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1 -x)Ba 0.5 Sr 0.5 TiO 3 (BST)-xMgO composite ceramics have been prepared in situ by a citrate-nitrate combustion process, and their crystal structure and effective dielectric response have been investigated systematically. Results reveal that MgO particles homogeneously disperse in BST particles. Rietveld refinement of X-ray diffraction data shows that the amount of the incorporation of MgO into BST lattice is increased with increasing the MgO volume fraction (ƒ M ). It is identified that the incorporation of Mg 2+ into B site gives rise to the formation of F centers in the composites. Related with the incorporation of Mg 2+ into BST lattice, the permittivity of BST in the composite is strongly dependent on ƒ M , which is defined by the relation e BST ¼ a3e b3fM . Also, the high percolation threshold of ƒ M = 0.60 is observed, which is well in agreement with the theoretical predictions of the modified effective medium approximation coupled with the spherical inclusion model.

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