Competition between P n O 3 n +1 chains and Zn–O network size allows tuning of thermal conductivity.

Low-temperature specific heat of multi-walled carbon nanotubes (MWCNTs) with different grinding was studied. Two sets of modified, milled and oxidized/milled MWCNTs with an average outer diameter of 9.4nm were used. The experimental results were compared with literature data for different carbon systems: bundles of single-walled carbon nanotubes (SWCNTs), graphite and other MWCNTs. The contributions of phonon spectrum characteristics and intertube interactions were found to be significant factors influencing the heat capacity both in the case of MWCNTs and bundles of SWCNTs. The grinding effect, associated with the reduction of the size of MWCNTs agglomerates, leads to an increased heat capacity. It was demonstrated that the lowest-temperature heat capacity consists of two main contributions: the Debye (C3T3) and the dispersive (C5T5) one. The obtained negative C5 parameter indicated flexural dispersion for phonons. The magnitudes of Debye and flexural dispersive components depend on structural parameters of nanotubes: such as the diameter of individual nanotubes, the average diameter of the bundle and the size of agglomerates. A monotonic proportional correlation was observed between C3 and |C5| parameters: |C5| increases following a power law with an exponent of 1.5 with the increase of C3. The maximum values of C3 and |C5| correspond to SWCNT systems, while the minimum values correspond to MWCNTs. These results show that the heat capacity of the nanotube system increases if the interaction forces between neighbouring SWCNTs in the bundle or between the walls inside of MWCNTs decrease. This dependence is confirmed by the grinding effect in the MWCNTs.

The temperature dependencies of the thermal conductivity coefficient, κ (T), in 1-fluoroadamantane were investigated for two different orientationally-disordered states. In the first one, κ (T) exhibits a crystalline character with a typical maximum and two anomalous regions marked at higher temperatures. Here, the low-temperature power-law dependence is reduced due to a relatively large contribution of dislocations and defects in the polycrystalline sample. Above 196 K, there is an anomalous plateau in thermal conductivity resulting from the presence of an intermediate centrosymmetric state. Further, a significant increase in κ occurs above 234 K, resulting from a phase transition to a high-temperature disordered cubic phase. Subsequent cooling of the sample leads to an irreversible transition towards a disordered state resulting in a κ (T) dependence corresponding to an amorphous material behavior. The primary anomaly at 196 K disappears, while the second one associated with the phase transition at 227 K shifts towards lower temperatures. The irreversibility of this transformation is also verified by scanning microscopy images. Thermal conductivity in both phases has an additional contribution of an Arrhenius type in their temperature dependences.

We present low-temperature specific heat (Cp) measurements of a monoclinic P2_1/c crystal formed by quasiplanar molecules of tetrachloro-m-xylene. The dynamic disorder frozen at low-temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a three-fold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear in contribution in Cp ascribed to two-level systems and a broad maximum around 6.6 K in Cp/T^3 (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable number of in-plane molecular orientations We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak and Debye temperatures. We find that a linear correlation between the boson peak and Debye temperatures holds for many – but not all – structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.

The thermal conductivity of pure ABS polymer and ABS polymer composite with 0.5 wt% of the thermally reduced graphene oxide (trGO) was measured in a wide temperature range from 2 to 100 K. Adding 0.5 % trGO enhanced the thermal conductivity of ABS polymer by 1.5 times over the entire temperature range. A comparison of the thermal conductivity of ABS and epoxy-resin amorphous polymers and structural glasses shows that it is closely related to the concept of minimal thermal conductivity, which is determined by the intrinsic phonon scattering and the coherence contribution to the thermal conductivity in the material. The temperature dependence of the coherence contribution to the thermal conductivity, related to wave-like tunneling and loss of coherence between different vibrational eigenstates, was approximated by the exponential function of an Arrhenius type with characteristic energy E and a pre-exponential coefficient kappa 0. A proportional correlation was found between the low-temperature anomaly of the heat capacity, named the calorimetric Boson peak, and the high-temperature behavior of thermal conductivity of amorphous polymers and structural glasses. Thus, this study provides new physical information about the thermal conductivity in disordered materials and indicates a universality of its temperature dependence.

