The optical absorption edge of Ag0.05Cu0.95InP2Slayered crystals


Purpose. AgхCu1-хInP2S6 crystals belong to the family of CuInP2S6 layered crystals characterized by the presence of ferrielectric and piezoelectric properties and mixed electron-ion conductivity, which determines the prospect of their use as materials for functional electronics.
Methods. Сrystals of Ag0.05Cu0.95InP2S6 were grown using the Bridgman technique. For measurements, samples of size of 4×4×0.06 mm were used, splitted from the grown sample perpendicular to the axis c. The sample temperature was controlled by a copper-constantan thermocouple with an accuracy of 0.1 K. The hydrostatic pressure was created using a high pressure chamber and controlled with an accuracy of 1 MPa. Absorption spectra were investigated using the diffraction monochromator MDR-2 and the USB400 spectrometer “Ocean optics” in the interval of wavelength 190-1100 nm. For optical measurements we used an optical three-window chamber with a continuous change in pressure.
Results. The spectral dependences of the absorption coefficient of Ag0.05Cu0.95InP2S6 crystals for different temperatures were studied in the range of hydrostatic pressures up to 350 MPa. It was found that in the investigated temperature range the absorption edge has an exponential shape. In the studied region of temperature and pressure for Ag0.05Cu0.95InP2S6 crystals the Urbach's rule is not fulfilled. The dependences of α1/2(hν) at atmospheric pressure and at different temperatures for Ag0.05Cu0.95InP2S6 crystals were built. Breaks on the dependencies α1/2(hν) correspond indirect interband transitions. Based on the performed studies of the absorption edge of Ag0.05Cu0.95InP2S6 crystals, the temperature and pressure behavior of the energy position of the absorption edge of Ag0.05Cu0.95InP2S6 crystals were studied.
Conclusions. Optical absorption spectra of Ag0.05Cu0.95InP2S6 layered crystals under hydrostatic pressure up 350 MPa were investigated in the temperature range of 200-400 K. It was found, that the absorption edge is increasing exponentially and is not explained using Urbach rule. The optical absorption edge of layered crystals of Ag0.05Cu0.95InP2S6 is formed by indirect allowed interband transitions. Pressure and temperature behavior of the characteristic parameters of the optical absorption edge in the phase transition regions of the paraelectric-ferrilectric phases were obtained

Keywords: ferrіelectrics, phase transitions, absorption spectra, hydrostatic pressure