To the theory of rare gas alloys: heat capacity

Abstract

Low-temperature heat capacity of cryocrystals, which contain impurity clusters has been investigated theoretically and experimentally. Such defects might essentially enrich low-frequency part of the phonon spectrum by introducing both localized and delocalized vibrations. The effect of both types of the vibrations on the temperature dependence of the heat capacity is analyzed. Heat capacity of the disordered solid solution Kr-Ar (Ar concentration is~25%) is studied as an example of the effect of the light weakly connected impurities on the low-temperature thermodynamic characteristics of the system. The mass defect of such an impurity induces «phonon pumping» from the low-frequency part of the spectrum into the high-frequency part and decreasing the low-temperature heat capacity, while the weakened interaction between the impurity and the host atoms combined with even weaker interaction between the impurities leads to the formation of the low-temperature maximum on the heat capacity temperature dependence. The analysis performed shows that at rather high Ar concentrations, the nonmonotonous temperature dependence of the relative change in the heat capacity of solid Kr Ar 1-p p solutions is determined by excitation of delocalized high-dispersion low-frequency phonons. PACS: 63.20.-e Phonons in crystal lattice; 63.20.Mt Phonon-defect interaction; 63.50.+x Vibrational states of disordered systems; 63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions.

The Features of Low Frequency Atomic Vibrations and Propagation of Acoustic Waves in Heterogeneous Systems

“…Argon is 2.09  times lighter than krypton, and the interaction of the impurity of argon with krypton atoms is slightly weaker than the interaction of krypton atoms between each other, so an isolated Ar impurity in the Kr matrix behaves almost like a light isotope. At the same time, in a krypton matrix the interaction of argon impurities between each other is more than five times weaker than the interaction between the krypton atoms (Bagatskii et al, 2007). Fig.…”

“…which can not be described by the expansion of the impurity concentration. Thus the weakening of bonds between the argon impurities in krypton matrix leads to a characteristic "two-extreme" behavior of the temperature dependence of the relative change in the lowtemperature heat capacity unexplained by the superposition of contributions of isolated impurities, impurity pairs, triples and etc., without taking into account the restructuring of the entire spectrum (Bagatskii et al, 2007). The restructuring of the phonon spectrum of the crystal and the delocalization of QLV at finite concentrations of impurities in the coherent potential approximation was considered in (Ivanov, 1970;Ivanov & Skripnik, 1994).…”

Low-temperature vibration characteristics in InSe single crystals intercalated by Ni

“…With the increasing concentration x (in our experiment x = 3 and 7 at.%), the scattering of long-wavelength acoustic phonons on quasi-localized vibrations levels of the impurity atoms can be interpreted as the formation of a so-called boson peak [18,26,27,37] in the phonon spectrum. Note that the formation of BP in the phonon spectrum of even much simpler compounds (e.g., solid solution Kr-Ar) can be lead to a multiextremal dependence ∆C ph /C ph [25]. In our case the intercalation of InSe by nickel influences a large number of force constants, leading to more extremes in the temperature dependence of the relative change of the heat capacity.…”

“…It is still a matter of discussion. A lot of theories have been constructed to explain the microscopic origin of the BP [18][19][20][21][22][23][24][25][26][27], which attributed it to different types of vibrational excitations. Some of them ascribe the BP to a strongly localized excitations induced by the intrinsic disorder, whereas others attribute it to the collective propagating modes.…”

Unusual morphology of equimolar Ar–Kr alloys

“…Analysis brought the authors to the conclusion that the Kr matrix contains Ar clusters, which means that the inferable of the complete solubility of both components has to be but to doubt. The boson peak discovered in calorimetric studies on the Kr-25%Ar alloy [6] was another evidence against the homogeneity of Ar-Kr alloys within the entire range of mutual concentrations. A similar conclusion about the complete solubility was made for the N 2 -Ar alloys (a close analog of the Ar-Kr system) [7,8].…”