Stability of tantalum-doped Li₇La₃Zr₂O₁₂ (LLZO) via cluster expansion and Monte Carlo

Materials Modelling Centre, University of the Limpopo, South Africa Kemeridge.malatji@ul.ac.za, refiloekhomotso@gmail.com, cliffton.masedi@ul.ac.za, phuti.ngoepe@ul.ac.za, raesibe.ledwaba@ul.ac.za

^* Corresponding author: kemeridge.malatji@ul.ac.za

Abstract

This study employs first-principles calculations and cluster expansion to explore new stable phases of Ta-doped LLZO (Li5La3Zr2-xTaxO12) to enhance the conductivity of the tetragonal structure. Monte Carlo simulations further analyze the temperature-dependent behavior of these phases under the canonical ensemble and determine phase transition of mixed systems. The structures were selected by identifying those with the lowest energy at various compositions from the cluster expansion, where the low cross-validation score (CVS < 5 meV) guarantees the predicted energies are highly accurate and physically meaningful, allowing for a reliable assessment of thermodynamic stability. The structures were selected by identifying those with the lowest energy at various compositions from the cluster expansion. The cluster expansion reveals 28 miscible multi-component structures, all thermodynamically stable with negative formation enthalpies. Monte Carlo results indicate no phase separation, with optimal mixing at ~900 K. These findings suggest Ta doping improves structural stability without compromising miscibility, offering potential pathways for higher ionic conductivity.