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Title: The Structural Phase Transition in Deuterated Benzil, C 14D 10O 2

Abstract

Neutron inelastic scattering has been used to examine the structural phase transition in deuterated benzil C{sub 14}D{sub 10}O{sub 2}. The transition in benzil, in which the unit cell goes from a trigonal P3{sub 1}21 unit cell above T{sub c} to a cell doubled P2{sub 1} unit cell below T{sub c}, leads to the emergence of a Bragg peak at the M-point of the high temperature Brillouin zone. It has previously been suggested that the softening of a transverse optic phonon at the {lambda}-point leads to the triggering of an instability at the M-point causing the transition to occur. This suggestion has been investigated by measuring the phonon spectrum at the M-point for a range of temperatures above T{sub c} and the phonon dispersion relation along the {lambda}-M direction just above T{sub c}. It is found that the transverse acoustic phonon at the M-point is of lower energy than the {lambda}-point optic mode and has a softening with temperature as T approaches T{sub c} from above that is much faster than that of the {lambda}-point optic mode. This behavior is inconsistent with the view that the {lambda}-point mode is responsible for triggering the phase transition. Rather the structural phase transition inmore » benzil appears to be driven by a conventional soft TA mode at the M-point.« less

Authors:
 [1];  [1];  [2];  [2]
  1. Australian National University, Canberra, Australia
  2. ORNL
Publication Date:
Research Org.:
High Flux Isotope Reactor; Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
978163
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review B; Journal Volume: 73; Journal Issue: 13
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ACOUSTICS; BRAGG CURVE; BRILLOUIN ZONES; DISPERSION RELATIONS; INELASTIC SCATTERING; INSTABILITY; NEUTRONS; OPTICS; PHONONS

