Influence of strontium on the cubic to ordered hexagonal phase transformation in barium magnesium niobate

Oxides of the type Ba3-xSrxMgNb2O9 were synthesized by the solid state route. The x = 0 composition (Ba3MgNb2O9) was found to crystallize in a disordered (cubic) perovskite structure when sintered at 1000C. For higher Sr doping (x greater than or equal to 0.5), there was clearly the presence of an ordered hexagonal phase indicated by the growth of superstructure reflections in the powder X-ray diffraction patterns. In all the compositions there was the presence of a minor amount of Ba5-xSrxNb4O15 phase which increased with Sr substitution up to x = 1 and then it remained nearly constant at about 5%, Samples sintered at 1300C showed the hexagonally ordered phase for the entire range of composition (0 less than or equal to x less than or equal to 3 ). The degree of ordering being considerably greater than in the 1000C heated samples as evidenced by several superstructure reflections.