CRYSTAL STRUCTURES OF THE HYDROTHERMALLY SYNTHESIZED CHROMATES KSc3(CrO4)2(OH)6, KIn3(CrO4)2(OH)6, RbIn3(CrO4)2(OH)6, AND AgIn3(CrO4)2(OH)6: A CONTRIBUTION TO THE CRYSTAL CHEMISTRY OF THE ALUNITE SUPERGROUP

Four new alunite-type chromates, KSc3(CrO4)(2)(OH)(6), KIn3(CrO4)(2)(OH)(6), RbIn3(CrO4)(2)(OH)(6), and AgIn3(CrO4)(2)(OH)(6), have been prepared by mild hydrothermal synthesis at T = 220 degrees C as well-developed, thick tabular to pseudo-octahedral crystals with maximum dimensions between approximately 0.5 and 1 mm. The crystal structures were refined from single-crystal intensity data (MoK alpha X-radiation, CCD area detector, 293 K, 20(max) = 70 degrees). The new members adopt the alunite parent structure-type (space group R3m, no. 166), with a = 7.763(1)/7.813(1)/7.817(1)/7.845(1), c = 17.575(3)/17.682(3)/18.075(3)/16.997(3) angstrom, V = 917.2(2)/934.8 (2)/956.5(2)/905.9(2) angstrom(3) (Z = 3), and R(F) = 1.36 / 1.21 / 1.23 / 1.33%, respectively. The H atoms could be located in each compound. Hydrogen bonds are all within a very close range (O3...O1 = 2.959-3.020 angstrom). All alkali and Sc/In sites are fully occupied, and the alkali atoms do not show any positional disorder, unlike Ag which is distinctly off-origin in AgIn3(CrO4)(2)(OH)(6). Average bond-lengths are as follows: K-[12]-O = 3.003, Sc-[6]-O = 2.106, Cr-[4]-O = 1.653 angstrom (KSc member); K-[12]-O = 3.000, (InO)-In-[6] = 2.145, Cr-[4]-O = 1.653 angstrom (KIn); Rb-[12]-O = 3.051, In-[6]-O = 2.147, Cr-O = 1.653 angstrom (RbIn); and Ag-[9]-O = 2.826, In-[6]-O = 2.142, Cr-[4]-O = 1.648 angstrom (AgIn). The origin and possible meaning of a small, but conspicuous residual-density peak at (0, 0, 0.5) in the In members is discussed. These chromates represent the first structurally characterized Sc and In members of the large alunite supergroup, and may serve as analogues in future studies of naturally occurring Fe or sulfate members. Alunite-type Cs analogues could not be synthesized hydrothermally, in agreement with the fact that no natural or synthetic Cs compounds with the alunite topology have been reported so far. Instead, the syntheses yielded orthorhombic CsSc(CrO4)(2) [CsCr3+(Cr6+O4)(2)-type] and monoclinic CsIn(CrO4)(2) [(NH4)Fe(CrO4)(2)-type].