ABSTRACT

A polycrystalline sample of the Sn_20.5□_3.5As₂₂I₈ clathrate was synthesised by a standard ampoule method from elements. Temperature dependencies of the heat capacity C_p(T) and unit cell parameter a(T) experimentally studied at the temperatures of 2–300 K revealed abnormal behaviour of C_P(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.

摘要

Sn _20.5 □ _3.5 As ₂₂ I ₈包合物的多晶样品通过标准安瓿法从元素合成。在 2-300 K 温度下实验研究的热容量C _p ( T ) 和晶胞参数a ( T ) 的温度依赖性揭示了C _P ( T ) 和a ( T) 在最低 (2–30 K) 和升高 (200–300 K) 温度下的依赖性。这归因于包合物基质中大量空位导致的包合物晶体结构的无序。在 Debye-Einstein 方法中分析了所研究的相关性，同时考虑了软原子势理论、最低温度下的两能级系统 (TLS) 的振动以及高温下晶格振动的非谐性，这允许确定不同贡献的参数所研究的包合物的热力学特性。根据实验数据确定的 Grüneisen 参数的温度依赖性比较，计算出的 Grüneisen 模式参数的温度依赖性揭示了特定模式，这些模式对研究温度范围的不同部分中标题包合物的热力学特性的异常行为具有决定性影响。无量纲 Grüneisen 参数达到γ  = 4 在中等高温下，这表明显着程度的结构无序及其对包合物热力学性质的影响，并且与标题包合物的低热导率密切相关。