We investigate lattice dynamics in LaCoO3 using inelastic neutron and x-ray scattering over T=2\mbox−650K, spanning the spin-state crossover at T1≈100K and the insulator--metal transition at T2≈550K. Comparison with quasi-harmonic ab−initio lattice-dynamical calculations helps reveal anomalous softening of a ≈10meV oxygen phonon, confined to the temperature interval T1≤T≤T2 and localized in momentum space at qSSO=(21,21,21)c. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co3+ states in LaCoO3, linking spin-state fluctuations to anomalous phonon renormalization.
@article{arxiv.2602.22725,
title = {Phonons reflect dynamic spin-state order in LaCoO$_3$},
author = {Alsu Ivashko and Taishun Manjo and Maximilian Kauth and Yuliia Tymoshenko and Adrian M. Merritt and Klaus-Peter Bohnen and Rolf Heid and Michael Merz and Andreas Eich and John-Paul Castellan and Alexandre Ivanov and Nathaniel Schreiber and Hong Zheng and J. F. Mitchell and Martin Meven and Jitae T. Park and Daisuke Ishikawa and Yuiga Nakamura and Alfred Q. Baron and Frank Weber},
journal= {arXiv preprint arXiv:2602.22725},
year = {2026}
}