Study of the volume and spin collapse in orthoferrite LuFeO 3 using LDA + U

Rare-earth $(R)$ orthoferrites $R{\text{FeO}}_{3}$ exhibit large volume transitions associated with a spin collapse. We present here ab initio calculations on ${\text{LuFeO}}_{3}$. We show that taking into account the strong correlation among the $\text{Fe-}3d$ electrons is necessary. Indeed, with the $\text{LDA}+\text{U}$ method in the projector augmented wave, we are able to describe the isostructural phase transition at 50 GPa, as well as a volume discontinuity of 6.0% at the transition and the considerable reduction in the magnetic moment on the Fe ions. We further investigate the effect of the variation in $U$ and $J$ and find a linear dependence of the transition pressure on these parameters. We give an interpretation for the nonintuitive effect of $J$. This emphasizes the need for a correct determination of these parameters especially when the $\text{LDA}+\text{U}$ is applied to systems (e.g., in geophysical investigations) where the transition pressure is a priori unknown.