Open Access. Powered by Scholars. Published by Universities.®

Physics Commons

Open Access. Powered by Scholars. Published by Universities.®

Articles 1 - 30 of 73

Full-Text Articles in Physics

Dielectric Properties And Maxwell-Wagner Relaxation Of Xompounds Acu3Ti4O12 (A=Ca,Bi2/3,Y2/3,La2/3), Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy Jan 2005

Dielectric Properties And Maxwell-Wagner Relaxation Of Xompounds Acu3Ti4O12 (A=Ca,Bi2/3,Y2/3,La2/3), Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

We have studied the frequency and temperature dependences of permittivity and impedance of the compounds ACu3Ti4O12 (A=Ca, Bi2/3, Y2/3, Las2/3) in the ranges of 10−1–106 Hz 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 ...


Phase Transition In Single Crystal Cs2Nb4O11, Jianjun Liu, E. P. Kharitonova, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy Jan 2005

Phase Transition In Single Crystal Cs2Nb4O11, Jianjun Liu, E. P. Kharitonova, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

We studied temperature dependence of complex capacitance, impedance, and polarized Raman spectra of single crystal Cs2Nb4O11. First, we observed a sharp λ-shaped peak at 165 °C in the complex capacitance, then found drastic changes in the Raman spectra in the same temperature range. Utilizing the pseudosymmetry search of structure space group, we attributed the observed anomalies to a structural change from the room temperature orthorhombic Pnn2 to another orthorhombic Imm2. We also measured room temperature polarized Raman spectra in different symmetries of normal vibrations and assigned high wavenumber Raman bands to the internal ...


Superionicity In Na3Po4: A Molecular Dynamics Simulation, Wei-Guo Yin, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy Jan 2004

Superionicity In Na3Po4: A Molecular Dynamics Simulation, Wei-Guo Yin, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

Fast ionic conduction in solid Na3PO4 is studied by use of molecular dynamics simulation based on the modified Lu-Hardy approach. We obtain reasonable agreement with experiment for the structural transition and diffusion of the sodium ions. All the sodium ions are found to contribute comparably to the high ionic conductivity. The results of the simulation are discussed in terms of the relative magnitude of the two proposed transport mechanisms: percolation and paddle-wheel. It appears to us that the percolation mechanism dominates the sodium diffusion.


Large Dielectric Constant And Maxwell-Wagner Relaxation In Bi2/3Cu3Ti4O12, Jianjun Liu, Chun-Gang Duan, Wei-Guo Yin, Wai-Ning Mei, R. W. Smith, John R. Hardy Jan 2004

Large Dielectric Constant And Maxwell-Wagner Relaxation In Bi2/3Cu3Ti4O12, Jianjun Liu, Chun-Gang Duan, Wei-Guo Yin, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

We studied frequency and temperature dependences of impedance, electric modulus, and dielectric permittivity of Bi2/3Cu3Ti4O12 in the ranges of 10−1–106 Hz and −150–200 °C, respectively. We first observed two electrical responses in the impedance and modulus formalisms. Then we detected a Debye-like relaxation in the permittivity formalism. Most interestingly, we found that the large dielectric constant of Bi2/3Cu3Ti4O12 is independent of the temperature and frequency below 150°C. The results are interpreted in terms of a two-layer model with conducting ...


Electronic Properties Of Nacdf3: A First-Principles Prediction, Chun-Gang Duan, Wai-Ning Mei, Jianjun Liu, Wei-Guo Yin, John R. Hardy, R. W. Smith, M. J. Mehl, L. L. Boyer Jan 2004

Electronic Properties Of Nacdf3: A First-Principles Prediction, Chun-Gang Duan, Wai-Ning Mei, Jianjun Liu, Wei-Guo Yin, John R. Hardy, R. W. Smith, M. J. Mehl, L. L. Boyer

John R. Hardy Papers

Based on first-principles total energy calculations, we predict that NaCdF3 could be formed in a ferroelectric crystal structure. Using a symmetry guided search with structure optimization, we found two ferroelectric structures, nearly degenerate in energy, competing for the ground state: a rhombohedral structure with space group R3c and an orthorhombic structure with space group Pna21. The energies of both structures are ≈60 meV lower than the sum of those of the constituents, NaF and CdF2, implying chemical stability.


Dielectric Permittivity And Electric Modulus In Bi2Ti4O11, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy Aug 2003

Dielectric Permittivity And Electric Modulus In Bi2Ti4O11, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

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 ...


