Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free-energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behavior above the transition temperature.

The near-edge structure in energy-loss spectroscopy: many-electron and magnetic effects in transition metal nitrides and carbides

We investigate the ability of the local density approximation (LDA) in density functional theory to predict the near-edge structure in electron energy-loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find that anion K-edge threshold energies are systematically overestimated by 4.22 +/- 0.44 eV in twelve transition metal carbides and nitrides in the rock-salt (B1) structure. When we apply this 'universal' many-electron correction to energy-loss spectra calculated within the transition state approximation to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approach using a projected Mulliken density which honours the dipole selection rule, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignments of ther sublattice.

Energy levels and transition probabilities for nitrogen-like Fe xx

Energies of the 700 lowest levels in Fe XX have been obtained using the multiconfiguration Dirac-Fock method. Configuration interaction method on the basis set of transformed radial orbitals with variable parameters taking into account relativistic corrections in the Breit-Pauli approximation was used to crosscheck our presented results. Transition probabilities, oscillator and line strengths are presented for electric dipole (E1), electric quadrupole (E2) and magnetic dipole (M1) transitions among these levels. The total radiative transition probabilities from each level are also provided. Results are compared with data compiled by NIST and with other theoretical work.

Relativistic allowed and forbidden transition probabilities for fluorine-like Fe XVIII

Energy levels and the corresponding transition probabilities for allowed and forbidden transitions among the levels of the ground configuration and first 23 excited configurations of fluorine-like Fe XVIII have been calculated using the multiconfigurational Dirac-Fock GRASP code. A total of 379 lowest bound levels of Fe XVIII is presented, and the energy levels are identified in spectroscopic notations. Transition probabilities, oscillator strengths and line strengths for electric dipole (E1), electric quadrupole (E2) and magnetic dipole (M1) transitions among these 379 levels are also presented. The calculated energy levels and transition probabilities are compared with experimental data.

Dirac-Fock energy levels and transition probabilities for oxygen-like Fe XIX

Multiconfigurational Dirac-Fock calculations are reported for 656 energy levels and the 214 840 electric dipole (E I), electric quadrupole (E2) and magnetic dipole (M1) transition probabilities in oxygen-like Fe xix. The spectroscopic notations as well as the total transition probabilities from each energy level are provided. Good agreement is found with data compiled by NIST.

North European last glacial-interglacial transition (LGIT; 15-9 ka) tephrochronology: extended limits and new events

High-precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9?ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time-parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high-precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST-C2) indicating a more northeasterly distribution of this fan than reported previously.

An exactly solvable four transition point model

Heavy particle collisions, in particular low-energy ion-atom collisions, are amenable to semiclassical JWKB phase integral analysis in the complex plane of the internuclear separation. Analytic continuation in this plane requires due attention to the Stokes phenomenon which parametrizes the physical mechanisms of curve crossing, non-crossing, the hybrid Nikitin model, rotational coupling and predissociation. Complex transition points represent adiabatic degeneracies. In the case of two or more such points, the Stokes constants may only be completely determined by resort to the so-called comparison- equation method involving, in particular, parabolic cylinder functions or Whittaker functions and their strong-coupling asymptotics. In particular, the Nikitin model is a two transition-point one-double-pole problem in each half-plane corresponding to either ingoing or outgoing waves. When the four transition points are closely clustered, new techniques are required to determine Stokes constants. However, such investigations remain incomplete, A model problem is therefore solved exactly for scattering along a one-dimensional z-axis. The energy eigenvalue is b(2)-a(2) and the potential comprises -z(2)/2 (parabolic) and -a(2) + b(2)/2z(2) (centrifugal/centripetal) components. The square of the wavenumber has in the complex z-plane, four zeros each a transition point at z = +/-a +/- ib and has a double pole at z = 0. In cases (a) and (b), a and b are real and unitarity obtains. In case (a) the reflection and transition coefficients are parametrized by exponentials when a(2) + b(2) > 1/2. In case (b) they are parametrized by trigonometrics when a(2) + b(2) <1/2 and total reflection is achievable. In case (c) a and b are complex and in general unitarity is not achieved due to loss of flux to a continuum (O'Rourke and Crothers, 1992 Proc. R. Sec. 438 1). Nevertheless, case (c) coefficients reduce to (a) or (b) under appropriate limiting conditions. Setting z = ht, with h a real constant, an attempt is made to model a two-state collision problem modelled by a pair of coupled first-order impact parameter equations and an appropriate (T) over tilde-tau relation, where (T) over tilde is the Stueckelberg variable and tau is the reduced or scaled time. The attempt fails because (T) over tilde is an odd function of tau, which is unphysical in a real collision problem. However, it is pointed out that by applying the Kummer exponential model to each half-plane (O'Rourke and Crothers 1994 J. Phys. B: At. Mel. Opt. Phys. 27 2497) the current model is in effect extended to a collision problem with four transition points and a double pole in each half-plane. Moreover, the attempt in itself is not a complete failure since it is shown that the result is a perfect diabatic inelastic collision for a traceless Hamiltonian matrix, or at least when both diagonal elements are odd and the off-diagonal elements equal and even.

