Tooru Atake

Molar heat capacity and thermodynamic properties of p-quaterphenyl☆

Abstract The molar heat capacity of p -quaterphenyl was measured by adiabatic calorimetry between 3 and 300 K. The phase transition associated with a molecular-conformation change is observed as a very broad anomaly with a maximum at (233.0±0.5) K. The molar enthalpy and entropy of transition are (414±20) J·mol −1 and (1.82±0.10) J·K −1 ·mol −1 , respectively. A comparison is made with the properties of biphenyl and p -terphenyl.

Heat capacity anomalies due to successive phase transitions in 1,1′-biphenyl☆

Abstract Precision heat capacity measurements on 1,1′-biphenyl revealed two broad anomalies at 11.0 K and 40.4 K. The excess enthalpies and entropies are 0.28 ± 0.02 J mol−1 and 0.026 ± 0.002 J K−1mol−1 for the anomaly extending from 7.5 to 14.0 K, and 5.02 ± 0.08 J mol−1 and 0.129 ± 0.003 J K−1mol−1 for the anomaly extending from 30.0 to 47.0 K, respectively. A possible molecular mechanism is suggested.

Rb2ZnCl4: three heat-capacity anomalies due to soft-mode and commensurate-incommensurate transitions☆

Abstract There are three phase transitions for Rb 2 ZnCl 4 . The transition temperatures, the enthalpies of transition, and the entropies of transition are (74.6±0.15) K, (30±1) J·mol −1 , (0.42±0.01) J·K −1 ·mol −1 ; (195.2±0.05) K, (6.2±0.9) J·mol −1 , (0.032±0.005) J·K −1 ·mol −1 ; (303.2±0.3) K, (222±7) J·mol −1 , (0.66±0.14) J·K −1 ·mol −1 . The second transition shows a very sharp heat-capacity peak but the other two are much broader, being consistent with the soft-mode mechanism. Thermodynamic functions of Rb 2 ZnCl 4 are tabulated. A comparison is made with the behaviour of ferroelectrics of the same type: K 2 SeO 4 and Rb 2 ZnBr 4 .

Heat capacity of solid carbon tetrachloride from 3 to 50 K

The heat capacity of solid carbon tetrachloride was measured between 3 and 50 K. The results compare favorably with previous measurements above 20 K. The standard entropy for the gas was revised to (74.2 ± 0.3) cal K−3 mol−1 in comparison with the spectroscopic entropy of 74.03 cal K−1 mol−1 at 25°C. From the analysis of low temperature heat capacities, the Debye temperature at 0 K was derived as (100.3 ± 2.7) K. The average molecular librational wavenumber in the Einstein approximation was 40 cm−1 in agreement with the predictions from other experiments. However, it is suggested that the librational spectrum has a distribution of frequencies towards lower frequencies. Approximate values of the positive moments of the frequency spectrum were estimated.

K2SeO4: calorimetric studies of two successive phase transitions

Abstract The heat capacity of K 2 SeO 4 was measured from 3 to 300 K. The commensurate-incommensurate transition (so-called lock-in transition) showed a rather broad anomaly at (95.1±0.1) K. The enthalpy and entropy of transition are (1.1±0.2) J·mol −1 and (0.012±0.002) J·K −1 ·mol −1 , respectively. The incommensurate-normal transition has a heat-capacity maximum at (127.7±0.1) K and appears to be of second order; its enthalpy and entropy changes are (110±5) J·mol −1 and (0.88±0.05) J·K −1 ·mol −1 . Some thermodynamic functions are derived.

Thermodynamic studies of p-polyphenyls: heat capacity of 4,4′-difluorobiphenyl☆

Abstract The heat capacity of 4,4′-difluorobiphenyl has been measured by adiabatic calorimetry between 3 and 300 K. No thermal anomaly was observed. The possibility of a phase transition due to twisting of the phenyl rings in the molecule is discussed and qualitative deductions have been made as to the properties of the potential hindering molecular twisting as compared with biphenyl and other p -polyphenylenes. Derived thermodynamic quantities including the third-law molar entropy are tabulated.

