I. E. Paukov

Polymorphism of glycine, Part II

The contribution summarizes the results of a systematic study of the three glycine polymorphs (a, b, g-forms), including: i) the controlled crystallization of a desirable form, ii) a comparative calorimetric study of the three forms in the temperature range between 5 K and the sublimation temperatures (»500 K).

Low-temperature heat capacity of α and γ polymorphs of glycine

Low-temperature heat capacity of two polymorphs of glycine (α and γ) was measured from 5.5 to 304 K and thermodynamic functions were calculated. Difference in heat capacity between polymorphs ranges from +26% at 10 K to -3% at 300 K. The difference indicates the contribution into the heat capacity of piezoelectric γ polymorph, probably connected with phase transition and ferroelectricity. Thermodynamic evaluations show that at ambient conditions γ polymorph is stable and α polymorph is metastable.

A low-temperature heat-capacity study of synthetic anhydrous Mg-cordierite (Mg2Al4Si5O18)

Abstract The heat capacity of synthetic anhydrous Mg-cordierite (Mg2Al4Si5O18) was measured using adiabatic calorimetry between 6 and 300 K. Mg-cordierite was synthesized from a glass of stoichiometric cordierite composition at 1250 °C for one week. The refined cell dimensions using powder X-ray diffraction give a = 17.0553(2) Å, b = 9.7167(1) Å, and c = 9.3375(1) Å with V = 1547.428(1) Å3 and a distortion index of Δ = 0.25 indicating complete Al-Si ordering. The absence of channel H2O was confirmed by powder IR spectroscopy, which did not show the presence of any bands located around 3600 cm-1. The CP results values agree with those obtained previously by Weller and Kelley (1963) who measured the CP of Mg-cordierite between 50 K and 298 K. The calculated entropy of Mg-cordierite at standard condition is S0298.15 = 404.0 ± 1.6 J/(mol·K) using the CP data from this study. This value is in agreement with the value presented by Weller and Kelley [407.2 ± 3.77 J/(mol·K)], but appears to be very slightly lower than estimates made from various modeling studies incorporating phase-equilibrium data on Mg-cordierite.

Thermodynamic properties of MnMoO4 and Mn2Mo3O8

The temperature dependencies of the heat capacity of MnMoO 4 and Mn 2 Mo 3 O 8 have been investigated from 5 K to 300 K with a vacuum-jacketed adiabatic calorimeter. The thermodynamic functions have been calculated on the basis of experimental results on C p,m ( T ). The standard values at the temperature 298.15 K for MnMoO 4 are: C p,m = 13.924 · R; S S o m = 17.992 · R ; H o m (T) − H o m (0) = 2556.7 · R · K; and Φ o m = 9.417 · R . For Mn 2 Mo 3 O 8 the same quantities are equal to 29.575 · R , 30.609 · R , 4797.7 · R · K and 14.517 · R , respectively. The λ-anomalies connected with the magnetic-ordering phase transitions were observed and studied in detail. The phase-transition temperatures for MnMoO 4 and Mn 2 Mo 3 O 8 correspond to 10.5 K and 39.44 K, respectively. A sharp increase of the thermal-relaxation time has been revealed in MnMoO 4 at T

Heat capacity of synthetic hydrous Mg-cordierite at low temperatures: Thermodynamic properties and the behavior of the H2O molecule in selected hydrous micro and nanoporous silicates

