Spin crossover plays a central role in the structural instability, net magnetic moment modification, metallization, and even in superconductivity in corresponding materials. Most reports on the pressure-induced spin crossover with a large volume collapse have so far focused on compounds with a single transition metal. Here we report a comprehensive high-pressure investigation of a mixed Fe-Mn perovskite ${\mathrm{La}}_2{\mathrm{FeMnO}}_6$. Under pressure, the strong coupling between Fe and Mn leads to a combined valence/spin transition: $\mathrm{F}{\mathrm{e}}^{3+}\left(S=5/2\right)\to \mathrm{F}{\mathrm{e}}^{2+}\left(S=0\right)$ and $\mathrm{M}{\mathrm{n}}^{3+}\left(S=2\right)\to \mathrm{M}{\mathrm{n}}^{4+}\left(S=3/2\right)$, with an isostructural phase transition. The spin transitions of both Fe and Mn are offset by $\sim 20$ GPa of the onset pressure, and the lattice collapse occurs in between. Interestingly, ${\mathrm{Fe}}^{3+}$ ion shows an abnormal behavior when it reaches a lower valence state (${\mathrm{Fe}}^{2+}$) accompanied by a +0.5 eV energy shift in the Fe K-absorption edge at 15 GPa. This process is associated with the charge-spin-orbital state transition from high spin ${\mathrm{Fe}}^{3+}$ to low spin ${\mathrm{Fe}}^{2+}$, caused by the significantly enhanced $t_{2g}\text{−}{e}_g$ crystal field splitting in the compressed lattice under high pressure. Density functional theory calculations confirm the energy preference of the high-pressure state with charge redistribution accompanied by spin state transition of Fe ions. Moreover, ${\mathrm{La}}_2{\mathrm{FeMnO}}_6$ maintains semiconductor behaviors even when the pressure reached 144.5 GPa as evidenced by the electrical transport measurements, despite the huge resistivity decreasing seven orders of magnitude compared with that at ambient pressure. The investigation carried out here demonstrates high flexibility of double perovskites and their good potentials for optimizing the functionality of these materials.

• Received 7 January 2019
• Revised 26 March 2019

DOI:https://doi.org/10.1103/PhysRevB.99.195115