Strain-induced heteronuclear charge disproportionation in EuMnO3

Abstract

Charge disproportionation transitions in complex oxides most commonly link high-temperature phases containing identical cations with the same oxidation state and crystallographic site, to lowtemperature phases in which charge transfer between these ions results in unequal oxidation states.Here we propose, based on density functional calculations, a related concept that we term heteronuclear charge disproportionation, in which charge transfer occurs between different elements on different crystallographic sites. We show for the case of EuMnO3 that such a transition from the experimentally observed Eu 3+ Mn 3+ O3 phase to the so far unknown Eu 2+ Mn 4+ O3 phase can be triggered by pressure and epitaxial strain. We then identify measurable signatures to aid in an experimental exploration of the complex pressure-and biaxial strain-dependent phase stability of EuMnO3 that we hope to motivate with our predictions. We suggest other candidate crystal chemistries for heteronuclear charge disproportionation, in which novel physics could emerge from the coexistence of instabilities. arXiv:1810.09204v1 [cond-mat.mtrl-sci]

Eu–Mn Charge Transfer and the Strong Charge–Spin–Electronic Coupling Behavior in EuMnO3

“…In other words, phase transitions are necessary to contribute to the stability of the EuMnO 3 system under high pressure (positive or negative) owing to the convenience of Eu/Mn valence transition. On the other hand, we predicted the bulk modulus of E2M4 to be 188 ± 5 GPa by using eq , in good agreement with the value of 182 GPa calculated by Aschauer et al As portrayed in the bottom panel of Figure b, the P – V trends with the E2M4 structure and the fitting data using eq match very well except for those with too large volume, which is attributed to the valence and structure change with volume, displayed in Figure a with light-green circular marks. In addition, the energy comparison under E2M4/FG and E3M3/FA settings elucidates that the E2M4 is indeed a metastable phase within an unrestrained condition but could be expected under high pressure if the structure and distortion conditions were met.…”

“…For instance, based on the optimized structure of 210 Å 3 with an initial structure of E3M3/FA, the self-consistent energy under FG magnetic ordering, in which all Eu ions are Eu 2+ , exhibits a higher energy than that with FA ordering, as shown in Figure a marked by pink hollow square. In addition, it is should be noted that the occurrence of charge transfer in E3M3 under hydrostatic pressure at the critical volume of 210 Å 3 actually causes lattice variation of −0.19% (axis- a ) and −9.20% (axis- b ), which present more difficult transfer conditions compared with the prediction reported by Aschauer et al It is attributed to the different choice of the U values in the DFT + U method and the critical behavior needs further experimental verification. Overall, there exist close couplings between magnetism, charge, and structure in the transition region of charge and structural evolutions, which would give rise to the regulation of magnetic couplings and electronic tailoring.…”

“…As a consequence, the values of U Eu = 11 eV and U Mn = 3 eV were determined as ultimate selected parameters, which were proved to be appropriate to reproduce the experimental results of EuMnO 3 , including lattice parameters, insulating A-type AFM nature, and electronic structure . Meanwhile, we also got the initial supposed structure of Eu 2+ Mn 4+ O 3 as predicted by Aschauer et al through U -value manipulation . The specific calculation settings in each code are given as follows.…”

“…However, the components of Eu and Mn could exhibit multiple valence states in different surroundings. Aschauer et al proposed the heteronuclear charge disproportionation transition from Eu 3+ Mn 3+ O 3 to Eu 2+ Mn 4+ O 3 though the latter has not been observed experimentally. , Theoretically, they predicted the feasibility of charge transfer under isostatic pressure as well as epitaxial strain by imposing specific oxidation states. In the meantime, they constrained magnetic states to AFM because of the favored ferromagnetic alignment of Mn when considering 4f electrons as valence states independently of the nature of Eu magnetic order.…”

Room-temperature valence transition in a strain-tuned perovskite oxide

“…In the RCoO 3 series this has been verified through density functional theory (DFT) calculations 33 , and active Pr-O bonding was deduced from anomalous structural behavior in bulk Pr 0.5 Sr 0.5 . The possibility of R valence changes vs. pressure and strain have also been discussed theoretically in RMnO 3 systems 35 . In terms of thermal valence transitions, the extraordinary behavior in cobaltites such as PYCCO is potentially analogous to transitions seen in certain Fe-based complex oxides 36 , as well as Sm monochalcogenides such as SmS, where a pressure-and temperature-dependent Sm valence transition arises 37 .…”