SHORT-RANGE ORDER MEASUREMENTS IN MGO-FEO AND MGO-LIFEO2 SOLID-SOLUTIONS BY DLS SIMULATION-ASSISTED EXAFS ANALYSIS

EXAFS analysis is here used to characterize short-range order (SRO) an d local structure in oxide solid solutions by direct measurement of th e identity and number of next nearest neighbor (NNN) cations. Precisio n is improved through the use of constraints, including simulation-der ived interatomic distances. EXAFS Fe-K edge measurements were made on seven MgO-FeO solid solution samples rapidly quenched from 1140-degree s-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 a gree with simulation results. Fourier deconvolution of Mossbauer spect ra of these samples resolves the fine structure, which further support s a random cation distribution. Computer simulations of alpha-LiFeO2 a nd solid solutions of alpha-LiFeO2 and MgO were carried out for random distributions and for locally ordered distributions in which local ch arge-balancing cation interchange (simulated diffusion) was permitted. Such interchanges decrease Li-Li and Fe-Fe NNN contacts relative to t he number of Li-Fe contacts but do not affect Mg distribution. Fe-K ed ge EXAFS measurements of two samples of alpha-LiFeO2 prepared at 1000- degrees-C gave 4.6 and 4.8 NNN Fe atoms, in agreement with the 4.89 pr edicted by a locally ordered simulation. Precise fitting of EXAFS spec tra of 12 MgO-alpha-LiFeO2 solid solution samples (also quenched from 1000-degrees-C) required the additional assumption of no Mg atom clust ering. EXAFS measurement of the remaining Li,Fe NNN atoms around probe Fe atoms also corresponded closely (within 2.6% of total NNN atoms) t o a random distribution. The change from locally ordered to random app eared between 100 and 80% LiFeO2. Several samples with very low Fe con tent gave erratic results attributable, we believe, to sample heteroge neity.