Band Structure, Phonon Spectrum and Thermoelectric Properties of Ag3CuS2

Abstract

Sulfides and selenides of copper and silver have been intensively studied, particularly as potentially efficient thermoelectrics. Ag3CuS2 (jalpaite) is a related material. However very little is known about its physical properties. It has been found that the compound undergoes several structural phase transitions, having the tetrahedral structural modification I41/amd at room temperature. In this work, its band structure, phonon spectrum and thermoelectric properties were studied theoretically and experimentally. Seebeck coefficient, electrical conductivity and thermal conductivity were measured in a broad temperature range from room temperature to 600 K. These are the first experimental data on transport properties of jalpaite. Ab initio calculations of the band structure and Seebeck coefficient were carried out taking into account energy dependence of the relaxation time typical for the scattering of charge carriers by phonons. The results of the calculations qualitatively agree with the experiment and yield large values of the Seebeck coefficient characteristic for lightly doped semiconductor. The influence of intrinsic defects (vacancies) on the transport properties was studied. It was shown that the formation of silver vacancies is the most probable and leads to an increase of hole concentration. Using the temperature dependent effective potential method, the phonon spectrum and thermal conductivity at room temperature were calculated. The measurements yield low lattice thermal conductivity value of 0.5 W/(m K) at 300 K, which is associated with the complex crystal structure of the material. The calculated room temperature values of the lattice thermal conductivity were also small (0.14–0.2 W/(m K)).

Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2

“…Moreover, Ag and Cu atom vibrations have predominant contributions in low-frequency (up to 100.2 cm −1 ) optical and acoustic modes. 28 Thus, a low-temperature heat capacity measurement confirms that low-energy localized vibrational modes are responsible for the extremely low κ lat . A similar observation was previously predicted for bulk Ag 3 CuS 2 , where strong anharmonicity in bonding arises due to non-Debye-like behavior of the heat capacity.…”

“…28 Overall, the measured thermal conductivity for the SPS-pressed sample is lower than that of the bulk Ag 3 CuS 2 sample, which has an average value of ∼0.5 W/mK. 28 Such ultralow κ lat has further been verified using room-temperature sound velocity measurements. The experimentally measured room-temperature average sound velocity (ν m ) in nanocrystalline Ag 3 CuS 2 is estimated to be as low as ∼1124 m/s, with the values of longitudinal (ν l ) and transverse (ν t ) components found to be ∼1350 and 1011 m/s, respectively.…”

“…The spectroscopically measured band gap of Ag 3 CuS 2 is ∼1.2 eV in the near-IR region (Figure 2), which is higher than for theoretical and bulk Ag 3 CuS 2 . 10,28 Size reduction to the nanodimension is responsible for the increase in the band gap. interconnected in nature (Figure 3a,b).…”

“…25 Such a strong bond anharmonicity in the system with a large number of optical phonons modes with low group velocity is responsible for low thermal conductivity in the system. 25,28 Furthermore, in the nanocrystalline materials, the presence of point defects (see Figure S7 and discussion in the Supporting Information) and nanoscale grain boundaries effectively scatter heat-carrying phonons to a further extent and reduce thermal conductivity significantly.…”

“…Upon further rise of temperature, the material transforms into a mixture of bcc and face-centered cubic (fcc) (space group Im 3̅ m + Fm 3̅ m at 483 K) and subsequently to a fcc phase (space group Fm 3̅ m at 549 K) with strong disorder in the Cu sublattice and ∼0.58% volume change. Both of the high-temperature phases are superionic in nature. ,, In previous reports, this compound was synthesized using high-temperature solid-state reactions and hydrothermal/solvothermal methods. ,− …”