Crystallographic studies of some mixed manganite spinels

Abstract

A crystallographic study has been made of solid solutions of normal, cubic copper manganite with normal, tetragonally distorted manganites of Mn²⁺, Co²⁺, Cd²⁺, Zn²⁺ and Mg²⁺. The compositions may be represented by A_x²⁺Cu_1-x²⁺[Mn³⁺Mn³⁺]O₄, and the minimum amounts of Cu²⁺ required in different cases for tetragonal to cubic transition have been determined as 58, 53, 48, 83 and 28 mole per cent, respectively. Considering these values as a measure of the stability of the distorted lattice, it has been concluded that the relative contribution of the A-sited cations in stabilizing tetragonal distortion at the B-site follows the order Zn²⁺ >Mn²⁺ >Co²⁺ >Cd²⁺ >Mg²⁺. A plot of the cube root of the volume of the unit cell against compositions shows an inflexion at the transition region indicating that in the cubic region the distortion of the individual octahedron is suppressed by the strain energy of the lattice. At the transition point, cooperative elongation and ordering of the C-axis take place overcoming the strain imposed by the surrounding cubic lattice and thus a tetragonal distortion appears.