Crystal structure of double chlorides, Na 2MCl 4 (M=Mg, Cr, Cd): Correlation with ionic conductivity

Abstract

Double chlorides with the Sr 2PbO 4-type structure, Na 2MCl 4 (M=Mg, Cr, Cd), were characterized by X-ray Rietveld structure refinement, differential thermal analysis, and electrical conductivity measurement. The orthorhombic structure was determined with lattice parameters, a=6.9067(3), b=11.8854(7), and c=3.8142(2) Å for Na 2MgCl 4 and a=7.0167(2), b=11.9622(3), and c=3.9356(1) Å for Na 2CdCl 4. The monoclinic distorted Na 2CrCl 4 has space group P2 1/c with lattice parameters a=3.9407(1), b=11.5905(4), c=6.9618(3) Å and ß=92.465(2)°; its structure consists of chains formed by edge-sharing CrCl 6 octahedra, which are elongated in the same direction in [ CrCl 4 2 ] ∞ plane as a consequence of the cooperative Jahn-Teller effect. The trigonal prismatic sodium sites sharing faces to the rhombic prismatic voids participate in the sodium ionic conduction. The activation energies for ionic conduction decreased with increasing average Na distance in Na 2MCl 4 with M=Mg, Mn, Cd. The distortion of NaCl 6 trigonal prism caused by the cooperative Cr 2+ Jahn-Teller ordering depressed the ionic conduction in monoclinic Na 2CrCl 4.