Structural phase transition, equation of state and phase diagram of functional rare earth sesquioxide ceramics (Eu1−xLax)2O3

Abstract

The intriguing functional nature of ceramics containing rare earth sesquioxide (RES) is associated with the type of polymorphic structure they crystallize into. They prefer to be in the cubic, monoclinic or hexagonal structure in the increasing order of cation size, R_RE. Since the functional properties of these ceramics varies with R_RE, temperature and pressure, a systematic investigation delineating the cation size effect is indispensable. In the present work we report the structural stability and compressibility behaviour of the RES ceramics, (Eu_1−xLa_x)₂O₃, of RESs with dissimilar structure and significant difference in cationic radii. The selected compositions of (Eu_1−xLa_x)₂O₃ have been studied using the in-situ high pressure synchrotron X-ray diffraction and the structural parameters obtained through Rietveld refinement. The cubic structure, which is stable for 0.95 Å $\le$ R_RE$<0.98$ Å at ambient temperature and pressure (ATP), prefers a cubic to hexagonal transition at high pressures. The biphasic region of cubic and monoclinic structure, which is stable for 0.98 Å $\le$ R_RE$<1.025$ Å at ATP, prefers a cubic/monoclinic to hexagonal transition at high pressures. Further, in the biphasic region of monoclinic and hexagonal structure, observed for 1.025 Å $\le$R_RE$<1.055$ Å, the monoclinic phase is found to be progressing towards the hexagonal phase with increasing pressure. The pure hexagonal phase obtained for 1.055 Å $\le$ R_RE$\le$ 1.10 Å is found to be structurally stable at high pressures. The bulk moduli are obtained from the Birch–Murnaghan equation of state fit to the compressibility data and its dependance on the cation size is discussed. The microstrain induced by the difference in cation size causes an internal pressure in the crystal structure leading to a reduction in the bulk modulus of $\mathit{x}=0.2$ and 0.6. A pressure–concentration (P–x) phase diagram upto a pressure of 25 GPa is constructed for (Eu_1−xLa_x)₂O₃. This would provide an insight to the fundamental and technological aspects of these materials and the RESs in general.