Citation Formats

Goosens, D. J., Welberry, T. R., Hagen, Mark E, and Fernandez-Baca, Jaime A. The Structural Phase Transition in Deuterated Benzil, C14D10O2. United States: N. p., 2006. Web. doi:10.1103/PhysRevB.73.134116.
Goosens, D. J., Welberry, T. R., Hagen, Mark E, & Fernandez-Baca, Jaime A. The Structural Phase Transition in Deuterated Benzil, C14D10O2. United States. doi:10.1103/PhysRevB.73.134116.
Goosens, D. J., Welberry, T. R., Hagen, Mark E, and Fernandez-Baca, Jaime A. Sun . "The Structural Phase Transition in Deuterated Benzil, C14D10O2". United States. doi:10.1103/PhysRevB.73.134116.
@article{osti_978163,
title = {The Structural Phase Transition in Deuterated Benzil, C14D10O2},
author = {Goosens, D. J. and Welberry, T. R. and Hagen, Mark E and Fernandez-Baca, Jaime A},
abstractNote = {Neutron inelastic scattering has been used to examine the structural phase transition in deuterated benzil C{sub 14}D{sub 10}O{sub 2}. The transition in benzil, in which the unit cell goes from a trigonal P3{sub 1}21 unit cell above T{sub c} to a cell doubled P2{sub 1} unit cell below T{sub c}, leads to the emergence of a Bragg peak at the M-point of the high temperature Brillouin zone. It has previously been suggested that the softening of a transverse optic phonon at the {lambda}-point leads to the triggering of an instability at the M-point causing the transition to occur. This suggestion has been investigated by measuring the phonon spectrum at the M-point for a range of temperatures above T{sub c} and the phonon dispersion relation along the {lambda}-M direction just above T{sub c}. It is found that the transverse acoustic phonon at the M-point is of lower energy than the {lambda}-point optic mode and has a softening with temperature as T approaches T{sub c} from above that is much faster than that of the {lambda}-point optic mode. This behavior is inconsistent with the view that the {lambda}-point mode is responsible for triggering the phase transition. Rather the structural phase transition in benzil appears to be driven by a conventional soft TA mode at the M-point.},
doi = {10.1103/PhysRevB.73.134116},
journal = {Physical Review B},
number = 13,
volume = 73,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • Neutron inelastic scattering has been used to examine the structural phase transition in deuterated benzil C{sub 14}D{sub 10}O{sub 2}. The transition in benzil, in which the unit cell goes from a trigonal P3{sub 1}21 unit cell above T{sub C} to a cell doubled P2{sub 1} unit cell below T{sub C}, leads to the emergence of a Bragg peak at the M-point of the high temperature Brillouin zone. It has previously been suggested that the softening of a transverse optic phonon at the {gamma}-point leads to the triggering of an instability at the M-point causing the transition to occur. This suggestionmore » has been investigated by measuring the phonon spectrum at the M-point for a range of temperatures above T{sub C} and the phonon dispersion relation along the {gamma}-M direction just above T{sub C}. It is found that the transverse acoustic phonon at the M-point is of lower energy than the {gamma}-point optic mode and has a softening with temperature as T approaches T{sub C} from above that is much faster than that of the {gamma}-point optic mode. This behavior is inconsistent with the view that the {gamma}-point mode is responsible for triggering the phase transition. Rather the structural phase transition in benzil appears to be driven by a conventional soft TA mode at the M-point.« less
  • Hydrothermal reactions of molybdenum-oxide precursors with polyalcohols in the presence of base yielded two series of mixed-valence oxomolybdenum clusters, the hexadecanuclear species [XH{sub 12}(Mo{sup VI}O{sub 3}){sub 4}Mo{sup V}{sub 12}O{sub 40}]{sup m{minus}}(X=Na{sup +}, m=7; X=2H{sup +}, m=6) and the superclusters [XH{sub n}Mo{sup VI}{sub 6}Mo{sup V}{sub 36}O{sub 109}(OCH{sub 2}){sub 3}CR{sub 7}]{sup m{minus}} (X=Na-(H{sub 2}O){sub 3}{sup +}, m=9, n=13; X=Na(H{sub 2}O){sub 3}{sup +}, m=7, n=15; X=MoO{sub 3}, m=9, n=14; X=MoO{sub 3}, m=10, n=13). In a representative synthesis for the hexadencanuclear class of materials, the hydrothermal reaction of a mixture of Na{sub 2}MoO{sub 4}{center_dot}2H{sub 2}O, MoO{sub 3}, Mo metal, and NH{sub 4}Cl produced (NH{submore » 4}){sub 7}[NaMo{sub 16}(OH){sub 12}O{sub 40}]{center_dot}4h{sub 2}O (1{center_dot}4H{sub 2}O) as red-orange crystals. The compound (Me{sub 3}NH){sub 4}K{sub 2}[H{sub 2}Mo{sub 16}(OH){sub 40}]{center_dot}8H{sub 2}O(2{center_dot}8H{sub 2}O(2{center_dot}8H{sub 2}O) was prepared in a similar fashion. The structure of the anion of 1 consists of an {epsilon}-Keggin core H{sub 12}Mo{sub 12}O{sub 40}, capped on four hexagonal faces by MoO{sub 3} units and encapsulating a Na{sup +} cation. The structure of the oxomolybdenum framework of 2 is essentially identical to that of 1; however, the central cavity is now occupied by 2H{sup +}.« less