A concealed anomaly at 117.5 K in the heat capacity of hexamethylbenzene

Heat capacities of solid hexamethylbenzene were measured in an adiabatic calorimeter between 3 and 300 K with particular attention to the phase transition at 117 K between phase III and phase II. The transition is very likely to be of first order with the temperature 117.5±0.1 K, the heat of the transition 0.99±0.02 kJ mol−1, and the entropy of transition 8.3±0.2 J K−1 mol−1. The reason why the transition has been referred to as a λ transition is because there is an additional thermal anomaly concealed behind the transition, which has a maximum as large as 50 J K−1 mol−1 at 128 K. This anomaly, or hump in the heat capacity curve, was interpreted in terms of rotatory oscillation/hindered rotation of the methyl groups in a potential barrier of the shape V = V3 cos 3ϑ+V6 cos 6ϑ+V9 cos 9ϑ with V3 = 4×10−21 J, V6 = 1×10−21 J, and V9 = 3×10−21 J. The effect of the transition on the hump is merely to cause a small jump and the hump is otherwise continuous beginning in phase III through phase II. A term, excitati...

Thermodynamic properties of polymorphic phases of tetramethylsilane

Abstract Heat capacities of tetramethylsilane in its most stable γ-phase and liquid were measured between 3 and 300 K. A metastable α-phase is shown to be a plastic crystal. Thermodynamic quantities were determined as follows: α-phase, T tr = (165.920 ± 0.010) K, ΔH f = (703 ± 2) J mol −1 , ΔS f = (4.24 ± 0.02) J K −1 mol −1 ; β-phase, T tr = (171.016 ± 0.003) K, ΔH f = (5.878 ± 0.005) kJ mol −1 , ΔS f = (34.37 ± 0.03) J K −1 mol −1 ; γ-phase, T tr = (174.074 ± 0.003) K, ΔH f = (6.874 ± 0.006) kJ mol −1 , ΔS f = (39.51 ± 0.04) J K −1 mol −1 . The stability relations among the phases were determined in terms of the Gibbs energies and existing confusion in the literature has been solved.

Calorimetric studies of successive phase transitions in crystalline biphenyl-d10☆

Abstract The heat capacity of biphenyl- d 10 was measured between 3 and 300 K by adiabatic calorimetry. The thermodynamic properties of the successive phase transitions were determined: T trs = (36.8 ± 0.2) K, Δ trs H m = (4.61 ± 0.08) J·mol −1 , and Δ trs S m = (0.128 ± 0.003) J·K −1 ·mol −1 for the twist transition, and T trs = (20.2 ± 0.1) K, Δ trs H m = (0.18 ± 0.04) J·mol −1 , and Δ trs S m = (0.009 ± 0.002) J·K −1 ·mol −1 for the partial lock-in transition. The properties of biphenyl- d 10 are compared with those of biphenyl. The apparatus used for calorimetry is described.

Thermodynamic properties of deuterated hexamethylbenzene and of its solid solutions with the hydrogenated analog. A large isotope effect on the phase transition at the temperature 117 K

The heat capacities of hexamethylbenzene-d 18 (HMB-d 18 ) were measured at temperatures T between 4 K and 300 K. The III-to-II transition occurs at T = 132.4 K and the molar entropy of transition is 13.1 J·K −1 ·mol −1 , which is 1.41 times as large as that of HMB-h 18 . The heat capacities of the solid solutions with n (D)/ n (H) = 1/9 and n (D)/ n (H) = 1 were also measured between T = 14 K and 300 K. Thermodynamic properties of the transition change continuously with the composition. Crystal structures of HMB-h 18 , HMB-d 18 , and their solid solutions are the same in phase II and phase III. The large isotope effect in the entropy of transition can be attributed to differences in the torsional modes of the methyl groups. Hysteresis phenomena for HMB-d 18 and HMB{ n (D)/ n (H) = 1} and the memory effect for HMB{ n (D)/ n (H) = 1 } in relation to the III-to-II transition were studied. The temperature dependence of the Raman spectrum is also reported.