M. I. Bagatskii

A simple low-temperature adiabatic calorimeter for small samples

A simple adiabatic calorimeter has been built to investigate the heat capacity of small (≤1 cm3) samples of carbon nanomaterials at temperatures ranging from 1 to 300 K. It enables (i) rapid mounting of samples (ii) doping of samples with gases directly in the calorimeter, and (iii) rapid cooling of samples to liquid helium temperatures. It can be placed in the helium vessel of a portable Dewar or in a helium cryostat. The heat capacity of a fullerite sample is measured in the temperature range 1–30 K.

Heat capacity of methane–krypton solid solutions. Conversion effect

The heat capacity of Kr–nCH4 solid solutions with the concentrations n=1; 5; 10% and of the solid solution Kr–1% CH4–0.2% O2 is studied at 0.7–8 K. The contributions Crot to the heat capacity of the solutions due to the rotation of the CH4 molecules are estimated. The deviations of the measured Crot from the values corresponding to the equilibrium distribution of the CH4 nuclear spin modifications are dependent on the correlation between the characteristic times of conversion and of the calorimetric experiment. The effects of temperature, O2 impurities, and CH4 clusters upon the conversion rate are studied. It is shown that the hybrid mechanism of conversion proposed by Berlinsky and Nijman, which takes into account both intramolecular and intermolecular interactions of the proton spins, is predominant.

Rotational Excitations in Concentrated Solid Kr-CH4 Solutions: Calorimetric Studies

The heat capacity of solid Kr-CH4 solutions with 30 and 60 mol.% CH4 has been studied at 0.8–20 K. The contribution of the rotational subsystem Crot to the heat capacity of the solutions is separated. The results obtained in this study and Ref. 4 were used to estimate the difference between the lowest-level energies εAT of the nuclear spin A and T modifications of CH4 and to find the characteristic conversion times τ for the solutions with 5–60 mol.% CH4 at low temperatures.

Thermodynamic Properties of Molecular Orientational Glasses with Indirect Interaction

The general regularities of the behaviour of heat capacity and thermal expansion of solutions of substances with simple linear molecules in solidified inert gases at concentrations of about 1 mol % when the direct quadrupole interaction between impurities does not play an important role are discussed. In such solutions indirect (static and dynamic) interactions between impurity molecules (rotators) are dominant. The static interactions are realized through deformation fields produced by impurities in a lattice. The dynamic ones are performed by the phonon exchange between impurity molecules. Under certain conditions the indirect interactions mentioned lead to stochasticity of an energy spectrum of the subsystem of rotators and the appearance of an orientational glass–like state. Special attention is paid to the behaviour of the systems in which an energy spectrum of rotational motion of impurity molecules is determined by intramolecular degrees of freedom. The experimental results of the studies of thermal properties of N2and O2in solid Ar and Kr are presented.

Heat capacity of solid solutions of deuteromethane in krypton. Nuclear spin conversion in CD4 molecules

A study is made of the heat capacity of the binary solid solutions (CD4)0.01Kr0.99 (in the temperature interval ΔT=0.9–7 K) and (CD4)0.05Kr0.95 (ΔT=0.7–20 K) and of the ternary solid solutions (CD4)0.01Kr0.988(O2)0.002 (ΔT=0.7–11 K) and (CD4)0.05Kr0.948(O2)0.002 (ΔT=0.8–4 K). The contribution of the rotational subsystem to the heat capacity of the solutions is separated out. The influence of temperature, oxygen impurities, and the interaction between the deuteromethane molecules on the effective conversion rate is studied. The energy differences between the lowest-lying levels of the nuclear spin species A and T of deuteromethane is determined, and the effective characteristic conversion times are found. Rapid conversion of isolated CD4 molecules is observed in CD4–Kr solutions. It is shown that in the CD4–Kr solutions, as in CH4–Kr solutions, a hybrid mechanism of conversion is dominant at low temperatures (T<1.4 K), and the “bottleneck” governing the conversion rate is the intermolecular effective octup...

