CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition. Issue 14 (16th April 2020) - Albrecht, Ralf
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 · 6H2 O and Cr(NO3 )3 · 9H2 O 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 R 3 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 Na2 CrO4 transform into the oxochromates(V) Ca5 (CrO4 )3 O0.5 and Ca3 (CrO4 )2, respectively. Abstract :