Negative Thermal Expansion in Ba0.5Sr0.5Zn2SiGeO7

Abstract

Solid solutions with the composition Ba0.5Sr0.5Zn2Si2-xGexO7 and BaZn2Si2-xGexO7 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. Ba0.5Sr0.5Zn2Si2-xGexO7 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.

Solid solutions based on BaZn2Si2O7 with thermal expansions from negative to highly positive – a review

“…In a BaZn 2 Si 2 O 7 lattice, up to 75 % Si 4+ can be replaced by Ge 4+ without changing the phase while larger Ge 4+ -concentrations lead to the formation of secondary phases, such as Zn 2 GeO 4 . 30,31 As shown in Fig. 10, the temperature of the phase transition increases linearly with increasing Ge concentration from 280 °C for BaZn 2 Si 2 O 7 to around 700 °C for BaZn 2 Si 0.5 Ge 1.5 O 7 an hence, in analogy to the substitution of Zn 2+ by other divalent cations, the monoclinic low temperature phase is increasingly stabilized.…”

Role of Tin Oxide as a Nucleating Agent with Low Solubility in BaO–SrO–ZnO–SiO2 Glasses Studied by Electron and X-ray Microscopy

“…From such alkaline earth zinc silicates, a crystalline solid solution with the general composition Ba 1– x Sr x Zn 2– y M y Si 2– z Ge z O 7 (M = Co, Cu, Mg, Mn, Ni) can be precipitated. − If the crystals contain comparatively small quantities of Sr and/or high quantities of M, a phase with the structure of low temperature BaZn 2 Si 2 O 7 will crystallize, which has an exceptional high coefficient of thermal expansion. Depending on the values of x , y , and z , as well as on the chosen element M, also a phase with the crystal structure of high temperature (HT-) BaZn 2 Si 2 O 7 may precipitate which exhibits an (anisotropic) very low or even negative coefficient of thermal expansion. ,,, Solid solutions with such a crystal structure were recently denoted as LEAZit TM , which is an acronym of L ow E xpansion A lkaline Earth Zi nc Silica t es. The main challenge in the development of a material containing such a phase is the synthesis of an almost crack-free bulk material.…”