Transition Entropy ΔS Research Articles

Phase transition and proton exchange in 1,3-diazinium hydrogen chloranilate monohydrate

In the hydrate crystal of 1:1 salt with 1,3-diazine and chloranilic acid (H2ca), (1,3-diazineH)·H2O·Hca, an unique hydrogen-bonded molecular aggregate is formed. There exists proton disorder in the N–H...O hydrogen bond between 1,3-diazinium ion and water (H2O) of crystallization. In order to reveal dynamic aspect of this disorder, 35Cl NQR measurements were conducted. Two resonance lines observed at 35.973 and 35.449 MHz at 321 K split into four lines below T c = 198 K clearly showing occurrence of a solid–solid phase transition; 36.565, 36.357, 36.011, 35.974 MHz at 77 K. Temperature dependence of spin-lattice relaxation time T 1 in high-temperature phase was observed to obey an Arrhenius-type relation with the activation energy of 8.5 kJ mol − 1. This result leads to the conclusion that proton exchange in the N–H...O hydrogen bond takes place in the high-temperature phase. Specific heat measurements by DSC resulted in the transition entropy ΔS = 1.3 J K − 1 per 1 mole [(1,3-diazineH)·H2O·Hca]2 which is far less than 2R ln2 = 11.5 J K − 1 mol − 1. It is expected that proton exchange in the two hydrogen bonds within the aggregate does not occur independently but concertedly with strong correlation in the high-temperature phase.

Dynamic Disorder of Bi82+ Clusters in the Plastic Phase (Bi8)3Bi[InI4]9

AbstractBlack, air‐insensitive crystals of the bismuth‐rich tetraiodido‐indate(III) (Bi8)3Bi[InI4]9 have been prepared from melts of the contributing elements. X‐ray diffraction on a single crystal at 293 K and 220 K revealed a cubic structure with [InI4]– tetrahedra, isolated Bi3+ ions and rotationally disordered Bi82+ square antiprisms ace group Fmc; a = 20.162(2) Å at T = 293(1)]. The principal arrangement of the complex ions corresponds to the AuCu3 type. The small bulk modulus B0 = 12(1) GPa and its large pressure derivative B0′ = 11(1) as well as the low Debye temperature θD = 52 K are in accordance with a plastic behavior. Cooling below 148(1) K induces the first order transition into an orderedphase with 0.43 % higher density but much larger unit cell and lower crystallographic point group. The transition enthalpy ΔH ≈ 3.8 kJ·mol–1 and transition entropy ΔS ≈ 3 R were determined from specific heat data. A similar reversible phase transition into a lower symmetric structure is observed at a pressure of approximately 5 GPa. (Bi8)3Bi[InI4]9 is a diamagnetic semiconductor.

Anomalous Optical Characteristics of Carrier Transfer Process in Quaternary AlInGaN Multiple Quantum Well Heterostructure

The carrier-transport characteristics of quaternary AlInGaN heterosystems are studied in-depth using photoluminescence measurements. Based on Singh's model, a higher degree of disorder in quaternary AlInGaN heterostructures is observed to manifest not only the extension of static microbarrier width, but also the enhancement of carrier localization effects. To provide a clear picture of the random configuration of the carriers photogenerated in quaternary AlInGaN heterosystems, the thermodynamic quantities, i.e., the transition enthalpy ΔH and the transition entropy ΔS, describing the spontaneous fluctuations in the irreversible generation-recombination processes increased with temperature. It is found that the anomalous temperature-dependent phenomena can be attributed to the carrier-thermalization processes. The narrow interlayer distance of an AlInGaN system facilitates thermally excited carrier redistribution. However, due to the inhibition of photocarrier transfers, AlInGaN heterostructures with wider interlayer spacing exhibit more temperature insensitivity.

The phase transitions of ferroelectric Sr2Ta2O7 crystals by MDSC, Brillouin and dielectric spectroscopy

The structural phase transitions of Sr2Ta2O7 single crystals have been studied by the modulated temperature differential scanning calorimetry (MDSC), Brillouin scattering and dielectric spectroscopy. The specific heat (Cp) was measured over a wide temperature range from −150°C to 25°C and from 100°C to 210°C. The Cp curve showed an anomaly at To = 166.7°C, indicating the phase transition Cmcm → P21/m. The transition enthalpy ΔH, the transition entropy ΔS and specific heat jump ΔCp at To were estimated to be 0.465 J g−1, 1.01 mJ g−1 K−1 and 9.78 mJ g−1 K−1, respectively. The Cp anomaly associated with the ferroelectric phase transition at Tc = −107°C has not been detected. However, both Brillouin and dielectric data showed the anomalies corresponding to the ferroelectric phase transition from P21/m to P21.

Myuller’s concept of the viscous flow: Prospects for its evolution

The salient points of Myuller’s concept of viscous flow are set forth. The concept is analyzed in terms of the basic principles of quantum mechanics and statistical thermodynamics. Particular emphasis is placed on the activation entropy of the viscous flow S η * and the glass transition entropy ΔS g . It is demonstrated that, when the tunnel penetration gives way to over-the-barrier passage, the temperature dependences are not described by the Arrhenius equation. The information aspect of the discrete transformations of chemical bonds is described in detail.

Ti-isotope effect on ferroelectric phase transition of PbTiO3

Ti-isotope effects on structural phase transition in PbTiO3 have been reported. The transition temperature, the transition entropy ΔS and the shape of the anomaly in the heat capacity curve changed by the Ti-isotope substitution. The phase-transition temperature of 46Ti-enriched PbTiO3 (756·3 K) was 7·6 K higher than that of 48Ti-enriched PbTiO3 (748·7 K). The Ti-isotope mass dependence of T C in PbTiO3 was somewhat weaker than that of T C in BaTiO3, although the positional change shift of Pb or Ti atom at the phase transition in PbTiO3 was larger than that of Ba or Ti in BaTiO3. The transition entropy ΔS of lighter 46Ti-enriched PbTiO3 was 1·08 times larger than that of 48Ti-enriched PbTiO3.

Orientation-Dependent Interactions in Polymer Systems. 5. Thermotropic Liquid Crystalline Transition of Tri-O-heptylcellulose

Fairly narrow fractions of tri-O-heptylcellulose (THC) covering a wide range of degrees of polymerization (DP) were prepared and studied by viscometry and light scattering to determine the wormlike model parameters of the polymer to be q= 43.2 exp(-0.0050T) nm and M L = 880 nm -1 , where q is the persistence length, M L is the shit factor, and T is the absolute temperature. The same fractions were studied with their thermal properties to determine the anisotropic-isotropic transition temperature T i and transition entropy ΔS as a function of DP. Both T i and ΔS increased with increasing DP to approach a limiting value for large enough DP. These trends were favorably compared with the predictions of the statistical models with the above-noted parameter values. It was shown that the main factor that causes the thermotropic transition and the chain-length dependence of T i in bulk THC is the temperature-dependent flexibility of the polymer

Thermodynamic values of the helix-coil transition of DNA in the presence of quaternary ammonium salt

The knowledge of the enthalpy and entropy of the helix-coil transition in DNA is necessary for the understanding of the stabilization of its native conformation in solution. Reported here is the transition temperature T m, the transition enthalpy ΔH, determined with the help of an adiabatic scanning calorimeter, the transition entropy ΔS and the breadth of the helix-coil transition as a function of tetramethyl and tetraethyl ammonium chloride concentration.