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Gittinsite-type M¹⁺-M³⁺-diarsenates (M¹⁺ = Li, Na; M³⁺ = Al, Sc, Ga): insights into an unexpected isotypy and crystal chemistry of diarsenates

¹ Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstr. 14, A-1090 Wien, Austria
² Mineralogisch-Petrographische Abt., Naturhistorisches Museum, Burgring 7, A-1010 Wien, Austria

^* E-mail: karolina.schwendtner{at}univie.ac.at

Hemimorphic crystals of four M¹⁺-M³⁺-diarsenates (M¹⁺ = Li, Na; M³⁺ = Al, Sc, Ga) were prepared by mild hydrothermal methods (T = 493 K) at very low pH values of ~1–1.5. The crystal structures of all compounds were determined using single-crystal X-ray diffraction data (Mo-K, CCD detector, 293 K) and were found to be isotypic with gittinsite (CaZrSi₂O₇), and the later described synthetic compound LiFeAs₂O₇. Crystal data for LiAlAs₂O₇, LiScAs₂O₇, LiGaAs₂O₇ and NaScAs₂O₇, respectively, are: space group C2 (no. 5), a = 6.583(1), 6.790(1), 6.638(1), 6.959(1), b = 8.007(2), 8.532(2), 8.181(2), 8.712(2), c = 4.635(1), 4.845(1), 4.696(1), 4.817(1) Å, ß = 104.13(3), 103.75(3), 104.01(3), 102.05(3)°, V = 236.92(9), 272.64(9), 247.43(9), 285.6(1) ų; Z = 2. The structure models were refined anisotropically to R1(F) = 0.0152, 0.0157, 0.0144, 0.0199, respectively.

The non-centrosymmetric framework structure contains isolated As₂O₇ groups (in staggered configuration; As–O–As angles show a narrow range between 130.32(16) and 139.8(2)°) which share their O ligands with MO₆ (M = Al, Sc, Ga) octahedra and distorted Li/NaO₄ tetrahedra. The strongly distorted LiO₄ tetrahedra exhibit an average Li–O bond length of 2.071 Å, but four longer Li–O bonds (between 2.781–3.042 Å) also contribute to give a reasonable bond-valence sum for the Li atoms, and reflect an effective [4+4]-coordination. The Na compound is also characterized by four short and four long Na–O bonds; the Na atom appears slightly overbonded. The average Sc–O bond length in the two Sc representatives is 2.083 and 2.097 Å.

Details of the topology and crystal chemistry are discussed in comparison with those of gittinsite and other diarsenates, and correlations are noted. A reevaluation of the geometry of As₂O₇ groups in 58 presently known and structurally characterized inorganic diarsenates shows a mean As–O–As angle of 124.2° (excluding 180° angles). The As–O bond lengths to the central bridging O atom are elongated to a mean value of 1.755 Å, whereas all other bond lengths are shortened (mean value 1.666 Å); the overall mean As–O bond length is 1.688 Å.

KEYWORDS: diarsenate, lithium, scandium, gittinsite, crystal structure, crystal chemistry

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