Session P20 - Structural Phase Transitions.
MIXED session, Wednesday afternoon, March 22
208A, MCC

[P20.001] Kinetics of Ordering in a Long-Period Superstructure Alloy Cu_0.79Pd_0.21

The Cu-Pd alloys are a classic long-period superstructure (LPS) system. We have used x-ray scattering to examine the equilibrium structure and ordering kinetics of a Cu_0.79Pd_0.21 single crystal sample. The equilibrium structure of the disordered phase exhibits a four-fold splitting symmetry indicative of antiphase correlations. The peak splitting distance decreases with decreasing temperature. From the temperature dependence of the peak width, a mean-field instability temperature is extrapolated at approximately 20^oC below the phase transition temperature. Following a rapid quench of the disordered sample into the LPS regime, the peak splitting initially undershoots the equilibrium value and then increases back to it slowly. In addition, the central superlattice peak grows more slowly than do the satellite peaks. Our interpretation of this behavior is that initially the modulated structures form with a variety of antiphase domain walls; the equilibrium low-energy antiphase domain walls dominate only at a later time.

[P20.002] Spectroscopic study of CaVO_3-x as a function of O content

We have measured the soft x-ray absorption spectra of stoichiometric, oxygen-deficient, and oxygen-excess CaVO_3-x for x from 0.5 to - 0.125. The polycrystalline samples were prepared by solid state reaction, with their oxygen content determined by weighing the reactants before preparation and confirmed by mass measurement after the reaction. All samples were characterized by measurement of electrical resistivity and magnetic susceptibility, which both showed clearly the influence of the oxygen stoichiometry. In addition, x-ray diffraction revealed that within the perovskite lattice structure of CaVO_3 the local V environment changes dramatically with oxygen content. These changes are reflected in prominent changes in the x-ray absorption spectra of the V L_2,3 edge as well as the Ca L_2,3 edge and the O K edge, indicating that the structural changes are associated with clear and systematic modifications of the electronic properties.

[P20.003] Structural studies of charge dispoportionation and magnetic order in CaFeO3

The contrasting structural, magnetic and electronic behavior of the high spin d^4 transition metal perovskites LaMnO_3, SrFeO_3, CaFeO_3 and SrRuO_3 have long been of interest to condensed matter scientists. Among these materials, CaFeO_3 is the less studied and, in order to be profitably compared with the other ones, several aspects on its crystal and magnetic structure need to be investigated. We addressed these questions by synchrotron x-ray and neutron powder diffraction experiments at 300 K and 15 K. At 300 K, CaFeO_3 belongs to s.g. Pbnm, a = 5.32630(4)Åb = 5.35270(4) Åc = 7.53986(6) ÅBelow 290 K two types of Fe sites develop lowering the symmetry to monoclinic. At 15 K (crystallographic s. g. P2_1/n, a = 5.31182(3) Åb = 5.34775(4) Åc = 7.52058 Åand \beta= 90.065^o) the data can be fit equally well by a screw spiral structure or by a sinusoidal amplitude-modulated structure. The magnetic moments at the two Fe sites are 2.5 and 3.5 \mu_B for the spiral structure and maximum amplitudes of 3.5 and 5.0 \mu_B for the sinusoidal structure.

[P20.004] Direct observation of structural transition in Y hydrides by time resolved x-ray diffraction

We have observed the evolution of structural transition in Y hydrides (YHx, x=0-3) using time-resolved x-ray diffraction. Y films capped with thin Pd layers were prepared under UHV. The chamber was then back filled with hydrogen to stimulate the transition. Pressure was carefully controlled to allow observation of intermediate stages. Upon H loading the films transformed from alpha-phase (Y-hcp) to beta-phase (dihydride-fcc), then to gamma-phase (trihydride-hex). The lattice expansion in alpha-phase and contraction in beta-phase were clearly observed. Gamma-phase films could be transformed back to beta-phase reversibly by annealing to 130C. Optical transmission was measured simultaneously and to monitor the metal-insulator transition between beta- and gamma-phase. Our results are to be compared with earlier results on transmission [1] and x-ray. [2] [1] J.N. Huiberts et al., Nature 380, 231 (1996). [2] A. Remholf et al., Phys. Rev. B 56, R1 (1997).

[P20.005] Theory and Simulation of Phase Transitions in Plutonium

We have investigated the nature of structural phase transitions in elemental plutonium, focusing on the role of a novel, self-induced Anderson localization [1]. Our picture conceptually explains the unusual sequential phase transitions and anomalously low melting point. The basic premise of the model is the existence of an entropy-driven transition from a uniformly delocalized phase to one characterized by a disordered distribution of two kinds of atomic sites associated, with equal probability, with either localized or delocalized 5f electrons. This locally fluctuating site behavior is due to the singlet 5f component of the two electron atomic ground state, which is randomly driven by hybridization to itinerancy. The significance of two electron dynamics and the existence of the Anderson localization were established earlier in uranium-based systems. The applicability of this picture to plutonium is examined with an LDA-LMTO approach. Progress in this direction, and in predicting relevant material properties from ab-initio computation will be presented. [1] B. R. Cooper et al, Phil. Mag. B 79, 683 (1999).