Equilibrium polymerization of cyclic carbonate oligomers. III. Chain branching and the gel transition

Ring-opening polymerization of cyclic polycarbonate oligomers, where monofunctional active sites act on difunctional monomers to produce an equilibrium distribution of rings and chains, leads to a "living polymer." Monte Carlo simulations [two-dimensional (2D) and three-dimensional (3D)] of the effects of single [J. Chem. Phys. 115, 3895 (2001)] and multiple active sites [J. Chem. Phys. 116, 7724 (2002)] are extended here to trifunctional active sites that lead to branching. Low concentrations of trifunctional particles c(3) reduce the degree of polymerization significantly in 2D, and higher concentrations (up to 32%) lead to further large changes in the phase diagram. Gel formation is observed at high total density and sizable c(3) as a continuous transition similar to percolation. Polymer and gel are much more stable in 3D than in 2D, and both the total density and the value of c(3) required to produce high molecular weight aggregates are reduced significantly. The degree of polymerization in high-density 3D systems is increased by the addition of trifunctional monomers and reduced slightly at low densities and low c(3). The presence of branching makes equilibrium states more sensitive (in 2D and 3D) to changes in temperature T. The stabilities of polymer and gel are enhanced by increasing T, and-for sufficiently high values of c(3)-there is a reversible polymer-gel transformation at a density-dependent floor temperature. (C) 2002 American Institute of Physics.

Current-induced transition in atomic-sized contacts of metallic alloys

We have measured conductance histograms of atomic point contacts made from the noble-transition-metal alloys CuNi, AgPd, and AuPt for a concentration ratio of 1:1. For all alloys these histograms at low-bias voltage (below 300 mV) resemble those of the noble metals, whereas at high bias (above 300 mV) they resemble those of the transition metals. We interpret this effect as a change in the composition of the point contact with bias voltage. We discuss possible explanations in terms of electromigration and differential diffusion induced by current heating.

Thickness independence of true phase transition temperatures in barium strontium titanate films

The functional properties of two types of barium strontium titanate (BST) thin film capacitor structures were studied: one set of structures was made using pulsed-laser deposition (PLD) and the other using chemical solution deposition. While initial observations on PLD films looking at the behavior of T-m (the temperature at which the maximum dielectric constant was observed) and T-c(*) (from Curie-Weiss analysis) suggested that the paraelectric-ferroelectric phase transition was progressively depressed in temperature as BST film thickness was reduced, further work suggested that this was not the case. Rather, it appears that the temperatures at which phase transitions occur in the thin films are independent of film thickness. Further, the fact that in many cases three transitions are observable, suggests that the sequence of symmetry transitions that occur in the thin films are the same as in bulk single crystals. This new observation could have implications for the validity of the theoretically produced thin film phase diagrams derived by Pertsev [Phys. Rev. Lett. 80, 1988 (1998)] and extended by Ban and Alpay [J. Appl. Phys. 91, 9288 (2002)]. In addition, the fact that T-m measured for virgin films does not correlate well with the inherent phase transition behavior, suggests that the use of T-m alone to infer information about the thermodynamics of thin film capacitor behavior, may not be sufficient. (C) 2004 American Institute of Physics.