Abstract
The room-temperature form (α) of colorless transparent synthetic Zn(Zn0.1Li0.6Si0.3)SiO4 has monoclinic symmetry with a = 6.340(1), b = 10.516(2), c = 5.011(1)A, β = 90.50(2)°, space group P21/n, and Z = 4. It transforms at about 400°C to a high-temperature form {β) which is orthorhombic with a = 6.406(3), b = 10.520(8), c = 5.043(2)A, space group Pmnb, and Z = 4. The crystal structures for both forms were determined by 3-dimensional Patterson analysis from X-ray intensity data collected at room temperature and at 450°C. The structures were refined by the least-squares methods to a final weighted R = 0.068 (unweighted R = 0.068) and 0.064 (unweighted R = 0.070) for the room-temperature and the high-temperature form respectively.
The high-temperature structure is a tetrahedral framework structure with a sharing coefficient of 3. There are two different tetrahedral sites: T1, an 8-fold site, and T2, a 4-fold site. The T1 site contains all Zn and Li and some Si; the T2 site is all Si. The T1 tetrahedra form puckered layers parallel to (010) and are composed of corner-shared tetrahedral chains. These chains are parallel to [100]. Unlike pyroxenes, all tetrahedra lie on the same side of the chain axis. The layers stack in antiparallel arrangement and are cross-linked by the T2 tetrahedra to constitute the framework structure.
Below the transition temperature, the 8-fold T1 site splits into two symmetry-independent 4-fold positions T1(o) and T1(m) sites, and the symmetry degenerates from Pmnb to P21/n. Zn atoms are completely ordered on T1(o). At the transition, Zn atoms may concentrate in either of the two T1 positions. This option results in a domain structure with the two related by reflection across (100). The order-disorder transformation also produces twin (α′) structure which has monoclinic symmetry with space group B21 and a = 13.01, b = 10.41, c = 10.07A, and β ≈ 90°.
