title: Structural, Raman spectroscopic and microwave dielectric studies on Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 ceramic compounds with wolframite structure.
abstract: Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 (x = 0.0-1.0) compositions were synthesized via conventional solid-state reaction method. Structural and lattice vibrational characteristics of these compositions were studied with the help of powder X-ray diffraction and Raman spectroscopic measurements. Rietveld refinements confirm the formation of all these compositions in monoclinic wolframite structure with P2/c space group. When moving towards Ni(2+)-poor compositions, splitting in the X-ray reflections was observed and is explained with lattice parameter variation. With increasing value of x, Raman spectra show two additional Raman active modes and the possible reasons for observing these modes are discussed in terms of electronegativity difference of the randomly distributed cations in B-site of these compositions. X-ray photoelectron spectroscopic measurements were done to understand the chemical bonding states of different elements in these compositions. Surface morphology studies reveal that the average grain size increases with increasing x. Microwave dielectric properties such as dielectric constant and quality factor were measured using Hakki-Coleman and reflection cavity techniques and enhancement in these values with x is correlated with intrinsic parameters such as polarizability and 3d electrons present in the constituent ions of these compositions. Temperature coefficient of resonant frequency was measured using an invar cavity attached to a programmable hot plate and is explained with B-site octahedral distortion of these compositions. Well dense Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 (x = 0.0-1.0) compositions possess good microwave dielectric properties.
authors.affiliation: Microwave Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai - 600 036, India. vrkm@physics.iitm.ac.in.
authors.forename: S D
authors.initials: SD
authors.lastname: Ramarao
authors.forename: V R K
authors.initials: VR
authors.lastname: Murthy