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Birefringence Measurements Of The Uniaxial-Stress Dependence Of The Incommensurate Phase Transition In K2Seo4, D. P. Billesbach, F. G. Ullman, John R. Hardy
Birefringence Measurements Of The Uniaxial-Stress Dependence Of The Incommensurate Phase Transition In K2Seo4, D. P. Billesbach, F. G. Ullman, John R. Hardy
John R. Hardy Papers
The uniaxial-stress dependence of the incommensurate-phase-transition temperature (at 130 K) in K2SeO4 was measured along all three crystal axes by observation of the anomalies in the birefringence caused by the structure change. The results for the c-axis-stress dependence (- 17.0±0.2 K/kbar) agree with previous studies of other workers. This experiment has also provided the first direct measurements of the a-axis- and b-axis-stress dependences (+ 3.1 ±0.2 K/kbar and +7.7± 0.2 K/kbar, respectively). Also, a value of -6.2±0.3 K/kbar was calculated from these data for the dependence of the transition temperature on hydrostatic pressure. This …
First-Principles Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity And Ferroelasticity, J. W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer
First-Principles Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity And Ferroelasticity, J. W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer
John R. Hardy Papers
We have made a systematic theoretical survey of the competition between ferroelastic and ferroelectric instabilities in the family of halide-based perovskites of formula ABX3, where A is an alkali-metal ion, B is a Be, Mg, or Ca ion, and X is a halide ion. Initially we surveyed the whole series of such compounds, making a theoretical lattice-dynamical study using first-principles interionic potentials composed of a long-range pure Coulomb interaction between the spherically symmetric free ions, and a short-range component calculated by the Gordon-Kim approach from the overlapping free-ion charge densities. We then proceeded to examine in more detail …
Beyond The Rigid-Ion Approximation With Spherically Symmetric Ions, L. L. Boyer, M. J. Mehl, J. L. Feldman, John R. Hardy, J. W. Flocken, C. Y. Fong
Beyond The Rigid-Ion Approximation With Spherically Symmetric Ions, L. L. Boyer, M. J. Mehl, J. L. Feldman, John R. Hardy, J. W. Flocken, C. Y. Fong
John R. Hardy Papers
Ab initio calculations show that a spherically symmetric charge relaxation of ions in a crystal, in response to the long-range electrostatic potential, is important for understanding the splitting between longitudinal- and transverse-optic-mode frequencies, and the violation of the Cauchy relations among elastic constants.