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Highly transparent X2O3 sesquioxide ceramics were obtained from a solid solution of five different oxides (Lu2O3, Y2O3, Yb2O3, Gd2O3, and Dy2O3), mixed in an equal molar ratio according to the principle of high-entropy. The fabricated (Lu, Y, Yb, Gd, Dy)2O3 ceramics achieved 99.97 % of the relative density and exhibited a high degree of optical transparency with the in-line transmittance of almost 80 % in the visible wavelength range. Emissions of Gd3+ (6PJ → 8S7/2 at 312 nm), Dy3+ (4F9/2 → 6H15/2 at 492 nm and 4F9/2 → 6H13/2 at 572 nm), and Yb3+ (2F5/2 → 2F7/2 at 1031 nm) suggested a potential application of the high-entropy ceramics as multi-wavelength emission phosphor transparent ceramics. High-entropy ceramics also exhibited lower specific heat and thermal conductivity compared to single-element sesquioxide ceramics. This work demonstrated that highly transparent oxide ceramics, with complex chemical compositions and good optical properties, could be obtained using the high-entropy principle.

The thermal conductivity of crystalline methane samples with spherical palladium nanoparticles of diameter 6, 8, 10, 12, 18 and 24 nm embedded in the CH4 structure was determined in the temperature range 2–35 K. The spherical nanoparticles featured a low dispersion of the sphere diameter and the volume fraction of the nanoparticles in the nanocomposites amounted to 0.15. Time relaxation approximation analysis shows that the matrix resistive phonon scattering processes are dominated by diffuse scattering of phonons by matrix-nanoparticle boundaries. Other considered intensities of resistive scattering mechanisms of phonons, i.e., scattering by point defects, by dislocation strain fields and by phonons in U-processes remain almost the same in nanocomposites and pure methane crystal. In transfer of the heat through the investigated nanocomposites non-resistive scattering of phonons by the nanoparticles, such as phonon specular scattering and multiple scattering of phonons show very high intensity. The nanocomposites formed of 18 and 24 nm nanospheres show anomalous power increase of the thermal conductivity coefficient dependence on temperature, exceeding the values known for a typical dielectric crystal.The thermal conductivity of crystalline methane samples with spherical palladium nanoparticles of diameter 6, 8, 10, 12, 18 and 24 nm embedded in the CH4 structure was determined in the temperature range 2–35 K. The spherical nanoparticles featured a low dispersion of the sphere diameter and the volume fraction of the nanoparticles in the nanocomposites amounted to 0.15. Time relaxation approximation analysis shows that the matrix resistive phonon scattering processes are dominated by diffuse scattering of phonons by matrix-nanoparticle boundaries. Other considered intensities of resistive scattering mechanisms of phonons, i.e., scattering by point defects, by dislocation strain fields and by phonons in U-processes remain almost the same in nanocomposites and pure methane crystal. In transfer of the heat through the investigated nanocomposites non-resistive scattering of phonons by the nanoparticles, such as phonon specular scattering and multiple scattering of phonons show very high intensity. The nanoco...

The results of experimental investigations into the thermoelectric properties (electrical conductivity, thermoelectric power, and thermal conductivity) of microtextured foils and single-crystal wires based on semimetal and semiconductor Bi1 –xSbx alloys are presented in the temperature range of 4.2–300 K. It is found that the band gap ΔE in Bi–17 at % Sb wires increases with decreasing wire diameter d, which is a manifestation of the quantum-size effect. At low temperatures (T < 50 K), in the wires with d < 400 nm, the electrical conductivity increases due to the significant contribution of highly conductive surface states characteristic of topological insulators. It is found for the first time that the thermal conductivity of semimetal Bi–3 at % Sb foils at low temperatures is two orders of magnitude lower, and that of semiconductor Bi–16 at % Sb foils one order of magnitude lower, than that in bulk samples of the corresponding composition due to significant phonon scattering at grain boundaries and surfaces. This effect leads to considerable enhancement of the thermoelectric figure-of-merit ZT and can be used in miniature low-temperature thermoelectric energy converters.

