Copper scandium zirconium phosphate: occupancy of the M1 and M2 sites in the temperature range 100-300 K

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 group Rc. 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/ZrO6 octahedra and PO4 tetrahedra. 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 symmetry, referred to as the M1 site in the NASICON‐type structure. The other Cu+ position is situated close to the twofold symmetric M2 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 the M1 site at room temperature, but no movement into or out of the M2 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 the M2 site.