Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X38: Structural Phase Transitions |
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Sponsoring Units: DCMP Chair: Kyle Shen, Cornell University Room: F149 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X38.00001: Simulation of structural phase transition in two dimensional ionic crystal Dongsheng Zhang, Graziano Vernizzi, Monica Olvera de la Cruz We investigate the structure of a two-dimensional monovalent ionic crystal observed in cationic-anionic molecules adsorbed into surfaces by molecular dynamics simulations. The pair interaction between ions include a short-range Lennard-Jones term and a long-range electrostatic term. When the dielectric constant is small, electrostatic interactions dominate and the crystal form a regular square lattice. At large values of the dielectric constant the Lennard-Jones attraction dominates, and the crystal form a triangular lattice. We study the phase diagram of this model and the properties of the structural transition. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X38.00002: A Model for Strain Glass Turab Lookman, Romain Vasseur We demonstrate that a strain discrete ``pseudo-spin'' model for martensitic alloys predicts a glass phase in the presence of disorder, consistent with recent experiments on binary and ternary alloys that have established the existence of such a phase above a critical composition. We find that the glass phase, as characterized by the Edwards-Andersen order parameter, exists even in the absence of elastic long-range interactions which compete with the disorder to shift the glass transition to higher values of the disorder. This approach, using mean-field analysis and Monte Carlo simulations, may be generalized to the study of glassy behavior in more complex structural transformations in two and three dimensions. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X38.00003: Multi-scale study of martensite stability in Fe-based solid solutions Alexander Udyansky, Johann von Pezold, Alexey Dick , J\"{o}rg Neugebauer Martensitic phases refer to tetragonal states of interstitial solid solutions. We study such Fe-based dilute phases by combining atomistic modeling with the reciprocal space microscopic elasticity theory (MET), which allows us to account for long-ranged elastic interactions between impurities. The short-range chemical interactions, as well as the parameters entering the MET are obtained by density functional theory (DFT) calculations. This approach allowed us to compute temperature/interstitial concentration phase diagrams and provided a direct insight into the stability limits and formation mechanisms of martensite: specifically, tetragonal states are predicted to be preferred even at low impurity concentrations of carbon, nitrogen and oxygen due to a thermodynamically driven orientational ordering of the interstitials. Moreover, both the structural type of the tetragonal state and the transition temperature were found to sensitively depend on the local strain state of the system. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X38.00004: X-ray diffraction studies of high temperature structural evolution of multiferroic hexagonal HoMnO3 Haiyan Chen, Jianming Bai, Yuhao Wang, Trevor Tyson, S.-W. Cheong Multiferroic rare earth manganites RMnO3 have attracted great attention due to the coexistence of ferromagnetism and ferroelectricity plus the giant coupling between them. High temperature structural evolution of hexagonal RMnO3 is of great interest in the elucidation of ferroelectric properties of this class of materials. In this study, synchrotron powder X-ray diffraction has been used to investigate the phase transitions of hexagonal HoMnO3 from room temperature to 1400 K. Preliminary results have shown positive thermal expansion for lattice a and negative thermal expansion for lattice c. A phase transition from non-centrosymmetric to centro-symmetric structure was observed around 1250 K. Rietveld analysis of the XRD data to reveal more structural detail is in progress. This work is supported by DOE Grant DE-FG02-07ER46402 and NSF Instrumentation grant DMR MRI-0722730. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X38.00005: Neutron diffraction studies on crystal and magnetic structures of BiFeO$_{3}$ at high pressures Xiao-Jia Chen, Malcolm Guthrie, Russell J. Hemley, Ho-kwang Mao, Yu-Jie Wu, Xiao-Kun Chen, Jamie J. Molaison, Antonio F. Moreira Dos Santos, Christopher A. Tulk As one of the most prominent examples of multiferroic materials, bismuth ferrite BiFeO$_{3}$ shows both electric polarization below $T_{C}$= 1100 K and long-range magnetic ordering below $T_{N}$= 640 K. The coexistence of both electric and magnetic order parameters at room temperature is particularly interesting for technological applications. However, the physical and structural properties of BiFeO$_{3}$ under pressure are still a matter of debate in the literature. Here we report neutron diffraction data on BiFeO$_{3}$ at 294 K up to 10 GPa. The evolution of both the crystal structure and magnetic structure of this material with pressure has been well determined. Our results shed insight on the long standing controversy surrounding the structural transformations at 3 GPa and 7-10 GPa reported by different laboratories. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X38.00006: Embedded Binary Eutectic Alloy Nanostructures as Phase Change Materials D.C. Chrzan, S.J. Shin, J. Guzman, C.W. Yuan, C.Y. Liao, C.N. Boswell-Koller, P.R. Stone, O.D. Dubon, A.M. Minor, M. Watanabe, J.W. Beeman, K.M. Yu, J.W. Ager, III, E.E. Haller Phase change materials are essential components of both optical data storage and emerging static random access memory technologies. We suggest a new approach to development of phase change memory materials: embedded binary eutectic-alloy nanostructures. The approach exploits the unique properties binary eutectic-alloys that emerge when they are embedded within nanoscale voids. The equilibrium, as-grown, morphology of GeSn nanostructures within SiO$_2$ is a phase separated, bi-crystalline, bi-lobed state. The rapid cooling following pulsed laser melting stabilizes an amorphous, homogeneously mixed state. Subsequent annealing recrystallizes the bi-lobed state. Further, the composition of the alloy can be used to tune the recrystallization temperature over the range of temperatures between 150C and 500C. Thus these nanostructures display the requisite crystalline-amorphous-crystalline transition, and enable tuning of the relevant transformation temperatures. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X38.00007: 90 K X-ray structure determination and vibrational spectroscopic investigation of L-alanine alaninium nitrate, a homologue of diglycine nitrate Matthew Hudson, Damian Allis, Wayne Ouellette, Bruce Hudson The structure of the 2:1 amino acid salt L-alanine alaninium nitrate (LAAN) was determined at 90 K by X-ray diffraction. The vibrational spectrum of LAAN was measured at 25 K by inelastic neutron scattering (INS) spectroscopy and simulated using solid-state density functional theory (DFT). In LAAN, a feature observed at approximately 450 cm$^{-1 }$in the experimental spectrum is noticeably absent in the calculation based on the 90 K X-ray structure. Further investigation of LAAN by Raman spectroscopy reveals spectral differences between the 78 and 293 K spectra. The nature of the spectral changes in LAAN and the disagreement between the INS spectrum and simulation are discussed in relation to a structural change at low temperature which involves the movement of hydrogen-atom(s). This is considered in relationship to the homologous diglycine nitrate, a well characterized ferroelectric material with a T$_{c}$ = 206 K. doi: 10.1039/b905070a [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X38.00008: Observing electronically induced structural transformations using ultrafast electron crystallography Ramani K. Raman, Ryan A. Murdick, Yoshie Murooka, Zhensheng Tao, Tzong-Ru T. Han, Richard J. Worhatch, Subhendra D. Mahanti, Chong-Yu Ruan We have observed, using ultrafast electron crystallography, the photoinduced fragmentation of isolated Silver nanocrystals excited at the surface plasmon resonance (SPR) at fluences far below their melting / ablation threshold. The fragmentation process at such low fluences is attributed to the strong non-linear coupling of SPR with interband transitions that lead to the creation of local valence instabilities, whose growth and percolation leads to the eventual fragmentation of the nanocrystals. Similar electronically induced structural rearrangement is also seen in graphite, where a fs photoexcitation at 800nm creates transient interlayer \textit{sp}$^{3}$ bonds beyond a threshold fluence (Phys. Rev. Lett. 101, 077401 (2008)). In both graphite and silver, we have observed the creation of strong transient surface voltage along with ejection of photoelectrons from the surface (Appl. Phys. Lett. 95, 181108 (2009)), pointing to the dominant role of charges in driving structural transformations at surface and interfaces. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X38.00009: On the Role of Electronic Polarization in Continuous Structural Peter Riseborough Martensitic transitions are defined to be diffusionless structural transitions that lower the crystal symmetry and in which the order parameter, usually strain and shuffle, changes discontinuously. It had been proposed that martensitic transitions are driven by the entropy, due to the soft phonons. Anderson and Blount have shown that it is highly improbable that structural transitions are second-order (continuous) and they have suggested that the apparently continuous structural transition in V$_3$Si is a ferroelectric transition. Recently, a continuous structural martensitic transition has also been identified in AuCd strain glass by Wang {\it et al.} and in single crystal AuZn by Lashley {\it et al.}. Furthermore, pressure measurements on AuZn show that the martensitic temperature can be depressed and indicate the existence of a quantum critical point~[5]. The exact nature of apparently continuous structural transition is still being debated. However, measurements of magneto-acoustic oscillations in the speed of sound of AuZn, indicate that the phonon softening may be driven by the polarization of the conduction electrons. This has been confirmed by recent measurements on AuZn and V$_3$Si in magnetic fields which have shown that the transitions are intimately linked to the dielectric response, and are in accord with the predictions of Dieterich and Fulde for V$_3$Si. Various theoretical scenarios concerning continuous structural transitions are discussed. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X38.00010: Mapping the magneto-structural phases of magnetodielectric Mn$_{3}$O$_{4}$ Minjung Kim, X.M. Chen, P. Abbamonte, S.L. Cooper We report temperature-dependent x-ray diffraction and temperature- and field-dependent Raman scattering studies of single crystal samples of Mn$_{3}$O$_{4}$, which allow us to elucidate the microscopic origins of the magneto-dielectric and --elastic behaviors of this material.