Anharmonic thermal motion of Ag in AgCrSe 2: A high-temperature single-crystal X-ray diffraction study

Abstract

The thermal motion of Ag in the two-dimensional silver ion conductor AgCrSe 2 (at 295 K; a = 3.6798(2) Å, c = 21.225(2) Å; Z = 3; spacegroup R3 m) is studied as a function of temperature by single-crystal X-ray diffraction. At room temperature the Ag atoms occupy only one-half of the available tetrahedral holes between the CrSe 2 sandwiches. Disordering of the silver sublattice over all tetrahedral sites proceeds second order with T c = 475 K: above T c the spacegroup is R 3m . Refinements were performed with data measured at six different temperatures in the range 295–673 K. Anharmonic temperature factors of Ag based on the Gram-Charlier expansion have been refined up to fourth order. R w values range from 0.022 to 0.049, the number of variables from 17 to 22. The refinements confirm the second-order nature of the order-disorder transition. Below T c the largest thermal vibrations are directed to the octahedral sites between the CrSe 2 sandwiches. The plane between the α- and β-lattice plays a crucial role. The joint probability density function (jpdf) of Ag is high in regions surrounding the projected α- and β-sites and the octahedral site; at these sites the jpdf shows a minimum. The diffusion of Ag from an α- to a β-site proceeds therefore via this intermediate plane.