Models of polaron fluctuations in LuFe₂O₄

The structure of LuFe2O4 below the charge ordering temperature of TCO = 320 K has been studied immensely due to the question of whether charge ordering on the mixed valence Fe sublattice leads to a macroscopic electric polarization. In contrast, the local structure associated with polaron fluctuations above TCO, from which the charge ordered structure emerges, is largely unknown. In this work, we characterize this local structure using the x-ray three-dimensional difference pair distribution function (3D-APDF). Atomic correlations extracted directly from the 3D-APDF are used to propose models for the local structure, which are simulated using Monte Carlo methods. It is found that the primary Fourier components of the diffuse scattering can be described by the large, correlated Lu displacements. The local structure of Lu displacements draws conceptual parallels to the classical models of triangular Ising antiferromagnets with the added complexity of interplane interactions. The weaker features of the 3D-APDF are modeled by displacement correlations involving Fe and O, the latter resulting in an increased coordination number of Lu compared with the average structure. Analysis of the Fe-coordination environments in the atomic configurations obtained from Monte Carlo simulations reveals a strong correlation between distortion of the coordination environment and the +II oxidation state of Fe, which suggests a JahnTeller polaronic nature of the charge carriers.