Synthesis and Characterization of Cu2ZrCl6: A Thermochromic van Vleck Paramagnet.

Abstract The solid state reaction of CuCl and ZrCl 4 yields the pale yellow crystalline solid of composition Cu 2 ZrCl 6 . This material crystallizes in the trigonal space group P3m 1 (#164) at room temperature with lattice constants a =10.9092(4) and c =6.1517, Z =3. The structure is shown to be related to ZrCl 3 , where every other Zr site has been replaced with Cu atoms that partially occupy the six tetrahedral interstices surrounding the now vacant octahedral interstice. At room temperature, two copper atoms are disordered about three crystallographically distinct sites. Two thermochromic transitions are observed for this material at 120 and 673 K, which appear to be related to the movement of copper from the center of a tetrahedral interstice to a trigonal planar face. A second-order Jahn–Teller distortion accounts for this structural variation. This along with the insertion of the empty Zr-4d orbitals into the already small copper (I) based HOMO–LUMO gap afford significant mixing between ground and excited states resulting in notable van Vleck paramagnetism.