Session X16 - Structural Phase Transitions.
ORAL session, Thursday afternoon, March 15
Room 303, Washington State Convention Center

[X16.001] Correlated Electrons in Metallic Plutonium: Self-Consistent Dynamical Mean-Field Calculations.

We incorporate dynamical mean field theory (DMFT) into the electronic structure calculation to study anomalious temperature behavior of volume in metallic plutonium connected to its well-known 25transition from the low-temperature alpha phase to the 600K delta phase. We develop a fully integrated approach for estimates of total energies in materials with strong correlations which adjusts all quantities such as hoppings, Greens functions, charge densities, etc. self-consistently. It is implemented as a double loop. The first internal loop solves many-body DMFT equations with given hoppings, and the second external loop updates the hoppings. It will be demonstrated that DMFT, applied to this material results in significant improvements over the standard approaches based on local density or generalized gradient approximations which give theoretical volume of the delta phase 30predict artificial long-range magnetic order. We obtained results for the volumes of both delta and alpha phases of Pu, as well as the photoemission and inverse photoemission spectra.

[X16.002] Simultaneous SHG amp; XRD monitoring of HMX and TATB solid-solid phase transitions*

In our accompanying talk (TA:4.5) we presented energy dispersive XRD results on the temperature-induced phase transitions of pure HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and TATB (1,3,5-triamino-2,4,6-trinitrobenzene) powder samples. Here we present the first ever simultaneous measurements of solid-solid phase transitions using real-time second harmonic generation (SHG) and angle resolved XRD techniques. These experiments were conducted at the Stanford synchrotron radiation laboratory beam line 10-2. Material structures, as determined from the XRD data, can now be correlated to corresponding SHG signal intensities during the entire solid-solid phase transition. This work represents the first quantitative justification for using the exceptionally sensitive albeit qualitative SHG diagnostic tool for monitoring solid phase transitions involving noncentrosymmetric to centrosymmetric, or the reverse, symmetry changes. We observe that HMX SHG signal intensities increase while TATB SHG intensities decrease when they undergo small volume to large volume structural changes at 1bar and high-temperature. The SHG/XRD results permit calculation of thermal expansion coefficients and activation energies.

[X16.003] Dielectric and piezoelectric enhancement in the tetragonal side of MPB of PMN-PT

Detailed study of structural and physical properties of polycrystalline (1-x)PMN-xPT near morphotropic phase boundary (MPB) shows the remarkable enhacement of dielectric and piezoelectric constant in the tetragonal side of MPB. Our results of the shifts of x-ray Bragg peak position and intensity under electric field (E) indicates that 90 degree domain rotation is the main extrinsic contribution to additional strain in the tetragonal side of MPB.

[X16.004] Electrical Properties of Praseodymium Metal at Structural Phase Transitions Under High Pressure

Electrical properties of rare earth metal Praseodymium (Pr) were studied to high pressures of 40 GPa in a diamond anvil cell using designer anvils with embedded electrical microprobes. This designer diamond anvil technology allows us to carry out precise four probe electrical reistance measurements at high pressures. Pr metal is known to undergo four structural phase transitions below 25 GPa giving rise to five distict crystallographic phases(Chesnut and Vohra, Phys. Rev. B62, 2965 (2000)). We document distinct resistance drop at 5 GPa at the dhcp to fcc transition and another resistance drop at the formation of alpha uranium phase at 20 GPa. This last transition at 20 GPa is associated with a 10-17 attributed to 4f-shell delocalization under high pressure in Pr. Our electrical data thus indicates that at the 4f-delocaliztion, electrical resistance drop, which is likely due to increase in the density of states at the Fermi level accompanying delocalization.

