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The temperature dependences of the heat capacity (Cp(T)) and crystal lattice parameters (a(T) and Vu(T)) of barium difluoride, BaF2, as well as the heterovalent (BaF2)0.59(TmF3)0.41 solid solution, in the temperature range of 2–300 K are experimentally studied. Anomalous deviations of the dependences of Cp(T), a(T) and Vu(T) of the solid solution at low temperatures (2–20 K) both in the direction of higher and lower values, due to the rearrangement of the crystal field, as well as the appearance of two-level systems in the metal sublattice of the solution, are revealed. At elevated temperatures (150–300 K), an anomalous feature of the studied characteristics with a deviation towards lower values is also observed, which we attribute to the increased influence of the anharmonicity of the vibrations of the solution lattice. It is established that the occurrence of the mechanisms of the revealed anomalies may be attributed to the structural disorder of the solid solution due to the random nature of the placement of Tm3+ ions when replacing Ba2+ ions in the metal sublattice, as well as the appearance of F− ions in the interstitial spaces of the barium difluoride structure. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Heat capacity CP(T) (2–300 K) and unit cell parameters a(T), b(T), and c(T) (5–300 K) of type‐VII clathrates SrNi2P4 and BaNi2P4 have been investigated experimentally. The studied compounds are prepared by a two‐stage ampoule synthesis. Diffuse maxima of CP(T) near 5 K along with anomalies in temperature dependencies of unit cell parameters below 10 K are observed and related to the influence of two‐level systems in guest‐atom substructures. Joint analysis of temperature dependencies of heat capacity and unit cell volume of both clathrates within the frames of the Debye–Einstein model allowed determining characteristic temperatures of the clathrate frameworks and guest atoms as well as parameters of the two‐level systems. The relationships of the obtained characteristics and the peculiarities of the crystal structures of SrNi2P4 and BaNi2P4 are discussed. [ABSTRACT FROM AUTHOR]