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 0.00less than or equal toxless than or equal to0.50. 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 500degreesC (AA=acetylacetone). The solid solutions of LixMg1-2xFe2+xO4 (0.10less than or equal toxless than or equal to0.45) were synthesized by conventional ceramic method. The samples with xgreater than or equal to0.40 belong to ordered spinels with primitive cubic cell, space group P4(3)32 and the samples with xgreater than or equal to0.35 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 Wyckoffs 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(Mg-8a(2+))=0.183(7) and N(Fe-8a(3+))=0.817(7). (C) 2002 Elsevier Science BY All rights reserved.