystallizes in the space group Pbam and is isotypic to M 3 Sb 5 O 14 (M = K, Rb, Cs). Two different settings of the Pbam space group were used to describe the structures: a > b by Hong (1974) and a < b by Hirschle et al. (2001). These are equivalent descriptions, because the (a 0 , b 0 , c 0 ) = (b, Àa, c) operation is an element of the affine normalizer of the Pbam space group. Herein we use the original setting and atom labeling of Hong (1974).
In structures of the M 3 Sb 5 O 14 type, the monovalent metal atoms M are located in channels of a triperiodic network formed by [SbO 6 ] octahedra. There are two distinct channels parallel to [010], both with p y b2 1 m symmetry (Fig. 1). In one channel, the M1 atoms are located in zigzag chains and bridged by the M3 atoms, which are located at the boundary of the channels (Fig. 2). In the second channel, the M2 atoms are likewise arranged in the form of zigzag lines (Fig. 2). All of the M atoms are located on or very close to the reflection plane of the channels. Additionally, channels with a smaller diameter extend in the [001] direction (Fig. 3). For K 3 Sb 5 O 14 , Hong (1974) reports excessive enlargement of the ADPs of the K1 and K2 atoms in the [010] and [001] directions of the channels, with the 'thermal motions' in these directions being 'eight times bigger' than in the [100] direction. The Tl1 and Tl2 atoms in the title compound show a much milder enlargement of the ADPs. The ratio of the mean-square displacement of the longest and shortest principal axes of the ADP tensor is 3.2 for Tl1 and 2.9 for Tl2. Note that the value for Tl2 is not directly comparable, since it was refined as disordered about the reflection plane. However, even when placing the atom on the reflection plane, the ratio increases to only 3.2. From these values, it appears that Tl 3 Sb 5 O 14 is not a prime candidate for ion conductivity, at least at the measurement temperature of 100 K. For Rb 3 Sb 5 O 14 and Cs 3 Sb 5 O 14 , similarly mild enlargement of the ADPs has been reported (Hirschle et al., 2001). In contrast to the Tl 3 Sb 5 O 14 title compound, these were derived from data collected at room temperature.
All Sb atoms are coordinated by six O atoms forming highly irregular [SbO 6 ] octahedra (Table 1) with O-Sb-O cis angles ranging from 73.37 (17) to 103.83 (13) and trans angles up to 150.66 (16) . As noted by Hirschle et al. (2001), the framework can be described as being composed of four distinct parts: two infinite octahedra chains and two edgeconnected pairs of octahedra. In general, these elements are connected via corners but there is an additional connection between a pair and a chain via an edge.
A quantitative comparison of Tl 3 Sb 5 O 14 and the alkalimetal analogues M 3 Sb 5 O 14 (M = K, Rb, Cs) was performed using the COMPSTRU (de la Flor et al., 2016) module of the Bilbao Crystallographic Server (Aroyo et al., 2006). The Tl2 atom was moved onto the reflection plane to make the sets of Wyckoff positions compatible. The degree of lattice distortion with respect to the Tl compound is S = 0.0042 (M = K), S = 0.0048 (M = Rb) and S = 0.0262 (M = Cs). This shows that the K, Rb and Tl compounds feature very similar cell parameters, with the volume increasing slightly according to K > Rb > Tl (Table 2). In contrast, the lattice of Cs 3 Sb 5 O 14 features a pronounced distortion with a ca 11% larger unit-cell volume. The enlargement affects foremost the a and b lattice parameters, whereas c is smaller than for the Tl compound. We      therefore presume that the unit-cell volume for the M = K, Rb, Tl compounds is mostly determined by the triperiodic antimonate network, which cannot contract any further. The minimum size of the channels may explain the collapse of the structure when attempting to replace K by Na, as reported by Hong (1974).
The degree of similarity likewise shows a close relationship of the M = K (Á = 0.022) and M = Rb (Á = 0.035) compounds with Tl 3 Sb 5 O 14 , whereas the atomic positions in Cs 3 Sb 5 O 14 differ distinctly (Á = 0.178). In particular, the positions of the O atoms that coordinate to the Tl2 atoms feature a strong deviation (d max = 0.6356 Å for the O4 atom) showing a distinct distortion of the [SbO 6 ] octahedra around the respective channels. Thus, it appears that the Tl2 channels are responsible for the distinct enlargement of the unit cell of Cs 3 Sb 5 O 14 .

Synthesis and crystallization
A mixture of 0.682 g TlNO 3 and 0.373 g Sb 2 O 3 (which makes for an approximate molar ratio of 1:1 for Tl:Sb) was heated in a corundum crucible at 1273 K for 12 h in air. From the reaction, a dark-orange powder was obtained. The single crystals formed as rectangular-prismatic plates. Crystals were isolated under a polarizing microscope and cut to an appropriate size for single crystal diffraction of a highly absorbing crystal.

Trithallium pentaantimonate(V)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.