Structural stability of orthorhombic and rhombohedral La_0.75Nd_0.25CrO₃: a high-temperature neutron diffraction study

Abstract

A high-temperature neutron diffraction study of La_0.75Nd_0.25CrO₃ compound has been carried out in the temperature range 300–1400 K. The sample has been studied for its structural stability of the two observed crystallographic phases at elevated temperatures. It was found to contain orthorhombic and rhombohedral phases with weight percentages 85% and 15%, respectively, at ambient temperature. It was observed that the two phases coexist even up to 1400 K; however, the fraction of the rhombohedral phase increases with increasing temperature at the expense of the orthorhombic phase. At 1400 K, the weight fractions of the orthorhombic and rhombohedral phases are about 11% and 89%, respectively. This study reveals a negative thermal expansion along the b-axis for the orthorhombic phase and along the c-axis for the rhombohedral phase from 950 K up to 1200 K, where a rapid transformation of one phase to the other occurs. Below 950 K as well as above 1200 K, both the phases show a normal positive thermal expansion. The La–La and Cr–Cr bond distances show an abrupt drop from 950 K up to 1200 K and a sharp rise after that, which corroborates the negative thermal expansion of the cell volume and b-axis of the orthorhombic phase. The Cr–Cr distance in the rhombohedral phase shows a reduction in magnitude in the temperature range 950–1200 K, explaining the negative thermal expansion of the c-axis of this phase. The coefficients of volume thermal expansion and linear thermal expansion have been determined for both the phases. The significance of the present study has been discussed for practical applications of the studied compound in solid oxide fuel cells.