Controlling the Order-Disorder Transition Temperature through Anion Substitution in CuCrX ₂ (X = S, Se, Te)

In solid-state ion conductors, order-disorder transitions often govern the onset of superionic behavior, making them a key target for tuning ionic mobility. Layered ACrX ₂ (A = Ag, Cu; X = Se, S) chalcogenides have high ionic conductivity enabled by cation site disorder associated with a high-temperature phase. In this work, we investigated alloying with S or Te at the anion site in CuCrSe₂ and the impact that alloying has on the degree of cation disorder and the temperature of the order-disorder transition. We prepared a series of polycrystalline CuCrSe_2‑x Te _x (x = 0, 0.1, 0.15, 0.175) and CuCrSe_2‑y S _y (y = 0, 0.1, 0.25, 0.5, 0.75, 1.0, 2.0) compounds by solid-state synthesis. X-ray diffraction analysis confirmed that the S-Se system exhibits complete solubility, whereas Te substitution at the anion site in CuCrSe₂ is limited to x = 0.15. Variable temperature X-ray diffraction and thermal diffusivity measurements were conducted to track the order-disorder and superionic transition temperature (T _c) of the compounds. The transition temperature was found to be highly composition-dependent, exhibiting a decreasing trend with the incorporation of larger anions; CuCrSe_1.85Te_0.15 had the lowest T _c at 282 K, which is the lowest reported T _c to date for bulk samples in this crystal structure type. We also investigated the elastic properties and speed of sound in the CuCrSe_2‑x Te _x series as functions of composition and temperature. We show that the samples soften sharply as the anion size increased. As a function of temperature, we see only a small inflection of the temperature coefficient of elasticity, dC _ij/dT, at the order-disorder phase transition, confirming prior findings that long-wavelength acoustic phonons are largely unaffected by the phase transition. Thermoelectric (TE) characterizations were also performed, revealing that the TE figure of merit of the compounds remains nearly unchanged at high temperatures (493 K). These findings demonstrate that tuning interatomic distances and bond stiffness through the anion site alloying can effectively tailor the behavior of solid-state ionic conductors.