Contributions to the stereochemistry of zirconium oxysalts—part V: syntheses and crystal structures of MZr(SeO4)3 (M = Mg, Mn, Co, Ni, Zn, Cd) and Li2Zr(XO4)3 (X = S, Se)

The new compounds M2+Zr(SeO4)3 (M2+  = Mg, Mn, Co, Ni, Zn, Cd) and Li2Zr(XO4)3 (X = S, Se) were synthesized at 220 °C by reaction of Zr2O2(CO3)(OH)2 with hydroxides or carbonates of M / Li and the respective acids H2SeO4/H2SO4. They form crystals up to several tenths of a mm and were investigated by single crystal X-ray diffraction. The framework structures of these selenates can be deduced from that of monoclinic Fe2(SO4)3 in space group P21/n, which is characterized by two types of isolated Fe3+O6 octahedra, corner-linked with three different sulfate groups: ferric iron is substituted in 1:1 ratio by Zr4+ and M2+ as already known for isotypic MZr(SO4)3 representatives. In the case of Li2Zr(XO4)3 members, one additional lithium atom occupies a tetrahedral vacancy of the Fe2(SO4)3 architecture.


Results and discussion
Selected individual and mean bond lengths as well as bond valences (calculated according to [12]) of the title compounds are listed in Table 1. The atomic arrangement of these phases is based on that of the monoclinic archetype structure Fe 2 (XO 4 ) 3 with X = S [5][6][7] or Se [8] in space group P2 1 /n (No. 14), Z = 4. These Fe 2 (XO 4 ) 3 framework structures feature two types of isolated Fe 3+ O 6 octahedra, corner-linked to three different XO 4 tetrahedra. The cations Fe(1) and Fe(2) are centred on general sites. By replacing ferric iron in 1:1 molar ratio with tetravalent zirconium and divalent M cations, balance of charges is maintained, giving rise to a series of isotypic M 2+ Zr(XO 4 ) 3 representatives. The respective sulfates with M 2+ = Mn, Co, Ni, Zn, and Cd [4] exhibit strict order of cations with Zr on the Fe(1) and M 2+ on the Fe(2) octahedral site. It was shown that for Fe 2 (XO 4 ) 3 compounds the mean Fe 3+ -O bond lengths of the Fe(1) site are somewhat shorter (1.978 and 1.986 Å) than those of the Fe(2) position (1.993 and 2.004 Å). The mean Zr [6] -O distance [4] is generally shorter (~ 2.06 Å), as compared to the respective mean M [6] -O bond lengths, which range from 2.06 Å (Ni) up to 2.26 Å (Cd). This might explain the site preference of zirconium for the less distorted and smaller Fe(1)O 6 octahedron. The same holds true for the now-studied selenates with M 2+ = Mg, Mn, Co, Ni, Zn and Cd (see Table 1). Partial cation disorder was only reported [4] for the mixed-valence Fe-dominant sulfate (Fe 3+ 2-2x , Fe 2+ x , Zr x )(SO 4 ) 3 with x ~ 0.105, assuming full site occupancies, where zirconium is preferentially substituting the Fe(1) site with 16% while the Fe(2) position shows only 5% Zr. A representative with x = 1, i.e. an ordered FeZrS phase containing only ferrous iron, could not be synthesized. Our present attempts to synthesize FeZrSe only led to a phase with even less incorporation of zirconium, i.e. only about 5% Zr at the Fe(1) site.
In the case of the new phases Li 2 Zr(XO 4 ) 3 with X = S or Se, one lithium atom is located on the six-coordinated M(2) position; an additional lithium-ion, necessary for charge balance, occupies a tetrahedral vacancy M(3) of the Fe 2 (XO 4 ) 3 framework as illustrated in Fig. 1b. The incorporation of lithium at the M(3) site has a strong influence on the details of the crystal structure. While the geometry of the ZrO 6 octahedron is only slightly affected, the Li(2) O 6 polyhedron shows severe bond-distance and bond-angle distortions, particularly evident in the strongly elongated Li(2)-O5 distances with around 2.67 Å, also allowing an alternative description as pyramidal Li(2)O 5 polyhedron (Figs. 1b and 2; Table 1). This distortion can be attributed, on the one hand, to the additional bond valence contribution of the shortest Li(3)-O5 bond to the bond valence sum ν of O5 (Table 1), thus reducing its attraction to Li(2). On the other hand, when adhering to the description as Li (2) (Fig. 2). Consequently, also the Li(3)O 4 tetrahedron exhibits strong bond-angle (and also bond-length) distortion. Generally, as quantified and discussed in [13], high distortion parameter values are not uncommon for various types of LiO x coordinations. It is, nevertheless, worth emphasizing that the present Lipolyhedra rank among the respective strongest distorted ones reported in [13].
In the compounds of the related Li 3-y V 2 (PO 4 ) 3 structure type [9], as well as LiZr 2 (PO 4 ) 3 [10] or LiZr 2 (AsO 4 ) 3 [11], lithium ions are found on one or more of the potential tetrahedral sites within the framework, while the vanadium or zirconium cations are incorporated, as expected, on the M(1) and M(2) sites. For a detailed description of Table 2 Relevant crystal data and details of the single-crystal intensity measurements and structure refinements for MZr(SeO 4 ) 3 (M = Mg, Mn, Co, Ni, Zn, Cd) and Li 2 Zr(XO 4 ) 3 (X = S, Se). Space group P2 1 /n, Z = 4  Fig. 3 illustrates the specific location of lithium in LiZr 2 P. As observed previously for MZrS compounds [4], the individual tetrahedral X-O distances in the present study are again clearly separated into two groups: oxygen atoms shared with ZrO 6 octahedra have X-O distances which are about 0.03-0.04 Å longer compared with those belonging to M 2+ O 6 polyhedra; for the Li 2 ZrX representatives this difference is even more pronounced (up to 0.06 Å).
In general, the crystal-chemical features of the present M 2+ ZrSe compounds closely resemble those of the respective sulfates reported previously [4]: the lattice parameters, compared in Fig. 4a, deviate from a linear relationship with the M 2+ cationic radii [14], whereas the mean M-O bond lengths (Fig. 4b) obviously behave according to Vegard's rule in both series of compounds. Compared to the sulfates, the larger selenate tetrahedra still increase the overall structural flexibility of the Fe 2 (XO 4 ) 3 structure type. Hence, in contrast to MgZrS, crystallizing in a related superstructure type-a fact attributed in [4] to a tendency towards comparatively larger Mg-O-S angles at the bridging oxygen atoms-the MgZrSe phase now also crystallizes in the 'standard' monoclinic Fe 2 (XO 4 ) 3 structure type. Nevertheless, the tendency to larger Mg-O-X angles is still observable also in the present title compounds: the overall mean M-O-Se and Zr-O-Se angles in the MZrSe phases (without Mg and Li, including Fe 2 (SeO 4 ) 3 [8]) are 135.7° and 145.7°, respectively, those in

