Manganese Disulfide (Hauerite) and Manganese Ditelluride. Thermal Properties from 5 to 350°K and Antiferromagnetic Transitions

The heat capacities of manganese disulfide and manganese ditelluride were determined by adiabatic calorimetry in the range 5–350°K. Lambda‐type transitions are present in both compounds with maxima at 47.93°K for MnS2 and at 83.0°K for MnTe2. Entropies, enthalpies, and Gibbs energy function values are calculated and tabulated. At 298.15°K they are: S°  =  23.88cal/mole⋅°K,H° − H0°  =  3384cal/mole,− [(G° − H0°) / T]  =  12.258cal/mole⋅°KS°=23.88cal∕mole⋅°K,H°−H0°=3384cal∕mole,−[(G°−H0°)∕T]=12.258cal∕mole⋅°K for MnS2 and 34.66, 4416, and 19.847 for MnTe2. The clearly cooperative entropy increments are only 0.71 cal/mole⋅°K for MnS2 and 0.80 for MnTe2. Available magnetic susceptibility data are interpreted in terms of zero‐field splitting of the 6S5/26S5∕2 state of the manganese 3d53d5 electrons. The resulting contributions to the heat capacity are evaluated. At 298°K the combined λ‐transitional and Schottky contributions to the entropy are 2.6 and 2.4 cal/mole⋅°K for MnS2 and MnTe2, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69763/2/JCPSA6-52-7-3820-1.pd

Heat Capacities and Thermodynamic Properties of Two Tetramethylammonium Halides

Heat capacities of tetramethylammonium chloride and bromide were determined by low‐temperature adiabatic calorimetry from 5° to 350°K. Derived thermodynamic properties were then calculated. Two transitions were found in the chloride: a sharp, apparently first‐order transition occurs at 75.76°K with an entropy of transition of 0.37 cal mole—1 °K—1 and a lambda‐shaped transition at 184.85°K with an entropy increment of 0.14 cal mole—1 °K—1. No anomaly has been observed in the bromide. Molal values of heat capacity, entropy, and free energy function at 298.15°K for the chloride and the bromide are: 37.51, 38.64, 45.58, 47.99, and —23.36, —25.36 cal mole—1 °K—1, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69935/2/JCPSA6-36-9-2420-1.pd

Molecular Freedom of the Ammonium Ion. Heat Capacity and Thermodynamic Properties of Ammonium Perchlorate from 5°–350°K

The heat capacity of NH4ClO4 has been determined by adiabatic calorimetry from 5°–350°K and found to be of simple sigmate character without thermal anomalies. The heat capacity (Cp)(Cp), entropy (S°)(S°), enthalpy function (H°−H°0) / T(H°−H°0)∕T, and Gibbs energy function (G°−G0°) / T(G°−G0°)∕T evaluated at 298.15°K from these data are 30.61, 44.02, 20.24, and −23.78 cal/(gfm °K). Combination of these values with aqueous NH4ClO4 thermochemical data suggests the absence of zero‐point entropy. Comparison with the heat capacity of isostructural KClO4 permits resolution of the molecular dynamics of the ammonium ions and leads to the conclusion that these ions are restricted rotators, prevented from freely rotating by comparatively low‐energy barriers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70216/2/JCPSA6-50-12-5083-1.pd

Methanol: Heat Capacity, Enthalpies of Transition and Melting, and Thermodynamic Properties from 5–300°K

Thermal properties of methanol were studied by adiabatic calorimetry. The first‐order nature of the phase transition at 157.4°K with an entropy increment of 0.97 cal mole−1⋅°K−1 was confirmed. The heat capacity of the crystalline phase stable just below the triple point was defined and shown to be extremely sensitive to impurity. No evidence for a second previously‐reported phase transition could be detected. The standard entropy (S°)(S°) and Gibbs energy function (− [G° − H°0] / T)(−[G°−H°0]∕T) for the liquid at 298.15°K are 30.40 and 15.18 cal mole−1⋅°K−1, respectively. The proposed classification of methanol as a plastic crystal on the basis of its small entropy of melting (4.38 cal mole−1⋅°K−1) is considered with respect to hydrogen bonding in the liquid phase.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70007/2/JCPSA6-54-4-1464-1.pd

