Thermal hysteresis and vibrational excitations in NiO containing NaNbO3

Abstract

A systematic study of the dielectric hysteresis and temperature-dependent vibrational excitations of NiO containing antiferroelectric (AFE) NaNbO3 composites is reported. The Mott insulator NiO plays a significant role in the crystal structure of NaNbO3. In particular, for low and moderate substitutions of NiO, a transition from orthorhombic to monoclinic symmetry is evident at room temperature. The temperature dependence of the relative dielectric permittivity, , exhibits an anomalous change in its magnitude across three distinct phase transitions of NaNbO3 at 449 K, 665 K and 751 K. These are, respectively, associated with a second-order phase transition occurring due to incommensurate phase between two orthorhombic AFE orderings, change from the orthorhombic P- to R-phase () without altering the AFE structure and an orthorhombic R- to S-phase transition accompanied by an AFE to paraelectric transition. A large thermal hysteresis ( K) was evident in and ac resistivity () between the temperatures 473 K and 673 K signifying the first-order nature of the transition. At room temperature, for higher NiO content, a new mode centred at 838 cm−1 was observed in the Raman spectra along with a two-magnon mode at 1496 cm−1, a broad longitudinal optical mode (2LO) at 1096 cm−1 and a weak translational optical mode (2TO) at 726 cm−1. At low temperatures, down to 80 K, several peculiar features are evident in the Raman spectra of NbO6 octahedra which are associated with the structural and antiferromagnetic transition of NaNbO3 and NiO, respectively.