Copper scandium zirconium phosphate: occupancy of theM1 andM2 sites in the temperature range 100–300 K

Abstract

The title compound, with nominal formula Cu2ScZr(PO4)3, has a beige coloration and displays fast Cu+cation conduction at elevated temperatures. It adopts a NASICON-type structure in the space groupR\overline{3}c. The examined crystal was an obverse–reverse twin with approximately equal twin components. The [ScIIIZrIV(PO4)3]2−framework is composed of corner-sharing Sc/ZrO6octahedra and PO4tetrahedra. The Sc and Zr atoms are disordered on one atomic site on a crystallographic threefold axis. The P atom of the phosphate group lies on a crystallographic twofold axis. Nonframework Cu+cations occupy three positions. Two of the Cu+positions generate an approximately circular distribution around a site of \overline{3} symmetry, referred to as theM1 site in the NASICON-type structure. The other Cu+position is situated close to the twofold symmetricM2 site, displaced into a position with a distorted square-based pyramidal coordination geometry. The structure has been determined at 100, 200 and 300 K. Changes in the refined site-occupancy factors of the Cu+positions suggest increased mobility of Cu+around the circular orbit close to theM1 site at room temperature, but no movement into or out of theM2 site. Free refinement of the Cu site-occupancy factors suggests that the formula of the crystal is Cu1.92(1)ScZr(PO4)3, which is consistent with the low-level presence of Cu2+exclusively in theM2 site.