Redetermination of the crystal structure of NaTcO4 at 100 and 296 K based on single-crystal X-ray data

The title compound, NaTcO4, forms tetragonal crystals both at 100 and 296 K with a thermal volumic expansion coefficient of 1.19 (12) × 10−4 K−1.

The redetermination of the title compound, sodium pertechnate, from singlecrystal CCD data recorded both at 296 and 100 K confirms previous studies based on X-ray powder diffraction film data [Schwochau (1962). Z. Naturforsch. Teil A, 17, 630; Keller & Kanellakopulos (1963). Radiochim. Acta, 1, 107-108] and neutron powder diffraction data using the Rietveld method [Weaver et al. (2017). Inorg. Chem. 12, 677-681], but reveals a considerable improvement in precision. The standard uncertainties of the room-temperature structure determination are about seven times lower than those of the neutron diffraction structure determination and about 13 times lower at 100 K, due to the decrease in the amplitude of librations. The crystal expansion could be approximated linearly with a thermal volumic expansion coefficient of 1.19 (12) Â 10 À4 K À1 . NaTcO 4 adopts the scheelite (CaWO 4 ) structure type in space group type I4 1 /a with Na and Tc atoms (both with site symmetry 4) replacing Ca and W atoms, respectively.

Chemical context
Sodium pertechnetate, NaTcO 4 , refers to a group of d 0 -tetroxide anion salts. Since the inception of quantum chemistry, compounds of this type have been models (generally with respect to the MnO 4 À anion) for which the validity of the proposed equations and approximations for the case of delectrons are verified. It was believed that, owing to the d 0 electronic state, they define the least complex class of compounds of d-elements. Such simplicity, due to the absence of d-electrons and their pseudospherical symmetry, does by far not imply that any of these compounds show no complex behavior under changing environmental conditions, e.g. by changing temperature and/or the strength of the crystal field, and publications on the discovery of a more complex behaviour and properties appeared periodically. For example, for sodium (German et al., 1987b(German et al., , 1993, potassium (German et al., 1993;Gafurov & Aliev, 2005) and caesium (Tarasov et al., 1991(Tarasov et al., , 1992 tetraoxidotechnates, the existence of phase transitions was noted at high temperatures, while for the rhenium analogue, caesium tetraoxidorhenate, the ability of laserexcited second harmonic generation has been observed (Stefanovich et al., 1991). Differences for these systems are also observed in the crystal structures. Potassium permanganate crystallizes in the orthorhombic system (Palenik, 1967), whereas the pertechnetate and perrhenate of the same cation crystallize in the tetragonal system (Hoppe et al., 1999;Schwochau, 1962). Next to the interest for the TcO 4 À anion in its sodium salt, sodium cations in general are worth being investigated in detail. For example, sodium salts are known to form hydrates with different hydration numbers and various coordination numbers for the sodium cation. The change in these numbers often occurs in the vitally important temperature range of 309-313 K (German et al., 1987b;Tarasov et al., 2015). Precise structural data of such systems are important for the analyses of transmutation rates in homogeneous systems as noted by Kuo et al. (2017) and in this respect, are more useful than the data of previously determined structures (Kuo et al., 2017;Ackerman et al., 2016;German et al., 1987a;Spitsyn et al., 1987;Tarasov et al., 1983Tarasov et al., , 1991. Likewise, Ackerman et al. (2016) have shown that precise structural data are needed for the estimation of the incorporation possibility for 99 Tc into stable scheelite matrices of different compositions. Another aspect for obtaining more precise structure data on pertechnates is to clarify if pseudo-Jahn-Teller distortions of d 0 -tetraoxide anions really take place when compared with previous determinations (German et al., 1987a;Spitsyn et al., 1987;Tarasov et al., 1983Tarasov et al., , 1991. In this context we have reinvestigated the crystal structure of NaTcO 4 that is known from powder diffraction data only, namely by inspection of its X-ray powder diffraction pattern (Schwochau, 1962;Keller & Kanellakopulos, 1963) and Rietveld refinement of neutron powder diffraction data (Weaver et al., 2017).

Structural commentary
The structure of anhydrous NaTcO 4 , determined here on the basis of X-ray diffraction data of a single crystal recorded both at room and low temperature, belongs to the CaWO 4 structural type (space group type I4 1 /a). The obtained bond lengths and angles are similar to those obtained from previous X-ray powder (Keller & Kanellakopulos, 1963;Schwochau, 1962) and neutron powder diffraction studies (Weaver et al., 2017) Lattice parameters determined here with the precision of 0.0002-0.0005 Å at 296 K (Table 1) are close to those of a = 5.342 (3) Å , c = 11.874 (6) Å given by Weaver et al. (2017). The lattice parameters at 100 K are a = 5.2945 (2) Å , c = 11.7470 (5) Å (single crystal measurement). These values represent the thermal volumic expansion coefficient of 1.19 (12) Â 10 À4 K À1 . The c/a ratio in this structure changes from 2.2187 (7) to 2.2223 (4) as a function of the temperature change from 100 to 296 K.
1992; Kuo et al., 2017;Ackerman et al., 2016). The elongation of bonds ( Fig. 1), while decreasing the temperature from 296 K to 100 K, can be attributed to a decrease in the libration effect (German et al., 1987a). A similar phenomenon has previously been observed in the structure of anilinium pertechnetate (Maruk et al., 2010). The greatest distortion of the TcO 4 À anion from an ideal tetrahedral configuration reported by Weaver et al. (2017) is confirmed by our analysis of the O-Tc-O angles in the NaTcO 4 structure, but the difference is not as high as in the model from the neutron diffraction experiment (Weaver et al., 2017). The maximum deviation values are 3.12 at 100 K and 3.08 at 296 K for the sodium salt and are larger in comparison with the potassium and rubidium salts, because the sodium cation has the smallest ionic radius compared to K + and Rb + and hence has the highest polarizing ability. This distortion is insensitive to the temperature change from 100 K to 296 K.
The packing of Na + cations and TcO 4 À anions in the crystal is presented in Fig. 2. Each Na + cation is coordinated by eight The coordination polyhedron of the sodium cation (data from 100 K measurement).

Figure 2
View of the crystal packing of the title compound. oxygen atoms that are belonging to four TcO 4 À anions. The resulting coordination polyhedron can be described as a distorted dodecahedron (Fig. 3). The two dihedral angles between pairs of two triangular faces sharing an edge that connects two five-edged vertices of the dodecahedron are equal to 21.2 and 30.3 , respectively. The corresponding faces should form an angle of 29.5 for a dodecahedron and 0 for a square anti-prism according to the Aslanov-Porai-Koshits criterion (Porai-Koshits & Aslanov, 1972). Hence the coordination polyhedron of the sodium cation is closer to a dodecahedron than to a square anti-prism. Each of the four oxygen atoms of an individual TcO 4 À anion is in contact with two sodium cations, so that each TcO 4 À anion is directly contacted with eight sodium cations.

Synthesis and crystallization
The synthesis of the title compound was carried out based on neutralization of an aqueous solution of freshly prepared HTcO 4 with an equivalent quantity of 1 M aqueous solution of chemically pure sodium hydroxide. The HTcO 4 solution was made by dissolution of Tc 2 O 7 sublimed from TcO 2 in an oxygen flow at 973 K.