Harold T. Stokes

First Principles Phase Diagram Calculations for the Octahedral-Interstitial System ZrOX, 0 ≤X ≤1/2

First principles based phase diagram calculations were performed for the octahedral-interstitial solid solution system αZrO_X (αZr[ ]_(1-X)O_X; [ ] = vacancy; 0 ≤X ≤1/2). The cluster expansion method was used for ground state analysis, and to calculate the phase diagram. The predicted diagram has four ordered ground-states in the range 0 ≤X ≤1/2, but one of these, at X=5/12, is predicted to disproportionate at T≈20 K, well below the experimentally investigated range T ≈420 K. Thus, at T≳420 K, the first-principles based calculation predicts three ordered phases rather than the four that have been reported by experimentalists.

Development of a Kohn-Sham like potential in the self-consistent atomic deformation model

Abstract This is a brief description of how to derive the local ‘atomic’ potentials from the Self-Consistent Atomic Deformation (SCAD) model density functional. Particular attention is paid to the spherically averaged case.

Ab Initio Calculation of Spin Gap Behavior in CaV4O9

Second neighbor dominated exchange coupling in CaV 4 O 9 has been obtained from ab initio density functional (DF) calculations. A DF-based self-consistent atomic deformation model reveals that the nearest neighbor coupling is small due to strong cancellation among the various superexchange processes. Exact diagonalization of the predicted Heisenberg model yields spin-gap behavior in good agreement with experiment. The model is refined by fitting to the experimental susceptibility. The resulting model agrees very well with the experimental susceptibility and triplet dispersion.

Using symmetry in frozen phonon calculations

Software has been developed which applies group-theoretical methods to the calculation of phonon spectra. The software draws on a large data base which includes information on each of the 230 crystallographic space groups and their irreducible representations. The software finds the optimal phonon modes for the frozen-phonon energy calculations.

Self-Consistent potential induced breathing model calculations for longitudinal modes in MgO

The self-consistent potential-induced breathing model is applied to calculate vibrational modes in MgO using the frozen phonon method. The model gives the frequency difference between longitudinal and transverse optic modes expected for a non-polarizable ion system, which removes a deficiency in the earlier non-self-consistent potential-induced breathing model. For large amplitudes the longitudinal optic mode energy is lowered from its harmonic value by charge transfer.