Low-temperature acanthite-like phase of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cu</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi mathvariant="normal">S</mml:mi></mml:mrow></mml:math>: Electronic and transport properties
Ho Ngoc Nam, Katsuhiro Suzuki, Tien Quang Nguyen, Akira Masago, Hikari Shinya, Tetsuya Fukushima, Kazunori Satō
Abstract
The mobility and disorder in the lattice of Cu atoms as liquidlike behavior is an important characteristic affecting the thermoelectric properties of ${\mathrm{Cu}}_{2}\mathrm{S}$. In this study, using a theoretical model called an acanthite-like structure for ${\mathrm{Cu}}_{2}\mathrm{S}$ at a low-temperature range, we systematically investigate the electronic structure, intrinsic defect formation, and transport properties by first-principles calculations. Therefore, previous experimental reports on the indirect band-gap nature of ${\mathrm{Cu}}_{2}\mathrm{S}$ were confirmed in this work with an energy gap of about 0.9--0.95 eV. As a result, the optical absorption coefficient estimated from this model also gives a potential value of $\ensuremath{\alpha}>{10}^{4}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the visible spectrum range. According to the bonding analysis and formation energy aspect, Cu vacancy is the most preferred defect to form in ${\mathrm{Cu}}_{2}\mathrm{S}$, which primarily affects the conductive behavior as a $p$ type, as experimentally observed. Finally, the transport properties of ${\mathrm{Cu}}_{2}\mathrm{S}$ system were successfully reproduced using an electron-phonon scattering method, highlighting the important role of relaxation time prediction in conductivity estimation instead of regarding it as a constant.