A first-order phase transition in zektzerite, NaLiZrSi6O15: high-temperature single-crystal X-ray diffraction study

Abstract

The crystal structure of zektzerite, NaLiZrSi6O15, orthorhombic (space group Cmca), consists of six-tetrahedral-repeat corrugated double silicate chains connected into a three-dimensional framework by edge-sharing Li tetrahedral–Zr octahedral chains. At room temperature, the Na+ ions occur at the Na(1) site located within the cylindrical channels created by the corrugation of the silicate double chains. The second sodium site, Na(2), occurs above and below the Zr octahedron and is nominally vacant at room temperature. Above 773 K, vacancies are created at the Na(1) site due to the migration of Na+ ions from the Na(1) to the Na(2) site. Crystal structure refinements at 300, 773, 973, 1223 and 1323 K indicate a progressive increase in the sodium occupancy at the Na(2) site from 0.04 at 773 K to 0.35 at 1323 K. Induced by increasing Na+-ion migration, a first-order high-temperature phase transition occurs above 1123 K as evidenced by a break in the slopes of the thermal expansion curves of the unit cell dimensions and a substantial increase in the electrical conductivity. The unit cell measurements as a function of temperature show marked hysteresis effects during the heating and cooling cycles.