Low-temperature thermal diffusivity of molecular cryocrystals (O₂, N₂, CO, N₂O and CO₂) has been investigated and some anomalies were found. The thermal diffusivity of these crystals vary by more than 4–6 orders of the magnitude in the temperature range from 1 K up to their triple points. The thermal diffusivity displays jumps at the phase transition points. It has been found that the thermal diffusivity of solid oxygen is nonmonotonic. Below 4 K, the temperature dependence shows a plateau which is due to the fact that both the heat capacity and the thermal conductivity have the cubic temperature dependence in this temperature range. For the cryocrystals of the nitrogen group, a similar plateau is shifted to lower temperatures. The temperature dependence of the equalization time for these crystals is studied. Досліджено низькотемпературну температуропровідність молекулярних кріокристалів (O₂, N₂, CO, N₂O і CO₂). Температуропровідність цих кристалів змінюється більш ніж на 4–6 порядків в області температур від 1 К до їх потрійних точок. В області фазових перетворень спостерігаються стрибки температуропровідності. Виявлено, що температуропровідність твердого кисню є немонотонною. Нижче 4 К на температурній залежності спостерігається плато, яке обумовлено тим, що як теплоємність, так і теплопровідність в цьому температурному діапазоні мають кубічну температурну залежність. Для кріокристалів азотної групи подібне плато зміщується в бік більш низьких температур. Вивчено температурну залежність часів вирівнювання температури для цих кристалів. Исследована низкотемпературная температуропроводность молекулярных криокристаллов (O₂, N₂, CO, N₂O и CO₂). Температуропроводность этих кристаллов изменяется более чем на 4–6 порядков в области температур от 1 К до их тройных точек. В области фазовых переходов наблюдаются скачки температуропроводности. Обнаружено, что температуропроводность твердого кислорода является немонотонной. Ниже 4 К на температурной зависимости наблюдается плато, обусловленное тем, что как теплоемкость, так и теплопроводность в этом температурном диапазоне имеют кубическую температурную зависимость. Для криокристаллов азотной группы подобное плато смещается в сторону более низких температур. Изучена температурная зависимость времен выравнивания температуры для этих кристаллов.

The detailed investigation on the specific heat in manganites of Pr0.6Sr0.4−xAgxMnO3 (x = 0.05 and x = 0.1) has been reported. The experimental conditions were: the temperature range 1.8–330 K and applied magnetic fields of 0, 1, 2 and 3 T. Specific heat results confirmed two phase transitions, a structural one at Tstr = 43 K for both samples and a paramagnetic–ferromagnetic one near TPM–FM = 288 K and 284 K for the x = 0.05 and x = 0.1 sample, respectively. For the more doped sample the magnetic field causes the shift of the PM–FM transition temperature towards higher temperatures. A thorough analysis of particular contributions to the specific heat was presented. The indirect magnetocaloric effect was deduced for the more doped sample (x = 0.1). Under an applied magnetic field of 3 T magnetic entropy changes and adiabatic temperature exhibits a maximum. The absolute ΔSmax reaches the value of 1.74 J/mol K, resulting in adiabatic cooling of ΔTad = 0.95 K. Such result confirms presented samples as promising candidates for the magnetic cooling technology at room temperature.

The temperature dependencies of thermal conductivity χ(Т), electrical conductivity σ(Т), and thermopower α(Т) in foils of Bi1–xSbx alloys in the semimetal and semiconductor states, in a temperature range of 4.2–300 K, were experimentally studied. Foils of Bi1–xSbx alloys were prepared by high-speed crystallization of a thin layer of the melt on the polished inner surface of a rotating copper cylinder. High crystallization rates (v = 5 × 105 m/s) enabled a uniform distribution of the components throughout the volume. The thickness of the foils was 10–30 μm with the texture 101¯2 parallel to the foil plane and the С3 axis coinciding with the normal to the foil surface. It was shown that, in the low-temperature range (T

The preliminary results of experimental investigations of the thermal conductivity of nanocomposites built of amorphous silica nanoparticles embedded in the structure of crystalline methane are presented. The investigations were carried out on the samples containing the particles of linear dimensions of 5, 10–20 and 42 nm. The measurements were performed with steady-state heat flow method in the temperature interval 2–35 K. A very simple thermal conductivity model shows that the dependence of the thermal conductivity coefficient of the investigated nanocomposites on temperature can be successfully described by taking into account only two mechanisms of the heat carrier dissipation: scattering of phonons by methane crystal grain boundaries and phonon-phonon interaction in U-processes. It was found that the low-temperature phonon mean free path is inversely proportional to the linear dimension of the nanopowder particles embedded in the methane crystal structure.

