Neutron diffraction study of phase transitions in the superionic conductor Li0.25Cu1.75Se

Some Thermoelectric Phenomena in Copper Chalcogenides Replaced by Lithium and Sodium Alkaline Metals

“…At 152-177 °C, when the peak characteristic of the low temperature phase is almost diminished, some peaks that are not present in the high temperature phase persist. These peaks can be indexed together with the other strong peaks using a monoclinic unit cell (a = 6.379 Å; b = 5.815 Å; c = 6.155 Å; β = 97.91°, which is similar to what has been suggested from an earlier study on Li0.25Cu1.75Se by Bickulova et al [78] and Balapanov et al [23]. The compound LiCuS has been studied quite well.…”

Phase transitions and transport phenomena in Li0.25Cu1.75Se superionic compound

“…In a previous paper [2] it was assumed that the cubic phase of Li 0.25 Cu 1.75 Se earlier not observed might exist at temperatures higher than 673 K. To check this supposition we performed neutron diffraction structural investigations in a wider temperature interval using a diffractometer with higher resolution. The neutron diffraction patterns at 300, 433, 533 and 773 K are presented in Fig.…”

“…Chemically similar ACu 7-x S 4 (A = K, Rb, Tl) compounds [3 -5] are incommensurate modulated crystals, but for Li 0.25 Cu 1.75 Se ordinary low-symmetry structures are observed at low temperatures [2]. Probably, this is explained by the smallness of the Li atom or the different synthesis conditions.…”

“…Of course, Li ions are rather light compared to K, Rb and Tl ions, but nevertheless some general tendencies should exist for chemical interactions in the crystal lattice. Structural investigations [2] by means of neutron diffraction have revealed that Li 0.25 Cu 1.75 Se undergoes a series of consecutive phase transformations (PTs), the origin of which is associated with gradual disordering of the cationic sublattice as the temperature increases. The gradual disordering of the cationic sublattice should result in an increase of the ionic conductivity and a change of the corresponding activation energy.…”

“…The investigation of the electrophysical and diffusion properties depending on composition has shown that the Li 0.25 Cu 1.75 Se composition stands out among other Li x Cu 2-x Se (0 ≤ x ≤ 0.25) compounds with regards the temperature dependencies of ionic conductivity, of electronic Seebeck coefficient and of chemical diffusion coefficient. The crystal structures of these compounds have been rarely investigated [2], but chemical analogy with known ACu 7-x S 4 (A = K, Rb, Tl) compounds [3 -5] allows one to suppose that the order-disorder phenomena cause various anomalies in Li 0.25 Cu 1.75 Se. Of course, Li ions are rather light compared to K, Rb and Tl ions, but nevertheless some general tendencies should exist for chemical interactions in the crystal lattice.…”