Elucidating the Nature of Pseudo Jahn–Teller Distortions in LixMnPO4: Combining Density Functional Theory with Soft and Hard X-ray Spectroscopy

Abstract

A combination of soft and hard synchrotron-based spectroscopy with first-principles density functional theory within the GGA + U framework is used to investigate the distortion of the Mn local environment of LixMnPO4 as a function of electrochemical delithiation (x = 1.0, 0.75, 0.5, 0.25) and its effect on the electron and hole polaron formation. Analysis of the soft X-ray absorption spectroscopy (XAS) of the Mn L3,2-edges confirmed the evolution from the Mn2+ to the Mn3+ charge state as a two-phase reaction upon delithiation; the corresponding Mn K-edge extended X-ray fine structure measurements clearly revealed a splitting of the Mn–O nearest-neighbor distances with increasing Mn3+ character. In addition, the O K-edge absorption and emission spectra confirmed the corresponding orbital lifting of degeneracy accompanying the distortion of the MnO6 octahedra in the Mn3+ state. Our GGA + U calculations show that the distortion is not a strict Jahn–Teller distortion but is instead a preferential elongation of two of the equatorial Mn–O bonds (edge-sharing with the PO4), which results in a Mn–O–P induction driven hybridization of the unoccupied states (i.e., a pseudo Jahn–Teller distortion). Excellent agreement between the calculated electronic structure and our soft X-ray measurements of the electrochemically delithiated LixMnPO4 nanoparticles verifies the link between the preferential structural distortion and the resultant hybridization of the unoccupied 3d dxz and dx2–y2 orbitals. Our analysis of the corresponding calculated electron and hole polaron supports claims that the elongation of the equatorial bonds (edge-sharing with the PO4) in the Mn3+ charge state (i.e., the pseudo Jahn–Teller distortion) is responsible for increasing the activation energy for polaron migration and the formation energy of the electron (hole) lithium ion (vacancy) complex of the Mn olivine compared to the Fe olivine.