Synthesis
The investigated zirconium oxysalts MZr(XO 4 ) 3 were obtained by low-hydrothermal synthesis from stoichiometric amounts of Zr 2 O 2 (CO 3 )(OH) 2 with M(OH) 2 (M = Mg, Co, Ni), Zn 5 (CO 3 ) 2 (OH) 6 or MCO 3 (M = Mn, Cd or Li 2 ), dissolved in access of concentrated H 2 SeO 4 resp. H 2 SO 4 . As soon as the first reactions had calmed down, the mixtures, filled in Teflon-lined steel autoclaves of about 5 cm 3 volume (filling level ≤ 25%), were heated up to a maximum of 220 °C within several hours, kept at this temperature for one week and finally slowly cooled down to room temperature. The reaction products, commonly platy crystals of several tenth of a mm in size, were separated from the remaining liquid and embedded in silicone grease to avoid decomposition.
Single crystal X-ray diffraction data and structure refinement Suitable crystals were selected by optical microscopy and prepared for single-crystal X-ray investigations. Data collections were performed at 200 K on a Bruker APEXII diffractometer equipped with a CCD area detector, an  [14] along the MZrS [4] and MZrSe (present data) series. Note Fe*=Fe 2+ 0.21 Fe 3+ 0.74 Zr 0.05 (see [4]) Incoatec Microfocus Source IµS (30 W, multilayer mirror, Mo-K α ), and an Oxford Cryosystems Cryostream 800 Plus LT device. Several sets of phi-and omega-scans with 2° scan-width were combined at a crystal-detector distance of 35 mm to achieve respective full-sphere data. For data handling including integration and absorption correction (evaluation of multi-scans) the Bruker Apex4 suite was used [15]. The atomic coordinates given for MZrS compounds [4] in space group P2 1 /n with Z = 4 were taken as starting parameters and the crystal structures were refined by full-matrix least-squares techniques (Shelxl [16]). Contrary to MgZrS, the basic structure and not a larger unit cell in space group Pc with Z = 8 was observed for MgZrSe. In case of Li 2 ZrX, subsequent difference Fourier syntheses yielded the positions of the additional lithium atoms. Crystal parameters as well as a summary on the data collections and structure refinements are given in Table 2, final atomic positions and equivalent isotropic displacement parameters are compiled in Table 3. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https:// www. ccdc. cam. ac. uk/ struc tures/ by quoting the CSD deposition numbers CSD 2215922 (MgZrSe), CSD 2215923 (MnZrSe), CSD 2215924 (CoZrSe), CSD 2215925 (NiZrSe), CSD 2215926 (ZnZrSe), CSD 2215927 (CdZrSe), CSD 2215928 (Li 2 ZrSe), and CSD 2215929 (Li 2 ZrS). An extended Table S3 including anisotropic displacement parameters is available as supplementary information.