[P20.006] Ultrafast Electronic Disorder in Heat-induced Structural Phase Transitions in Metal

Heat-induced structural phase transitions of noble metals silver and gold are studied using 1.5 eV, 120 fs ultrashort laser pulses. The dynamics of heating process is monitored by measuring the time evolution of the dielectric constants following optical excitation, which shows behavior consistent with that reported previously. Fluence-dependent dielectric constants in silver and gold show similarities with the photon-energy-dependent dielectric constants, which in fact indirectly map out the degree of the heat-induced electronic disorder in noble metals. The results demonstrate, for the first time, that the heat-induced structural phase transition in metal is in fact tied to the electronic disorder and band structure collapse.

[P20.007] Structural Phase Transition of Aluminum Induced by Electronic Excitation

The dynamics of a structural phase transition induced by direct interband electronic excitation in aluminum is studied, for the first time, by determining the time evolution of the dielectric constant through the measurement of the transient reflectivity induced by an 800-nm ultrashort (\sim 120 fs) laser pump pulse. The threshold fluence for this transition is found to be significantly less than the value necessary for ultrafast heat-induced melting indicating that this phase change is caused by band structure collapse and lattice instability resulting from strong electronic excitation, which is fundamentally different from the traditional heat-induced melting.

[P20.008] Simulation of superionic phase transition in Na_3PO_4

The superionic phase transition in Na_3PO_4 has been studied by using parameter-free Gordon-Kim potentials. With the interionic potentials calculated from ab initio electron charge densities of the ions, we performed first-principles static structural relaxation and supercell molecular-dynamics simulation. We found superionic phase transition in Na_3PO_4 which is related to the orientational disorder of phosphate ions and positional disorder of sodium ions. The phosphate ions show partial disorder below the phase transition temperature T_c. The sodium ion sublattice remains intact until the phosphate ions are completely disordered at T_c.

[P20.009] Ab initio molecular dynamics simulations of melting in the NaSn alloy

The NaSn alloy shows a phase transition at 756K to a high-temperature rotationally disordered solid. A model involving rapid reorientations of the Sn_4 tetrahedra present in the solid together with jump migrations of the Na cations has been proposed. We report ab initio molecular dynamics simulations of the two-step melting of NaSn, which give insight into the detailed behavior of the atoms, as well as accompanying changes in the electronic properties. Several simulation temperatures have been considered in order to distinguish and clarify the features of each distinct step in the melting process: low-temperature, high-temperature solid and liquid. The effect of the sudden volume change at the phase transition from the ordered to the disordered solid will be discussed. Acknowledgements: Work supported by Junta de Castilla y Leon (VA 28-99). MJS acknowledges support of NSERC of Canada.

[P20.010] Effect of isovalent and heterovalent Yb-site substitutions on the valence transition in YbInCu_4

YbInCu_4 undergoes a first-order isostructural valence transition at T_v=42 K from a high-temperature Yb^+3 state to a low-temperature intermediate valence state. Not surprisingly, diluting the Yb site with non-magnetic and isovalent Y or Lu causes the valence transition to move to lower temperature. Isovalent, magnetic substitutions (e.g., Gd or Tb) have a similar effect. The extent to which the transition remains first order as a function of doping depends on the size of the substituted ion. However, when either Ca^+2 or Th^+4 is substituted for Yb, the valence transition temperature increases with doping. The dramatic difference between isovalent and heterovalent dopants emphasizes the importance of conduction electron density in stabilizing the valence transition in YbInCu_4.

[P20.011] Landau theory of tetragonal-orthorhombic ferroelastics

A Landau expansion of the elastic energy in the strains is used to study 2D structures and their time evolution in tetragonal-orthorhombic ferroelastics with boundary constraints. A complex energy surface with many bulk metastable states results purely from boundary conditions, without bulk inhomogeneities (such as impurities) of any sort. In the static limit, some settings require twin walls in only one set of tetragonal 110-type planes and only two variants appear. Other settings yield both 110 and 1\bar11 walls and so all four variants appear. To study the time dependence of the structures, we derive general equations of motion which include three viscosity coefficients. We describe the motion of domain walls, including their collisions. Understanding of the structures requires study of the local rotation; an example is the alternate narrowing and broadening of variants which occurs at the collision of walls with the orthogonal family of walls.

Part P of program listing