Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P38: Relaxor Ferroelectrics and Piezoelectrics |
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Sponsoring Units: DCMP Chair: Nicola Spaldin, University of California, Santa Barbara Room: Morial Convention Center 230 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P38.00001: Structure and Energetics of Bi(Zn,Ti)O$_{3}$ Tingting Qi, Ilya Grinberg, Andrew Rappe Lead-free piezoelectric materials are drawing more and more attention during recent years due to the environmental issues. Bi based materials are considered as quite promising alternative to Pb due to the 6s$^{2}$ ``inert pair'' electron configuration. Experiments have proved that BZT is an good analogue of PT with a high c/a ratio and a large cations' displacements with respect to oxygen cage centers. However, there has been no theoretical examination of this material. We apply DFT calculations for different B-cation arrangements of BZT' using a 40-atom supercell. The large supercell allows local structure information to be extracted from our computational results. We find extremely large ($\sim $1 {\AA}) Bi displacements as well as $\sim $0.5 {\AA} B-site displacements. Due to the low tolerance factor, large octahedral tilts are also present. The combination of large displacements and large octahedral tilts is in contrast to the Pb-based materials, where these structural motifs are mutually exclusive. The large ionic displacements give rise to a high polarization in the BZT material. This should lead to an extremely high Curie transition temperature (T$_{c})$, as the T$_{c}$ is proportional to the square of the polarization. Examination of the relaxed structures also shows that the ionic displacements from their ideal positions are inline with the Bond Valence Theory. Surprisingly, comparing the different B-cation arrangements, we find that rocksalt structure is not the most preferred contrary to the prediction of electrostatic model of atomic ordering. Although the rocksalt arrangement has a lower and favored Ewald energy, it's deviation from the ideal bond valence is high. This means that the energy of covalent bonding in the rocksalt structure is smaller than in the other structures. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P38.00002: Search for New High Performance Piezoelectrics P. Ganesh, Ronald E. Cohen In an effort to design new high performance piezoelectric materials, we have performed first-principles calculations to study the energetics of several ABO$_{3}$ type materials, with the `A' site being occupied by two types of atoms. The motivation comes from recent findings of a morphotropic-phase boundary, which gives rise to large electromechanical coupling, at high pressure and low temperatures in pure PbTiO$_{3}$[1]. This prompted us to substitute the `A' site with a smaller atom compared to Pb to apply ``chemical pressure'' and tune the morphotropic-phase boundary to lower pressures. We have discovered (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$, (Pb$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ and (Sn$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ to be promising new piezoelectric materials. (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$ shows lower energy monoclinic phases (space groups Cm with polarization along [xxz] and Pm with polarization along [x0z]) compared to the layered tetragonal phase (space group P4mm with polarization along [001]) while the remaining two compounds have a rhombohedral (space group R3m with polarization along [111]) ground-state compared to the tetragonal phase in the rock-salt pattern (space group I4mm with polarization along [001]).~Our results also suggest ease of polarization rotation and large electromechanical strain. [1] Zhigang Wu and Ronald E. Cohen, PRL, \textbf{95}, 037601, 2005 [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P38.00003: High-pressure x-ray diffraction study of Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-32{\%}PbTiO$_{3}$. Muhtar Ahart, Ronald E. Cohen, Russell J. Hemley Motivated to determine and understand PMN-32{\%}PT's behavior under pressure, we employed the angular dispersive x-ray diffraction methods (Advanced Photon Source, Argonne National Laboratory) to investigate PMN-32{\%}PT in a diamond anvil cell up to 15 GPa. The x-ray diffraction results show changes in Bragg peaks at 4 GPa which indicate that PMN-32{\%}PT undergoes a ferroelectric rhombohedral to a paraelectric cubic phase transition. In addition, we investigated the pressure dependence of domain structure of PMN-32{\%}PT up to 10 GPa; rhombohedral domains decrease with pressure and disappear above 4 GPa. These results are qualitatively consistent with earlier Raman study of B. Chaabane, {\{}Phys. Rev. B 70, 134114, 2004{\}}. We suggest a phase diagram for PMN-PT system which is slightly different from the one predicted by B. Chaabane et al. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P38.00004: Aging mechanisms in field cooled PMN-PT 12{\%} Matthew Delgado, Eugene Colla, Michael Weissman In the relaxor ferroelectric PMN-PT 12{\%} ((PbMg$_{1/3}$Nb$_{2/3}$O$_{3})_{0.88}$ (PbTiO$_{3})_{0.12})$ the dielectric susceptibility ($\varepsilon \prime $, $\varepsilon \prime \prime )$ and pyroelectric current I$_{P}$ were measured after different field-temperature histories. As previously reported [1], when the sample was cooled in zero field, standard spinglass-like aging of $\varepsilon \prime \prime $ was found in the glassy relaxor regime. Cooling in a DC electric field of 0.67 kV/cm drove a transition into a polarized ferroelectric-like state, which retained its polarization after the field was removed at low temperature. During 10 hours of subsequent zero-field aging at 160K, $\varepsilon \prime \prime $ showed almost no change. This suggests that the mechanisms responsible for glassy aging in the relaxor state are absent in the ferroelectric state. [1] L. Chao, et. al. \textbf{PRB} 74, 014105 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P38.00005: Hyper Piezoelectric Nano-Electromechanical Systems Dustin Kreft, Robert Blick, Seung-Hyub Baek, Chang-Beom Eom, V. Vaithyanathan, Darrell Schlom, Vladimir Aksyuk Piezoelectric materials are extremely important for applications in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS). Here, we will be presenting PMN-PT (Lead Magnesium Niobate-Lead Titanate) on SOI as a hyper piezoelectric material for NEMS devices. The main advantages are the extremely high piezoelectric coefficient, higher electromechanical coupling, and larger attainable bandwidth. This will lead to enhanced agility of NEMS/MEMS devices and lower energy consumption. The work will also have considerable impact for nanoscale mechanics. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P38.00006: Zone Boundary Soft Modes in Relaxor PMN Peter Gehring, Ian Swainson, Chris Stock, Guangyong Xu, Haosu Luo The lattice dynamics of PbMg$_{1/3}$Nb$_{2/3}$O$_3$, a prototypical relaxor, have been studied using neutron TOF techniques at the NIST Center for Neutron Research. Unusual ``columns'' of inelastic scattering are seen at the M and R-point zone boundaries at 300\,K that extend from 5\,meV to the elastic line. The columns weaken substantially upon heating to 600\,K, suggesting the existence of soft, zone boundary modes. This would imply a dynamical origin to the superlattice peaks previously observed via TEM and x-ray diffraction techniques. Preliminary neutron structure factor calculations indicate that the corresponding ionic displacements involve the Mg/Nb and Pb atoms. This picture is consistent with x-ray studies according to which the superlattice peaks result from $<$110$>$ correlated, anti- parallel Pb displacements [1]. The potential relationship between the columns and superlattice peaks is intriguing because the temperature dependence of the M-point superlattice peak tracks that of the soft, zone center mode [2], which is associated with the development of short-range ferroelectric correlations. [1] A. Tkachuk and H. Chen, AIP Conf. Proc., 677, p. 55 (2003); [2] P. M. Gehring, S. Wakimoto, Z.-G. Ye, and G. Shirane, Phys. Rev. Lett. 87, 277601 (2001). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P38.00007: Polar nanoregions and phase instability in relaxors Guangyong Xu, Jinsheng Wen, Christopher Stock, Peter Gehring Local polar clusters called polar nano-regions (PNR) are believed to play important roles in the electronic properties of relaxor ferroelectrics. Recent work has shown that the presence of PNRs in relaxor systems could also be related to the high piezoelectric response in these materials. Diffuse scattering studies on PNRs in PMN-xPT and PZN-xPT systems will be presented and their electric field response will be discussed. Our results suggest that there is an intrinsic phase instability in relaxor systems induced by the PNRs, which makes it easier for an (electric field) induced strain to develop. Possible scenarios for the further enhancement of the piezoelectric response near the morphotropic phase boundary (MPB) will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P38.00008: First-Principles Investigations of Pb Anti-Site Defects in PbZrO$_3$ and Pb(Zr, Ti)O$_3$ Perovskites Ricardo Kagimura, David J. Singh Lead zirconate (PZ) and lead zirconate titanate (PZT) have the perovskite type structure, ABO$_3$. Bivalent lead (Pb$^{+2}$) ions occupy the A site, while tetravalent titanium and zirconium (Zr$^{+4}$, Ti$^{+4}$) ions occupy the B site at random of the PZT solid solution. Also, lead can be tetravalent (Pb$^{+4}$), such as in PbO$_2$ structure. Recent experimental work has reported that tetravalent Pb ions can locate at the B site of the PZT perovskite forming a lead zirconate-titanate-plumbate solid solution. The experimental results suggest that, based on a PbZrO$_3$-PbTiO$_3$-PbPbO$_3$ ternary solution phase diagram [G. Suchaneck \emph{et al.