Low-temperature Cp Research Articles

K8CaSi10O25 – Synthesis, in-situ high-temperature single-crystal diffraction and heat capacity

Different sol-gel routes were tested to synthesize polycrystalline material of K8CaSi10O25. The best result in terms of phase purity was obtained using combustion synthesis with glycine as fuel. Single crystals were obtained from the devitrification of a glass. Cyclic single-crystal diffraction studies were carried out within the temperature range of ambient conditions to 873 K with different cooling rates. The crystal structure (space group R3c) at ambient temperature was consistent with previous investigations (a = 11.1135(6) Å, c = 21.9286(14) Å, V = 2345.5(2) Å3). A new high-temperature polymorph was discovered, showing the following crystallographic data at 873 K: space group R3‾m, a = 11.2251(8) Å, c = 11.1789(9) Å, V = 1219.9(2) Å3. On heating, an incommensurately modulated structure was observed between the two aforementioned modifications. The standard third-law entropy was determined from low-temperature Cp’s measured by relaxation calorimetry and amounts to S°(298 K) = 898.7 ± 6.3 J mol−1 K−1.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Low-Temperature Heat Capacity Anomalies in Ordered and Disordered Phases of Normal and Deuterated Thiophene.

We measured the specific heat Cp of normal (C4H4S) and deuterated (C4D4S) thiophene in the temperature interval of 1 ≤ T, K ≤ 25. C4H4S exhibits a metastable phase II2 and a stable phase V, both with frozen orientational disorder (OD), whereas C4D4S exhibits a metastable phase II2, which is analogous to the OD phase II2 of C4H4S and a fully ordered stable phase V. Our measurements demonstrate the existence of a large bump in the heat capacity of both stable and metastable C4D4S and C4H4S phases at temperatures of ∼10 K, which significantly departs from the expected Debye temperature behavior of Cp ≈ T3. This case study demonstrates that the identified low-temperature Cp anomaly, typically referred to as a “Boson-peak” in the context of glassy crystals, is not exclusive of disordered materials.

Are universal “anomalous” properties of glasses at low temperatures truly universal?

The specific heat Cp and other properties of glasses (ranging from amorphous solids to disordered crystals) at low temperatures are well known to be markedly different from those in fully-ordered crystals. For decades, this qualitative, and even quantitative, universal behavior of glasses has been thoroughly studied. However, a clear understanding of its origin and microscopic nature, needless to say, a closed theory, is still lacking. To shed light on this matter, I review the situation in this work, mainly by compiling and discussing measured low-temperature Cp data of many glasses and disordered crystals, as well as highlighting a few exceptions to that “universality rule”. Thus, one can see that, in contrast to other low-temperature properties of glasses, the magnitude of the “glassy” Cp excess at low temperature is far from being universal. Even worse, some molecular crystals without a clear sign of disorder exhibit linear coefficients in Cp larger than those found in many amorphous solids, whereas a few of the latter show negligible values.

Prediction of heat capacity for crystalline substances

An algorithm is derived that estimates the trend of heat capacity with temperature based on zero-Kelvin properties (V0,B0), the Debye temperature and the thermal expansion coefficient at the Debye temperature. The algorithm predicts not only the trend of heat capacity but also the temperature trend of the volume and the bulk modulus, which can be also included in new thermodynamic databases. Calculations of thermophysical properties take less then two minutes on a standard desktop CPU for a temperature range between 0 K and 2000 K. The algorithm is used to assess thermophysical properties of the intermetallic phases η (Fe2Al5), ϵ (Fe5Al8) and τ4 (FeAl3Si2) in comparison to calculated DFT results and experimental data. A method is shown how the melting point of a substance can be predicted using our algorithm in combination with low temperature cp and volume data.

Dense Kondo behavior in the low-temperature resistivity and specific heat for amorphous Ce50Al50 alloy

We measured the low-temperature specific heat Cp, resistivity ρ, and magnetoresistance Δρ(H)/ρ(0) for amorphous Ce50Al50 synthesized by a DC high-rate sputter method. The low-temperature Cp (T < 2 K) decreases rapidly toward 0 K and has no indication of phase transition. The value of γ0 which is extrapolated down to 0 K of the Cp/T is 117 mJ/molK2. The temperature dependence of ρ increases with decreasing temperature down to 0.6 K. We found that both a weak localization effect and a coherent Kondo state might be realized in the low-temperature region for the present alloy from the conductivity analysis. Furthermore, in the low-temperature ρ, a T2 term with a very large coefficient A was observed. The ratio of A/γ02 is 0.63 ×10-5 (μΩcm/K2)/(mJ/molK2)2 and is near the value of typical Ce-based heavy-fermion compounds. The magnetoresistances Δρ(H)/ρ(0) at 0.5 K and 2 K are almost constant in the magnetic field region of H < 10 kOe. We considered that the negative magnetoresistance effect is due to the weak localization and the positive magnetoresistance to the heavy-fermion state in the present alloy.

