Role of entropy and structural parameters in the spin-state transition of LaCoO3

Abstract

The spin state transition in LaCoO3 has eluded description for decades despite concerted theoretical and experimental effort. In this study, we approach this problem using fully charge self-consistent Density Functional Theory + Embedded Dynamical Mean Field Theory (DFT+DMFT). We show from first principles that LaCoO3 cannot be described by a single, pure spin state at any temperature. Instead, we observe a gradual change in the population of higher spin multiplets with increasing temperature, with the high spin multiplets being excited at the onset of the spin state transition followed by the intermediate spin multiplets being excited at the metal insulator transition temperature. We explicitly elucidate the critical role of lattice expansion and oxygen octahedral rotations in the spin state transition. We also reproduce, from first principles, that the spin state transition and the metal-insulator transition in LaCoO3 occur at different temperature scales. In addition, our results shed light on the importance of electronic entropy in driving the spin state transition, which has so far been ignored in all first principles studies of this material.

Manifold of spin states and dynamical temperature effects in LaCoO3 : Experimental and theoretical insights

“…The transition occurs in a rather narrow range of p-d parameter variation (less than 0.1 eV), which indicates a high sensitivity of the cobalt ion spin state to the relative positions of the d and p states. These results are in good agreement with previous reports [42][43][44]59,60]. We also found that the weights of d 7 L states make a significant contribution to the total spin state, while the statistical weight of the intermediate d 6 state is about 10% and it remains almost constant through the spin-state transition.…”

“…It was also found that a slight change in the degree of hybridization due to the proximity of the surface layer can induce a spin-state transition at any temperature in the observed region. This scenario agrees well with recently published works [43,59]. The experimental data obtained above 550 K can be well explained assuming a partial spin-state transition from a configuration with a dominant IS state into a configuration with a prevailing HS state.…”

“…It was previously shown that even in the case of the bulk crystal of LaCoO 3 , its electronic structure cannot be properly described by a pure d 6 spin state at any temperature [42,43,62,63]. Correct description of the spin-state transition mechanism requires taking into account the charge and spin fluctuations arising from strong pd hybridization and Coulomb correlations, which lead to the appearance of charge-transfer multiplet states, in addition to states with a formal valence of Co [44,45].…”

“…Dynamic mean-field theory (DMFT) calculations were carried out using the AMULET package (amulet.org) with the state-of-the-art continuous-time quantum Monte Carlo solver [38][39][40][41] applied to all five 3d orbitals of the Co ion. A Hubbard U = 6 eV and Hund's coupling J = 0.8 eV were used [42,43]. The crystal structure data obtained from the experiment at the temperatures T = 20, 300, and 700 K were used as a starting point for the DFT calculations.…”

Исследование Твердотельной Структурной Динамики С Помощью Сверхбыстрой Электронной Дифракции И Микроскопии

“…Так, для иона кобальта в этом перовските возможны одновременно несколько типов спинового состояния и несколько типов зарядового состояния. Экспериментальные данные указывают на две важные модификации электронной структуры LaCoO 3 , происходящие по мере роста температуры системы: диамагнитный-парамагнитный спиновый переход около 100 K и фазовый переход полупроводник-металл между 350 и 650 К. Несмотря на большое количество работ в области физики кобальтита лантана, единое понимание электронных свойств и изменений магнитной конфигурации этой системы продолжает разрабатываться [25,26].…”