Influence of temperature on spectroscopic and thermal properties of Tm 3+ and Er 3+ -doped GdVO 4 and LuVO 4 crystals was examined. Polarized absorption and emission spectra as well as excited state luminescence lifetimes have been measured and analyzed especially at cryogenic temperatures in the 1.5–1.6 μm and 1.8–1.9 μm spectral ranges to assess laser qualities of these materials. The emission cross sections for 4 I 132 → 4 I 15/2 (Er) and 3 F 4 → 3 H 6 (Tm) transitions at T = 77 K were determined and confronted to the results obtained at room temperature. Effective emission cross sections estimated for aforementioned erbium and thulium transitions indicate that effective laser operation at cryogenic temperatures can be achieved in the gadolinium and lutetium vanadate crystals. Efficiency of laser material depends on its ability to heat distribution upon high-power energy pumping process. Accordingly, it has been found that the anisotropy of thermal conductivity for the undoped crystals is weakly influenced by the temperature albeit conductivity values for the GdVO 4 are higher than those for LuVO 4 at all temperatures. Incorporation of erbium or thulium ions results in a decrease of thermal conductivity in the two vanadate crystals and an anomalous temperature dependence of thermal conductivity in the LuVO 4 was observed. Anisotropy of temperature dependence of thermal conductivity was found for the LuVO 4 :3.2 at%Tm 3+ crystal. In this case the thermal conductivity along c-axis is inferior to that along a-axis of the crystal.

The preliminary results of investigations of heat transfer in nanocomposites consisting of nanoparticles randomly distributed in solid carbon monoxide matrix are presented. In the experiment the thermal conductivity coefficient dependence on temperature for CO crystal with silica and palladium nanoparticles of different size embedded in the crystal structure was determined over the temperature range 2.2–35 K by steady-state heat flow method. The results of the measurements were analyzed within the frame of relaxation time approximation. The analysis shows that lowering of the thermal conductivity of the nanocomposites relative to pure carbon monoxide crystal observed for both types of the investigated nanoparticles, palladium and silica, is caused mostly by scattering of phonons by boundaries of the nanoparticles. Additionally, the presence of the nanoinclusions promotes higher density of dislocations and influences the matrix lattice dynamics. Наведено попередні результати досліджень теплопереносу в нанокомпозитах, які містять наночастинки, що випадково розподілені у твердій матриці монооксиду вуглецю. Методом стаціонарного теплового потоку в температурному інтервалі 2,2–35 К визначено експериментальні залежності коефіцієнта теплопровідності від температури для кристалевого СО, що містить наночастинки оксиду кремнію та паладію різних розмірів. Результати вимірювань проаналізовано в рамках апроксимації часу релаксації. Аналіз показує, що зниження теплопровідності нанокомпозитів щодо чистого кристала монооксиду вуглецю, яке спостерігається для досліджених наночастинок паладію та оксиду кремнію, обумовлено головним чином розсіюванням фононів на границях наночастинок. Крім того, наявність нановключень сприяє більш високій щільності дислокацій і впливає на динаміку гратки матриці. Приведены предварительные результаты исследований теплопереноса в нанокомпозитах, содержащих наночастицы, случайным образом распределенные в твердой матрице монооксида углерода. Методом стационарного теплового потока в температурном интервале 2,2–35 К определены экспериментальные зависимости коэффициента теплопроводности от температуры для кристаллического СО, содержащего внедренные наночастицы оксида кремния и палладия различных размеров. Результаты измерений проанализированы в рамках аппроксимации времени релаксации. Анализ показывает, что понижение теплопроводности нанокомпозитов относительно чистого кристалла монооксида углерода, наблюдаемое для исследуемых наночастиц палладия и оксида кремния, обусловлено главным образом рассеянием фононов на границах наночастиц. Кроме того, наличие нановключений способствует более высокой плотности дислокаций и влияет на динамику решетки матрицы.

