Session P45: Structural and Ferroelectric Phase Transitions

Structural symmetry of Cd$_2$Re$_2$O$_7$ from nonlinear optics

Cd$_2$Re$_2$O$_7$, a superconducting metallic pyrochlore, undergoes a second-order structural phase transition at 200~K from a cubic to tetragonal lattice. Landau theory predicts that any second-order cubic-to-tetragonal phase transition must also possess an order parameter associated with broken inversion symmetry. By observing optical second-harmonic generation, we provide a direct demonstration that the 200~K transition in Cd$_2$Re$_2$O$_7$ involves broken inversion symmetry. Moreover, we have used the polarization dependence of SHG to refine the crystal structure. We find that the low-temperature crystal symmetry is that of the $F\overline{4}2m$ space group.   

X-ray absorption spectroscopy on the phase transition of Cd$_{2}$Re$_{2}$O$_{7}$

Pyrochlore Cd$_{2}$Re$_{2}$O$_{7}$ has received considerable attention and investigated extensively because of its manifold phase transitions accompanied by profound physical properties. In this study, the relations among the electronic structure, phase transition, and some physical properties of Cd$_{2}$Re$_{2}$O$_{7}$ single crystals have been investigated. We conducted the X-ray diffraction as well as Re $L_{2,3}$-edge extended X-ray absorption fine structure (EXAFS) at different temperatures to investigate the local structure of Cd$_{2}$Re$_{2}$O$_{7}$ and temperature-dependent O $K$-edge X-ray absorption near edge structure (XANES) to probe its electronic structure during the phase transition. We also performed the band structure calculations to understand the DOS near the Fermi level of Cd$_{2}$Re$_{2}$O$_{7}$ at different temperatures.   

Pressure effects on a low carrier density metal's infrared response

We have performed pressure and temperature dependent infrared studies of semi-metal bismuth. Along with other semi-metals such as graphite and antimony, bismuth is a material of much current interest due to the interesting properties resulting from an extremely low carrier density, small effective mass, and long mean free path. A novel optical setup has been developed to measure low temperature infrared responses at pressures of up to 17kbar. We observe massive changes in bismuth's optical and infrared conductivity as a function of temperature and pressure. We discuss the origin of these changes both in terms of simple band structure effects, as well as in terms of the correlation effects that are expected in this very low-carrier density metal.   

Ferroelectric phase transition in the incommensurate phase of the K$_{2}$SeO$_{4}$ crystal

It is shown that in the incommensurate (IC) phase of the K$_{2}$SeO$_{4}$ crystal a phase transition takes place to the ferroelectric IC phase, prior to the lock-in transition to the triple-period commensurate phase. Such a behavior is due to the Lorentz microscopic electric field, which is induced by the IC domains on approaching the lock-in transition temperature. The proposed behavior of the K$_{2}$SeO$_{4}$ crystal explains the second-harmonic generation, observed in the IC phase, near the lock-in transition. It explains also the observed drop in the crystal's elastic constant c55 near the lock-in transition, which corresponds to a significant decrease of the z-polarized sound velocity in the x-direction. Continuous increasing of the polarization in the ferroelectric IC phase manifests itself as a continuous decreasing of the sound velocity. In the same model one can explain also the observation of an overdamped Raman scattering in the z(xz)y geometry in the low temperature range of the IC phase existence. Dielectric properties of the ferroelectric crystals with artificially fabricated domains are discussed.   

