Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D20: Focus Session: Ferroelectric and Other Oxides |
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Sponsoring Units: DMP Chair: C. Stephen Hellberg, Naval Research Laboratory Room: Colorado Convention Center 105 |
Monday, March 5, 2007 2:30PM - 2:42PM |
D20.00001: Electric Field Gradient Comparisons in Perovskites Dandan Mao, Eric J. Walter, Henry Krakauer Piezoelectric and dielectric properties of Pb-based complex ferroelectric alloys with the A(B'B''B''')O$_3$ perovskite structure depend on composition and the local ordering of the B-site cations. A prototypical example is Pb(Sc$_{1/2}$Ta$_{1/2}$)O$_3$ (PST), which is a normal ferroelectric when the B atoms are ordered and becomes a relaxor when they are disordered. Electric field gradients (EFG) are sensitive to variations in local structure, and they can be probed using high-field nuclear magnetic resonance experiments. Calculations of EFGs are presented using the LAPW method within the local density approximation. We examine trends in B-site EFGs as a function of composition and order in PST, Pb(Zr$_{1-x}$Ti$_{x}$)O$_3$ (PZT), Pb(Sc$_{2/3}$W$_{1/3}$)O$_3$ (PSW), and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ (PMN). We discuss these results in terms of B- and Pb-atom off-centerings, and B-atom ordering. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D20.00002: Comparative study of the properties of BaTiO$_3$ and PbTiO$_3$ using DFT (LDA, GGA), HF and hybrid (B3LYP) functionals D. I. Bilc, P. Hermet, Ph. Ghosez, J. Iniguez The study of ferroelectrics (FE) requires to be able to reproduce accurately not only the structure but also in some cases their energy gap $E_g$. DFT underestimates $E_g$ of a typical FE by a factor of about two, while hybrid Hartree Fock (HF)-DFT functionals such as B3LYP are known to give improved values for $E_g$. Therefore, we performed B3LYP and HF study of BaTiO$_3$ and PbTiO$_3$ and compared our results to those of LDA and GGA. For the cubic phase, B3LYP gives very good agreement with the experimental lattice constants and $E_g$. For the tetragonal FE phase, if the atomic positions are relaxed at the experimental lattice constants then LDA, GGA and B3LYP give comparable results for the atomic distortions. However, full relaxation of the tetragonal phase using B3LYP gives a supertetragonality comparable to GGA. We performed hybrid calculations for different values of the three Becke's parameters and found that the supertetragonality is introduced by the non-local part of the Becke GGA exchange. This suggests that a hybrid functional generated from a better GGA$^1$ might provide simultaneously better structural and electronic properties of FE. 1. Z. G. Wu, and R. E. Cohen, PRB {\bf 73}, 235116, (2006). [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D20.00003: Nuclear Magnetic Resonance Chemical Shielding Calculations of Bulk Oxides Daniel Pechkis, Eric J. Walter, Henry Krakauer We will present calculations of nuclear magnetic resonance (NMR) chemical shielding in oxides, modeled using embedded clusters. NMR spectroscopy is an important probe of local structure in disordered materials, such as ferroelectric perovskite solid solutions. Combined with electric field gradient (EFG) calculations, a complete interpretation of high magnetic field NMR spectra is possible in principle. Determination of NMR parameters within the embedded cluster method allows the study of both periodic and disordered systems. Moreover, this approach can take advantage of mature chemical shielding methods found in standard quantum chemistry electronic structure packages at several levels of theory, including different forms of density functionals as well as more correlated approaches. Results will be presented for several materials including ferroelectric perovskites. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D20.00004: Structure and Antiferroelectric Properties of Cesium Niobate, Cs$_{2}$Nb$_{4}$O$_{11}$ Jianjun Liu, Wai-Ning Mei, Robert W. Smith, Chunhua Hu, Kuan-Jiuh Lin We determined the crystal structure of Cs$_{2}$Nb$_{4}$O$_{11}$ by using single crystal X-ray diffraction. The structural refinements at both 100 and 297 K show it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. We also studied the electric-field dependence of the polarization and observed double hysteresis loops which we interpret as the manifestation of antiferroelectricity. We then elucidated the origin of the antiferroelectricity by using symmetry analysis of the structural space groups. