Negative Thermal Expansion in Ba_0.5Sr_0.5Zn₂SiGeO₇

Solid solutions with the composition Ba_0.5Sr_0.5Zn₂Si_2-xGe_xO₇ and BaZn₂Si_2-xGe_xO₇ were prepared with different values of x using a conventional mixed oxide route. Both compounds exhibit very different thermal expansion, which is due to the different crystal structures. Ba_0.5Sr_0.5Zn₂Si_2-xGe_xO₇ solid solutions exhibit the structure of high-temperature BaZn₂Si₂O₇ and show negative thermal expansion, which was proven via high-temperature X-ray diffraction. Up to around x = 1, the crystal structure remains the same. Above this value, the low-temperature phase becomes stable. The Sr-free solid solutions have the crystal structure of low-temperature BaZn₂Si₂O₇ and show also a limited solubility of Ge. These Sr-free compositions show transitions of low- to high-temperature phases, which are shifted to higher temperatures with increasing Ge-concentration.