Heat capacity of Fe-Al intermetallics: B2-FeAl, FeAl2, Fe2Al5 and Fe4Al13

The heat capacity of three Fe-rich bcc-based Fe-Al alloys (35, 40 and 45 at.% Al) and of the three Al-rich intermetallics ζ (FeAl2), η (Fe2Al5) and θ (Fe4Al13) were measured by DSC between 120 K and their respective melting points. For the Fe-rich bcc-based alloys, the heat capacity was measured in the temperature range of the A2/B2 transition which reaches a maximum value of 54.7 and 86.3J/K⋅mol for the samples with compositions of 35 and 40 at.% Al close to the order/disorder transition temperature of 1456 and 1555 K, respectively. The Debye temperature and the standard entropy values of the Al-rich intermetallic phases ζ (ΘD=455K, S298.15=24.45J/K⋅mol), η (ΘD=484K, S298.15=23.00J/K⋅mol) and θ (ΘD=469K, S298.15=23.65J/K⋅mol) were estimated based on low temperature cp values. All three intermetallics and the 45 at.% Al sample show a non-linear increase in cp by approaching the melting point which probably originates from the creation of thermal vacancies in the respective intermetallic phase. The enthalpy of formation of vacancies within the three phases ζ (Hf=156±4kJ/mol), η (Hf=181±3kJ/mol) and θ (Hf=95±2kJ/mol) was estimated based on the results of the high temperature cp measurements.

Unprecedented Integral-Free Debye Temperature Formulas: Sample Applications to Heat Capacities of ZnSe and ZnTe

Detailed analytical and numerical analyses are performed for combinations of several complementary sets of measured heat capacities, for ZnSe and ZnTe, from the liquid-helium region up to 600 K. The isochoric (harmonic) parts of heat capacities, CVh(T), are described within the frame of a properly devised four-oscillator hybrid model. Additional anharmonicity-related terms are included for comprehensive numerical fittings of the isobaric heat capacities, Cp(T). The contributions of Debye and non-Debye type due to the low-energy acoustical phonon sections are represented here for the first time by unprecedented, integral-free formulas. Indications for weak electronic contributions to the cryogenic heat capacities are found for both materials. A novel analytical framework has been constructed for high-accuracy evaluations of Debye function integrals via a couple of integral-free formulas, consisting of Debye’s conventional low-temperature series expansion in combination with an unprecedented high-temperature series representation for reciprocal values of the Debye function. The zero-temperature limits of Debye temperatures have been detected from published low-temperature Cp(T) data sets to be significantly lower than previously estimated, namely, 270 (±3) K for ZnSe and 220 (±2) K for ZnTe. The high-temperature limits of the “true” (harmonic lattice) Debye temperatures are found to be 317 K for ZnSe and 262 K for ZnTe.

Non-Debye heat capacity formula refined and applied to GaP, GaAs, GaSb, InP, InAs, and InSb

Characteristic non-Debye behaviors of low-temperature heat capacities of GaP, GaAs, GaSb, InP, InAs, and InSb, which are manifested above all in form of non-monotonic behaviors (local maxima) of the respective Cp(T)/T3 curves in the cryogenic region, are described by means of a refined version of a recently proposed low-to-high-temperature interpolation formula of non-Debye type. Least-mean-square fittings of representative Cp(T) data sets available for these materials from several sources show excellent agreements, from the liquid-helium region up to room temperature. The results of detailed calculations of the respective material-specific Debye temperature curves, ΘD(T), are represented in graphical form. The strong, non-monotonic variations of ΘD(T) values confirm that it is impossible to provide reasonable numerical simulations of measured Cp(T) dependences in terms of fixed Debye temperatures. We show that it is possible to describe in good approximation the complete Debye temperature curves, from the cryogenic region up to their definitive disappearance (dropping to 0) in the high temperature region, by a couple of unprecedented algebraic formulas. The task of constructing physically adequate prolongations of the low-temperature Cp(T) curves up to melting points was strongly impeded by partly rather large differences (up to an order of 10 J/(K·mol)) between the high-temperature data sets presented in different research papers and/or data reviews. Physically plausible criteria are invoked, which enabled an a priori rejection of a series of obviously unrealistic high-temperature data sets. Residual uncertainties for GaAs and InAs could be overcome by re-evaluations of former enthalpy data on the basis of a novel set of properly specified four-parameter polynomial expressions applying to large regions, from moderately low temperatures up to melting points. Detailed analytical and numerical descriptions are given for the anharmonicity-related differences of isobaric vs. isochoric (harmonic) parts of heat capacities. Relevant sets of empirical parameters and representative collections of heat capacity and Debye temperature values for all materials under study are presented in tabulated form.