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D20.00005: First-principles study of polarization and piezoelectric properties of Zn$_{1-x}$Mg$_x$O Andrei Malashevich, David Vanderbilt Wurtzite ZnO can be substituted with up to $\sim$30\% MgO to form a metastable Zn$_{1-x}$Mg$_x$O alloy while still retaining the wurtzite structure. Because this alloy has a larger band gap than pure ZnO, Zn$_{1-x}$Mg$_x$O/ZnO quantum wells and superlattices have been much studied recently as promising candidates for applications in optoelectronic and electronic devices. Here, we report the results of an {\it ab-initio} study of the spontaneous polarization of Zn$_{1-x} $Mg$_x$O alloys as a function of their composition. We perform calculations of the crystal structure based on density-functional theory in the local-density approximation, and the polarization is calculated using the Berry-phase approach. We decompose the changes in polarization into purely electronic, lattice-displacement mediated, and strain mediated components, and quantify the relative importance of these contributions. We consider both free-stress and epitaxial-strain elastic boundary conditions, and show that our results can be fairly well reproduced by a simple model in which the piezoelectric response of pure ZnO is used to estimate the polarization change of the Zn$_{1-x}$Mg$_x$O alloy induced by epitaxial strain. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D20.00006: Ferroelectricity in CsPbF$_3$ Marco Fornari We discuss the structural and electronic features of ABX$_3$ halides with perovskitic structure. In particular we analyze the instabilities that drive CsPbF$_3$ to a rhombohedral phase and study the effect of halogen substitutions. The properties of CsPbF$_3$ are considered from the point of view of ferroelectricity and compared with prototypical oxides. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D20.00007: Enhanced Piezoelectricity from Polarization Rotation in Perovskites Invited Speaker: R.E. Cohen Theoretical and experimental studies show that the large electromechanical response of PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN)-PbTiO$_3$ (PT) and related perovskite materials is due to the ease of rotating the polarization with an electric field applied oblique to the polarization. The applied field rotates the polarization from a rhombohedral phase towards tetragonal through monoclinic. Underlying the large coupling is the large c/a strain (about 6\%) of ferroelectric PT. The best materials, including the most commonly used piezoelectric material, PZT (PbZrO$_3$ (PZ)-PT) tend to be solid solutions with PT. The other endmember is typically a relaxor, such as PMN or PZN. Is the relaxor behavior crucial to obtaining large coupling transducer materials? First-principles total energy and linear response computations for PT show the surprising prediction of a pressure induced morphotropic phase transition from tetragonal to monoclinic to rhombohedral, and finally cubic. In the transition regions the dielectric constant and piezoelectric constants become very large, in fact larger than those of the new single crystal piezoelectrics PMN-PT and relatives. This shows that large strain piezoelectricity in solutions with PT is due to the behavior of PT itself, and the other components simply tune the transition to zero pressure. This suggests that the key to finding new materials is in finding new pure compounds with pressure induced morphotropic phase transitions. First-principles and multiscale simulations will be discussed for relaxor ferroelectrics. This work was done in collaboration with A. Asthagiri, Y. Lei, M. Sepliarsky., Z. Wu and X. Zeng. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D20.00008: Theoretical study of ferroelectric KNO$_3$ Oswaldo Dieguez, David Vanderbilt We present a detailed study of the structural behavior and polarization reversal mechanism in phase III of KNO$_3$, an unusual ferroelectric material in which the nitrate groups rotate during polarization reversal. This work extends a preliminary study presented as an example in our earlier paper on the mapping of the energy ($E$) versus polarization ($P$) in insulators.\footnote{O.~Di\'eguez and D.~Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 96}, 056401 (2006).} Here we analyze in detail a two-parameter model in which the energy of the system is written as a low-order expansion in the polarization and in the nitrate group orientation. Apart from confirming that this model reproduces very well the first-principles results for KNO$_3$ presented in Ref.~[1], we construct its parameter-space phase diagram, and in particular we describe regions of parameter space in which the $E(P)$ curves have an unusual triple-well structure. We also present first-principles calculations of KNO$_3$ under pressure, finding that as the material is compressed its $E(P)$ curves change character, going from having continuous to having discontinuous first derivatives at zero polarization. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D20.00009: Distortions in charge-ordered LuFe$_2$O$_4$ Takeshi Nishimatsu, David Vanderbilt, Karin M. Rabe, Yoichi Horibe, Sang-Wook Cheong, Claude Ederer Experimental evidence\footnote{Nature {\bf 436}, 1136 (2005)} suggests that LuFe$_2$O$_4$ develops ferroelectricity via a novel charge-ordering mechanism in which the Fe sites (of average valence 2.5) disproportionate into Fe$^{2+}$ and Fe$^{3+}$ sublattices in such a way as to break inversion symmetry. However, the precise nature of the charge and magnetic order, and the structural distortions that accompany them, remain poorly understood. With this motivation, we have undertaken a first-principles study of LuFe$_2$O$_4$ using DFT-based methods (especially LDA+U). We search for the ground state consistent with a variety of supercell choices and symmetry constraints, and thereby investigate whether the system is unstable to several possible charge and spin orderings. Despite the limitations of a DFT-based approach, it is argued that the results provide useful guidance in the quest to develop an understanding of the novel form of ferroelectricity displayed by this material. