Structure‐Related Thermal Properties of Type‐VII Clathrates SrNi₂P₄ and BaNi₂P₄ at Low Temperature

Abstract

Heat capacity C P (T) (2-300 K) and unit cell parameters a(T), b(T), and c(T) (5-300 K) of type-VII clathrates SrNi 2 P 4 and BaNi 2 P 4 have been investigated experimentally. The studied compounds are prepared by a two-stage ampoule synthesis. Diffuse maxima of C P (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 SrNi 2 P 4 and BaNi 2 P 4 are discussed.

Clathrate BaNi2P4: An Interplay of Heat and Charge Transport Due to Strong Host–Guest Interactions

“…19 Recently, a heat capacity investigation of BaNi 2 P 4 was reported which qualitatively agrees with our studies. 32 The reported positive slope for C P /T vs T 2 in BaNi 2 P 4 at T 2 < 25 K 2 is distinct from our observations. No such anomalies were observed in our studies (Figure 5E), which indicate that the reported low-temperature features may not be intrinsic for BaNi 2 P 4 .…”

“…No such anomalies were observed in our studies (Figure 5E), which indicate that the reported low-temperature features may not be intrinsic for BaNi 2 P 4 . The discussed low-temperature features affect the estimation of Debye temperatures which were reported as high as θ D = 500 K for BaNi 2 P 4 , 32 in contrast to our results of 205 K (Table 1). We cross-verified the Debye temperatures derived from the heat capacity measurements with the Debye temperatures estimated from the variable-temperature diffraction experiments (Table 1).…”

“…Recently, a heat capacity investigation of BaNi 2 P 4 was reported which qualitatively agrees with our studies . The reported positive slope for C P / T vs T 2 in BaNi 2 P 4 at T 2 < 25 K 2 is distinct from our observations.…”

EuNi2P4, the first magnetic unconventional clathrate preparedviaa mechanochemically assisted route

“…Synthesis. According to literature [10,23], samples of SrNi2P4 can be easily prepared from the elements by several subsequent annealing steps at 1123 K. Contrary to that, when a stoichiometric mixture of Eu, Ni, and P is subjected to the same synthetic procedure, the yield of the target crystalline phase of EuNi2P4 does not exceed 80 %, with all samples containing sizeable amounts of EuNi2P2 and phosphorus as by-products. Reducing the temperature to 973 K does not improve the sample quality, heating above 1123 K leads to an irreversible decomposition observed by DTA measurements, whereas below 873 K reaction kinetics becomes too slow.…”

“…In the current paper, we report a mechanochemical synthesis as well as structural and thermodynamic study of the first magnetic unconventional clathrate, EuNi2P4, and compare it with the Sr analogue [10,23].…”