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

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.

Hydrothermal Synthesis, Crystal Structure, and Magnetism of Na2[Ir(OH)6] and its Dehydration to Na2IrO3

“…Na 2 [Ir(OH) 6 ] crystallizes as a hettotype of Mg(OH) 2 (brucite) and is isostructural to Na 2 [Sn(OH) 6 ] (Figure 2). A detailed structural comparison, including a Bärnighausen tree, of the aristotype Mg(OH) 2 and structurally related hexahydroxidometalates A 2 [ M (OH) 6 ] can be found in our publication on CaNa[Cr(OH) 6 ] [21] …”

“…]. [21] In Na 2 [Ir(OH) 6 ], the oxygen atoms are arranged in a hexagonal close-packing, Na + and Ir 4 + ions occupy the octahedral voids in every second layer, the voids of the next layers remain empty. The rhombohedral centring (3R stacking sequence) originates from the cation ordering in the layers.…”

The Hydrogarnets Sr3[RE(OH)6]2 (RE = Sc, Y, Ho – Lu): Syntheses, Crystal Structures, and their Thermal Decomposition to Ternary Rare‐Earth Metal Oxides

“…Only a few publications state precise values for the O–H bands of katoite type hydrogarnets; recently we reported the O–H valence vibration of hydrogarnets with transition metals: Ca 3 [Cr(OH) 6 ] 2 (3616 cm –1 ), Sr 3 [Cr(OH) 6 ] 2 (3625 cm –1 ) and Sr 3 [Rh(OH) 6 ] 2 (3592 cm –1 ) …”

“…At 655 °C the formation of Sr 4 Fe 3 O 10–δ starts in a second reaction step . Hydrogarnets with main group and transition metal ions were thoroughly investigated . However, only little is known about rare‐earth metal containing hydrogarnets.…”

Ba3[Rh(OH)6]2 ⋅ H2O – a Precursor to Barium Oxorhodates with One‐dimensional Hydrogen Bonding Systems

“…The octahedra layers perpendicular to all < 111 > directions are plane, as had been found in many other hydroxometalates and metal hydroxides, e.g., for the [Cr(OH) 6 ] 3À octahedra in the brucite-like structure of CaNa[Cr(OH) 6 ]. [23] The barium cations and the water molecules fill all tetrahedral voids in this ccp packing, which corresponds to the anti-CaF 2 or Li 2 O structure type ðFm � 3mÞ. Its decorated variant K 2 PtCl 6 ðFm � 3mÞ might be more suitable for a structural comparison, as the calcium atoms are replaced by [PtCl 6 ] 2À octahedra (Figure S5, Supporting Information).…”