Spectral Functions Of The Falicov-Kimball Model With Electronic Ferroelectricity, Wei-Guo Yin, Wai-Ning Mei, Chun-Gang Duan, Hai-Qing Lin, John R. Hardy Jan 2003

Spectral Functions Of The Falicov-Kimball Model With Electronic Ferroelectricity, Wei-Guo Yin, Wai-Ning Mei, Chun-Gang Duan, Hai-Qing Lin, John R. Hardy

John R. Hardy Papers

We calculate the angular resolved photoemission spectrum of the Falicov-Kimball model with electronic ferroelectricity where d- and f-electrons have different hoppings. In mix-valence regimes, the presence of strong scattering processes between df excitons and a hole, created by emission of an electron, leads to the formation of pseudospin polarons and novel electronic structures with bandwidth scaling with that of df excitons. Especially, in the two-dimensional case, we find that flat regions exist near the bottom of the quasiparticle band in a wide range of the d- and f-level energy difference.


Molecular Dynamics Simulation Of The Order-Disorder Phase Transition In Solid Nano2, Wei-Guo Yin, Chun-Gang Duan, Wai-Ning Mei, Jianjun Liu, R. W. Smith, John R. Hardy Jan 2003

Molecular Dynamics Simulation Of The Order-Disorder Phase Transition In Solid Nano2, Wei-Guo Yin, Chun-Gang Duan, Wai-Ning Mei, Jianjun Liu, R. W. Smith, John R. Hardy

John R. Hardy Papers

We present molecular dynamics simulations of solid NaNO2 using pair potentials with the rigid-ion model. The crystal potential surface is calculated by using an a priori method which integrates the ab initio calculations with the Gordon-Kim electron gas theory. This approach is carefully examined by using different population analysis methods and comparing the intermolecular interactions resulting from this approach with those from the ab initio Hartree-Fock calculations. Our numerics show that the ferroelectric-paraelectric phase transition in solid NaNO2 is triggered by rotation of the nitrite ions around the crystallographical c axis, in agreement with recent x-ray experiments [Gohda ...


Polymorphous Transformations In Alkaline-Earth Silicates, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy Mar 2002

Polymorphous Transformations In Alkaline-Earth Silicates, Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, John R. Hardy

John R. Hardy Papers

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 -α’H and β-α’L - α’H - α in Ca2SiO4, and the transition β-α’ in Sr2SiO4. We find that the α’L phase of Ca2SiO4 is an ax3b x c superstructure of the α’H phase, while the α’H phase has a β-K ...


Order-Disorder Phase Transitions In Kno2 , Csno2, And Tlno2 Crystals: A Molecular Dynamics Study, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, Jianjun Liu, M. M. Ossowski, John R. Hardy Jan 2001

Order-Disorder Phase Transitions In Kno2 , Csno2, And Tlno2 Crystals: A Molecular Dynamics Study, Chun-Gang Duan, Wai-Ning Mei, R. W. Smith, Jianjun Liu, M. M. Ossowski, John R. Hardy

John R. Hardy Papers

The order-disorder phase transitions of KNO2 , CsNO2, and TlNO2 have been studied using parameter-free molecular dynamics simulation. It is found that the phase transitions in nitrites investigated are driven by the rotations of the NO2- ions about different axes together with displacements of cations and anions. We successfully reproduce the high-temperature phases of these nitrites, i.e., the NaCl-like structure for KNO2 and CsCl-like structure for Cs(Tl)NO2. Based on the investigation of the radial distribution function of the cations and anions, we explain why KNO2 and Cs(Tl)NO2 form ...


Molecular-Dynamics Study Of Phase Transitions In Alkali Thiocyanates, M. M. Ossowski, John R. Hardy, R. W. Smith Aug 2000

Molecular-Dynamics Study Of Phase Transitions In Alkali Thiocyanates, M. M. Ossowski, John R. Hardy, R. W. Smith

John R. Hardy Papers

An account is presented of our studies of the order-disorder phase transitions in KSCN, RbSCN, and CsSCN. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions. We performed static structural relaxations and supercell molecular dynamics and predicted with reasonable accuracy the temperatures for the onset of the transitions. In particular, we address the question of how the SCN- ions disorder to yield subsequent structural transformations. We found high-temperature phases of average Fm3m symmetry for both KSCN and RbSCN. We argue that in reality the full appearance of these ...


