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A simple model for calculating the compressibility of the transition-metal carbides and nitrides alloys ZrxNb1−xC and ZrxNb1−xN

100%

Katsika-Tsigourakou V.

Open Physics

2011

tom 9

nr 5

1309-1314

The 4d-transition-metals carbides (ZrC, NbC) and nitrides (ZrN, NbN) in the rocksalt structure, as well as their ternary alloys, have been recently studied by means of a first-principles full potential linearized augmented plane waves method within the local density approximation. These materials are important because of their interesting mechanical and physical properties, which make them suitable for many technological applications. Here, by using a simple theoretical model, we estimate the bulk moduli of their ternary alloys ZrxNb1−xC and ZrxNb1−xN in terms of the bulk moduli of the end members alone. The results are comparable to those deduced from the first-principles calculations.

Defect entropies and enthalpies in BaF2

63%

Katsika-Tsigourakou V. , Skordas E.

Open Physics

2010

tom 8

nr 6

900-904

Various experimental techniques have revealed that the predominant intrinsic point defects in BaF2 are anion Frenkel defects. Their formation, enthalpy and entropy, as well as the corresponding parameters for the fluorine vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements in BaF2doped with uranium leads to the parameters τ0, E in the Arrhenius relation τ = τ0 exp(E/k B T) for the relaxation time τ. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced from the pre-exponential factor τ0 is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of the bulk material which have been recently studied by employing density-functional theory and density-functional perturbation theory.