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CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

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Ruck,  Michael
Michael Ruck, Max Planck Fellow, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Citation

Albrecht, R., Hunger, J., Doert, T., & Ruck, M. (2020). CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition. Zeitschrift für anorganische und allgemeine Chemie; Verlag Johann Ambrosius Barth / Wiley-VCH, Leipzig, Weinheim, 646(14), 1130-1137. doi:10.1002/zaac.202000052.


Cite as: http://hdl.handle.net/21.11116/0000-0006-4DF5-0
Abstract
Green single-crystals of the hydroxochromate(III) CaNa[Cr(OH)6] were grown under highly alkaline hydrothermal conditions at about 200 °C. The starting materials Ca(NO3)2·6H2O and Cr(NO3)3·9H2O were reacted in a mixture of water and sodium hydroxide with the molar ratio of 2.8:1. CaNa[Cr(OH)6] crystallizes in the non-centrosymmetric trigonal space group R3 with the lattice parameters a = 583.86(2) pm and c = 1428.73(6) pm [T = 100(1) K]. Characteristically, the crystals are reverse-obverse as well as inversion twins. The crystal structure is a stack of uncharged metal hydroxide layers, which can be regarded as a cation-ordered rhombohedral variant of the Mg(OH)2 (brucite) structure type. The oxidation state of chromium(III) and its coordination by hydroxide groups was confirmed by UV/Vis and IR spectroscopy, respectively. Temperature-dependent magnetic measurements revealed paramagnetic behavior with an effective moment of 3.82 μB per chromium atom. The thermal decomposition of CaNa[Cr(OH)6] takes place at about 225 °C, where the fast elimination of 1.5 equivalents of water is followed by the oxidation of chromium(III) to chromium(VI). Upon further heating to 1000 °C and 1200 °C, the intermediate decomposition products CaCrO4 and Na2CrO4 transform into the oxochromates(V) Ca5(CrO4)3O0.5 and Ca3(CrO4)2, respectively. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.