Analysis of temperature dependences of the heat capacity of the phosphorus and nickel VII-type clathrates in the temperature range 2 – 300 K

The article is devoted to the experimental study of the heat capacity of CP (T) inorganic VII-type clathrates: SrNi2P4 and BaNi2P4 in a low-temperature range 2 – 300 K. The studied compounds were synthesized using a two-stage ampoule method. The observed diffuse maximums CP(T) about 5 k are associated with the influence of two-level systems in the sublattice of guest atoms. 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 relationship of the obtained parameters with the features of the crystalline structures of the VII-type clathrates is also discussed.