Biquadratic coupling between two ordering parameters in 1 : 3 B-site ordered spinels

The Letter presents an analysis of the order-disorder phase transition mechanism, Fd (3) over bar LT ---- GT P4(3)32, in 1:3 B-site ordered spinels, Li1.33xCo2-2xTi1+0.67xO4 and Li1-0.5xFe2.5xMn2-2xO4, by using the Landaus theory of phase transitions. Two ordering parameters were used to describe cation distribution: Q(1) for cation distribution between tetrahedral and octahedral sites. and Q(2) for cation distribution between two octahedral sites. Analysis of the topology of the order parameter vector space indicates biquadratic coupling between Q(1) and Q(2). For the Li1.33xCo2-2xTi1+0.67xO4 (x = 0.50) in the low-temperature ferroelastic phase both ordering parameters differ from zero and they both influence on the spontaneous strain scalar values. (C) 2004 Elsevier B.V. All rights reserved