The effective thermal conductivity of samples of cryocrystal nanocomposite obtained from argon and SiO2 nanopowder was determined in the temperature interval 2–35 K using the steady-state method. The thermal conductivity of crystalline argon with nanoparticles of amorphous silica oxide embedded in its structure shows a weak dependence on particle linear dimension in the interval 5–42 nm. The temperature dependence of the thermal conductivity of the nanocomposites can be well approximated by taking into account only the two mechanisms of heat carrier scattering: phonon-phonon interaction in U-processes and scattering of phonons by dislocations.

Study on half-Heusler compounds RENiSb, RE = Sc, Dy, Ho, Er, Tm, Lu, was performed in perspective of high-temperature thermoelectric application. Relatively small electrical resistivity (4–60 μ Ω m in 300–1000 K) combined with fairly high positive Seebeck coefficient (40–160 μV/K) yielded for the RENiSb ternaries rather large thermoelectric power factors at 750 K, namely 14 μW K−2cm−1 for ScNiSb and 11 μW K−2cm−1 for TmNiSb. At room temperature the thermal conductivity measured for the two materials was 8.1 W K−1m−1 and 4.4 W K−1m−1, respectively. The value obtained for the latter compound is smaller than those reported for unoptimized half-Heusler phases based on d-electron elements. This finding may be explained by strong point defect scattering occurring due to intrinsic disorder in crystal structure of TmNiSb, or to smaller extent by enhanced scattering on incoherent grain boundaries with the impurity phases. Nevertheless, achieving good thermoelectric performance of the RENiSb phases at high temperatures requires proper tuning of carrier concentration, as well as further reduction in their thermal conductance by means of alloying and/or nanostructurization.

The measurements of thermal conductivity coefficient of a solid carbon monoxide crystal containing argon and nitrogen admixtures at different concentrations (1.5, 3, 6, 12.5, 25% for N2 and 0.5, 1, 1.25, 2, 4% for Ar) were performed in the temperature range from 1.5 to 40 K by steady-state heat flow method. For analysis of the experimental data the Callaway method in the framework of the Debye model was used. The contribution of various mechanisms of phonon scattering, including scattering by disordered dipoles of the CO matrix, to the thermal conductivity of CO–N2 and CO–Ar solid solutions were taken into account.

The low-temperature dependence of thermal conductivity coefficient of dielectric crystals on temperature spreads over a vast region, both in terms of the absolute value of the coefficient as well as its functional dependence on the temperature. However, we were able to notice a group of simple molecular polycrystals which show their thermal conductivity very close to each other. What is more, the similarity of the thermal conductivity of the polycrystals for many reasons resembles known and studied for over 30 years the universality observed for low-temperature thermal conductivity of amorphous solids. Here, utilizing already developed thermal conductivity models, we try to understand what phenomena could result in the similarity of the thermal conductivity of the polycrystals and therefore explain the finding. Provided that some other conditions are met at least each of three different phonon scattering mechanisms could lead to the similarity.

Order or disorder in subsystem of dipolar constituents of a solid often significantly influences its physical properties. The results of investigation of the effect of glassy state in subsystem of carbon monoxide molecules on thermal conductivity of CO crystal in its equilibrium vapor pressure are reported. The thermal conductivity of a high quality carbon monoxide crystal was specified over the temperature range 1.2–45 K. The results of these measurements were analyzed within the frame of relaxation time approximation. It was shown that at low temperatures phonon scattering by subsystem of disordered CO molecules is the most significant dissipative mechanism of heat transfer in the investigated crystal.

The results of measurements of the thermal conductivity dependence on temperature of pure ammonia crystal obtained in the temperature range 1.3–44 K have been shown. Quantitatively, the sample displays the thermal conductivity typical for a dielectric crystal. Below 2 K a singularity of the dependence of thermal conductivity coefficient on temperature was observed. The singularity was linked to the existence of TLS in the crystal. The analysis of the temperature dependence of the phonon mean free path in the NH3 crystal was also done.

