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 lanthanum chromite. Sample expansion was dependent on the concentrations of the acceptor dopant and CaCrO4, a secondary phase observed in samples with greater than 25 mole% calcium, as well as measurement temperature and the oxygen partial pressure. The instability of yttrium chromite in reducing environments was directly related to the loss of lattice oxygen and the simultaneous reduction of Cr4+ to Cr3+ to maintain electroneutrality. The evolution of oxygen and the reduction of chromium resulted in a lattice expansion, caused by the size change associated with the Cr4+ to Cr3+ transition and electrostatic repulsion between adjacent oxygen octahedra. The measured expansion of yttrium chromites was 42% less than that for similarly doped lanthanum chromites. The reduction in expansion is a result of the decreased size of the unit cell (6.5% smaller than calcium-doped lanthanum chromite) which suppresses expansion by hindering the reduction of Cr4+ and oxygen evolution during reduction.