Abstract The heat capacity, CP, of a synthetic hydrous cordierite of composition Mg1.97Al3.94Si5.06O18·0.625H2O was measured for the first time using precise adiabatic calorimetry in the temperature range from 6 to 300 K. Hydrous Mg-cordierite was obtained by hydrothermal treatment of anhydrous Mg-cordierite (Paukov et al. 2006) at 4 kbar and 600 °C for 24 hours. The synthetic product was characterized using X-ray diffraction and powder IR spectroscopy. Rietveld refinement gives a = 17.060(2) Å, b = 9.721(1) Å, and c = 9.338(1) Å with V = 1548.7(3) Å3 and Δ = 0.25, and the IR spectrum shows only the presence of Class I-Type I H2O in the channel cavities. Small CP anomalies were observed at 272.98 ±0.03 K and 239.43 ±0.13 K, which are thought to be related to very small amounts of H2O occurring in tiny fluid inclusions and to surface H2O, respectively. From the heat-capacity data on hydrous Mg-cordierite, various thermodynamic functions were calculated and are presented in table form. The calculated partial molar entropy for one mole of H2O in hydrous Mg-cordierite at 298.15 K and 1 bar is 80.5 J/(mol·K). The partial molar volume for H2O in hydrous Mg-cordierite at 298 K and 1 bar is zero. The CP results, together with published heat-capacity data on three different zeolites, permit a comparison and analysis of their heat-capacity behavior. The heat-capacity behavior of H2O molecules in zeolites is more similar to that of ice at T < 300 K and not to gaseous H2O, which can be attributed to the presence of hydrogen-bonded H2O molecules. In contrast, the heat-capacity behavior for the .quasi-free. H2O molecule in cordierite is more similar to that of a free H2O molecule in the gaseous state between approximately 100 and 300 K. At T < 100 K, the energies of low-energy modes, especially external H2O translations, determine heat-capacity behavior. Model heat capacities for H2O in cordierite were calculated using the Einstein model and using as input data the results from inelastic neutron-scattering measurements on hydrous Mg-cordierite (Winkler and Hennion 1994). Reasonable agreement between experiment and calculations can be achieved using three H2O translational modes, one of which is hypothetical, and two librational H2O modes. The experimental spectra do not appear to show all six external H2O modes and further vibrational spectroscopic study is required to determine their energies. At T > 300 K, the heat capacity for H2O is the smallest in steam with values increasing in hydrous beryl and cordierite, to H2O in various zeolites, and finally to liquid H2O. This behavior may reflect the nature of the hydrogen bonding and the energies of internal H2O stretching modes, which decrease in energy with increasing hydrogen-bonding strength in the various systems

Thermodynamic properties of dysprosium from 7 to 300 K

The heat capacity of high-purity dysprosium was measured over the temperature range 7 to 300 K by adiabatic calorimetry. Results of mass-spectroscopic and gas analyses as well as the ratio of resistivities R(300 K)R(4.2 K) ≈ 200 confirmed the high quality of the sample. Values of thermodynamic functions have been calculated: Cp(T), So(T), {Ho(T)−Ho(0)}, and −{Go(T)−Ho(0)}T at T = 298.15 K are respectively 27.73 J·K−1·mol−1, 75.55 J·K−1·mol−1, 8973 J·mol−1, and 45.45 J·K−1·mol−1. The phase transitions in dysprosium were found to be at 92.1 K and at 180.2 K. The influence of the purity of a sample on the thermodynamic functions is discussed.

Low temperature heat capacity of the coordination compound: Nickel(II) nitrate with 4-amine-1,2,4-triazole at temperatures from 11 to 317 K

Low-temperature heat capacity of the coordination compound of nickel(II) nitrate with 4-amine-1,2,4-triazole was measured in the temperature range from 11 to 317 K using a computerized vacuum adiabatic calorimeter. The thermodynamic functions have been derived from the smoothed experimental data over the whole temperature interval covered and at standard conditions. At 298.15 K, the heat capacity is 574.7±1.2 J K-1 mol-1, the entropy is 599.2±1.2 J K-1 mol-1, the enthalpy is 91070±200 J mol-1, and the reduced Gibbs energy is 293.7±1.2 J K-1 mol-1. The results on Cp(T) were compared with those for Cu(NH2trz)3(NO3)2·0.5H2O. It was revealed that the slope of the curve dCp/dT (T) changes essentially for both compounds at 110-120 K. It implies that additional degrees of freedom appear in the heat capacity at these temperatures.

Heat capacity measurements under continuous heating and cooling using vacuum adiabatic calorimetry

Abstract An automated apparatus for measuring heat capacity by the method of vacuum adiabatic calorimetry is described. A procedure is also described for measuring heat capacity under continuous heating and cooling, which has been made possible with this apparatus. The possibilities of the apparatus are illustrated by a study of first- and second-order phase transitions in the rare earth metals terbium and dysprosium.

Phase transition at 204–250 K in the crystals of β-alanine: kinetically irreproduceable, or an artefact?

The crystals and the powder samples of β-alanine were studied in the wide temperature range by adiabatic calorimetry, differential scanning calorimetry, IR- and Raman spectroscopy and X-ray diffraction. No phase transitions could be observed. A small anomaly observed at about 256 K in the Cp(T) dependences measured for the samples re-crystallized from water was shown to be due to the presence of a small (about 0.14% of the total sample mass) amount of solvent inclusions. This anomaly was not observed in the solvent-free samples, either powders or single crystals.

Boundary line of the transition into the pseudogap state in thulium cuprates

Precision measurements of the heat capacity of 1-2-3 thulium cuprates with oxygen content ranging from 6.3 to 6.92 were performed in the temperature range 6–300 K. Analysis of the experimentl data showed anomalies in the temperature dependence of the electronic heat capacity. It is conjectured that the anomalies are due to a transition from the normal metallic into the pseudogap state.