  • We present {sup 29}Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl{sub 2}Si{sub 2}O{sub 8}-SrAl{sub 2}Si{sub 2}O{sub 8}, from which the composition dependence and coupling of order parameters describing Si-Al order and the triclinic-monoclinic displacive transition were determined. Spectra of SrAl{sub 2}Si{sub 2}O{sub 8} contain narrow peaks for the two crystallographic Si sites, plus additional peaks for Si having three and two Al nearest neighbors, indicating the presence of approximately 0.14 Al-O-Al linkages per formula unit and a value of {sigma} = 0.93 for the short-range order parameter. For the triclinic feldspar samples, short-rangemore » Si-Al order increases continuously with Sr content from {sigma} = 0.89(3) for CaAl{sub 2}Si{sub 2}O{sub 8} to 0.97(1) for Sr{sub 0.80}Ca{sub 0.20}Al{sub 2}Si{sub 2}O{sub 8} but then decreases discontinuously to 0.93(2) for the monoclinic samples, Sr{sub 0.85}Ca{sub 015}Al{sub 2}Si{sub 2}O{sub 8} to SrAl{sub 2}Si{sub 2}O{sub 8}. The variation of peak positions with composition is consistent with a structural phase transition near Sr{sub 0.85}Ca{sub 0.15}Al{sub 2}Si{sub 2}O{sub 8} from I1 to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the Tlmz crystallographic site, and its composition dependence gives an order-parameter critical exponent of {beta} - 0.49(2), indicating classical second-order behavior. 11 refs., 4 figs., 1 tab.« less
  • An alcoholysis exchange between tris(hydroxymethyl)ethane (THME-H{sub 3}) or tris(hydroxymethyl)propane (THMP-H{sub 3}) and group IV metal isopropoxides yields compounds of the general formula (THMR){sub 2}M{sub 4}(OCHMe{sub 2}){sub 10}[M = Ti (R = E, 1; P, 2); Zr (R = E, 3; P, 4)]. 1 and 2 are formed in toluene, at ambient glovebox temperatures, and adopt a typical fused-M{sub 3}O{sub 12} structure where each titanium atom is surrounded by six oxygens in a slightly distorted face-shared bioctahedral arrangement. All of the oxygens of the central core are from the THMR ligand, present as {mu}-O and {mu}{sub 3}-O oxygen bridges. Generation ofmore » 3 or 4 requires heating in toluene at reflux temperatures. The zirconium atoms of 3 possess an extremely distorted edge-shared bioctahedral geometry where the central core consists of a Zr{sub 4}O{sub 8} ring (eight oxygens: six from THME ligands and two from isopropoxide ligands). Each of the zirconium atoms is six-coordinated with four bridging oxygens and two terminal isopropoxide ligands. Spincast deposited films generated from toluene solutions of 1 and 3 indicate that increased uniformity of the films and decreased hydrolysis occur in comparison to the cases of Ti(OCHMe{sub 2}){sub 4}, 5, and [Zr(OCHMe{sub 2}){sub 4}{center_dot}HOCHMe{sub 2}]{sub 2}, 6, respectively.« less
  • K{sub 3}InF{sub 6} is synthesized by a sol-gel route starting from indium and potassium acetates dissolved in isopropanol in the stoichiometry 1:3, with trifluoroacetic acid as fluorinating agent. The crystal structures of the organic precursors were solved by X-ray diffraction methods on single crystals. Three organic compounds were isolated and identified: K{sub 2}InC{sub 10}O{sub 10}H{sub 6}F{sub 9}, K{sub 3}InC{sub 12}O{sub 14}H{sub 4}F{sub 18} and K{sub 3}InC{sub 12}O{sub 12}F{sub 18}. The first one, deficient in potassium in comparison with the initial stoichiometry, is unstable. In its crystal structure, acetate as well as trifluoroacetate anions are coordinated to the indium atom. Themore » two other precursors are obtained, respectively, by quick and slow evaporation of the solution. They correspond to the final organic compounds, which give K{sub 3}InF{sub 6} by decomposition at high temperature. The crystal structure of K{sub 3}InC{sub 12}O{sub 14}H{sub 4}F{sub 18} is characterized by complex anions [In(CF{sub 3}COO){sub 4}(OH{sub x}){sub 2}]{sup (5-2x)-} and isolated [CF{sub 3}COOH{sub 2-x}]{sup (x-1)-} molecules with x=2 or 1, surrounded by K{sup +} cations. The crystal structure of K{sub 3}InC{sub 12}O{sub 12}F{sub 18} is only constituted by complex anions [In(CF{sub 3}COO){sub 6}]{sup 3-} and K{sup +} cations. For all these compounds, potassium cations ensure only the electroneutrality of the structure. IR spectra of K{sub 2}InC{sub 10}O{sub 10}H{sub 6}F{sub 9} and K{sub 3}InC{sub 12}O{sub 12}F{sub 18} were also performed at room temperature on pulverized crystals.« less