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D20.00010: High-pressure x-ray diffraction of PbTiO$_{3 }$at low temperature Muhtar Ahart, Maddury Somayazulu, Ronald Cohen, Russell Hemley We combined the angular and energy dispersive x-ray diffraction methods to investigate the structural behaviors of PbTiO$_{3}$ (PT) in a diamond anvil cell (pressure up to 23 GPa) at 10 K. The energy dispersive x-ray diffraction results show drastic change in Bragg peak intensities at 16 and 20 GPa which indicate that lead titanate undergoes successive phase transitions with pressure. The results of angular dispersive x-ray diffraction indicate that the lattice parameters a and c decrease with pressure and crossover between 10 and 11 GPa. Pressure induced phase transitions at low temperature are reversible. The experimental results confirm theoretical calculations, including the predicted the phase diagram. This work is supported by the ONR under the contract number N000140210506 and the Carnegie/Department of Energy Alliance Center (CDAC) (DF-FC03N00144). [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D20.00011: Phase diagram of PZT solid solutions near the morphotropic phase boundary from first principles I. Kornev, L. Bellaiche, P.-E. Janolin , B. Dkhil , E. Suard A first-principles-derived scheme, that incorporates ferroelectric and antiferrodistortive degrees of freedom, is developed to study finite-temperature properties of Pb(Zr$_{1-x} $Ti$_{x}$)O$_3$ solid solutions near its morphotropic phase boundary [1]. The use of this numerical technique (i) resolves controversies about the monoclinic ground-state for some Ti compositions, (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points, that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions. [1] Igor A. Kornev, L. Bellaiche, P.-E. Janolin, B. Dkhil, and E. Suard, {\em Phys. Rev. Lett.} {\bf 97}, 157601 (2006) This work is supported by ONR grants N00014-04-1-0413, N00014-01-1-0600 and N00014-01-1-0365, by NSF grant DMR- 0404335, and by DOE grant DE-FG02-05ER46188. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D20.00012: Dynamic fluctuations and static speckle in critical X-ray scattering from SrTiO$_{3}$ Martin Holt, Mark Sutton, Paul Zschack, Hawoong Hong, T.-C. Chiang We report a study of critical x-ray scattering from SrTiO$_{3}$ near the antiferrodistortive structural phase transition at $T_C \approx 105\mbox{K}$. A lineshape analysis of the thermal diffuse scattering results in the most precise experimental determination to date of the critical exponent $\gamma =1.38\pm 0.08$. The microscopic mechanism behind the anomalous ``central peak'' critical scattering component is clarified here by the first-ever observation of a static coherent diffraction pattern (speckle pattern) within the anomalous critical scattering of SrTiO$_{3}$. This observation allows us to directly attribute the origins of the central peak to Bragg diffraction from remnant static disorder above $T_C$. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D20.00013: Theoretical investigation of phase transitions in hafnia Xuhui Luo, Alexander A. Demkov Transition metal (TM) oxides find applications in ceramics, catalysis and semiconductor technology. In particular, hafnium dioxide or hafnia will succeed silica as a gate dielectric in advanced CMOS devices. However, the thermodynamics properties of thin TM oxide films are not well understood, despite their technological importance. We study theoretically phase transitions in hafnia using density functional theory. We find that the cubic phase of hafnia transforms without a barrier into a tetragonal phase \textit{via} a soft-phonon mode. The direct calculation of the cubic phase phonon dispersion confirms the existence of a zone edge soft mode in the cubic phase. Using the nudged elastic band method (NEBM) we find a barrierless transition path between the cubic and tetragonal phases which coincides with the same soft-mode path. In addition we identify the pathway for the tetragonal to monoclinic phase transition, and find a 0.045 eV/mol barrier. We construct the effective Hamiltonian for zone center distortions correct to the fourth order including the strain renormalization. The energy surface found correctly explains the symmetry of the phase structure observed at low temperature. We find that there are two sides to the phase transition driving forces. First there are local distortion modes; secondly there is strong coupling between the local modes. The coupling determines the transition temperature. We calculate the coupling and estimate the 3-D cubic to tetragonal transition temperature in fair agreement with experiment. [Preview Abstract] |
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