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process of superconductors, however, a low value of T c is required. Here we report and analyze in detail the thermal properties of bulk Ti-doped Cu–Zn ferrite ceramics Cu0.3Zn0.7Ti0.04Fe1.96O4 and Mg0.15Cu0.15Zn0.7Ti0.04Fe1.96O4. They are characterized by a Curie temperature in the range 120–170 K and a maximum DC magnetic susceptibility exceeding 20 for the Cu0.3Zn0.7Ti0.04Fe1.96O4 material. The temperature dependence of both the specific heat C p and of the thermal conductivity κ, determined between 2 and 300 K, are found not to exhibit any peculiar feature at the magnetic transition temperature. The low-temperature dependence of both κ and the mean free path of phonons suggests a mesoscopic fractal structure of the grains. From the measured data, the characteristics of thermally actuated waves are estimated. The low magnetic phase transition temperature and suitable thermal parameters make the investigated ferrite ceramics applicable as magnetic wave producers in devices designed for magnetization of high-temperature superconductors.

Undoped Li3Lu3Ba2(MoO4)8 monoclinc (C2/c) single crystals, as well as crystals doped with Tm or Ho and codoped with Tm and Ho, have been grown by the top seeded solution growth method using Li2MoO4 as the solvent. A convenient growth flux was found for the solute/solvent molar composition of 1:5; for this, the growth temperature interval was ≈1073–1053 K, with a cooling rate of 0.05 K/h. The crystal shape is controlled by the seed orientation, and a- and b-oriented seeds give rise to near rectangular cross sections while c-oriented seeds produce rhombohedral cross sections. The grown crystals are deficient in Li with regard to the nominal formula. Li and Lu (Tm, Ho) ions share the same 8f crystal site with occupancy factors near 0.2 and 0.8, respectively. The relationships between the crystallographic and physical frames of some crystal properties (thermal expansion, refractive indices, and optical absorption) are determined as well as the specific heat and thermal conductivity (along a- and b-crystal axe...

The thermal conductivity of solid CO is investigated in the temperature range 1–20K. The experimental temperature dependence of thermal conductivity of solid CO is described using the time-relaxation method within the Debye model. The comparison of the experimental temperature dependences of the thermal conductivity of N2 and CO shows that in the case of CO there is an additional large phonon scattering at temperatures near the maximum. Analysis of the experimental data indicates that this scattering is caused by the frozen disordered dipole subsystem, similar to a dipole glass. The scattering is described by resonant phonon scattering on tunneling states and on low-energy quasi-harmonic oscillations within the soft potential model.

Measurements of thermal conductivity of solid methane-deuteromethane solutions at equilibrium vapor pressure in the temperature range 1.2÷20 K are reported. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4-CD4 system. The contribution of the orientational defect scattering to the thermal conductivity is discussed in frame of the model of local changes in the moments of inertia of molecules.

The thermal conductivities of CO2 and N2O solids are investigated in the low-temperature range 1–40K. The thermal conductivities of CO2 and N2O are large compared with those of simple molecular crystals such as N2, CO, or O2 in the whole investigated temperature range. Analysis of the experimental data by the Callaway method shows that a relatively large size of the crystalline grains, low density of dislocations, and weak phonon–phonon interaction might be the reasons for the good thermal conduction in these crystals at temperatures near the maxima. A comparison between calculated values of the intensity of normal phonon scattering processes and experiment gives evidence that in N2O there is an additional (in comparison with CO2) giant scattering of phonons. This scattering is described in the frameworks of the soft potential model by the resonance phonon scattering on tunneling states and low-energy vibrations.

The temperature dependence of the magnetic susceptibility of polycrystalline oxygen samples is measured in the range from 1to54K. The measurements are performed using our home-designed SQUID-based gradiometer. The sensitivity of the instrument with respect to susceptibility reaches values of the order of 10−13. The results are compared with data in the literature.

Some mixed crystals show a low-temperature thermal-conductivity dependence on temperature similar to that of amorphous solids. This phenomenon is usually ascribed to a disorder of another kind. In the current paper we report results of measurements of the thermal conductivity of CH4–CD4 solid solution which also displays a glass-like behaviour for some concentrations of deuterated methane. Here, however, the disorder appearing at these concentrations of CD4 cannot account for the characteristic plateau. Instead, we explain it in terms of an enhancement of the phonon-rotation interaction resulting from lowering the symmetry of the molecular field.

