PHYSICAL PROPERTIES OF TRANSITION METAL SUBSTITUTED ORTHOVANADATES

Abstract

Rare-earth orthovanadates have emerged as an important class of multifunctional oxide materials because of their interesting structural, dielectric, optical, and electrical properties. Among these materials, lanthanum orthovanadate LaVO4 and its niobium substituted compositions have gained considerable attention due to their structural flexibility and wide range of technological applications in dielectric ceramics, sensors, photocatalysis, fuel cells, and electronic devices. The substitution of Nb5+ ions at V5+ site introduces compositional strain and local lattice distortion within the crystal framework, which significantly influences the phase stability, defect chemistry, dielectric behaviour, and electrical transport properties of the material system. In view of these aspects, the present work investigates the effect of Nb substitution on the structural, dielectric, and impedance characteristics of LaV1-xNbxO4 ceramics synthesized by conventional solid state reaction route. Polycrystalline LaV1-xNbxO4 samples with different Nb concentrations (0 ≤ x ≤ 1) were prepared using high-temperature solid-state synthesis followed by calcination and sintering under controlled conditions. The structural evolution of prepared ceramics was examined using X-ray diffraction (XRD) and Rietveld refinement analysis. XRD results confirmed gradual phase transformation from the monoclinic monazite structure P21/n pure LaVO4 towards the tetragonal scheelite phase I41/a, followed by the appearance of the monoclinic fergusonite phase (I2/a) at higher Nb concentrations. This structural transition was mainly associated with the ionic size difference between V5+ and Nb5+, which produces internal chemical pressure inside the lattice and alters the crystal symmetry. Raman spectroscopic studies further supported the phase evolution and indicated noticeable changes in vibrational modes, local symmetry, and tetrahedral distortion with increasing Nb incorporation. The microstructural behaviour of the synthesized ceramics was investigated using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). The results showed that Nb substitution affects grain growth, particle morphology, and grain boundary distribution. Increase in dopant concentration x Ph.D. Thesis (Ashok Kumar) introduced lattice strain and microstructural disorder, which subsequently influenced the electrical and dielectric response of the ceramics. X-ray photoelectron spectroscopy (XPS) analysis was carried out to examine oxidation states and local chemical environment of constituent elements. The spectra confirmed the presence of V5+, Nb5+ and oxygen vacancy related defect states, indicating the important role of defect chemistry in governing the conduction and polarization mechanisms. The dielectric properties of LaV1-xNbxO4 ceramics were studied as a function of frequency and temperature. The dielectric response exhibited strong dependence on Nb content, applied frequency, and thermal conditions. An enhancement in dielectric permittivity was observed for Nb-rich compositions due to increased lattice polarizability and defect assisted interfacial polarization. The dielectric dispersion behaviour was analyzed using Maxwell–Wagner polarization and space charge polarization models. In addition, relaxation behaviour suggested non-Debye type characteristics, indicating the presence of distributed relaxation processes within the material. Complex impedance spectroscopy was used to investigate conduction mechanism and electrical relaxation processes in the prepared ceramics. Nyquist plots revealed separate grain and grain boundary contributions, confirming the electrically heterogeneous nature of polycrystalline samples. Frequency-dependent conductivity behaviour was found to obey Jonscher’s universal power law. The decrease in activation energy with increasing Nb concentration indicated improved charge carrier transport and enhanced carrier hopping probability. The electrical conduction mechanism was mainly governed by small polaron hopping assisted by oxygen vacancies and local structural distortion. The combined impedance and modulus studies also revealed thermally activated relaxation behaviour associated with defect-induced charge transport processes. The effect of Nb substitution on the electronic structure and optical behaviour of the ceramics was also examined. The incorporation of Nb modified the conduction band structure through hybridization of Nb-4d, V-3d, and O-2p orbitals, resulting in bandgap reduction and improved electronic delocalization. These changes contributed to enhanced visible light absorption and influenced the dielectric as well as conductivity behaviour of the material system. The study clearly demonstrates that structural distortion and defect xi Ph.D. Thesis (Ashok Kumar) engineering through controlled Nb incorporation provide an effective route for tuning the functional properties of LaVO4-based ceramics. The overall results indicate that LaV1-xNbxO4 ceramics possess improved dielectric response, better conductivity behaviour, good thermal stability, and tunable structural properties, making them suitable for multifunctional applications. The coexistence of monazite, scheelite, and fergusonite phases provides additional flexibility for tailoring material performance according to specific technological requirements. These materials may therefore be considered promising candidates for high-k dielectric capacitors, photocatalytic systems, impedance-based sensors, and advanced electrochemical devices. In conclusion, the present investigation provides a detailed understanding of the relationship between structural modification, defect chemistry, dielectric relaxation, and electrical conduction in Nb substituted lanthanum orthovanadate ceramics. The findings of this work contribute to the development of advanced rare-earth oxide ceramics with improved multifunctional properties for future electronic and energy-related applications.