Structural investigations on the (Ba,Sr)(Zr,Ti)O3 system

Structural investigations of the compounds in the series Ba0.27Sr0.73ZryTi1-yO3 have been carried out. The crystal structure goes to lower symmetry with increasing Zr content and the tolerance factor decreases. The variation of V-a/V-b is linear with the tolerance factor. The tilt system is a(-)a(-)c(0) for compositions with y = 0.50 and 0.65. The tilt system is a(-)a(-)c(+) for compositions with y = 0.80, 0.90 and 0.97 and for BSZT (Ba0.29Sr0.71Zr0.97Ti0.03O3) and BSZTTa (Ba0.29Sr0.71Zr0.95Ti0.04Ta0.01O3). The tilt angle with respect to the pseudocubic [100] and [001] directions increases with the Zr content. For compositions with y = 0, 0.25 and 0.40, the coordination of the A ion (Ba or Sr) is 12, whereas the A-ion coordination number is 11 for compounds with y = 0.50 and 0.65 and it is lower for compositions with y = 0.80, 0.90 and 0.97 and for the compounds BSZT and BSZTTa. The decrease in coordination number indicates that the compounds are becoming more and more covalent with increasing Zr content. A graph of \90 degrees - gamma(pc)\ against the tolerance factor shows that \90 degrees - gamma(pc)\ falls sharply at a tolerance factor corresponding to the compositions used in the fabrication of dielectric resonators. TCf values show that the sign of this parameter changes at a tolerance factor near 0.965 and has a minimum for compounds with orthorhombic Pbnm/Pnma structures.