The change of crystal symmetry and cation ordering in Li–Mg ferrites

Abstract

In this work we present results of synthesis and structural investigation of the spinels with general formula LixMg1−2xFe2+xO4, where 0.00≤x≤0.50. Ultrafine powder of MgFe2O4 (x=0.00) was prepared by thermal decomposition of a mixture of complex compounds [Mg(AA)2] and [Fe(AA)3] at 500°C (AA=acetylacetone). The solid solutions of LixMg1−2xFe2+xO4 (0.10≤x≤0.45) were synthesized by conventional ceramic method. The samples with x≥0.40 belong to ordered spinels with primitive cubic cell, space group P4332 and the samples with x≥0.35 are spinels with face-centered cubic cell, space group Fd-3m. Crystal structures of samples with x=0.00, 0.40 and 0.45 were refined by the Rietveld method using DBWS-9411 software package. In ordered spinels (x=0.40, 0.45) Li+ ions occupy both 4b (preferentially) and 12d octahedral sites. At tetrahedral 8c positions Li+ ions were not found (within the experimental error). Mg2+ ions occupy octahedral 12d and tetrahedral 8c sites and Fe3+ ions occupy all three different cationic sites. Cation ordering in 1:3 type ordered spinel (the occupancy ratio of the Wyckoff’s positions 4b and 12d) of Li+ and Fe3+ at octahedral sites is not complete as in Li0.5Fe2.5O4. In MgFe2O4 spinel, cation distribution in the tetrahedral sites was determined and given by occupation numbers N: N(Mg2+8a)=0.183(7) and N(Fe3+8a)=0.817(7).