To the theory of rare gas alloys: heat capacity

The low-temperature heat capacity of cryocrystals containing impurity clusters is investigated theoretically and experimentally. Such defects might essentially enrich the low-frequency part of the phonon spectrum by introducing both localized and delocalized vibrations. The effect of both types of vibrations on the temperature dependence of the heat capacity is analyzed. The heat capacity of the disordered solid solution Kr–Ar (Ar concentration ∼25%) is studied as an example of the effect of the light, weakly coupled impurities on the low-temperature thermodynamic characteristics of a 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 decreases 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 a low-temperature maximum on the heat capacity temperature dependence. The an...

Phase transitions in Kr–CH4 solid solutions and rotational excitations in phase II

The heat capacity CP of Kr-nCH4 solid solutions with CH4 concentrations n=0.82, 0.86, and 0.90 and of solutions with n=0.90, 0.95 doped with 0.002 O2 impurity is investigated under equilibrium vapor pressure over the interval 1–24K. The (T,n) phase diagram of Kr-nCH4 solid solutions is refined, and the region of two-phase states is determined. The contribution of the rotational subsystem, Crot, to the heat capacity of the solutions is separated. Analysis of Crot(T) at T<3K makes it possible to estimate the effective conversion times τ and the energy gaps E1 and E2 between the tunneling levels of the A,T and A,E nuclear-spin species of CH4 molecules in the orientationally ordered subsystem and to determine the effective energy gaps E1 between the lowest levels of the A and T species. The relations τ(n) and E1(n) stem from changes of the effective potential field in result of the replacement of CH4 molecules by Kr atoms at sites of the ordered sublattices. The effective gaps EL between a group of tunneling ...

Heat capacity of a p-H2–p-D2–Ne solid solution: Effect of (p-D2)Ne clusters

The heat capacity of a solid solution of 1% p-D2 and 0.25% Ne in p-H2 is investigated in the interval ΔT=0.5–4 K. An excess heat capacity ΔCNe of this solution exceeding the heat capacity of the solution of 1% p-D2 in p-H2 is detected and analyzed. It is found that below 2 K the dominant contribution to the heat capacity ΔCNe is made by the rotation of the p-D2 molecules in (p-D2) Ne-type clusters. The number of (p-D2) Ne clusters in the solid sample is strongly dependent on the conditions of preparation. The splitting of the J=1 level of the p-D2 molecules in the (p-D2) Ne clusters, Δ=3.2 K, is consistent with the theoretical estimate.

Quantum diffusion in solid H2−Ne solutions

Quantum diffusion in solid hydrogen containing 0.02–0.25 mol.% neon has been investigated by the calorimetric method in temperature range 1–3 K. The concentrations of orthohydrogen were 0.23; 0.5 and 1 mol. %. The parameter studied was characteristic configurational relaxation time τ. Heat capacity is very sensitive to space distribution of orthohydrogen molecules. Therefore, the determination of configuration relaxation rate has been performed by observing the time dependence of heat capacity. A neon impurity in the indicated concentration is observed to accelerate quantum diffusion in hydrogen. The magnitude of the effect diminishes as the temperature increases.

The effect of n‐H2 impurity on the heat capacity of solid Ne

The alloy Ne-5.1% n‐H2 is investigated calorimetrically under equilibrium vapor pressure in an interval of 0.9–25K. The magnitude and temperature dependence of the heat capacity Csol(T) of the alloy at T=11–20K are found to be strongly dependent on the conditions of sample preparation. The temperature dependence of Csol(T) for the sample prepared in a calorimeter by direct condensation of a gas mixture at T≈15K exhibits a smeared maximum near the triple-point temperature of hydrogen and a phase transition at T=17.1K. The detected features of Csol(T) indicate that preparation of solid Ne–n‐H2 from the gas phase leads to the formation of a long-lived nonequilibrium phase of Ne with high H2 concentrations and a small portion of H2 inclusions with low Ne concentrations. The phase transition is caused by decomposition of this phase. The phase does not recover on cooling of the sample after the phase transition. It is found that the rate of ortho-para conversion of the H2 molecules in the Ne–n‐H2 solid solution...