Structural properties and the pressure-induced C → A phase transition of lanthanide sesquioxides from DFT and DFT + U calculations

Abstract

Density functional theory (DFT) calculations were performed to analyze the structural properties of rare-earth lanthanide sesquioxides (Ln2O3) with the hexagonal A and the cubic C phases. The calculations were performed with the DFT-based Augmented Plane Wave plus local orbital (APW + lo) method, using both the local density approximation (LDA) and the Wu and Cohen parametrization of the generalized gradient approximation (WC-GGA) for the exchange and correlation energy. We also considered the addition of the Hubbard U term in the WC-GGA approximation (GGA + U approach), to take into account the strongly correlated Ln-4f electrons involved. The predicted equilibrium properties for both phases of each Ln2O3 system, i.e. crystal equilibrium volume, bulk modulus and its first pressure derivative are in good agreement with the corresponding experimental results. The obtained predictions with LDA and WC-GGA reveal that the A phase is stable for the lighter systems of the Ln2O3 sesquioxide series, and the C phase is favored for the heavier Ln2O3, in a qualitatively good agreement with the experimental data. However, the predicted C to A phase transition pressure PC→A, as well as the sesquioxide equilibrium volumes corresponding to the lanthanides of the middle of the series, underestimates the experimental values. The implementation of the U term gives a correct description of the insulating ground state of these systems, and also improves the predictions for the studied structural properties with respect to those predicted by LDA and WC-GGA. However, PC→A is extremely sensitive to the U parameter, giving the GGA + U approach method very poor predictions for this quantity. The reasons of these effects are analyzed in detail.