2D-3D transition in Cu–TiS2 system

Abstract

The phase diagram of the quasi-binary Cu-TiS system has been studied in situ in the temperature-concentration plane for the first time. Phase transitions between 2D (layered) and 3D (cubic) phases in CuTiS (x = 0-0.5) intercalation compounds have been studied by the X-ray diffraction technique in the temperature range 20-1000 °C. It has been found that the stability of the layered phase is determined by the distribution of copper atoms between the octahedral and tetrahedral crystallographic sites. The occupation of octahedral sites dominates at low temperatures. Upon heating, tetrahedral site occupation is limited due to elastic lattice distortion and the layered phase becomes unstable and transforms to the spinel. Further heating allows the distribution of copper between octahedral and tetrahedral sites; the layered phase becomes stable again.

Thermal stability of the layered modification of Cu0.5ZrTe2 in the temperature range 25–900 °C

“…At the same time, the layered Cu x ZrTe 2 phase becomes depleted in copper content, reaching a composition close to ZrTe 2 , with a residual copper amount of about 4 mol%. This means that in the present case, a phenomenon similar to the retrograde solubility is observed, which was also reported for the Cu-TiTe 2 (Titov et al, 2014), Fe-TiSe 2 (Shkvarina et al, 2016) and Fe-TiTe 2 (Titov et al, 2013) systems. As is shown in these studies, the retrograde solubility in the intercalation compounds is caused by the thermal broadening of the band of hybridized states of titanium and the intercalated atoms.…”

“…Thus, a change in the copper coordination, which causes a change in the stability of the two-dimensional layered and three-dimensional cubic (Shkvarina et al, 2017) or rhombohedral structures can have the same nature as the retrograde solubility which was observed in the other intercalation compounds. However, the factors which determine the phase diagram remain unclear.…”

“…The structural data obtained from this procedure became the basis for the refinement of the crystal structure of the annealed sample. It can be seen that this annealing did not reach the equilibrium state, although in the previously studied systems of Cu x TiS 2 and Fe x TiSe 2 (Shkvarina et al, 2017;Titov et al, 2016), this procedure generated an equilibrium state of the sample. It seems that the reason for the observed slow rate of establishing equilibrium in Cu x ZrTe 2 might be due to a low vapour pressure of tellurium in comparison to the vapour pressure of sulfur and selenium.…”

“…Our previous experiments, carried out with similar materials, i.e. Cu x TiS 2 (Shkvarina et al, 2017) and Fe x TiSe 2 (Shkvarina et al, 2012), showed that this annealing method is sufficient to achieve an equilibrium state.…”

“…Intercalation compounds with copper and silver, based on ZrTe 2 , demonstrate a superconducting state (Generoso et al, 2012;Santana et al, 2012). On the other hand, lattice instability in the intercalation compounds with copper and silver does not allow the existence of a layered phase with these interesting properties, as was observed, for example, in the Cu-TiS 2 system (Shkvarina et al, 2017) and in the hightemperature region of the Ag-TiS 2 system (Titov & Titova, 1995). Our previous study of the Cu-TiS 2 compounds allowed a correlation of the instability of the layered phase with the ISSN 2053ISSN -2296 # 2018 International Union of Crystallography presence of octahedral and tetrahedral coordination of the intercalated atoms in the interlayer space to be made.…”