In this study, we propose and validate a universal temperature-dependent model for characterizing the thermal conductivity of amorphous polymers over a wide temperature range. Our approach captures key features in the thermal conductivity data, including a plateau, an inflection point, and the subsequent increase and saturation with rising temperature. Importantly, this model proves effective not only for pristine amorphous polymers but also for polymers subjected to external influences. We investigate the temperature-dependent thermal conductivity of amorphous polymer materials under various external conditions, such as hydrostatic pressure, radiation exposure, and the incorporation of fillers. Our analysis reveals novel insights into the dual-channel heat transfer mechanisms within amorphous polymers. Specifically, we observe a linear relationship between the logarithm of the “coherence” conductivity pre-factor and the characteristic energy, consistent with the Meyer–Neldel rule governing thermal conductivity. This research advances our understanding of thermal transport in amorphous polymers and underscores the applicability of the proposed universal model in describing complex thermal behavior across different conditions.

We present low-temperature specific heat (Cp) measurements of a monoclinic P 2 1 / c crystal formed by quasi- planar molecules of tetrachloro-m-xylene. The dynamic disorder frozen at low temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a threefold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear contribution in C p ascribed to two-level systems and a broad maximum around 6.6 K in Cp/T 3 (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable numbers of in-plane molecular orientations. We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak (TBP) and Debye (OD) temperatures. We find that a linear correlation between T BP and O D holds for many-but not all-structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.

The nonlinear electric conductivity of κ-(BEDT-TTF)4Hg2.89Br8, which is known as a hole-doped spin liquid, has been observed by single crystal transport measurements using dc and ac excitations. This compound is known as a charge transfer complex with strong magnetic fluctuations accompanied by electron mass enhancement at ambient pressure. We discuss the nonlinear dc conductivity drastically emerging below 100 K. We also performed ac impedance measurements of this compound and compared the results with those of a typical dimer-Mott compound of deuterated κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br, located just near the Mott boundary. The analyses of the Nyquist plots of κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br and κ-(BEDT-TTF)4Hg2.89Br8 reveal qualitatively different features. The former shows a behavior pertinent to the inhomogeneous distribution of domains due to a mixing of metal and insulating phases, taking into account the influence of the proximity effect, while the doped spin liquid has a distinct semicircle-type frequency dependence in its Nyquist plot in the whole temperature range studied. We conclude that the nonlinear conductivity is intrinsically peculiar to the doped dimer Mott system, where the charge degrees of freedom dominate the itinerancy. We attribute the anomalous features, such as non-Fermi liquid behavior, heat capacity enhancement, and strong antiferromagnetic fluctuations in κ-(BEDT-TTF)4Hg2.89Br8, to a kind of charge confinement effect retained in the hole-doped spin-liquid state.

This paper presents a detailed analysis of the thermal conductivity behaviors exhibited by a diverse array of nanostructured materials, ranging from multilayer graphene nanocomposites to semiconductor-based nanostructures such as Bi0.5Sb1.5Te3 and In0.53Ga0.47As composites. The investigation extends to superlattices, nanowires, and hybrid nanostructures, encompassing materials like hexagonal boron nitride flakes, iron oxide nanoporous films, and organic-inorganic hybrid materials. The thermal conductivity of these materials is characterized by distinct trends, with some showcasing crystal-like behavior and others demonstrating glass-like characteristics. The analysis employs empirical expressions to discern the contributions of phonons and diffusons in crystal-like materials and incorporates Peierls contributions and Arrhenius-type terms for glass-like behavior. Noteworthy observations include deviations in fitting certain materials at lower temperatures and the identification of negative diffuson contributions in specific cases. These findings contribute to a nuanced understanding of thermal transport in nanostructured materials and have implications for applications in advanced thermal management systems and thermoelectric devices. The extracted parameters provide valuable insights for researchers exploring the thermal conductivity of diverse nanostructured materials.

