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On the Crystal Chemistry of Alkali Metal Oxocobaltates
Author(s) -
MüllerBuschbaum Hanskarl
Publication year - 2012
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.201200076
Subject(s) - valence (chemistry) , alkali metal , chemistry , cobalt , crystallography , oxidation state , coordination number , inorganic chemistry , coordination complex , metal , bond length , crystal chemistry , oxygen , crystal structure , ion , organic chemistry
This Review on the crystal chemistry of oxocobaltates of alkali metals is presented according to the chemical composition. Classification of alkali oxocobaltates mainly depends on an increasing oxygen content of the compounds. Identical chemical compositions are sorted with increasing atomic numbers of the alkali metals. Cobalt is observed in the valence states Co + , Co 2+ , Co 3+ and Co 4+ . No other metal shows such a close dependence of coordination and valence state. The oxygen coordination of Co + in A 3 CoO 2 ( A = Na, K, Rb) and K 2 CsCoO 2 is dumbbell‐shaped, the bond angles O–Co–Co being close to 180°. Coordination number two for the alkali metals ( A ) in these compounds is not less interesting, with the bond angles showing large deviations from a stretched O– A –O configuration. Cobalt in the valence state Co 2+ (K 2 CoO 2 ; Rb 2 Co 2 O 3 ; Na 4 CoO 3 ; Rb 2 K 4 Co 2 O 5 ; K 2 Na 4 Co 2 O 5 ; RbNa 7 Co 2 O 6 ; Na 10 Co 4 O 9 ) prefers a trigonal planar coordination. There is only one compound (Na 7 Co 2 O 6 ) showing mixed valences Co 2+ /Co 3+ together with a triangular coordination. Tetrahedral coordination is typical for cobalt in the valence state Co 3+ (K/RbCoO 2 ; Na 6 Co 2 O 6 ; Na 5 CoO 4 ; Na 2 Li 3 CoO 4 ). Few compounds (K/Cs/Rb 2 CoO 3 ; Na 4 CoO 4 ; Li 8 CoO 6 ; K 6 Co 2 O 7 ) show Co 4+ beside Co 3+ in tetrahedral coordination. A small amount of oxocobaltates show octahedral coordination with cobalt in the valence states Co 3+ (LiCoO 2 ; NaCoO 2 ) and higher (NaCo 2 O 4 ; K 4 Co 7 O 14 ). Finally, one mixed alkali rare earth oxocobaltate, Nd 18 Li 8 Co 4 O 39 , is presented. Several oxocobaltates reported in the ICSD database show cobalt on different point positions. In these cases it is of interest to calculate M adelung P arts of L attice E nergie (MAPLE) established by Rudolf Hoppe . MAPLE allows calculation of the differences between the Coulomb‐terms of the lattice energy, which exclusively depends on the valences of cobalt on different point positions. In all comparable cases identical positional configurations of the atoms prevent influences of the repulsion term of the lattice energy. The energy balance therefore exclusively depends on the valence states of cobalt and its distribution on the point positions, with Rb 2 Co 2 O 3 , Na 4 CoO 3 , Na 8 Co 2 O 7 , Na 10 Co 4 O 9 , and Na 10 Co 4 O 10 being examples.