~ Among the key results:~ we find that the T$_{2g}$ Raman mode exhibits a clear splitting below T$\sim $33K, indicating a tetragonal-to-monoclinic structural change that coincides with the commensurate magnetic transition at T$_{2}$=33K; we also show evidence for the magnetic-field induced monoclinic distortions sensitive to magnetic field direction as well as magnitude, and provide evidence for a novel field-induced phase in the intermediate field regime of H$\vert \vert [1\bar {1}0]$. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X38.00011: High-temperature crystal structures and chemical modifications in RbH$_{2}$PO$_{4}$ Ronald J. Tackett, Heber Martinez, Russel R. Chianelli, Jianzhong Zhang, Yusheng Zhao, Cristian E. Botez We have used labortatory and synchrotron x-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline RbH$_{2}$PO$_{4}$ upon heating within the 30-250 $^{\circ}$C temperature regime. We report no evidence of the previously reported onset of partial polymerization at $T = 96 ^{\circ}$C which was proposed as an explanation for the observed high-temperature several-order-of-magnitude increase in proton conductivity observed in phosphate-based solid acids. Instead, we found that a tetragonal-to-monoclinic polymorphic transition initiates at $T \sim 90 ^{\circ}$C. The transition is complete at $T \sim 130 ^{\circ}$C, and the new monoclinic RbH$_{2}$PO$_{4}$ polymorph is stable upon further heating to $T = 200 ^{\circ}$C. Moreover, this reported monoclinic phase is {\it isomorphic} to that of monoclinic CsH$_{2}$PO$_{4}$. This remarkable similarity suggests that the microscopic structures and dynamics responsible for the high-temperature superprotonic behavior of RbH$_{2}$PO$_{4}$ could be the same as those of its Cs-based counterpart. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X38.00012: One-way-featured phase transition of confined oxygen (O$_{2}$) in porous materials with different geometry restrictions Hu Cao, Shah Valloppilly, Timothy Prisk, Paul Sokol The confinement effect of oxygen adsorbed in porous materials (MCM-48, MCM-41 and vycor) has been investigated by means of susceptibility measurements. A one-way-featured phase transition has been found at the phase transition of \textit{$\alpha $}$\to $\textit{$\beta $} on heating in MCM-41 and vycor. Our studies has clearly revealed that the confinement effect of oxygen confined in porous materials is dependent on the geometry restrictions: MCM-48 with ordered three-dimensionally (3-D) interconnected pore systems shows a complete bulk feature of oxygen, whereas MCM-41 with 1-D long nonintersecting arrays and vycor with randomly oriented 3-D connected pores show typical confinement effects of oxygen. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X38.00013: Competition of Structural Orderings in Presence of Magnetic Interactions Michael Kaplan, George Zimmerman The competition of two types of structural phase transitions in crystals with triple degenerate ground electronic states of transition metal ions is considered in the framework of the cooperative Jahn-Teller effect. The signs of the spontaneous deformations are different for the two orderings under discussion: the ferroelastic ordering corresponds to the elongation of all octahedrons in the same z- direction, while the XY-ordering of the octahedrons forms the tetragonal compression around the ions with orbital degeneracy. It is shown that the magnetic interactions orienting the magnetic moments in the z-direction (external magnetic fields or molecular magnetic fields) support the XY type of ordering. As a result of that when the two structural interactions are of the same order, it is possible that the initial ferroelastic ordering is switched to the XY-ordering as a result of a phase transition induced by the magnetic interactions. On another hand, if the magnetic interactions are stronger than both Jahn-Teller interactions responsible for the structural transitions, the transition following the magnetic one is of XY-type. This is what could explain the experimental situation in the MnV$_{2}$O$_{4}$ crystals. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X38.00014: Network Destabilizations with Inverted Mechanical Responses Zack Nicolaou, Adilson Motter A solid material can be regarded as a large mechanical network, with nodes representing the constituent particles and links representing interactions between nearby particles. Assuming the temperature is sufficiently low, the total force on each particle is nearly zero. In response to an applied force, the network rearranges itself so that the force exerted by the surface of the material counteracts the applied force. This reaction can be described as a flow of forces through the network. When the applied force changes, we expect a deformation to result in the direction of the change in applied force. In elastic materials, this behavior is guaranteed. However, when the assumptions of elasticity fail, as is often the case when the material undergoes finite strains, this intuition need not hold. We show that under a finite change in applied force, a stress-induced phase transition can occur which results in a deformation of the network from its initial configuration which is \em opposite \em the direction of the change in force. A potential applications is the design of new materials which \em expand under compression \em or \em contract under tension\em. [Preview Abstract] |
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