Normal and superconducting state properties of the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system

Our previous experimental studies of the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system revealed that: (1) superconductivity and ferromagnetism from both end compounds were suppressed almost monotonically toward $x = 0.55$, (2) retention of the antiferroquadrupolar order phase in (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ to higher values of $x$ than in Pr(Os$_{1-x}$Ru$_{x}$)$_4$Sb$_{12}$, (3) two possible CEF energy level schemes in NdOs$_4$Sb$_{12}$, in which the ground state is either the $\Gamma_{6}$ doublet or $\Gamma_{8}^{(2)}$ quartet. Recent ultrasonic measurements in the NdOs$_4$Sb$_{12}$ sample ($x=1$) revealed softening of the C$_{44}$ mode, which indicated that the CEF ground state in this compound is more likely to be the $\Gamma_{8}^{(2)}$ quartet. The lattice parameter in (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ seems to increase slightly from PrOs$_4$Sb$_{12}$ ($x = 0$) toward NdOs$_4$Sb$_{12}$ ($x = 1$). The T-x and H-x phase diagrams related to superconductivity, ferromagnetism, antiferroquadrupolar order, and the CEF energy level scheme for the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system for $0 \le x \le 1$ will be discussed.   

Elastic properties of ferromagnetic heavy fermion system SmOs$_4$Sb$_{12}$

We report the elastic constants of the heavy fermion system SmOs$_4$Sb$_{12}$ by means of an ultrasonic measurement. A steep decrease associated with the ferromagnetic transition was observed at around 2 K in elastic constants C11, (C11-C12)/2 and C44. Furthermore a characteristic increase, possibly due to the ``rattling-motion'' was observed around 15 K in the elastic constants. The variation of the onset temperature and a degree of the increase as a function of the ultrasonic frequency is reasonably reproduced in terms of the Debye-type dispersion. The obtained parameters describing the rattling-motion such as a relaxation time, an activation energy and a mean square displacement will be discussed as compared with those of an isostructural compound PrOs$_4$Sb$_{12}$.   

High-Energy X-Ray Study of Short Range Order and Phase Transformations in Ti-V

Phase transformations, especially precipitation processes, are a key factor in alloy design. Understanding these processes in the framework of statistical thermodynamics requires knowledge about the atomic interaction potentials between the alloy constituents. Experimentally, these parameters can be accessed via the diffuse x-ray scattering caused by the configurational short range order and lattice distortions. We employ a bulk sensitive high energy technique to study both phenomena simultaneously in situ, probing macroscopic single crystals in transmission geometry. The data recorded by a 2D detector reveal Bragg reflections from the precipitates superimposed on the diffuse scattering of the matrix. We present a detailed study of bcc Ti-V, a typical titanium $\beta$-alloy. The diffuse scattering is mainly due to lattice distortions induced by the atomic size mismatch. Depending on the annealing temperature, growth and dissolution of hcp $\alpha$-Ti precipitates and minute fractions of TiC are observed. HRTEM experiments have been conducted to complement our results.   

Reversible Magnetostriction with Temperature in Single Crystal Tb$_{5}$Si$_{2.2}$Ge$_{1.8}$

The magnetostriction changes that accompany the phase transitions of single crystal Tb$_{5}$(Si$_{2.2}$Ge$_{1.8})$ have been investigated at temperatures between 20 K and 150 K, by measurements of the reversible component of the magnetostriction along the crystallographic ``a'' axis. Over this temperature range the shape and slope of the magnetostriction curves change, which are indicative of changes in the magnetic state, crystal structure and magnetic anisotropy. Results show a phase transition that occurs near 106 K (onset-completion range 100 - 116 K). The abrupt nature of the strain transition, its unusual hysteresis, and its temperature dependence appear to indicate a first order phase transition which can be activated by applied magnetic field or temperature. Magnetostriction measurements at temperatures below the transition region show a magnetostriction of small overall magnitude but with a high, temperature dependent anisotropy. Funded by USDoE-Office of Basic Energy Sciences   

