Chemical manipulation of phase stability and electronic behavior in Cu4−xAgxSe2

Superionic chalcogenides have gained renewed research interest, within the last decade, as emerging thermoelectric materials due to attractive properties, such as glass-like phonon transport coupled with crystal-like carrier transport. Of particular interest has been p-type coinage metal-based materials (Cu2Se, CuAgSe), which have demonstrated figures-of-merit, ZT, exceeding unity through a broad temperature range. However, the lack of n-type counterparts within this class of compounds limits potential module deployment. Here we show that careful stoichiometry control of the Cu4−xAgxSe2 series enables the formation of stable n-type materials throughout the measured temperature range upon substitution of Cu by Ag (1 ≤ x ≤ 3). Thermopower data show that the sample with x = 1 (Cu3AgSe2) undergoes a transition from n- to p-type conducting behavior, whereas samples with x = 2 (CuAgSe) and x = 3 (CuAg3Se2) exhibit n-type character in the whole measured temperature range. The post superionic transition n-type conductivity of CuAgSe is quite surprising and is contrary to the n- to p-type transition previously reported for this composition. Room temperature X-ray diffraction studies indicate the formation of a two-phase mixture for samples with x = 1 (Cu3AgSe2 = α-Cu2Se + α-CuAgSe) and x = 3 (CuAg3Se2 = α-Ag2Se + α-CuAgSe), whereas a single-phase α-CuAgSe is observed for the sample with x = 2. At 523 K, X-ray diffraction patterns show that Cu3AgSe2 (x = 1) and α-CuAgSe (x = 2) transform into single phase structures with the space group Fmm, while the CuAg3Se2 (x = 3) sample remains a two-phase system (CuAg3Se2 = β-Ag2Se + β-CuAgSe) in contrast to previous studies. This structural study is consistent with the observed gradual evolution of the conduction type of Cu4−xAgxSe2 samples between the p-type character of Cu2Se (x = 0) and the n-type semiconducting behavior of Ag2Se (x = 4). This suggests that the conducting behavior in the Cu4−xAgxSe2 is modulated by the Cu : Ag ratio. All Cu4−xAgxSe2 samples exhibit extremely low thermal conductivity after their phase transitions (<0.5 W m−1 K−1), which result in modest ZT values (∼0.45 at 625 K).