Studies of optical, dielectric, ferroelectric, and structural phase transitions in [KNbO₃]_0:9[BaNb_0:5Ni_0:5O_3-δ]_0:1

Abstract

[KNbO₃]_0:9[BaNb_0:5Ni_0:5O_3-δ]_0:1 (KBNNO) is a robust eco-friendly (leadfree) ferroelectric perovskite, has diverse applications in electronic and photonic devices. In this work, we report the dielectric, ferroelectric, and structural phase transitions behavior in KBNNO compound using X-ray diffraction, Raman and Dielectric studies at ambient and as a function of temperature. Rietveld analyses on X-ray diffraction (XRD) data at room temperature (RT) revealed orthorhombic phase (Amm2) of the compound with a minor secondary NiO cubic phase (Fm3m) (0.18 %). A direct optical band gap Eg of 1.66 eV was estimated at RT from the UV-Vis reflectance spectrum analysis. Observation of nonsaturated electric polarization loops were attributed to leakage current effects pertaining to oxygen vacancies in the compound. Temperature dependent magnetization studies revealed that KBNNO have a lightly tendency to the magnetization due to the low presence of Nickel. XRD studies on KBNNO at elevated temperatures revealed orthorhombic to tetragonal, and tetragonal to cubic phase transitions between (500-535) K and (675-725) K, respectively. Temperature dependent dielectric studies on KBNNO identified two dielectric anomalies at ∼ 225 K, ∼ 590K and ∼ 690 K, correspond to the rhombohedral to orthorhombic, orthorhombic to teragonal and tetragonal to cubic phase transitions, respectively. Electrical conductivity data as a function of temperature obeys Jonscher power law and satisfied the correlated barrier hopping model indicating dominance of hopping conduction mechanism. Temperature dependent Raman spectros-copy studies over a wide range of temperature (82-1,025) K inferred the rhombohedral to orthorhombic, orthorhombic to tetragonal and tetragonal to cubic phase transitions at ∼ 250 K, 550 K and 675 K, respectively. Several Raman bands were found to disappear; while a few Raman modes such as at 225, 270, 289, and 831 cm^-1 exhibited discontinuity across the phase transitions at 250 and 550 K.