Pbnm to R3-c phase transformation in (1-x)LaFeO3.xLaMnO3 solid solution due to modifications in structure, octahedral tilt and valence states of Fe-Mn

TL;DR

This study investigates the phase transition from Pbnm to R3-c in LaFeO3-LaMnO3 solid solutions, revealing how structural, electronic, and vibrational properties are interconnected through experimental and theoretical analyses.

Contribution

It provides the first experimental and theoretical detailed analysis of the phase transition mechanism in LaFeO3-LaMnO3 solid solutions, linking structural distortions to electronic and vibrational changes.

Findings

Phase transition at x=0.625 from Pbnm to R3-c structure.

Octahedral tilting and distortion correlate with phase change.

Valence states and cation sizes influence octahedral tilting.

Abstract

A theoretically supported experimental study of the (1-x)LaFeO3.xLaMnO3 (LFO-LMO) solid solution is being reported for the first time which reveals a phase transformation from the Pbnm and R3-c phase at a chemical composition of x=0.625. Correlation of octahedral distortion and phase transition was extensively investigated using x-ray photoelectron spectroscopy (XPS), Raman and x-ray diffraction (XRD) measurements and density functional theory (DFT) calculation. A detailed study of the structural lattice parameters, bond lengths, bond angles have been done, supported by valence state and electronic properties studies. All the above parameters show a correlated modification to the phase transition. The distortion and tilting of the BO6 octahedra has been studied as a function of different Fe:Mn content and expressed by Glazer representation from the refined Crystallographic Information Files (CIF). The angle of tilting from the central non-tilted position also shows a correlated modification with the phase transformation. The valence state and size of cations influences the octahedral tilting. Octahedral volume is reduced as the entire perovskite structure is relatively flattened with increasing Mn-content implying a flattening of both the BO6 octahedra and the La8O6 cage. The vibrational properties were studied experimentally and supported by DFT phonon calculations, detailing the displacement pattern (eigen vectors) revealing considerable insight into the lattice dynamics of the compounds. The optoelectronic modifications in the band properties were studied experimentally and supported with theory. Hence, this manuscript is a in-depth analysis of the structure correlated phase transition of the LFO-LMO solid solution.