CRYSTALLOGRAPHIC AND AL-27 NMR-STUDY ON PREMELTING PHENOMENA IN CRYSTALS OF SODIUM TETRACHLOROALUMINATE

The crystal structure of NaAlCl4 was determined in the orthorhombic space group P2(1)2(1)2(1) (D2(4), No. 19; Z = 4) at temperatures 138, 150, and 154 +/- 0.2-degrees-C, resulting in the lattice parameters a = 10.442 (4), 10.449 (3), 10.455 (2) angstrom, b = 9.973 (3), 9.993 (2), 10.002 (2) angstrom, and c = 6.202 (2), 6.206 (2), 6.204 (2) angstrom, respectively. Samples of the compound were investigated by differential scanning calorimetry. Al-27 NMR spectra were obtained on NaAlCl4 and LiAlCl4 solids as a function of temperature up to and above the melting points, at 157 and 146-degrees-C, respectively. In accordance with the enhanced premelting heat contents, the observed phenomena indicate the existence of a specific dynamical behavior of the components in the two crystals, involving reorientational noncontinuous movements of the AlCl4- ions and translational jumps of the alkali-metal cations.