Correlation between thermal expansions β(T) and heat capacity C(T) of atomic and molecular crystals, amorphous materials with a structural disorder, carbon nanomaterials (fullerite C60, bundles SWCNTs of single-walled carbon nanotubes) was analyzed. The influence of the contribution to the coefficient of linear thermal expansion αXe(T) of Xe atoms adsorbed on the SWCNTs bundles is considered. The proportional correlation was found between the contribution to the coefficient of linear thermal expansion αXe(T) and the normalized to the gas constant heat capacity C Xe(T)/R of Xe atoms adsorbed on the SWCNTs bundles. The proportional correlation (β/β*) ∼ (CV/R) with the parameter β* for the bulk thermal expansion coefficient for cryocrystals is proposed. In the case of atomic crystals such as Xe and Ar, the proportional correlation (β/β*) ∼ (CV/R) is observed in the temperature range from the lowest experimental to temperatures where CV/R ≈ 2.3. The correlation is not observed in the temperatures where 2.3 < C V/R < 3 (classical Dulong-Petit law). It was found that the universal proportional correlation is also observed for molecular crystals with linear symmetry, such as CO2, CO, and N2O if the normalized heat capacity below the values CV/R ≈ 3 ÷ 3.5. It indicates that the proportional correlation between thermal expansions (β/β*) and heat capacity (CV/R) is related not only to the translational, but also to the rotational degrees of freedom of the molecule in the crystal. In the case of the C0, molecular crystal with translational and rotational degrees of freedom and intramolecular vibrations, the discussed above correlation occurs below the values of normalized heat capacity CV/R ≈ 7.5. In strongly anisotropic systems, such as systems of compacted bundles of single-walled carbon nanotubes and SWCNTs bundles with adsorbed Xe atoms, this universal dependence appears in a limited temperature range that does not include the lowest temperatures. A qualitative explanation of the observed correlation is proposed.

The increase in thermal conductivity with temperature in the dynamically orientationally disordered phases of cyclic hydrocarbons is explained by the recently proposed thermal activation mechanism of heat transfer. It is shown that high-temperature thermal conductivity is well described by the Arrhenius dependence. Activation energies and pre-exponential factors are determined. It is shown that the activation energies do not depend on the sample density. The dependence of the pre-exponential factor on the activation energy of a number of cyclic hydrocarbons was analyzed.

The construction of a thermal conductivity measurement system designed for tiny molecules-based compounds is reported. We introduce complementary usage of chip-type RuO2 thermometers and E-type thermocouples in the sample part by using thin (ϕ 13μm) constantan and chromel wires. Two pairs of the constantan and chromel wires are used as lead wires for the four-terminal measurement of the resistance of RuO2 thermometers in the low-temperature region below about 20 K. Also, in the higher temperature region above 10 K up to room temperature with the overlapping range of 10-20 K, they are used as thermocouples for detecting temperature differences from that of the heat sink. We also compare a kind of resolution parameter of several sensors as a function of temperature to discuss the rational reason to select suitable sensors depending on the temperature region. Using the constructed apparatus, we report temperature dependences of the thermal conductivity of deuterated κ-(d8:BEDT-TTF)2Cu[N(CN)2]Br in a wide temperature range between 2 and 250 K. The result provides convincing evidence for the validity of the newly developed system for the thermal measurements of molecular crystals.

The low-temperature properties of guest-host crystals, such as clathrates and skutterudites, offer a rich play-ground for discovering novel physical phenomena and developing new materials with unique properties. The temperature dependence of thermal conductivity in these materials can exhibit both crystal-like and glass-like behavior, which reflects the properties of the phonon excitations and various scattering mechanisms. The ultra-low thermal conductivity of clathrate crystals is closely related to the concept of minimal thermal con-ductivity, which is determined by the intrinsic phonon scattering in the material. In this work, the temperature dependence of thermal conductivity for both crystal-like and glass-like behavior of different structural types of clathrates and skutterudites was analyzed using the "Unified theory of thermal transport in crystals and glasses" of M. Simoncelli, N. Marzari & F. Mauri. A method was proposed and tested for the coherence contribution related to wave-like tunneling and loss of coherence between different vibrational eigenstates. The temperature dependence of the coherence contribution to thermal conductivity was approximated by the exponential function of an Arrhenius type with characteristic energy E and characteristic minimal thermal conductivity parameter kappa 0. The coherence contribution is intertwined with other phonon scattering mechanisms, and over a wide temper-ature range, its temperature dependence is universal with parameters depending on the crystal structure, posi-tional disorder, and impurity doping. This work provides insights into the temperature dependence of thermal conductivity in guest-host materials and its importance for designing and optimizing their properties for various applications, such as thermoelectric generators.

