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

Based on the experimental structures reported for the Cu _x Te (1  ⩽  x  ⩽  2) system, a theoretical study on stability and phase transitions has been performed. Three theoretical structures derived from rickardite (Cu_1.5Te) 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 Cu_1.5Te structures are energetically and dynamically stable (unlike Cu₂Te), and that their vibrational modes may play an important role in the reported Raman spectra for Cu _x Te 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.