Anomalous lattice compression in the hexagonal La2O3– A high pressure X-ray diffraction, Raman spectroscopy and first principle study

Abstract

High pressure behavior of hexagonal rare earth sesquioxide La2O3 have been investigated using X-ray diffraction, Raman spectroscopy and first principle calculations. Though the hexagonal structure is stable up to 26.5 GPa, incompressibility along the a axis is visible above 9.7 GPa. Rietveld structure refinements in conjunction with the Stephans anisotropic strain broadening model revealed the onset of an atypical bond compression and modification in the relative intensities of the diffraction peaks at ∼5.6 GPa. A significant increase in the intensity of 100 reflection and a clear reduction in the intensity of 103 reflection above 5.6 GPa suggests that pressure induces sliding of the adjacent La–O layers in opposite directions rather than compressing along the a axis. Further, a negative correlation of in-plane strain with pressure above 5.6 GPa indicate the layer sliding is accompanied by a reduction of in-plane strain in the hexagonal structure. Raman studies show the lifting of the degeneracy of the two stretching modes, in-plane Eg and out of plane A1g, and the change of slope of ω vs. P curve for the two in-plane Eg modes at 3.6 GPa. First principle density functional theory calculation substantiates our experimental observations that the pressure induced anisotropic strain distribution is driving the system into low energy configurations. The Birch –Murnaghan equation of state fit to the experimental pressure-volume data yielded a bulk modulus value of B0 = 102(5) GPa with B′0 = 9.8(1.0) for P ≤ 9.7 GPa, in good agreement with the 114 GPa of density functional theory calculations.