CRYSTAL-CHEMISTRY AND TRANSPORT-PROPERTIES OF ND2-XAXNIO4 (A = CA, SR, OR BA, 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.4)

Structural and electrical properties of the system crystallizing in the K2NiF4 structure, Nd2-xAxNiO4 (A = Ca, Sr, or Ba, 0≤x≤1.4), were studied. The solid solution limits of alkaline earth were 0.6, 1.4, and 0.6 for Ca, Sr, and Ba, respectively. Oxygen content was controlled by annealing under various oxygen pressures below 150 atm. Powder X-ray diffraction data were analyzed by the Rietveld method to determine the bond lengths. In the case of the Sr-substituted system, tetragonality in the lattice parameters, c/a, shows a maximum at x = 0.6, while distortion of a NiO6 octahedron, Ni-O(2) (//c-axis)/Ni-O(1) (//a-axis), monotonically decreases from 1.13 (x=0) to 1.04 (x = 1.4). No Jahn-Teller distortion appears in a Ni3+ region. A metal-semiconductor transition is observed for O<x<1.0, the transition temperature decreasing from 450 K (x = 0.1) to 100 K (x = 1.0), but it exhibits a change in character at x = 0.6 where the sign of the Seebeck coefficient also changes. © 1992 Academic Press, Inc.