Gd-4f Exchange Splitting and Mo-4d Crystal-Field Redistribution in Gd/W Co-doped La2Mo2O9: A DFT+U Study

Abstract

La2Mo2O9 (LAMOX) is a promising oxide-ion conductor for intermediate-temperature solid oxide fuel cells, but its practical application is limited by a first-order monoclinic-to-cubic (alpha to beta) phase transition. Here, we investigate the electronic structure of pristine La2Mo2O9 and the Gd/W co-doped composition La1.6Gd0.4Mo1.7W0.3O9 using spin-polarized density functional theory with an on-site Hubbard correction for the localized Gd-4f states. The projected density of states reveals that pristine La2Mo2O9 is an O-2p/Mo-4d charge-transfer oxide in which La contributes negligibly near the band edges. Gd/W co-doping introduces a strongly exchange-split Gd-4f manifold with a majority-minority separation of approximately 1011 eV, substantially redistributes the Mo-4d electronic states through a threefold increase in crystal-field splitting, and reduces the O-2p contribution near the valence-band maximum from 78% to 66%. These electronic signatures are consistent with the experimentally observed lattice contraction, MoO Raman-mode softening, and the non-monotonic evolution of oxide-ion conductivity across the co-doped series. In particular, the pronounced crystal-field splitting and localized Gd magnetism at the highest doping level provide a microscopic electronic explanation for the observed suppression of ionic conductivity. The present results establish an atomistic electronic-structure framework for understanding dopant-induced phase stabilization and oxide- ion transport in Gd/W co-doped LAMOX electrolytes, providing design principles for improved solid oxide fuel cell materials.