Low-temperature structural and dielectric phenomena in La1/3NbO3 and La1/3TaO3: Comparative study

The crystal structures of the perovskites La1/3NbO3 and La1/3TaO3 were studied between 10 and 350 K using high-resolution neutron powder diffraction and compared with their radio-frequency dielectric response over the same temperature range. The structure of La1/3NbO3 remains orthorhombic Cmmm, while La1/3TaO3 undergoes continuous transition from the high-temperature tetragonal P4/mmm to Cmmm phase at about 220 K. This transition is tricritical in nature and accompanied by no dielectric anomaly. In La1/3NbO3, the frequency-dependent peak of the dielectric permittivity is associated with an atypical increase in the lattice parameters below about 80 K.