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3.2 b. Neutron diffraction study of displacive phase transitions and strain analysis in Fe-doped CaTiO3 perovskites at high temperature (A.I. Becerro and F. Seifert, in collaboration with S.A.T. Redfern and M.A. Carpenter/Cambridge, UK, and K.S. Knight/Rutherford Appleton Laboratory, Chilton)

In view of the importance of silicate perovskites in the lower mantle and first indications that they might be defect-bearing (see above), we have embarked on a comprehensive study of the effects of oxygen vacancy concentration in defect perovskites and their clustering on physical and chemical properties (cf. also Annual Report 1999), using the system CaTiO3-CaFeO2.5 as a convenient starting point. This system also provides us with a model for temperature-dependent symmetry changes due to tilting of octahedra, which has also been postulated to occur also in silicate perovskites at high pressures and temperatures. In the titanate-ferrite system, two displacive phase transitions (from cubic Pm3m to tetragonal I4/mcm to orthorhombic Pnma) had been inferred in compositions CaFexTi1-xO3-x/2 with x < 0.25 on the basis of quench-experiments by studying the evolution of lattice parameters and strains as a function of composition at room temperature. The cubic Pm3m structure is the high-temperature phase and exhibits only short-range order of oxygen vacancies. On decrease of temperature, the rapid displacive transitions are not expected to change the disorder of the defects, but the perovskite lattice accommodates structural strains by tilting of octahedra. These transitions have now been studied by in situ high-resolution time-of-flight neutron diffraction and the transition temperatures have been determined as a function of composition. The Pm3m to I4/mcm and I4/mcm to Pnma transitions decrease in the temperature versus composition phase diagram of the system CaFexTi1-xO3-x/2 in a quasi-linear manner with increasing Fe content up to x = 0.2. A plateau effect is weak to absent, indicating a strong interaction of relaxation spheres around substituting atoms. At higher Fe concentrations, the transition temperatures drop sharply, probably due to incipient long-range ordering of oxygen vacancies.

The existence of a second orthorhombic ( Cmcm) phase, which had been postulated for CaTiO3, is ruled out in the Fe-doped perovskites in view of the behaviour of specific diffraction peaks. Finally, a strain analysis of the temperature-dependent data (Fig. 3.2-2) shows a first order thermodynamic character for the Pnma to I4/mcm transition, while the character of the Pm3m to I4/mcm transition is either second order or tricritical. The shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models, while the volume strain has an anomalous coupling with the order parameter components, which appears to be temperature dependent.

Fig. 3.2-2: Temperature-evolution of spontaneous strains for perovskite of composition CaTi0.80Fe0.20O2.90, calculated from lattice parameters determined by in situ neutron diffraction. eais the volume strain, and tx,tzare the tetragonal strains in the orthorhombic and tetragonal phase, respectively. |e4| = |(a-c)/ao2|, where ao is the reference parameter of the cubic structure extrapolated into the tetragonal and orthorhombic stability field.

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