Intriguing Interconnections Among Phase Transition, Magnetic Moment, and Valence Disproportionation in 2H-Perovskite Related Oxides

In this paper we report the crystal growth, structure determination, and magnetic properties of the 2H-perovskite related oxides, Sr5Co4O12 and Sr6Co5O15, as well as the charge disproportionation and associated phase transition of Sr5Co4O12. Sr5Co4O12 and Sr6Co5O15 are the (m = 2, n = 3) and (m = 1, n = I) members of the A(3m+3n)A'nB3m+nO9m+6n family, respectively. Sr6Co5O15 crystallizes in the space group R32 with lattice parameters of a = 9.5020(10) angstrom and c = 12.379(8) angstrom. The structure solution shows that Sr6Co5O15 is isostructural with Sr6Rh5O15. Magnetic measurements do not indicate any long-range magnetic order, although the Weiss temperature of -248 K indicates the presence of dominant antiferromagnetic interactions. Sr5Co4O12 crystallizes in the space group P-3c1 with lattice parameters of a = 9.4705(10) angstrom and c = 20.063(5) angstrom at room temperature. The single crystal structure solution revealed that the cobalt ions in the trigonal prismatic sites of Sr5Co4O12 undergo a structural transition at similar to 170 K, where the cobalt atoms are in the center of the trigonal prisms below this temperature and move partially toward the faces above this temperature. This structure transition is accompanied by a change in the magnetic moment of the oxide and can be related to a valence disproportionation of the cobalt ions and a concomitant Jahn-Teller distortion. In addition, specific heat, Seebeck coefficient, electric conductivity, and magnetic measurements as well as bond valence sum calculations were carried out for Sr5Co4O12. Sr5Co4O12 exhibits strong magnetic anisotropy but no long-range magnetic order.