MnWO4, calorimetric study of the bifurcated antiferromagnetic anomaly

The heat capacity of a powdered MnWO4 sample has been measured from 5–350 K. The data show three anomalies below 20 K: a small peak at 6.8±0.1 K, then two large sharp peaks at 12.57±0.05 and 13.36±0.05 K. The magnetic entropy was measured as R ln 6. The data between 5 and 11.5 K obeys a power law dependence Cmag=ATB where B=1.73. The sharp double peak is similar to the bifurcated anomaly in MnCl2 which originates from two distinct antiferromagnetic phases, reported by R. B. Murray et al. The double anomaly is discussed in terms of the superexchange properties of MnWO4.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87559/2/445_1.pd

Low‐Temperature Thermal Properties of Calcium Tungstate

The heat capacity of a single crystal of CaWO4 was determined by adiabatic calorimetry from 5° to 350°K and found to be without transitions or thermal anomalies. Deviation of the curve from normal sigmate shape is shown to be due to internal vibrations of the WO4= ions. Apparent Debye θθ's for the lattice‐only heat capacity and for that of the acoustical spectrum show “normal” deviation from simple Debye theory. Values of the heat capacity (Cp)(Cp), entropy (S°)(S°), enthalpy function [(H°–H0°) / T][(H°–H0°)∕T], and Gibbs function [(G° / H0°) / T][(G°∕H0°)∕T] at 298.15°K are 27.28, 30.21, 16.02, and −14.19, in calories per gram formula mass⋅degree Kelvin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69875/2/JCPSA6-49-8-3374-1.pd

Thermodynamics of Nonstoichiometric Nickel Tellurides. II. Dissociation Pressures and Phase Relations of Tellurium‐Rich Compositions

Dissociation pressures of tellurium over liquid and solid nickel telluride solutions have been measured with a silica Bourdon gauge at compositions corresponding to NiTe1.5, NiTe1.7, NiTe1.9, NiTe2.0, and NiTe9 at temperatures up to 780°. Modifications of the manometric technique are described which permit accuracies of 0.1 mm pressure and 0.1° at high temperatures with corrosive substances where the pressure is sensitive to impurities or to composition changes. The results, together with data on the vapor pressure of pure tellurium, define the partial molal free energies and entropies of tellurium and, together with direct eutectic temperature measurement, delineate features of the phase diagram for compositions with more than 60 atomic percent tellurium.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69856/2/JCPSA6-29-4-824-1.pd

Uranium Mononitride: Heat Capacity and Thermodynamic Properties from 5° to 350°K

The low‐temperature heat capacity of UN was determined by adiabatic calorimetry and found to have a normal sigmate temperature dependence, except for the presence of an anomaly near 52°K associated with antiferromagnetic ordering of the electron spins. At 298.15°K the heat capacity (CP), entropy (S°), enthalpy function [(H°—H°0)/T], and Gibbs energy function [—(G°—H°0)/T] are, respectively, 11.43, 14.97, 7.309, and 7.664 cal/(gfm⋅°K).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70037/2/JCPSA6-45-2-635-1.pd

Heat Capacities and Thermodynamic Properties of the Iron Tellurides Fe1.11Te and FeTe2 from 5 to 350°K

Heat capacities of the two iron telluride phases, Fe1.11 Te and FeTe2, were measured in the range 5 to 350°K. In Fe1.11Te a cooperative type of transformation was observed at about 63°K involving an entropy increment of 0.57 cal/mole °K. The thermodynamic functions were evaluated and the values of Cp, S°—S0°, and (H°—H0°)/T at 298.15°K are 13.15, 21.272, and 9.575 cal/mole °K, respectively, for Fe1.11Te, and the values of Cp, S°—S0°, H°—H0°, and — (F°—H0°)/T for FeTe2 are 17.60 and 23.940 cal/mole °K, 3567.4 cal/mole, and 11.975 cal/mole °K, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70044/2/JCPSA6-30-3-761-1.pd

Recent thermophysical developments on nuclear materials

The current status of chemical thermodynamics of the actinide and lanthanide chalcogenides including high-temperature adiabatic calorimetry of these important compounds (with stress on their electronic, magnetic, order-disorder, disproportionation transitions), as well as the spin-wave magnetic contributions, is reviewed. Schottky anomalies (and the information they yield on crystal-field level splitting), definitive resolution of lattice and magnetic contributions for both first- and second-order phase transitions, and other unusual aspects of phase behavior are considered. Unpublished work on the uranium trioxides, the non-stoichiometric and metastable tetrauranium octaoxides, the uranates, etc. is included. The close parallel between actinide and lanthanide behavior evidenced by our recent unpublished thermal studies on the actinide and lanthanide trichlorides, and hexaborides is emphasized.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22358/1/0000804.pd