Ba(1−x)Sr(x)Zn(2)Si(2)O(7) - A new family of materials with negative and very high thermal expansion

The compound BaZn(2)Si(2)O(7) shows a high coefficient of thermal expansion up to a temperature of 280 °C, then a transition to a high temperature phase is observed. This high temperature phase exhibits negative thermal expansion. If Ba(2+) is successively replaced by Sr(2+), a new phase with a structure, similar to that of the high temperature phase of BaZn(2)Si(2)O(7), forms. At the composition Ba(0.8)Sr(0.2)Zn(2)Si(2)O(7), this new phase is completely stabilized. The crystal structure was determined with single crystal X-ray diffraction using the composition Ba(0.6)Sr(0.4)Zn(2)Si(2)O(7), which crystallizes in the orthorhombic space group Cmcm. The negative thermal expansion is a result of motions and distortions inside the crystal lattice, especially inside the chains of ZnO(4) tetrahedra. Dilatometry and high temperature X-ray powder diffraction were used to verify the negative thermal expansion. Coefficients of thermal expansion partially smaller than −10·10(−6) K(−1) were measured.