Temperature-dependent influence of disorder on the thermodynamic properties of Sn20.5□3.5As20I8, a vacancy-driven superstructure of the type-I clathrate

Abstract

ABSTRACT A polycrystalline sample of the Sn20.5□3.5As22I8 clathrate was synthesised by a standard ampoule method from elements. Temperature dependencies of the heat capacity Cp (T) and unit cell parameter a(T) experimentally studied at the temperatures of 2–300 K revealed abnormal behaviour of CP (T) and a(T) dependencies at the lowest (2–30 K) and elevated (200–300 K) temperatures. This was attributed to disorder in the clathrate crystal structure due to significant number of vacancies in the clathrate host matrix. The dependencies studied were analysed within the Debye–Einstein approach taking into account soft atomic potential theory, vibrations of two-level system (TLS) at the lowest temperatures, and anharmonicity of lattice vibration at elevated temperatures, which allows determining parameters of different contributions to thermodynamic characteristics of the studied clathrate. A comparison of the temperature dependence of Grüneisen parameter, which was determined from experimental data, with the temperature dependence of the calculated Grüneisen mode parameters revealed specific modes that have a decisive influence on the anomalous behaviour of the thermodynamic characteristics of the title clathrate in different parts of the studied temperature range. The dimensionless Grüneisen parameter reaches γ = 4 at moderately high temperatures, which indicates a significant degree of structural disorder and its influence on thermodynamic properties of clathrates and correlates well with low thermal conductivity of the title clathrate.