STRUCTURAL PHASE-TRANSITIONS OF FAST LITHIUM ION CONDUCTORS LI2M11CL4 (M11 = MG, MN, CD)

The order-disorder phase transitions of spinel-type (I) chlorides Li2MIICl4 (MII =Mg, Mn, Cd) to Li2MnBr4cF56 (II) and disordered, deficient NaCl structure (III) at elevated temperatures are characterized by X-ray diffraction, dilatometry, thermoanalytical, and Raman spectroscopic studies, and are discussed with respect to cation distribution, lithium ion motion, anharmonic interaction of vibrational modes, and order parameters. Full formation of the ordered NaCl superstructure (II) occurs at 360°C for both Li2MgCl4 and Li2MnCl4. Mn2+-rich Li2-2xMn1+xCl4 solid solutions probably possess a Li2MnBr4cF56-type structure even at ambient temperatures. In the case of Li2CdCl4, disorder of II to III starts before complete depopulation of the tetrahedral8a site of the spinel structure is reached. Disorder to III is finished at 540, 465, and 380°C, respectively. Because of the high defect concentrations and coupling of several ordering processes, one-component Landau theory gives only an unsatisfactory description of the microscopic processes. © 1992 Academic Press, Inc. All rights reserved.