The preliminary results of thermal conductivity experiment carried out on solid CD4 at equilibrium vapor pressure in the temperature range 1.2–30 K are reported. The dependence of the thermal conductivity on temperature shows a clear change at the temperatures 27.0 and 22.1 K, which correspond to the phase transitions, from phase I to II and from phase II to III, respectively. In the orientationally ordered phase III of the deuterated methane, the thermal conductivity of CD4 depends on thermal history of the sample.

Results of measurements of thermal conductivity κ of bulk GaN crystals in the temperature interval 4.2-300K are reported. Experiments were performed on three types of GaN material: (i) single, high-pressure grown GaN crystals showing highly n-type conductivity, (ii) on bulk Mg doped GaN crystals with slightly p-type conductivity, (iii) on homoepitaxial GaN layer grown by hydride vapour phase epitaxy (HVPE) on bulk GaN crystal. For the n-GaN crystals, the record thermal conductivity value κ max is equal to 1600 W/m x K at T max = 45K, and κ ≅ 230 W/m K at 300K. It is suggested that for this crystal and for T ≥ T max the contribution of Umklapp phonon scattering processes dominate in the heat transport processes. A heavily Mg doped crystal and HVPE GaN layer on bulk GaN show much lower thermal conductivity in the whole applied temperature region. The temperature dependence of κ shows the importance of point defects (impurities, vacancies) in determination of the thermal resistance of GaN bulk crystals.

Results of NMR studies of the nuclear spin-spin relaxation times at low temperatures are reported for commensurate monolayers of 3 He physisorbed on hexagonal boron nitride. Temperature independent relaxation rates are observed for 0.11< T 3 He atoms. The relaxation times and the inferred effective exchange rates were observed to change significantly on replacing the 3 He atoms with relatively immobile Ne atoms. This result is understood in terms of a large 3-spin exchange in addition to 2-spin exchange.

Thermal conductivity investigations in phase II of solid methane with oxygen and nitrogen impurities were carried out in the temperature range from 1.4 to 20 K. The concentration of impurity molecules did not exceed 1%. A significant reduction of thermal conductivity of doped crystals as compared to pure methane has been discovered. Analysis shows that the reduction of thermal conductivity is connected with the deformation of the spectrum of methane molecules due to a crystal field of symmetry Oh. The results obtained for different impurities are discussed in connection with the intrinsic properties of the molecules involved.

Results are reported for high frequency pulsed NMR studies of 3He adsorbed on hexagonal boron nitride. The commensurate coverage, corresponding to a 0.75 fraction of a monolayer was studied. The temperature dependence of the nuclear spin-spin relaxation time was measured in the range of 0.1-2 K. From the relaxation times, the quantum mechanical exchange rates are calculated for a triangular array of 3He atoms.

Preliminary results are reported on the kinetics of the molecular spin conversion in solid methane (CH4) and deuterated methane (CD4), obtained through thermal conductivity measurements in the temperature range 2–10K.

Thermal conductivity of solid CO was investigated over the temperature range 1.3–37 K by steady state flow method. The thermal conductivity coefficient reaches its maximum value 28 mW/cm K at about 6 K. Problem of the dipolar ordering at low temperatures has been discussed.

Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er3+ levels up to 4G7/2 multiplet have been determined by the combination of experimental low (, This work has been partially supported by the USAITC W911NF-10-1-0234 contract and by the Spanish Ministry of Economy and Competitiveness under project MAT2011-29255-C02-01. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

The thermal conductivity of solid parahydrogen crystal with methane admixtures has been measured in the temperature range 1.5 to 8 K. Solid samples were grown from the gas mixture at 13 K. Concentration of CH4 admixture molecules in the gas varied form 5 to 570 ppm. A very broad maximum of thermal conductivity with absolute value of about 110 W/(m K) is observed at 2.6 K. The data are interpreted by Callaway model considering phonons resonant scattering on quasi-local vibrations of CH4 molecules, phonon-grain boundary and phonon-phonon scattering processes. The increase of grain boundary scattering leads to the decrease of the maximum broadening. The analysis shows that the solid mixture of p-H2 and CH4 is a heterogeneous solution for CH4 concentration higher than 0.1 ppm., 9 pages. to be published in LTP (2003)

This paper reports the results of the first measurements of the thermal conductivity of the \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} phases of solid oxygen over the temperature range of 1--52 K. A simple qualitative analysis was performed to explain the observed anomalies in thermal conductivity, which manifested themselves by a jump at the \ensuremath{\alpha}-\ensuremath{\beta} transition, the anomalously weak temperature dependence in the \ensuremath{\beta} phase and an increase of the conductivity with temperature in the \ensuremath{\gamma} phase.

