Compositional phase stability of strongly correlated electron materials within DFT+ U

Abstract

Predicting the compositional phase stability of strongly correlated electron materials is an outstanding challenge in condensed matter physics. In this work, we employ the density functional theory plus U (DFT+U ) formalism to address the effects of local correlations due to transition metal d electrons on compositional phase stability in the prototype phase stable and separating materials LixCoO2 and olivine LixFePO4, respectively. We exploit a new spectral decomposition of the DFT+U total energy, revealing the distinct roles of the filling and ordering of the d orbital correlated subspace. The on-site interaction U drives both of these very different materials systems towards phase separation, stemming from enhanced ordering of the d orbital occupancies in the x = 0 and x = 1 species, whereas changes in the overall filling of the d shell contribute negligibly. We show that DFT+U formation energies are qualitatively consistent with experiments for phase stable LixCoO2, phase separating LixFePO4, and phase stable LixCoPO4. However, we find that charge ordering plays a critical role in the energetics at intermediate x, strongly dampening the tendency for the Hubbard U to drive phase separation. Most relevantly, the phase stability of Li 1/2 CoO2 within DFT+U is qualitatively incorrect without allowing charge ordering, which is problematic given that neither charge ordering nor the band gap that it induces are observed in experiment. We demonstrate that charge ordering arises from the correlated subspace interaction energy as opposed to the double counting. Additionally, we predict the Li order-disorder transition temperature for Li 1/2 CoO2, demonstrating that the unphysical charge ordering within DFT+U renders the method problematic, often producing unrealistically large results. Our findings motivate the need for other advanced techniques, such as DFT plus dynamical mean-field theory, for total energies in strongly correlated materials.

Magnetism of (LaCoO3)n+(LaTiO3)n superlattices ( <

“…9(b). While the change of ∆E AFM fill as a function of U Co is relatively weak, ∆E AFM ord is the dominating contribution, similar to prior studies [41,42]. Therefore, the U Co behavior is dictated largely by the difference in σ 2 of the two respective spin configurations.…”

“…9(a). To explain this strong dependence, we analyze the difference E[HS AFM]−E[LS AFM] by rewriting the DFT+U total energy as a spectral decomposition [41,42]…”

Electronic structure and polar catastrophe at the surface of LixCoO2 studied by angle-resolved photoemission spectroscopy

“…The dispersion of the incoherent component would be related to incoherent hopping transport of Co 4+ species in background of low-spin Co 3+ as suggested in other triangular-lattice Co oxides. [11] This picture is consistent with the observation of Co 4+ and Co 3+ peaks in the O 1s XAS spectra [32] as well as the strong charge fluctuation suggested by recent theoretical [37] and optical [38] studies. The present ARPRES study shows that the x=0.46 sample exhibits homogeneous electronic states compared to the in homogeneous…”

Magnetism of (LaCoO$_3$)$_n$+(LaTiO$_3$)$_n$ superlattices with $n=1,2$

“…6(b). While the change of ∆E AFM fill as a function of U Co is relatively weak, ∆E AFM ord is the dominating contribution, similar to other prior cases [38,39]. Therefore, the U Co behavior is dictated largely by the difference in σ 2 of the two respective spin configurations.…”