John R. Hardy

Dielectric properties and Maxwell-Wagner relaxation of compounds ACu3Ti4O12 (A=Ca,Bi2∕3,Y2∕3,La2∕3)

We have studied the frequency and temperature dependences of permittivity and impedance of the compounds ACu3Ti4O12 (A=Ca,Bi2∕3,Y2∕3,La2∕3) in the ranges of 10−1–106Hz and −150–200°C. All compounds investigated display similar dielectric properties. Specifically, they all have a Debye-like relaxation and their dielectric constants are independent of frequency and temperature over a wide range. They all have two electrical responses in impedance formalism, indicating that there are two distinct contributions. We attribute them to grains and grain boundaries in the ceramic samples and explain the dielectric behaviors by Maxwell-Wagner relaxation arising at the interfaces between grains and their boundaries.

Dielectric permittivity and electric modulus in Bi2Ti4O11

Frequency and temperature dependences of dielectric permittivity and electric modulus of pure and Ba-doped Bi2Ti4O11 were studied in the ranges of 10−1–106 Hz and −150–350 °C, respectively. We found that the antiferroelectric phase transition temperature of Bi2Ti4O11 decreases with Ba doping. In the permittivity studies, we also observed dielectric relaxation peaks shift to higher temperature with increasing frequency. Furthermore, in the electric modulus formalism, conducting peaks were uncovered above 150 °C in addition to the dielectric relaxation peak. We discussed the mechanisms for the dielectric relaxation and conduction processes based on TiO6 octahedra distortion and a space-charge model.

Strain Induced Half-Metal to Semiconductor Transition in GdN

We investigate the electronic structure and magnetic properties of GdN as a function of unit cell volume. Based on the first-principles calculations of GdN, we observe that there is a transformation in the conduction properties associated with the volume increase: first from half-metallic to semimetallic, then ultimately to semiconducting. We show that applying stress can alter the carrier concentration as well as mobility of the holes and electrons in the majority spin channel. In addition, we found that the exchange parameters depend strongly on lattice constant, thus the Curie temperature of this system can be enhanced by applying stress or doping impurities.

The lattice dynamics and statics of alkali halide crystals

I. Introduction.- 1. Historical Background.- II. General Theory.- 2. Introduction of Normal Coordinates.- 3. The Adiabatic Approximation.- III. Dipolar Models.- 4. Long-Wave Optical Vibrations of Cubic Ionic Lattices...- 4.1. Macroscopic Theory.- 4.2. Microscopic Theory.- 5. Description and Justification of the Various Dipolar Models.- 6. Derivation of the Dipolar Coupling Coefficients.- IV. Theoretical and Experimental Single-Phonon Data.- 7. Comparison of Theoretical and Experimental Debye-Waller and Specific-Heat Data.- 7.1. Debye-Waller Factors.- 7.2. Specific-Heat Data.- 8. Direct Measurement of Phonon-Dispersion Curves.- V. Two-Phonon Data.- 9. Interpretation of the Infrared Spectra of Perfect Alkali Halides.- 10. Second-Order Raman Spectra of Alkali Halide Crystals.- VI. Dynamic and Static Defects.- 11. Theory of Impurity Vibrations.- 12. The Method of Lattice Statics 7..- VII. Conclusions.- References.

Structural phase transitions, ferroelectricity and high temperature superconductivity

Abstract Calculations based on a local double well potential show the near coincidence of a ferroelectric transition and a high temperature superconducting phase transition. Using double well parameters consistent with the Abrahams-Kurtz-Jamieson (AKJ) relation for ferroelectric phase transitions in oxide perovskite ferroelectrics we find that the superconducting transition temperature falls above or below the ferroelectrics transition temperature depending on modest changes in the height of the unrenormalized double well barrier. For the ReBa2Cu3O7-δ superconductors, this ferroelectric transition appears to fall above the superconducting transition temperature. The shallowness of the double-well in this case is consistent with our previous suggestion that the ReBa2Cu3O7-δ are relaxor ferroelectrics with a ferroelectric transition temperature that depends on the specific rare earth and a more or less constant superconducting transition temperature in the 90–100°K region. This result has important implicat...

Theoretical studies of shock-initiated detonations☆

Abstract The dynamics of shock propagation have been studied theoretically for a variety of two-dimensional lattices. The approach used is based on molecular dynamics and hinges on the exact numerical solution by computer of the equations of motion for the individual atoms or molecules in each lattice. Shocks have been launched into the lattices under study by methods designed to simulate flyer-plate impact. Two different interatomic potentials have been used, one endothermic and one net-exothermic. For both types of potential, a shock launched at one side of the lattice will spall a group of atoms off the other side. However, the subsequent behavior of the two types of lattice is very different. For endothermic potentials, after the initial atomic spall, the residual lattice is quiescent with little further activity. For net-exothermic potentials, the initial atomic spall injects additional energy into the system in such a manner that subsequently further spall occurs at both sides. Once this new spall is initiated, it leads rapidly to further bond breaking and explosive disintegration of the system.

Stress dependence of Tc for the incommensurate phase transition in K2SeO4

Abstract We have measured the uniaxial stress dependence of the frequency of the zone center A1 soft mode in the low temperature phase of K2SeO4. The frequency shows a strong decrease with stress. We infer from this a similar decrease of the incommensurate phase transition temperature Tc. This behavior is explained by detailed lattice dynamical calculations for K2SeO4 which reveal the unusual balance between Coulomb and short-range forces in this structure. Our calculations show that this balance is changed by stress in such a way that the quasi-harmonic normal mode frequencies harden drastically. Tc is reduced by this purely volume dependent effect. This unusual balance between Coulumb and short-range forces also provides a natural explanation for the occurence of an incommensurate phase transition in K2SeO4. We infer that similar transitions are likely in isomorphous structures.

Theoretical study of the magnetic ordering in rare-earth compounds with face-centered-cubic structure

We present a detailed theoretical study of the magnetic ordering in heavy rare-earth compounds with a face-centered-cubic structure. In addition to the exchange interactions which are counted up to the third nearest neighbors, the effect of the dipolar interactions and magnetic anisotropic effect are also included in our model Hamiltonian. The interactions parameters are obtained from first-principles band-structure calculations by fitting the total energies of different magnetic configurations to the Heisenberg Model. Thus from utilizing the Monte Carlo simulations, we explained the formation of different magnetic structures in the rare-earth compounds.

Polymorphous transformations in alkaline-earth silicates

Structural phase transitions in Ca2SiO4 and Sr2SiO4 are investigated by molecular dynamics simulations. The simulations are based on the potentials calculated from the Gordon–Kim modified electron gas formalism extended to molecular ions. We successfully reproduced the transition γ−αH′−α and β−αL′−αH′−α in Ca2SiO4, and the transition β−α′ in Sr2SiO4. We find that the αL′ phase of Ca2SiO4 is an a×3b×c superstructure of the αH′ phase, while the αH′ phase has a β-K2SO4 structure, and the α phase of Ca2SiO4 has a disordered structure with space group P63/mmc.

Determination of the origin of the 10.6‐μm absorption in CO2 laser window materials

Theoretical studies are presented which show that it should be possible to determine whether the absorption at 10. 6 μm in alkali halides is due to impurities or whether it is intrinsic. These studies show that the temperature dependence of the measured absorption should be very different for intrinsic multiphonon absorption as compared with defect‐activated absorption.