Phase transition of tetragonal copper sulfide Cu2S at low temperatures

Abstract

The low-temperature behavior of tetragonal copper sulfide, Cu 2 S, was investigated by powder and singlecrystal x-ray diffraction, calorimetry, electrical resistance measurements, and ambient temperature optical absorption spectroscopy. The experiments were complemented by density-functional-theory-based calculations. High-quality, polycrystalline samples and single crystals of tetragonal copper sulfide were synthesized at 5 GPa and 700 K in a large volume multianvil press. Tetragonal Cu 2 S undergoes a temperature-induced phase transition to an orthorhombic structure at around 202 K with a hysteresis of ±21 K, an enthalpy of reaction of 1.3(2) kJ mol −1 , and an entropy of reaction of 6.5(2) J mol −1 K −1. The temperature dependence of the heat capacity at the transition temperature indicates that the transition from the tetragonal to the low-temperature polymorph is not a single process. The structure of the low-temperature polymorph at 100 K was solved in space group P na2 1. The structure is based on a slightly distorted cubic close packing of sulfur with copper in threefold coordination similar to the structure of tetragonal copper sulfide. The electrical resistance changes several orders of magnitude at the transition following the temperature hysteresis. The activation energy of the conductivity for the tetragonal phase and the low-temperature polymorph are 0.15(2) and 0.22(1) eV, respectively. The direct band gap of the tetragonal polymorph is found to be 1.04(2) eV with the absorption spectrum following Urbach's law. The activation energies and the band gaps of both phases are discussed with respect to the results of the calculated electronic band structures.

Pressure-induced changes of the structure and properties of monoclinic α -chalcocite Cu2S

“…12. As observed in other phases of Cu 2 S [13] or Cu 2 O [41] and in the three electronic band structures shown in Fig. 12, in this compound the top of the valence band has a marked Cu d character, while the lowest part of the conduction band is mainly contributed by the Cu s orbitals.…”

“…After this analysis the value of the band gap E g was obtained from each spectrum as the intercept of the high-energy linear part of (hνα) 2 with the energy axis. We also plotted ln(α) as a function of energy for the three phases, observing that different from the tetragonal [13] polymorph of Cu 2 S, in our case the absorption edge cannot be explained with an Urbach tail [40].…”

“…According to Machani et al [12], nanoparticles of tetragonal copper sulfide exhibit optical properties similar to those of monoclinic α-chalcocite, drawing attention to nonambient phases of copper sulfides in general. However, * zimmer@kristall.uni-frankfurt.de a recently described temperature-induced phase transition of tetragonal copper sulfide at moderately low temperatures [13] might be disadvantageous for applications in solar cells.…”

Effect of the Annealing Atmosphere on Crystal Phase and Thermoelectric Properties of Copper Sulfide

“…Both chalcocite and djurleite are very common phases at ambient temperature and are often intermixed or intergrown. 6 Roxbyite Cu58S32 or Cu1.81S is closely related to chalcocite and djurletite, but it has a triclinic structure consisting of a distorted hexagonal framework of 32 S atoms with 58 independent Cu atoms, all having triangular coordination. 7 Roxbyite is uncommon in bulk, but can be readily synthesized in the form of nanocrystals.…”

“…Furthermore, several metastable phases also exist and are commonly obtained at ambient temperature, such as a tetragonal Cu1.96S phase 6,[15][16][17] and a rhombohedral Cu1.8S structure. 18,19 At relatively low temperatures, just above 70-100 ºC depending on stoichiometry, Cu atoms become highly mobile, which makes it difficult to determine their exact positions.…”