ABSTRACT
The lattice dynamics of La(1/2)Na(1/2)TiO(3) single crystal have been investigated by far-infrared and Raman scattering spectroscopy. Far-IR reflectivity spectra revealed Slater, Last and Axe modes near 140, 190 and 555 cm(-1), respectively. The frequency of the Slater mode decreases by about 10% on cooling, which is enough to account for the observed increase of the low-frequency permittivity. The results allowed us to discuss the reasons for the suppressed ferroelectricity of the material. Temperature evolution of the Raman scattering intensity of an oxygen octahedra bending mode near 455 cm(-1) suggests a second-order phase transition towards the perovskite aristotype phase near 870 K.
Subject(s)
Calcium Compounds/chemistry , Lanthanum/chemistry , Oxides/chemistry , Sodium/chemistry , Spectrophotometry, Infrared , Spectrum Analysis, Raman , Titanium/chemistry , Models, Molecular , Molecular Structure , Phase Transition , Temperature , ThermodynamicsABSTRACT
An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i) the symmetry of the ground state, (ii) the Néel and Curie temperatures, and (iii) the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a) an overlooked phase above Tc approximately 1100 K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b) improperlike dielectric features above Tc, and (c) that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases.
