A Low-Temperature Structural Transition in Canfieldite, Ag₈SnS₆, Single Crystals

Area of Science:

• Solid-state chemistry and materials science
• Crystallography and phase transitions
• Semiconducting materials research

Background:

• Canfieldite (Ag₈SnS₆) is a known semiconductor with applications in ionic conductivity, photosensitivity, and thermal conductivity.
• It exhibits a cubic to orthorhombic phase transition at approximately 460 K upon cooling.

Purpose of the Study:

• To report the solution growth of large single crystals of Ag₈SnS₆.
• To investigate the structural and physical properties of Ag₈SnS₆ at low temperatures.
• To explore the relationship between structure and unit cell volume in argyrodite analogues.

Main Methods:

• Solution growth of Ag₈SnS₆ single crystals from a ternary Ag-Sn-S melt.
• Magnetization and thermal expansion measurements between 5-300 K.
• Single crystal X-ray diffraction analysis at 90 K.

Main Results:

• A previously undiscovered structural transition in Ag₈SnS₆ at approximately 120 K was identified.
• The low-temperature phase adopts an orthorhombic structure (space group Pmn2₁), isostructural to related selenides.
• This 120 K transition is first-order with significant thermal hysteresis, and the room-temperature phase can be kinetically arrested into a metastable state.

Conclusions:

• A new low-temperature phase and associated transition mechanism in Ag₈SnS₆ have been characterized.
• A trend linking preferred argyrodite structures (Ag₈TQ₆) to unit cell volume was established, favoring the Pna2₁ arrangement for larger volumes.
• The transition to the Pmn2₁ phase is suppressed in Ge-alloyed samples and pure Ag₈GeS₆, supporting the identified structural trend.