The spin conversion of methane molecules in pure deuterated methane crystals and CD(4)-Kr solid solution for a wide range of concentrations of krypton was investigated in the temperature range 1.5-10 K. The experiment was performed by use of a steady-state heat flow experimental setup for determination of the thermal conductivity, utilized in an unconventional way. The obtained results were discussed in the frame of the spin conversion model taking into account direct one-phonon processes and indirect librationally-activated processes. It was found that the conversion, both for pure and krypton doped crystals, is dominated by the one-phonon mechanism. However, the importance of the indirect processes increases rapidly with the temperature. The obtained results indicate that the krypton admixture does not change the values of energy levels of the spin-librational (spin-rotational) spectrum of the crystal. The presence of Kr in the structure of CD(4) enhances the intensity of the direct one-phonon spin conversion processes and weakens the indirect librationally-activated ones.

A hysteresis of the thermal conductivity of $({\mathrm{CD}}_{4}{)}_{1\ensuremath{-}c}({\mathrm{CH}}_{4}{)}_{c}$ solid solutions has been detected in the vicinity of the phase transition from partially orientationally ordered phase II to completely orientationally ordered phase III. The temperature interval of the hysteresis increases with the ${\mathrm{CH}}_{4}$ concentration increase. An explanation is proposed, which attributes the hysteresis to the existence of a steady mixed two-phase state near the first-order phase transition. The state is observed in a wide range of temperatures.

The thermal conductivity of (CH4)1–c(CD4)c solid solutions with c = 0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 has been measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II) and completely ordered (phase III). The temperature range is 1.3–30 K. It is shown that the thermal conductivity has different temperature dependences k(T) in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in k(T) is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the k(T) curve of phase II approaches the dependence k(T) typical of phase I. There is a hysteresis in the vicinity of the II↔III phase transition. In phase III the impurity effect in k(T) can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH4 and CD4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4–CD4 system.

Thermal conductivity investigations of pure and oxygen doped argon crystals have been carried out over the temperature range 1.3--26 K. Samples with oxygen concentration of 0.008, 0.024, 0.063, 0.225, and 0.390 mol % were measured. The result is that the interaction of phonons with the spin-rotational motion of oxygen molecules in an argon matrix is the most significant among possible mechanisms of phonon scattering by the impurities. In the thermal conductivity of the samples, no mutual interaction of the oxygen impurities has been observed over the investigated impurity concentration range.

Thermal conductivity of ${\mathrm{CO}}_{2}$ and ${\mathrm{N}}_{2}\mathrm{O}$ solids has been investigated over the temperature range $1--40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The thermal conductivity coefficient of ${\mathrm{CO}}_{2}$ and ${\mathrm{N}}_{2}\mathrm{O}$ exhibits, in the whole investigated temperature range, surprisingly high value when compared with other ${\mathrm{N}}_{2}$-type molecular crystals. Analysis of the experimental data, in framework of the Debye model, indicates that relatively big size of the crystal grains, low density of dislocations and weak phonon--phonon interaction might be reasons for the good thermal conduction in these crystals at temperatures near the maximum of the thermal conductivity. It has been found that there is an additional (in comparison with ${\mathrm{CO}}_{2}$) significant mechanism of scattering of phonons in ${\mathrm{N}}_{2}\mathrm{O}$. Supposedly this scattering occurs on the end-to-end disordered ${\mathrm{N}}_{2}\mathrm{O}$ molecules.