Molecular-Dynamics Study Of Phase Transitions In Alkali Azides, M. M. Ossowski, John R. Hardy, R. W. Smith Dec 1999

Molecular-Dynamics Study Of Phase Transitions In Alkali Azides, M. M. Ossowski, John R. Hardy, R. W. Smith

John R. Hardy Papers

An account is presented of our studies of the order-disorder phase transitions in KN3, RbN3, and CsN3. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron-gas formalism extended to molecular ions. We performed static structural relaxations and supercell molecular dynamics and predicted with reasonable accuracy the temperatures for the onset of the transitions. In particular, we address the question of how the N3- ions reorient to yield the transitions. We found the existence of NaCl-type high-temperature phases in disordered KN3 and RbN3 and argue that this restructuring is preempted by ...


Centimeter-Wave Reflection In The Nitrates And Nitrites Of Sodium And Potassium: Experiment And Theory, Steven G. Cornelison, Arthur Guass, Jr., J. Krane, John R. Hardy Feb 1997

Centimeter-Wave Reflection In The Nitrates And Nitrites Of Sodium And Potassium: Experiment And Theory, Steven G. Cornelison, Arthur Guass, Jr., J. Krane, John R. Hardy

John R. Hardy Papers

Temperature-dependent centimeter-wave reflection is studied in powdered samples of potassium nitrate (KNO3), potassium nitrite (KNO2), sodium nitrate (NaNO3), and sodium nitrite (NaNO2). Temperature-dependent reflection measurements at centimeter-wave frequencies were performed on an HP8510B Network analyzer based reflectometer. These measurements are compared to calculations utilizing a Debye relaxation model. Reflection losses seen in KNO2 and NaNO2 are expected to be due to the presence of permanent dipoles that are excited to ‘‘hopping’’ modes as the temperature is raised. Although NaNO3 shows little reflection losses, KNO3 shows significant losses as the temperature is raised ...


Raman-Scattering Study Of The Order-Disorder Phase Transition In Kscn, R. Li, John R. Hardy Jan 1997

Raman-Scattering Study Of The Order-Disorder Phase Transition In Kscn, R. Li, John R. Hardy

John R. Hardy Papers

Raman measurements were performed from room temperature to above the phase transition at 413 K on KSCN crystals. The internal mode spectra reveal a sideband above the internal CN stretching vibration which shows softening in frequency approaching the phase transition, while the CN mode shifts toward higher frequencies. Given that the sideband is a two-phonon process, involving an off-zone-center CN stretching mode and a lattice phonon, the substantial softening in its frequency shift indicates the possibility of an off-zone-center lattice instability related to the order-disorder phase transition. An anomalous temperature dependence was also found for the intensity of both the ...


Donald J. Montgomery, Sitaram Jaswal, John Hardy, Paul M. Parker Jan 1996

Donald J. Montgomery, Sitaram Jaswal, John Hardy, Paul M. Parker

John R. Hardy Papers

Donald J. Montgomery, long a research professor at Michigan State University, died suddenly from a stroke on 19 January at the age of 78.


Lattice- And Molecular-Dynamics Studies Of Rbliso4, V. Katkanant, John R. Hardy Jan 1995

Lattice- And Molecular-Dynamics Studies Of Rbliso4, V. Katkanant, John R. Hardy

John R. Hardy Papers

Using our previously developed method for calculating parameter-free potential-energy surfaces for ionic molecular crystals, specifically sulfates, we study phase transitions in RbLiSO4 by means of lattice and molecular dynamics. We found that the high-temperature phase I (Pnam) is highly unstable and transforms to the observed lower temperature phase VI (P1121/n) at about 475–525 K. Compared with isomorphous CsLiSO4, there are more branches of unstable modes in the Pnam phase for RbLiSO4. The maximum instability of these modes occurs away from the zone center, q=(0.118a*,0,0), which implies that a high-order incommensurate ...


Molecular-Dynamics Simulations Of Some Baxf4 Compounds, John Flocken, Z. Mo, Wai-Ning Mei, John R. Hardy, Dorian Hatch Jan 1994

Molecular-Dynamics Simulations Of Some Baxf4 Compounds, John Flocken, Z. Mo, Wai-Ning Mei, John R. Hardy, Dorian Hatch

John R. Hardy Papers

We have carried out molecular-dynamics simulations on BaXF4 compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A21am space group reported experimentally. We were able to reverse polarization in BaMgF4 at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of ...


Arnold M. Karo, John R. Creighton, John R. Hiskes, Fredrick Mcmurphy, John R. Hardy Sep 1992

Arnold M. Karo, John R. Creighton, John R. Hiskes, Fredrick Mcmurphy, John R. Hardy

John R. Hardy Papers

Arnold M. Karo died on 16 June 1991 at age 63, after a yearlong battle with leukemia. He was a theoretical chemist and solid-state physicist with the chemistry and materials science department at Lawrence Livermore National Laboratory.


