I , 0 3 / Ca0-Zr0,/02,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rocksalt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu20-Cu0-AI2O3 at specified 0, pressures calculated from the present results are discussed in reference to available phase diagrams. I. IntroductionHERMODYNAMIC data for CuAIO, and CuAI,O, are of interest T in determining the conditions under which Cu melts will react with refractories containing AI,O,. Rigby and Hamilton' identified CuA10, during petrological examination of high-AI20, bricks used in a Cu refining furnace. Von Wartenberg and Reusch, presented a phase diagram for the system Cu,O-AI,O,, and Misra and Chaklader? studied phase relations in the systems CuO-AI,O, and Cu,O-AI2O3 in air. Gadalla and White,4 who provided the most extensive information available for the system CU,O-CUO-A~~O,, used a thermobalance to measure the dissociation temperatures of prefired CuO+ Al2O3 mixtures underp,,'s from 0.21 to 1 atm. The thermodynamic information derived from these measurements is valid for only a limited temperature range (= 50°C) and cannot be used to derive accurate free energies of formation for the aluminates of Cu. There are significant discrepancies in the results of the phase diagram studies2-l; e.g. at 1 100"C, Von Wartenberg and Reusch identified no compounds in the system Cu20-A120, in air, whereas Misra and Chaklader reported CuA10, and Gadalla and White both CuA10, and CUAI,~,. The crystal structure of CuAIO, beloBgs to the rhoTbohedral system (D2d-R%n),l*5*6 with a = 3.489 A andc= 16.98 A. Cupric aluminate has a spinel structure similar to the structures of the aluminates and chromites of Mn, Fe, Co, Ni, and Zn. When spinel phases are formed from component oxides having rock-salt and corundum structures, the entropy of formation can be expressed as -1.75+AS"cal "C-' mol-I, wherehS" represents theentropyof cation mixing in the interstitial sites of the spinel structure, if the spinel phases do not exhibit tetragonal distortion at low temperat~res.~ Therefore, the entropy of the formation of spinel CuAI,O, from CuO and AlzO,, which have the tenorite and corundum structures, respectively, is of interest.Navrotsky and Kleppa8 measured the heat of formation of CuA1,0, as 5.17k0.2 kcal mol-l at 723°C by high-temperature oxide-melt-solution calorimetry. The free energy of formation of CuA10, was measured by Schmalzried9 at 1000°C using an emf method and by Zalazinskii et a1.I" from 700" to 1000°C using an HZ/H20 gas equilibrium method. The oxygen potential corresponding to the CU~O-CUO equilibrium has been firmly established by the direct dissociation temperature measurements of Foote and Smith," Roberts and Srnyth,l2 Gadalla et al., Hochgeschwender and Ingraham,', and Schmahl and Mueller'j and...

No abstract

Vegard's law has been used extensively in mineralogy, metallurgy and materials science for the past six decades. According to the law, unit cell parameters should vary linearly with composition for a continuous substitutional solid solution in which atoms or ions that substitute for each other are randomly distributed. Although the law was postulated on empirical evidence, several cases of both positive and negative deviations from this law have been documented. Its theoretical foundations have not been critically explored. Presented in this communication is an analysis of the law within the framework of solution thermodynamics. It is shown that the deviation from Vegard's law is expected even for thermodynamically ideal solutions when there is a significant difference in lattice parameters of the pure components. The law should be reclassified as an approximation valid for specific conditions. The approximation is valid for ideal solutions when the lattice parameters of the pure components differ by less than 5 %. For solid solutions with positive deviations from ideality, there will always be positive deviations from Vegard's law. For solid solutions with moderately negative deviations from ideality, positive deviation from linearity of lattice parameters caused by size mismatch can be compensated for by the attractive interaction between the components, resulting in compliance with Vegard's law.

A process is demonstrated for the efficient separation of rare earth elements, using a combination of selective reduction and vacuum distillation of halides. The large differences in the redox chemistry of the rare earth elements and in the vapor pressures of rare earth di- and trihalides are exploited for separation. Experimental proof of concept is provided for the binary systems praseodymium-neodymium and neodymium-samarium. This process enhances the separation factor for the isolation of samarium and neodymium from their mixture by more than an order of magnitude.

No abstract

No abstract

Thermodynamic properties of three oxides of niobium have been measured using solid state electrochemical cells incorporating yttria-doped thoria (YDT) as the electrolyte in the temperature range T = (1000 to 1300) K. The standard Gibbs energies of formation of NbO, NbO2, and NbO2.422 from the elements can be expressed as: △ f G NbO o ± 547 / J · mol − 1 = − 414 986 + 86 .861 ( T / normalK ) △ f G NbO 2 o ± 548 / J · mol − 1 = − 779 864 + 164 .438 ( T / normalK ) △ f G NbO 2.422 o ± 775 / J · mol − 1 = − 911 045 + 197 .932 ( T / normalK ) The results are discussed in comparison with thermodynamic data reported in the literature. The new results refine data for NbO and NbO2 presented in standard data compilations. There are no data in thermodynamic compilations for NbO2.422 (Nb12O29). In the absence of the heat capacity and enthalpy of formation measurements, only the Gibbs energy of formation of NbO2.422 can be assessed. The free energy of formation of stoichiometric Nb2O5 is evaluated on the basis of measurements on NbO2.422 and information available in the literature on phase boundary compositions and isothermal variation of nonstoichiometric parameter with oxygen potential for Nb2O5−x . The results suggest a minor revision of data for Nb2O5. A minimum in the Gibbs energy of mixing for the system Nb−O occurs in the nonstoichiometric domain of Nb2O5−x with x = 0.036.

No abstract