[X16.005] Competing ferroelectric and structural transitions in SrTiO_3 at low temperatures

SrTiO_3 has a well known antiferrodistortive (AFD) cubic-to-tetragonal transition at 105 K, while the ferroelectric (FE) transition at lower T is believed to be solely prevented by zero-point quantum fluctuations. We present high resolution hyper Raman scattering data on the two soft polar modes obtained on oriented tetragonal SrTiO_3. The frequencies of these modes are shown to be affected by the AFD distortions and the associated strains. Our results confirm that the AFD and FE instabilities compete. The AFD strains are in fact essential in preventing the FE transition. Hence, in supported SrTiO_3 thin films, the FE transition can very well occur, which should be of importance in applications. Our very high resolution data on the FE soft modes also show no anomalies around 37 K. Hence, if a new effect at 37 K were confirmed, it could hardly be related with quantum paraelectricity.

[X16.006] Revisiting the TA-TO coupling of SrTiO_3 at low temperature with neutron scattering

The large number of anomalies that have been reported recently on SrTiO_3 prompted us to reinvestigate the lowest frequency TA modes with neutron scattering at small reduced wave vector \xi down to 0.01. We used crystals oriented in the tetragonal phase below 105 K. Results will be presented for the branches (2 0 \xi) and (\xi 0 2), were the third index refers to the c-axis. The (2 0 \xi) branch, corresponding to the softest TO mode E_u, behaves according to the TA-TO coupling described by Axe et al.^1. On simply rotating the same sample by 90 degrees, the results on the (\xi 0 2) branch are very different. An additional, very soft component appears below the normal TA. Scattering by the structural mode E_g, which should be nearly forbidden, is also clearly seen. A possible origin of these anomalies will be proposed.

^1 J.D. Axe et al Phys. Rev. B 1, 1227(1970)

[X16.007] ARPES investigation of the electronic structure of Titanium Diselenide

The band structure of TiSe2 near the Fermi Level was investigated with angle-resolved photoemission. The question regarding the indirect overlap of the Ti 3d and Se 4p bands was investigated both above and below the transition temperature. Also, the three dimensional nature of the system was explored, especially with regards to Se p-band folding in the charge density wave regime. Preliminary results indicate the phase transition is accompanied by a shift in the bands relative to the Fermi Level as well as a strong folding of the Selenium p bands.

[X16.008] Phase transition of CaSi_2 at high pressures and high temperatures

We have studied a pressure effect on alkaline-earth-metal disilicides because their structures have characteristic Si configurations. In situ x-ray diffraction measurements of BaSi_2 showed that the structure changes from orthorhombic to cubic, then to trigonal with increasing pressure. The cubic and the trigonal structures are the same as those of SrSi_2 and CaSi_2 at ambient conditions, respectively. Thus, the structures that appear at high pressure are the same as those at ambient conditions of the other alkaline-earth-metal disilicides with a smaller atomic number metal. This structural sequence is different from those known in elements and the other AB_2-type compounds such as dioxides of 14 group elements. For better understanding of the structural sequence, pressure experiments are necessary for the other alkaline-earth-metal disilicides. In this study, a pressure-temperature phase diagram of CaSi_2 is investigated by in situ x-ray diffraction measurements at pressures up to 10.6 GPa and temperatures from 290 to 1300 K. The in situ observation revealed that CaSi_2 has a low-temperature, high-pressure phase with a trigonal structure, and a high-temperature, high-pressure phase with a tetragonal structure. The results will be discussed in comparison with the results of BaSi_2. [1] M. Imai et al., Phys. Rev. B58, 11922 (1998).

[X16.009] Density Functional Calculations of Rubidium under Pressure

We will report on local density funcitonal calculations of the complex, high-pressure phases of solid Rubidium. Structural and electronic properties of solid Rb are calculated from 0 to 80 GPa at room temperature. The stablity of each individual phase throughout the appropriate pressure range will be studied. The calculated results will be compared with recent experimetal observations of the novel high\--pressure phases IV, V, and IV of solid Rb. We may also try to address the possibility of stable Phase III if our calculations allow us.

