Neutron and X-ray total scattering study on superionic thermoelectric AgCrSe

Li, B.; Feygenson, M.*; Ohara, Koji*; Zhang, Q.*; Vaknin, D.*; Yamada, Takeshi*; Miao, P.*; Kamiyama, Takashi*; Kawakita, Yukinobu  

Very recently, superionic AgCrSe was reported as a new thermoelectric material because of its record-low thermal conductivity. To understand the origin of such low thermal conductivity, we measured the powder sample of AgCrSe at NOMAD of Spallation Neutron Source and BL04B2 of SPring-8. The diffraction data and pair distribution functions were obtained as a function of temperature from both neutron and X-ray experiments. In this presentation, I will report the crystal structure and local structure of this compound. This compound undergoes a second-order transition at 170C from R-3m to R3m and can be described by 2D Ising model. The lattice constant c and Debye-Waller factor U11 of Ag exhibit significant change at the transition. The weaker Ag-Se bonding is seen from the pair distribution functions above this transition. At the same time, the pair distribution function beyond 80 Angstrom becomes very different above this transition. This implies there is mesoscale inhomogenousity and might be related to the low thermal conductivity.