THE EFFECT OF REDUCING ENVIRONMENTS ON THE STABILITY OF ACCEPTOR SUBSTITUTED YTTRIUM CHROMITE

The dimensional stability of calcium-substituted yttrium chromite was evaluated over a wide range of temperatures, oxygen partial pressures, and compositions, and compared to data collected for calcium-doped la nthanum chromite. Sample expansion was dependent on the concentrations of the acceptor dopant and CaCrO4, a secondary phase observed in samp les with greater than 25 mole% calcium, as well as measurement tempera ture and the oxygen partial pressure. The instability of yttrium chrom ite in reducing environments was directly related to the loss of latti ce oxygen and the simultaneous reduction of Cr4+ to Cr3+ to maintain e lectroneutrality. The evolution of oxygen and the reduction of chromiu m resulted in a lattice expansion, caused by the size change associate d with the Cr4+ to Cr3+ transition and electrostatic repulsion between adjacent oxygen octahedra. The measured expansion of yttrium chromite s was 42% less than that for similarly doped lanthanum chromites. The reduction in expansion is a result of the decreased size of the unit c ell (6.5% smaller than calcium-doped lanthanum chromite) which suppres ses expansion by hindering the reduction of Cr4+ and oxygen evolution during reduction.