Heat capacity and thermodynamic properties of ditungsten carbide, W2C1-x, from 10 to 1000 K

Thermodynamic properties of tungsten carbide, W2C0.833, have been derived from heat capacities measured by adiabatic calorimetry in the range 10-1000 K on a sample rich in this phase. The standard entropy of W2C0.833 was found to be 75.80 J K-1 mol-1 at 298.15 K and 159.8 J K-1 mol-1 at 1000 K. Thermodynamic formation values for W2C0.833 were deduced from the reported coexistence of this phase with tungsten and tungsten monocarbide at about 1550 K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27259/1/0000268.pd

Heat capacity of MnAs0.88P0.12 from 10 to 500 K: Thermodynamic properties and transitions

The heat capacity of MnAs0.88P0.12 has been measured by adiabatic shield calorimetry from 10 to 500 K. It is shown that very small energy changes are connected with two magnetic order-order transitions, indicating that these can be regarded as mainly "noncoupled" magnetic transitions. At higher temperatures contributions to the excess heat capacity arises from a magnetic order-disorder transition, a conversion from low- to high-spin state for manganese, and a MnP- to NiAs-type structural transition. The observed heat capacity is resolved into contributions from the different physical phenomena, and the character of the transitions is discussed. In particular it is substantiated that the dilational contribution, which includes magnetoelastic and magnetovolume terms as well as normal anharmonicity terms, plays a major role in MnAs0.88P0.12. The entropy of the magnetic order-disorder transition is smaller than should be expected from a complete randomization of the spins, assuming a purely magnetic transition. Thermodynamic functions have been evaluated and the respective values of Cp, {SOm(T) - SOm(0)}, and -{GOm(T) - HOm(0)}/T at 298.15 K are 68.74, 72.09, and 32.30 J K-1 mole-1, and at 500 K 56.05, 108.12, and 56.64 J K-1 mole-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26552/1/0000091.pd

Oxide-Ion Disorder Within the High Temperature delta Phase of Bi2O3

The delta phase of Bi2O3, which adopts an anion-deficient fluorite structure, has the highest known oxide-ion conductivity. Using a combination of neutron powder diffraction and Born-Oppenheimer molecular dynamics, the preferred local anion environment around the Bi3+ within delta-Bi2O3 is shown to be highly irregular, resembling the asymmetric "lone-pair" coordination found within many (fully ordered) oxides of Bi3+ under ambient conditions. The asymmetric electron density around the Bi3+ plays a central role in promoting the extreme anion disorder within delta-Bi2O3, with the ion diffusion facilitated by extensive relaxations of both the surrounding anions and a "soft" cation sublattice. The validity of previously proposed structural models based on a cubic environment in which O2- vacancies are aligned in pairs in , , and directions is discussed in light of these conclusions

Oxide-Ion Disorder Within the High Temperature delta Phase of Bi2O3

The delta phase of Bi2O3, which adopts an anion-deficient fluorite structure, has the highest known oxide-ion conductivity. Using a combination of neutron powder diffraction and Born-Oppenheimer molecular dynamics, the preferred local anion environment around the Bi3+ within delta-Bi2O3 is shown to be highly irregular, resembling the asymmetric "lone-pair" coordination found within many (fully ordered) oxides of Bi3+ under ambient conditions. The asymmetric electron density around the Bi3+ plays a central role in promoting the extreme anion disorder within delta-Bi2O3, with the ion diffusion facilitated by extensive relaxations of both the surrounding anions and a "soft" cation sublattice. The validity of previously proposed structural models based on a cubic environment in which O2- vacancies are aligned in pairs in , , and directions is discussed in light of these conclusions

Thermodynamics of copper sulfides IV. Heat capacity and thermodynamic properties of Cu1.90S from 5 K to 750 K, Cu1.95S from 5 K to 1000 K, Cu1.98S from 300 K to 1000 K, and Cu1.995S from 300 K to 750 K

The heat capacities of Cu1.90S, Cu1.95S, Cu1.98S, and Cu1.995S have been measured by adiabatic-shield calorimetry. All samples have been characterized by powder X-ray diffraction in the temperature interval studied. A revised version of the copper-rich part of the phase diagram is presented. All transitions are characterized by hysteresis in the attainment of equilibrium and a greater or lesser dependence on thermal history and/or thermal recycling. Thermodynamic functions have been evaluated and selected values are, for R = 8.3144 J[middle dot]K-1[middle dot]mol-1:Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28934/1/0000771.pd

Heat capacities of the wustites Fe0.9379O and Fe0.9254O at temperatures T from 5 K to 350 K. Thermodynamics of the reactions: xFe(s) + (1/4)Fe3O4(s) = Fe0.7500+xO(s) = Fe1-yO(s) at T [approximate] 850 K, and properties of Fe1-yO(s) to T = 1000 K. Thermodynamics of formation of wustite

Thermodynamic properties of wustites prepared from iron and iron(III) oxide, have been studied by adiabatic calorimetry. Heat-capacity measurements of metastable Fe0.9379O and Fe0.9254O from T = 5 K to 350 K yielded the following integrated values at T = 298.15 K: [[formula]] The wustites were decomposed to iron and iron(II, III) oxide at T [approximate] 800 K and then recombined in the calorimeter. The enthalpy absorption started at T [approximate] 850 K. It needed increased temperature and several days for completion: up to T = 932 K and 7.1 d total for Fe0.9254O, up to T = 898.4 K and 2.1 d for Fe0.9379O, and up to T = 948 K and 3.0 d for Fe0.9254O. By averaging the results for the two last determinations, the molar enthalpy of the eutectoid formation reaction:0.1817Fe+(1/4)Fe3O4 = Fe0.9317O, is [Delta]rHom = (9.04+/-0.25) kJ[middle dot]mol-1. The observed eutectoid formation temperature on heating was 854 K on the iron side and 844 K on the magnetite side. In order to delineate the composition range of wustite at T 0.90O were estimated and combined with available standard Gibbs free energies of formation for wustite and the neighboring magnetite phase at T = 1270 K. The resulting eutectoid composition is Fe0.932+/-0.004O and the calculated eutectoid temperature is (847+/-7) K.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30621/1/0000262.pd

Neutron total scattering study of the delta and beta phases of Bi2O3

The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm (3) over barm. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable a and metastable beta phases at ambient temperature, which is characteristic of the cation\u27s 6s(2) lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3

Phase stability and structural properties of Ni7 +/- [delta]S6 and Ni9S8 Heat capacity and thermodynamic properties of Ni7S6 at temperatures from 5 K to 970 K and of Ni9S8 from 5 K to 673 K

Two stable solid phases with composition in the range from (45 to 48) moles per cent of S exist for (nickel + sulfur): Ni9S8 which disproportionates to Ni1 - [delta]S and Ni7 +/- [delta]S6 above T = (709 +/- 5) K, and Ni7 +/- [delta]S6. The latter non-stoichiometric phase forms eutectoidally from Ni9S8 and Ni3S2 at T = (675 +/- 3) K. It is stable in a rather narrow temperature interval and disproportionates to Ni1 - [delta]S and Ni3 +/- [delta]S2 at T = (850 +/- 2) K. The phases are characterized structurally, and a revised phase diagram is presented. The heat capacity of a sample with composition Ni7S6 was determined over the temperature range T = 5 K to 970 K by adiabatic calorimetry. Effects from metastable modifications of the high-temperature phase were observed during thermal analysis, X-ray diffraction, and calorimetry on incompletely equilibrated samples. Thermodynamic-function values for Ni7S6 and Ni9S8 are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31376/1/0000289.pd