Magnetic phase transition and spin dynamics in Li(Ni1-xFex)PO4

Elastic and inelastic neutron scattering techniques were used to study the magnetic phase transition and spin dynamics in pure and Fe substituted LiNiPO4 single crystals. Pure LiNiPO4 undergoes a first-order magnetic phase transition from a long- range ordered incommensurate phase to an antiferromagnetic ground state at TN = 20.8 K. With the substitution of Fe for Ni, the magnetic phase transition changes from first-order to second-order, and moreover, the long-range ordered incommensurate phase of pure LiNiPO4 between 20.8 K to 21.5 K was suppressed in the LiNi0.75Fe0.15PO4 sample. Inelastic neutron scattering revealed a ~2 meV energy gap and an anomalous “soft mode” in the spin wave dispersion curve along the [010] direction for pure LiNiPO4. For LiNi0.8Fe0.2PO4, however, the energy gap was reduced to 0.9 meV and the anomaly along the [010] direction reduced. The spin-wave dispersion curves were simulated using a Heisenberg Hamiltonian with Dzyaloshinski-Moriya interactions.   

Phonon Anomalies in the Martensitic Phase of Ni$_{2}$MnGa

Ni$_{2}$MnGa is a cubic ferromagnetic shape memory alloy exhibiting phonon anomalies as precursors to the Martensitic phase transformation. By cooling in a small magnetic field (1.5T) through its transformation temperature it is possible to obtain a single domain of the tetragonal Martensite phase. One can, therefore, measure the phonon dispersion curves in the Martensite phase and compare them to the high temperature cubic phase. The TA branch propagating along the [qq0], with polarization along [q-q0], shows an anomaly near the wavevector of the charge density wave (CDW) peak at q=0.425, which differs from q=0.33 observed in the cubic phase. Most interesting is a new acoustic-like branch emerging from the CDW peak. This will be discussed in relationship to phasons and amplitude modes observed in other incommensurate systems.   

Medium-range Structure of La$_{2-x}$Sr$_x$CuO$_4$ (0.0 $<$ $x$ $<$ 0.3) by Pulsed Neutron PDF Analysis

We studied the local and medium-range atomic structure in high- temperature superconductor (HTSC) La$_{2-x}$Sr$_x$CuO$_4$ (0.0 $<$ $x$ $<$ 0.3) by the pulsed neutron pair-density function (PDF) analyses. The measurement was made with the NPDF of LANSCE, LANL. Because of the high $Q$-resolution of the NPDF the PDF was determined up to 20 $nm$. The measured PDF was compared with that calculated for the average structure determined by the Rietveld analysis of the same data. We found that the measured PDF deviates from the calculated PDF in two different ways. For $x$ = 0.16 - 0.3 deviations were seen up to about 3 $nm$, strongly related to the oxygen in the CuO$_2$ plane displaced along the $c$-axis. These deviations reflect local fluctuations between the orthorhombic and tetragonal phases. A more interesting deviations were seen for $x$ = 0.04 - 0.16 up to 2 $nm$ for all compositions. The lengthscale of this local fluctuation corresponds to the in-plane coherence length, suggesting that the structure that supports HTSC may be different from the average structure, and the size of the local domains is not limited by the dopant concentration. Further implications are discussed.   

Inelastic neutron scattering study of YBaFe$_2$O$_5$

YBaFe$_2$O$_5$ belongs to a new class of oxides with the chemical formula $R$Ba$M_2$O$_5$ ($R=$~rare-earth, $M=$~transition metal), based on the perovskite structure with a doubled unit cell pyramids of five-coordinated $M$-sites. The $M$-site is mixed valent in the stoichiometric formula unit (with an average valence of +2.5). Therefore, charge and orbital ordering phenomena can exist on the $M$-site and be studied without introducing disorder. The charge ordered phase of YBaFe$_2$O$_5$ is unusual, since it does not satisfy the Anderson criterion (i.e. it is not the lowest energy electrostatic arrangement of charges), but rather orders into alternating chains of 2+/3+. This indicates that other interactions, such as electron-phonon coupling, are necessary to arrive at the chain structure. Here, we present the results of an inelastic neutron scattering study of polycrystalline YBaFe$_2$O$_5$. We find the spectrum of phonon and magnetic excitations are clearly modified at the charge- and magnetic ordering temperatures: T$_{CO}$ = 308~K and T$_N$ = 430~K, respectively.