Raman Scattering And Lattice-Dynamical Calculations Of Crystalline Kno3, D. Liu, F. G. Ullman, John R. Hardy Jan 1992

Raman Scattering And Lattice-Dynamical Calculations Of Crystalline Kno3, D. Liu, F. G. Ullman, John R. Hardy

John R. Hardy Papers

The Raman spectrum of a KNO3 single crystal was measured at both room and liquid-nitrogen temperatures. Lattice-dynamical calculations, based on the rigid-ion approximation and empirical potentials, were performed. The possibility of a phase transition at 217 K was investigated by measuring the temperature dependence of the Raman spectrum.


Lattice- And Molecular-Dynamics Studies Of Phase Transitions In Csliso4, V. Katkanant Jan 1992

Lattice- And Molecular-Dynamics Studies Of Phase Transitions In Csliso4, V. Katkanant

John R. Hardy Papers

We report results of a simulation of the phase transitions in CsLiSO4. These are based on our previously developed method for calculating parameter-free potential-energy surface for ionic molecular crystals. Our lattice-dynamical and molecular-dynamics studies show that the roomtemperature (Pnam) phase is unstable and transforms to the observed low-temperature (P1121/n) phase over approximately 200–280 K. The unstable modes of the Pnam phase have maximum instability at the zone center, which indicates a possible phase transformation without a cell multiplication. The rotational ordering of tetrahedral SOM4 2- was found to be the driving mechanism of these ...


Raman Scattering And Lattice-Dynamical Calculations Of Crystalline Kno3, H. M. Lu Jan 1992

Raman Scattering And Lattice-Dynamical Calculations Of Crystalline Kno3, H. M. Lu

John R. Hardy Papers

The Raman spectrum of a KNO3 single crystal was measured at both room and liquid-nitrogen temperatures. Lattice-dynamical calculations, based on the rigid-ion approximation and empirical potentials, were performed. The possibility of a phase transition at 217 K was investigated by measuring the temperature dependence of the Raman spectrum.


Simulations Of Phase Transitions In Rb2Zncl4, H. M. Lu, John R. Hardy Jan 1992

Simulations Of Phase Transitions In Rb2Zncl4, H. M. Lu, John R. Hardy

John R. Hardy Papers

Structural relaxations, molecular-dynamics simulations, and lattice-dynamics calculations were performed to study the phase transitions in Rb2ZnCl4, using intermolecular and intramolecular potentials generated from ab initio quantum-chemistry calculations for the whole molecular ion ZnCl4 2-. Compared with an earlier treatment of the system by a polarizable-ion model, the present approach emphasizes the static effect of the electron covalency within the molecular ions that affects strongly both the intermolecular and intramolecular interactions. The calculations gave a close agreement with experiment on the static structures of the Pnam and the Pna21 phases and the transition temperature from ...


Fractal Character Of Two-Dimensional Fluid Mixing At Both Continuum And Atomic Levels, H. Z. Cao, John Hardy, R. W. Douglass, P. T. Dawkins, Steven Dunbar Jan 1992

Fractal Character Of Two-Dimensional Fluid Mixing At Both Continuum And Atomic Levels, H. Z. Cao, John Hardy, R. W. Douglass, P. T. Dawkins, Steven Dunbar

John R. Hardy Papers

Studies in two dimensions have been made of the evolution of an initially linear interface between two liquid areas by both continuum hydrodynamics and molecular dynamics. It was found that at both levels the interface evolves into a fractal curve. While the limiting fractal dimensionalities (D) of the continuum and molecular interfaces differ, this difference is no greater than that that can be induced in the hydrodynamic results by marked changes in the initial conditions and/or fluid parameters. There is, however, a marked qualitative difference in the initial values of D. This is discussed and shown to be a ...


Phase Transitions In K2Zncl4, H. M. Lu, John R. Hardy Jan 1992

Phase Transitions In K2Zncl4, H. M. Lu, John R. Hardy

John R. Hardy Papers

It is shown that the potential-energy surface in K2ZnCl4 contains a double-well structure, very similar to some of the other A2BX4 compounds (e.g., K2SeO4, Rb2ZnCl4), except that the double well is much deeper and broader, giving rise to a highly disordered high-temperature phase as observed experimentally. A lattice-dynamics study of the Pna21 structure shows an instability with the wave vector q=0.5b*+(0.5±δ)c*, providing an explanation to the incommensurate phase transition reported recently.