Measurements of thermal conductivity of solid deuterated methane at equilibrium vapor pressure in the temperature range $1.2\char21{}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are reported. Two phonon scattering mechanisms are sufficient to describe correctly the obtained dependence of thermal conductivity on temperature in the orientationally ordered phase III of the ${\mathrm{CD}}_{4}$. Unlike the situation in other simple dielectric crystals, the exponential decrease of thermal conductivity with increasing temperature is caused not by U processes, but rather by scattering of phonons on excitations of high-energy spin-librational levels of spatially oriented molecules. From comparison of these results with the measurements of thermal conductivity of methane $({\mathrm{CH}}_{4})$ some information on temperature dependence of phonon scattering on almost freely rotating methane molecule has been inferred.

Mutual solid solution of nitrogen and carbon monoxide is a unique object in terms of phonon spectroscopy investigations. The main reason for phonon interaction with foreign admixture molecule---the difference of masses of the host and the impurity molecule---is absent from that system. In the present paper the results of measurements of thermal conductivity of pure and carbon monoxide-doped nitrogen crystals, for samples containing up to 50% of CO, in the temperature range 1.2--26 K, are presented. For analysis of the experimental data the Callaway approach was applied. It has been found that the dependence of relaxation time of phonon in its interaction with varying force constants and deformation of lattice around the admixture molecule on the phonon frequency agrees with earlier theoretical considerations, however the effect is weak. It has also been found that, for concentration of the admixture molecules exceeding 1%, the mutual interaction of the impurity molecules becomes significant and lowers the thermal resistivity per unit concentration of the admixture molecule.

Thermal conductivity of crystalline nitrous oxide was investigated over the temperature range 1.2-40 K. Temperature dependence of the thermal conductivity has been determined at temperatures below 23 K. Thermal conductivity coefficient exhibits a surprisingly compared with simple molecular crystals such as N 2 , CO. or O 2 --high value in the whole investigated temperature range. Both the experimental results themselves and the method of Callaway applied to the experimental data analysis indicate that relatively low density of dislocations and weak phonon-phonon interaction might be reasons for the good thermal conduction in solid N 2 O. The problem of influence of possible low-temperature dipolar ordering on thermal conductivity of nitrous oxide crystal is speculated.

The results of measurements of the thermal conductivity of pure and carbon-monoxide-doped nitrogen crystals in the temperature range 1.2–26 K are presented for samples containing up to 0.7% CO molecules. It follows from a preliminary analysis that the interaction of phonons with an admixture molecule having the same mass as the host molecule is relatively weak and depends weakly on the admixture concentration within the investigated range of carbon monoxide concentrations in the nitrogen crystal.

We have measured thermal conductivity, κ, in the wide temperature range 4– 300 K of GaN bulk single crystals grown by high-pressure, high-temperature synthesis. Obtained results ( 1600 W/Km at 45 K ) are the highest κ values reported on GaN material. At the room temperature κ is about 210 W/Km . The contributions to the GaN thermal resistance of Umklapp process, mass point defects as well as phonon scattering on dislocations and sample boundary are discussed.

Thermal conductivity of crystalline natural nitrogen was investigated over the temperature range 1.2--38 K. The temperature dependence of thermal conductivity of \ensuremath{\alpha}-nitrogen has been determined in temperatures below 18 K. The thermal conductivity coefficient reaches the maximum value 250 mW/cm K at 3.6 K. The results of the experiment below 14 K can be well described by integral equation obtained by the method of Callaway. Relaxation times for various mechanisms of phonon scattering have been determined. The results of our experiments are in accord with earlier literature data for temperatures above 18 K.

Measurements of the 3He nuclear spin-spin relaxation time, T2, have been carried out for commensurate layers of 3He-Ne mixtures adsorbed on hexagonal boron nitride for temperatures 0.2< T

Thermal conductivities of solid oxygen doped with nonmagnetic impurities of nitrogen and argon were measured in the 1–35 K. The antiferromagnetic α-O2 doped with this impurities exhibits a strong thermal conductivity anomaly, which is assumed to be a result of supperpression of the magnon contribution to the heat transport by impurities. At the same time, the jump in the thermal conductivity at the α-β transition and th thermal conductivity in the β-O2 are insensitive to the doping effect, which we ascribe to strong magnetostriction effects. The halfwidth of the x-ray reflexes indicates that doping with Ar and Kr enhanced magnetostriction effects while doping with N2 weakens.