The low-temperature dependence of the heat capacity of acrylonitrile-butadiene-styrene (ABS) polymer and its composite with thermally reduced graphene oxide was studied. The existence of a so-called “boson peak” characteristic of orientational and structural glasses was demonstrated. The boson peak appears in the form of a local maximum in the heat capacity curve displayed as C/T3 vs T at Tmax = 3.52 K. It was found that for both ABS polymer and its composite, as well as for a number of other substances of a crystalline and amorphous nature, the manifestation of the anomaly of the boson peak in the heat capacity has a universal character that is described by an empirical function Δ*. The value of Δ* depends on the magnitude of the anomaly in the heat capacity and the temperature of the boson peak manifestation. Thus, this study provides new physical information about the possible causes of the boson peak appearance in disordered materials and indicates the universality of boson peak anomaly for substances with short- and long-range order.

We demonstrate that the heat capacity Boson peak (BP)-like anomaly appearing in fully ordered anharmonic molecular crystals emerges as a result of the strong interactions between propagating (acoustic) and low-energy quasi-localized (optical) phonons. In particular, we experimentally determine the low-temperature (<30 K) specific heat of the molecular crystal benzophenone and those of several of its fully ordered bromine derivatives. Subsequently, by means of theoretical first-principles methods based on density functional theory, we estimate the corresponding phonon dispersions and vibrational density of states. Our results reveal two possible mechanisms for the emergence of the BP-like anomaly: (i) acoustic-optic phonon avoided crossing, which gives rise to a pseudo-van Hove singularity in the acoustic phonon branches, and (ii) piling up of low-frequency optical phonons, which are quasi degenerate with longitudinal acoustic modes and lead to a surge in the vibrational density of states at low energies.

The thermal conductivity, κ, of solid triphenyl phosphite was measured by using the transient hot-wire method, and its temperature and pressure dependencies were analyzed to understand heat transfer processes in the solid polymorphic phases, as well as in the glass and the exotic glacial state. Phase transformations and the structural order of the phases are discussed, and a transitional pressure–temperature diagram of triphenyl phosphite is presented. The thermal conductivity of both the crystalline and disordered states is described within the theory of two-channel heat transfer by phonons and diffusons in dielectric solids. In the glass and glacial states, the weakly temperature-dependent (glass-like) κ is described well by the term associated with heat conduction of diffusons only, and it can be represented by an Arrhenius-type function. In the crystal phases, the strongly temperature-dependent (crystal-like) κ associated with heat transfer by phonons is weakened by significant heat transfer by diffusons, and the extent of the two contributions is reflected in the temperature dependence of κ. We find that the contribution of diffusons in the crystal phases depends on pressure in the same way as that in amorphous states, thus indicating that the same mechanism is responsible for this channel of heat transfer in crystals and amorphous states.

The thermal analysis and infrared (IR) spectroscopy were used to study cyclohexanol irreversible transformations from an orientationally disordered plastic state to metastable III and then to a stable II crystalline phase. It was found that the orientational glass transition in cyclohexanol occurs at Tg similar to 163.5 K. Solid-solid transition from orientationally disordered phase I to metastable phase III occurs at 195 K. Crystal phase III nucleates at Ta = 203 K and converts into phase I at T = 233.7 K. The stable crystal phase II is formed by nucleation at 213 K. It converts into phase I at T = 250 K. It is shown by IR spectroscopy that the phase transitions in solid cyclohexanol are correlated with conformational changes in the cyclohexanol molecule.

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

We review one-dimensional lattice models and the corresponding results that describe the low-temperature properties of quasi-one-dimensional lattice systems with long-range interaction. A widely known example is narrow-band low-dimensional conductors with long-range interelectron repulsion. The models deal with particles that live on the one-dimensional host lattice (chain), translation invariant or disordered, and interact via the long-range repulsive potential. The results are presented concerning the translation invariant host chain, in particular: the low-temperature thermodynamics incommensurable ground states and related devil-staircase form of various characteristics as functions of pertinent parameters, the self-localization of particles, a new branch of elementary excitations, etc. In the disordered case, where the sites of the host chain fluctuate randomly around the sites of the periodic chain, the low-temperature thermodynamics and the structure of the ground state are discussed in the framework of a certain model, which we call the cluster model and which seems to be a fairly reasonable approximation for low temperatures and small concentration of particles. Using analytical and numerical tools we analyze the thermodynamics and the ground state of the model. The latter proves to be a sequence of random domains and we study in detail their distribution.