Negative Thermal Expansion in Ba(0.5)Sr(0.5)Zn(2)SiGeO(7)

Solid solutions with the composition Ba(0.5)Sr(0.5)Zn(2)Si(2-x)Ge(x)O(7) and BaZn(2)Si(2-x)Ge(x)O(7) 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(0.5)Sr(0.5)Zn(2)Si(2-x)Ge(x)O(7) solid solutions exhibit the structure of high-temperature BaZn(2)Si(2)O(7) 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 BaZn(2)Si(2)O(7) 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.