[X16.010] All-Orbital LDA+FLEX Theory of Ce

We perform an ab initio strongly-correlated calculation of structural and electronic properties of Cerium. The local-density approximation (LDA) employing linear muffin-tin orbitals (LMTO) in the atomic-sphere approximation (ASA) is used to obtain the band structure first. Constrained-LDA methods are also applied to extract the Coulomb-interaction parameters. The resulting second-quantized Hamiltonians have 16 orbitals (1 s, 3 p, 5 d, and 7 f). The full k dependence of the band structure and the on-site density-density Coulomb-interaction terms between the f orbitals (U_f, 7^4=2401 terms) are retained. The conserving fluctuation-exchange (FLEX) approximation accelerated by numerical renormalization-group (NRG) techniques is used to obtain the strongly-correlated solutions. All four fluctuation channels (charge, spin, singlet, and triplet) are included, and the k dependence of the self energy is not neglected. Results for the total energy, spectral, and magnetic properties as a function of volume will be presented and analyzed in light of the volume-collapse phenomenon.

[X16.011] Absolute reaction rate for hcp to omega phase transition in titanium.

The equilibrium phase diagram of titanium contains two different hexagonal phases: hexagonal-closed-packed (hcp) and the high-pressure omega phase.(S.~K.~Sikka, Y.~K.~Vohra, and R.~Chidambaram, Prog.~Mater.~Sci.~27, 245--310 (1982).) These two phases are nearly degenerate in first-principles structural calculations, with the omega phase predicted to be lower in energy than the experimental room-temperature hcp structure. The pressure induced phase transition from hcp to omega is known to be martensitic, but very little is known about the pathway. We construct several possible pathways, and calculate the energy barrier for each---using a tight-binding model(Michael~J.~Mehl and Dimitrios~A.~Papaconstantopoulos, Phys.~Rev.~B 54, 4519--30 (1996).) reoptimized for titanium---to find the most energetically favorable one. We then calculate the phonon spectrum at the saddle point to determine an absolute rate using transition state theory.(R.~Marcelin, Ann. Phys. 3, 120 (1915).)

[X16.012] The spin-Peierls transition in CuGe0_3

The lowering of the energy of spin chains when dimerized induces a spontaneous lattice distortion in CuGeO_3 below 14 K. Applying mean-field theory and using the polarization pattern of the Peierls-active phonon modes the coupling constants of the magnetic exchange to the linear atomic displacements are obtained. The resulting pressure, temperature, and dimerization dependence of the magnetic exchange is consistent with various experimental results.

Treating the spin-phonon coupling in RPA explains the absence of a softening of the Peierls-active phonon modes at the transition. Instead, a new magneto-elastic mode appears. The correlation length and a real space interpretation of the magneto-elastic excitation are discussed.

[X16.013] MD Simulation of Structural Transitions in Cristobalite Silica

Molecular dynamics (MD) simulations, based on our charge-transfer potential, were used to simulate the phase transformation between alpha- and beta-cristobalite.* These simulations reveal the detailed transformation mechanism, in particular spatial correlations between transformed entities during incubation and progression of the transition. The transformation characteristics differ, depending on whether the transition is induced thermally or mechanically. The negative thermal expansion coefficient of beta-cristobalite has intriguing implications on the reverse transformation and the melting behavior of this substance. A better understanding of the thermo-mechanical anomalies of crystalline and amorphous silica is derived from careful analysis of the simulated atomic trajectories.

* L. Duffrène and J. Kieffer, "Molecular Dynamic Simulations of the alpha-beta Phase Transition in Silica Cristobalite," J. Phys. Chem. Solids 59, 1025-1037 (1998)

[X16.014] Nucleation Using an Automated Lag-Time Apparatus (ALTA)

The evolution of non-equilibrium systems into equilibrium has been studied for many years. Although all systems reach equilibrium eventually, a unified theory charecterizing this evolution in time is still lacking. The Automated Lag-Time Apparatus (ALTA) was constructed to gather statistics of nucleation on a single sample. By employing the ergodic hypothesis, one sample used repeatedly 500 times provides an advantage over trying to prepare and monitor 500 individual identical samples. ALTA repeatedly supercools a sample to a set supercooling point, measures the time until the onset of nucleation, heats the sample for four minutes to ensure melting of all residual ice crystals prior to the next run, and repeats the above steps as many times as possible in order to gain statistics of liquid to solid nucleation.

Part X of program listing