On the stability of Cu _x Te polytypes: phase transitions, vibrational and electronic properties

Abstract

Based on the experimental structures reported for the CuxTe (1 ⩽ x ⩽ 2) system, a theoretical study on stability and phase transitions has been performed. Three theoretical structures derived from rickardite (Cu1.5Te) were considered to represent different Cu/Te ratios (1, 1.5 and 2). The structural, electronic, and vibrational properties were calculated by density functional theory and compared to the experimental data available to date. This analysis showed that the proposed CuTe and Cu1.5Te structures are energetically and dynamically stable (unlike Cu2Te), and that their vibrational modes may play an important role in the reported Raman spectra for CuxTe films. As well, it was found that being vulcanite the most stable phase for x = 1, the addition of Cu atoms to this structure induces a gradual flattening of the Cu planes, producing significant changes in the electronic band structure. A thorough review of the experimental reports on the electrical properties of the system was carried out. The experimental data showed that, in agreement with the calculations, the electrical conductivity is higher for phases with x ⩽ 1.5, decreasing as x gets closer to 2. Hence, the low-copper concentration phases are the best choice for solar cell applications due to their electrical properties and stability.

Raman Spectroscopy Study of Phonon Liquid Electron Crystal in Copper Deficient Superionic Thermoelectric Cu2–xTe

“…The thermodynamical stabilities of various phases of Cu 2-x Te with temperature (T) and composition (x) have 26 CuTe contains four atoms in a unit cell; therefore, the irreducible representations of the vibrational optical modes of CuTe at the Brillouin zone center are Γ opt = 2A g + 2B 2g + 2B 3g + B 1u + B 2u + B 3u , where 2A g + 2B 2g + 2B 3g are Raman active modes. 37 Similar to earlier reports, 36,37 modes observed at ∼63, 75, 124, and 133 cm −1 are assigned to B 2g , B 3g , B 2g + A g , and A g , respectively, along with a low frequency mode ∼ 35 cm −1 , within the error bar of the measurements. A schematic of the Raman active vibrational mode for CuTe 37 is shown in Figure 3b.…”

“…Further, CT also shows a structural phase transition with temperature . Recently, theoretical studies on structural, electronic, and vibrational properties for various stoichiometries of Cu 2– x Te have been discussed to understand experimental observation. ,, The complex atomic arrangements of layered copper telluride with loosely bound Cu ions and several phase transitions can result in enhanced phonon and carrier scattering thus leading to an ultralow κ.…”

Lattice-Matched Metal–Semiconductor Heterointerface in Monolayer Cu2Te

“…Cu 2 Te, a member of group IB transition-metal chalcogenides (TMCs), is known for its applications in thermoelectrics and photovoltaics. − Bulk Cu 2 Te is a typical polymorphic material with several phase transitions, − while its two-dimensional form has been rarely explored. One difficulty hampering the synthesis of Cu 2 Te is that traditional growth methods induce severe Cu deficiency in products, leading to complex nonstoichiometry, Cu 2– x Te. , Recently, Tong et al .…”

Stoichiometric Effect of Sb2Te3 Thin Film on Thermoelectric Property

“…According to the literature, the formation energy of CuTe is −11.27 kJ/mol, which is lower than that of Cu 2 Te (25.44 kJ/mol). 34 Hence, the condition to form Cu 2 Te in this study was thermodynamically unfavorable. Notably, the formation of the CuTe compound requires the diffusing Cu to react with Te dissociated from the Sb 2 Te 3 matrix.…”

“…The layer was identified as NiTe IMC from the selected area electron diffraction pattern. The formation energies of CuTe and NiTe are −11.27 and −36.85 kJ/mol, respectively 34,37. Thermodynamically, NiTe IMC forms more easily than CuTe does under the same conditions.…”