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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Ionics

Background:

• Order-disorder transitions in solid-state ion conductors are crucial for superionic behavior and ionic mobility.
• Layered ACrX2 chalcogenides (A = Ag, Cu; X = Se, S) exhibit high ionic conductivity due to cation site disorder in their high-temperature phase.
• Tuning these transitions is key to developing advanced ionic materials.

Purpose of the Study:

• To investigate the impact of alloying with sulfur (S) or tellurium (Te) at the anion site of CuCrSe2 on cation disorder and the order-disorder transition temperature (Tc).
• To explore the relationship between anion size, interatomic distances, bond stiffness, and ionic conductivity in these materials.
• To assess the thermoelectric properties of the alloyed compounds.

Main Methods:

• Synthesis of polycrystalline CuCrSe2-xTex and CuCrSe2-ySy compounds using solid-state reactions.
• X-ray diffraction (XRD) and variable-temperature XRD to analyze crystal structure, solubility, and transition temperatures.
• Thermal diffusivity measurements, elastic property and speed of sound investigations, and thermoelectric characterizations.

Main Results:

• Complete solubility was observed in the S-Se system, while Te substitution was limited to x=0.15 in CuCrSe2-xTex.
• The transition temperature (Tc) decreased with the incorporation of larger anions, with CuCrSe1.85Te0.15 exhibiting the lowest Tc at 282 K, a record low for bulk samples of this crystal structure type.
• Samples softened significantly with increasing anion size, and thermoelectric properties remained largely unchanged at high temperatures.

Conclusions:

• Alloying at the anion site, particularly with larger elements like Te, effectively reduces the order-disorder transition temperature in CuCrSe2-based materials.
• This anion site alloying provides a viable strategy to tune interatomic distances and bond stiffness, thereby controlling the behavior of solid-state ionic conductors.
• The findings highlight a promising approach for optimizing ionic mobility in solid-state electrolytes.