Structural studies of Cr-doped mullite derived from single-phase precursors Original Research Article

Structural, electric and magnetic properties of Ba3Mg1−xCoxNb2O9 based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε (T )) and magnetic measurements (χ−1 (T )). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, View the MathML sourceP3¯m1 (no. 164), space group (SG) to a simple structure cubic-type, View the MathML sourcePm3¯m (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0–1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μeff) are close to the value that corresponds to Co2+ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative–positive–zero (NPO) type dielectric applications.