First-Principles Study Of The Lattice Dynamics Of K2So4, D. Liu, H. M. Lu, F. G. Ullman, John R. Hardy Jan 1991

First-Principles Study Of The Lattice Dynamics Of K2So4, D. Liu, H. M. Lu, F. G. Ullman, John R. Hardy

John R. Hardy Papers

Using a newly developed first-principles approach to simulations of ionic molecular crystals, we performed static relaxation, molecular-dynamics simulation, and lattice-dynamics calculations, and measurements of the Raman spectrum, for the Pnam structure of K2SO4. It was found that the structure does not have the zone-center instability present in isomorphous K2SeO42 found in an earlier study. This difference between the two systems is attributed to the different charge distributions in the molecular ions. The calculated Raman-active zone-center frequencies for the Pnam structure of K2SO4 were found to be in general agreement with the ...


Raman Scattering And Lattice-Dynamical Calculations Of Alkali-Metal Sulfates, D. Liu, H. M. Lu, John R. Hardy, F. G. Ullman Jan 1991

Raman Scattering And Lattice-Dynamical Calculations Of Alkali-Metal Sulfates, D. Liu, H. M. Lu, John R. Hardy, F. G. Ullman

John R. Hardy Papers

Raman-scattering measurements on single crystals of K2SO4, Rb2SO4, and Cs2SO4 have been made at both room and liquid-nitrogen temperatures. Lattice-dynamical calculations, based on a rigid-ion model using the Gordon-Kim method to calculate the short-range potentials, were performed. The influence of the alkali-metal ions on the lattice-dynamical properties of the crystals is discussed.


First-Principles Study Of Phase Transitions In Kno3, H. M. Lu, John R. Hardy Jan 1991

First-Principles Study Of Phase Transitions In Kno3, H. M. Lu, John R. Hardy

John R. Hardy Papers

We report a first-principles simulation study of phase transitions in KNO3, using our recently developed method for treating ionic molecular solids. With the interionic potentials calculated from ab initio electron charge densities of the ions, our structural static relaxation gave close fits to both the normal room-temperature (α-phase) and the ferroelectric (γ-phase) structures. Our supercell molecular-dynamics calculations closely simulated the transitions from the α phase and γ phase to the high-temperature disordered β phase, and successfully reproduced the abnormally large c-axis thermal expansion observed in experiment. Both transitions were found to be initiated by the rotations of the nitrate ...


Ab Initio Studies Of The Phase Transitions In K2Seo4, H. M. Lu, John R. Hardy Nov 1990

Ab Initio Studies Of The Phase Transitions In K2Seo4, H. M. Lu, John R. Hardy

John R. Hardy Papers

An ab initio model is developed for the potentials in ionic molecular solids in which the electron covalency within the molecular ions substantially affects the interionic interactions. By treating the intermolecular and intramolecular interactions on the basis of the true electron charge densities of the molecular ions, this new model leads to an accurate parameter-free description of the potentialenergy surfaces for such crystals. We performed first-principles static structural relaxation, supercell molecular-dynamics simulation, and lattice-dynamics studies for the room-temperature paraelectric phase and the lower-temperature ferroelectric superstructure of K2SeO4 and predicted with good accuracy the transition from the former ...


First-Principles Simulations Of Ionic Molecular Solids: The Phase Transitions In K2Seo4, H. M. Lu, John R. Hardy Feb 1990

First-Principles Simulations Of Ionic Molecular Solids: The Phase Transitions In K2Seo4, H. M. Lu, John R. Hardy

John R. Hardy Papers

We present a new approach to first-principles simulations of the statics and dynamics of ionic molecular crystals. It is shown that the new method gives very realistic simulations of the phase transitions in K2SeO4 and that a double-well type of structure in the potential-energy surface is the driving mechanism of these phase transitions.


Dielectric Response Spectrum Of A Damped One-Dimensional Double-Well Oscillator, J. W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer Dec 1989

Dielectric Response Spectrum Of A Damped One-Dimensional Double-Well Oscillator, J. W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer

John R. Hardy Papers

The characteristic features of the dielectric response spectrum associated with the condensation of a "soft" phonon mode during a ferroelectric phase transition can be reproduced by an oscillator moving in a damped linear double-well potential. The behavior of the response function below the transition temperature Tc can be simulated by introducing a linear mean-field coupling which destroys the symmetry of the well. Potentials derived from physically realistic parameters are shown to result in dielectric responses which are in order-of-magnitude agreement with experimental values.