Measurement and modelling of high temperature thermodynamic properties of URh3 alloy

The high temperature phase stability of arc-melted cubic URh3 intermetallic compound has been investigated using high temperature inverse drop calorimetry in the temperature range of 300–1273K. URh3 exists as a line compound with negligible solubility range. Room temperature XRD profile and elemental X-ray mapping experiments on 1273K/3h homogenized samples have confirmed the homogeneity and L12 (cF8; pm3m) crystal structure of URh3. The drop measurements yielded accurate values for the enthalpy increment ΔHT0 as a function of temperature, from which the specific heat CP has been estimated. The enthalpy data obtained in this study have been compared and combined with the reported data on low temperature CP and also with the ΔHT0 in the temperature range, 0–840K, for a comprehensive theoretical analysis using quasiharmonic Debye–Grüneisen formalism. It is found that this model with due allowance for thermal expansion effects can successfully account for the experimentally measured thermal property data in the entire temperature region spanning 0–1273K. Invoking a combination of measurement and modelling, a comprehensive set of thermodynamic quantities have been obtained for URh3.

Heat capacity and phase equilibria of almandine, Fe 3Al 2Si 3O 12

The heat capacity of a synthetic almandine, Fe 3Al 2Si 3O 12, was measured from 6 to 350 K using equilibrium, intermittent-heating quasi-adiabatic calorimetry and from 420 to 1000 K using differential scanning calorimetry. These measurements yield Cp 298 = 342.80 ± 1.4 J/mol · K and S 298 o = 342.60 J/mol · K. Mössbauer characterizations show the almandine to contain less than 2 ± 1% of the total iron as Fe 3+. X-ray diffraction studies of this synthetic almandine yield a = 11.521 ± 0.001 Å and V 298 o = 115.11 +- 0.01 cm 3/mol, somewhat smaller than previously reported. The low-temperature Cp data indicate a lambda transition at 8.7 K related to an antiferromagnetic-paramagnetic transition with T N = 7.5 K. Modeling of the lattice contribution to the total entropy suggests the presence of entropy in excess of that attributable to the effects of lattice vibrations and the magnetic transition. This probably arises from a low-temperature electronic transition (Schottky contribution). Combination of the Cp data with existing thermodynamic and phase equilibrium data on almandine yields ΔG f,298 o = −4938.3 kJ/mol and ΔH f,298 o = —5261.3 kJ/mol for almandine when calculated from the elements. The equilibrium almandine = hercynite + fayalite + quartz limits the upper T P for almandine and is metastably located at ca. 570°C at P = 1 bar, with a dP dT of +17 bars/°C. This agrees well with reversed experiments on almandine stability when they are corrected for magnetite and hercynite solid-solutions. In ‖ O 2- T space, almandine oxidizes near QFM by the reactions almandine + O 2 = magnetite + sillimanite + quartz and almandine + 02 = hercynite + magnetite + quartz. With suitable correction for reduced activities of solid phases, these equilibria provide useful oxygen barometers for medium- to high-grade metamorphic rocks.

GeO2 vs SiO2: Glass transitions and thermodynamic properties of polymorphs

Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2. The glass transition is very sensitive to the impurity content, with a Tg ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative Cp change at Tg of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two Cp cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO2 and SiO2 the coordination state markedly affects Cp and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S298 = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO2 and SiO2, respectively. The high-temperature Cp's of coesite and stishovite are likely similar to those of the low-pressure SiO2 forms. Finally, these results, low-temperature Cp data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO2 modifications.

Heat capacity and derived thermodynamic functions of the zeta phase in the Pu + Zr system between 15 and 373 K

Heat capacities of the zeta phase in the Pu + Zr system (2.55 moles per cent of Zr) have been determined between 15 and 373 K and values of the derived thermodynamic functions have been calculated. Low temperature Cp against T behavior indicates that the zeta phase is a stable phase below 373 K. A high density of states was suggested and a θD of 133 K was derived from the results. Values of Cp(T), So(T), and — {Go(T) — Ho(0)}/T were determined to be 7.29, 15.67, and 9.76 calth K−1 mol−1 and {Ho(T) — Ho(0)} to be 1761.4 calth mol−1 at 298.15 K, from smoothed results. Uncertainty in the different thermodynamic function values was judged to be ±0.7 per cent.