Abstract
EXAFS analysis is here used to characteize short-range order (SRO) and local structure in oxide solid solutions by direct measurement of the identity and number of next nearest neighbor (NNN) cations. Precision is improved through the use of constraints, including simulation-derived interatomic distances.
EXAFS Fe-K edge measurements were made on seven MgO-FeO solid solution samples rapidly quenched from 1140 °C. The number of Fe and Mg cations in the NNN shell about an average Fe atom were found to deviate from a random distribution by only 1.4% of total NNN atoms, on average. EXAFS-derived first shell distances agree with simulation results. Fourier deconvolution of Mössbauer spectra of these samples resolves the fine structure, which further supports a random cation distribution.
Computer simulations of α-LiFeO2 and solid solutions of α-LiFeO2 and MgO were carried out for random distributions and for locally ordered distributions in which local charge-balancing cation interchange (simulated diffusion) was permitted. Such interchanges decrease Li-Li and Fe-Fe NNN contacts relative to the number of Li-Fe contacts but do not affect Mg distribution. Fe-K edge EXAFS measurements of two samples of α-LiFeO2 prepared at 1000 °C gave 4.6 and 4.8 NNN Fe atoms, in agreement with the 4.89 predicted by a locally ordered simulation.
Precise fitting of EXAFS spectra of 12 MgO-α-LiFeO2 solid solution samples (also quenched from 1000 °C) required the additional assumption of no Mg atom clustering. EXAFS measurement of the remaining Li,Fe NNN atoms around probe Fe atoms also corresponded closely (within 2.6% of total NNN atoms) to a random distribution. The change from locally ordered to random appeared between 100 and 80% α-LiFeO2. Several samples with very low Fe content gave erratic results attributable, we believe, to sample heterogeneity.