}, Ferroelectrics {\bf 318}, 3 (2005)], the substitutional Pb atom prefers to occupy the Zr site instead of the Ti one. In this work, we report density functional supercell calculations for pure PbZrO$_3$ perovskite and for ordered Pb(Zr$_{1/2}$Ti$_{1/2}$)O$_3$ solid solution with different configurations for the Zr and Ti atoms. We investigate the anti-site defect energies and the effects on the electronic structure. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P38.00009: Stability of point defects in SrTiO$_{3}$ substrates during thin film deposition Emmanuel Towett, Shehnaz Jeddy, Gregg Janowski, Charles Brooks, Darryl Schlom, Mary Ellen Zvanut During deposition of multiferroic films on SrTiO$_{3}$, the substrate is subjected to ambients similar to those known to alter the oxygen vacancy concentration and conductivity. We have performed electron paramagnetic resonance (EPR) measurements on SrTiO$_{3}$ substrates at selected stages during film deposition as well as during controlled heat treatments. Fe$^{3+}$, Cr$^{3+}$ and an Fe-oxygen vacancy complex, Fe$^{3+}$V$_{o}$, were monitored during isochronal and isothermal vacuum (10$^{-6}$ Torr) and 1 atm O$_{2}$ heat treatments between 200 and 800 $^{\circ}$C. As expected, processing steps involving O$_{2}$ at 950 $^{\circ}$C reduces the concentration of Fe$^{3+}$V$_{o}$, consistent with the O$_{2}$ annealing study. Film deposition at 650 $^{\circ}$C, 5x10$^{-7}$ Torr with 10{\%} ozone returns the Fe$^{3+}$V$_{o}$ signal to the original intensity, consistent with vacuum treatments which follow 1 atm O$_{2}$ annealing. Surprisingly, isochronal and time-dependent vacuum and O$_{2}$ annealing produce the same trend for the Fe$^{3+}$V$_{o}$ signal for T $>$ 500 $^{\circ}$C. The results of all the studies suggest that electrostatic changes, as evidenced by variations in the intensity of Fe$^{3+}$ and Cr$^{3+}$, as well as oxygen vacancy migration can alter substrate characteristics during film growth. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P38.00010: Electron Beam Excitation Mechanism of Rare Earth Ions in LiNbO$_{3}$ S. Tafon Penn, V. Dierolf, G.S. Cargill III Many applications of ferroelectric materials in frequency conversion are based on precise engineering of the ferroelectric domains. Unfortunately, the quest for smaller feature sizes is slowed due to the lack of real-time local controls of the process. We explored the possibility to use as local probes RE ions in LiNbO$_{3}$ that exhibit in their excitation and emission spectra linear Stark shifts making them sensitive to local electric fields such as local charging and fields from defect dipoles. We studied the feasibility of this approach for the domain writing process induced by an e-beam in a SEM. A necessary first step is a good understanding of the excitation mechanisms of RE:LiNbO$_{3}$ that leads to CL under e-beam irradiation. We observe a pronounced saturation already for moderate e-beam currents indicating that not all RE ions can be excited. This excludes the direct excitation pathway as a major contributor towards the RE excitation. Our observations are explained by a defect trap mediated energy transfer between generated electron-hole pairs and the RE ions. Independent of the RE ion, we find that the energy transfer rate from the defect trap to the RE ion is less than (100$\mu $s)$^{-1}$. Under saturation, this small rate and the limited number of defect trap-RE ion pairs restricts the achievable CL intensity and the applicability of the proposed real-time probing scheme. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P38.00011: Raman study of the phonon modes in bismuth pyrochlores Daniel Arenas, Wei Qiu, Juan Nino, David Tanner, Lev Gasparov The Raman Spectra of the cubic bismuth pyrochlores Bi$_{3/2}$Zn$_{0.92}$Nb$_{1.5}$O$_{6.92}$, Bi$_{3/2}$ZnTa$_{3/2}$O$_{7}$, Bi$_{3/2}$MgNb$_{3/2}$O$_{7}$, and Bi$_{3/2}$MgTa$_{3/2}$O$_{7}$ were measured. The samples, in ceramic form, were measured from 50 to 1000 cm$^{-1}$ at room temperature. The Raman bands were tentatively assigned to specific vibrational modes. Overall, the Raman spectra were similar for all four samples and the number of modes was affected by the displacement disorder in the bismuth-based compounds. The results will also be compared to published infrared data to gain insight into these additional modes. Last, the existence of an 860 cm$^{-1}$ mode in BZN and BMN will be discussed as it suggests influence of the lone pair character on the displacement disorder. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P38.00012: A Low-Temperature Specific Heat Study of the Giant Dielectric Constant Materials C. P. Sun, J. Y. Lin, H. D. Yang, Jianjun Liu, Chun-gang Duan, W. N. Mei Low-temperature specific-heat study has been performed on the insulating giant dielectric constant material CaCu$_{3}$Ti$_{4}$O$_{12}$ and two related compounds, Bi$_{2/3}$Cu$_{3}$Ti$_{4}$O$_{12}$ and La$_{0.5}$Na$_{0.5}$Cu$_{3}$Ti$_{4}$O$_{12}$, from 0.6 to 10 K. From analyzing the specific heat data at very low-temperature range, 0.6 to 1.5 K, and moderately low-temperature range, 1.5 to 5 K, in addition to the expected Debye terms, we noticed significant contributions originated from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurred at the grain boundaries and induced by local defects. Together with the findings on electronic and mechanical properties, a phenomenological model is proposed to explain the high dielectric constant behaviors at both low and high frequency regions. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P38.00013: First Principles Investigation of Electronical and Lattice Dynamical Properties of High Dielectric Constant Material Na$_{1/2}$Bi$_{1/2}$Cu$_3$Ti$_4$O$_{12}$ Cihan Parlak, Resul Eryigit We will report the results of a first principles investigation of the electronic and lattice dynamical properties of so-called giant dielectric compound Na$_{1/2}$Bi$_{1/2}$Cu$_3$Ti$_4$O$_{12}$ which was found to have a very high 'extrinsic' as well as 'intrinsic' dielectric constant. The calculations have been carried out within the local spin density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The ground state is found to be antiferromagnetic direct-band gap semiconductor. Lattice dynamical properties, such as Born effective charge tensors, dielectric permittivity tensors, and phonon frequencies at the Brillouin zone center were calculated using density functional perturbation theory and found to be similar to more studied CaCu$_3$Ti$_4$O$_{12}$ and CdCu$_3$Ti$_4$O$_{12}$ compounds. The calculated electronic ($\epsilon_{\infty}\approx 11.5$) and static ($\epsilon_{0}\approx 150$) dielectric constants indicate that the observed high dielectric constant is extrinsic in origin. The main contribution to the static dielectric constant is found to be due to a low frequency (50 cm$^{-1}$) IR-active mode which has a large mode effective charge. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P38.00014: Transition temperature of martensitic transformations in hafnia and zirconia Xuhui Luo, A.A. Demkov Transition metal oxides find applications in ceramics, catalysis and semiconductor technology. In particular, hafnium dioxide or hafnia will succeed silica as a gate dielectric in advanced transistors. However, thermodynamic properties of thin hafnia films are not well understood, despite their technological importance. We use density functional theory to investigate the tetragonal to monoclinic phase transition in hafnia and zirconia. We find that unlike the case of the cubic to tetragonal transition, this phase transition is not driven by a soft mode. We use transition state theory to identify the minimum energy path (MEP) employing first principle calculations for hafnia and zirconia, sow that both transformations are martensitic, and obtain the transition barriers. Martensitic transformations include both the internal coordinate transformation and deformation of the cell lattice vectors (``strain and shuffle''), therefore the potential energy surface and MEP are function not only of the internal atomic coordinates but also of the unit cell lattice vectors. Considering the simplest case of uniform strain the transition temperatures we then relate the barrier height to the transition temperature. As a self-consistency check, assuming the equality of thermodynamics potentials of the tetragonal and monoclinic phases during the transition, and using the difference in the internal energy calculated from first principles we estimate the entropy change associated with the transition which is found in good agreement with that calculated form the phonon spectra. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P38.00015: Correlation effects in charge-density wave insulator BaBiO$_3$ Cesare Franchini, Martijn Marsman, Georg Kresse The negative-U nature of $\rm BaBiO_3$ leads to a charge-ordered insulating state in which pentavalent $\rm Bi^{5+}$ coexists with trivalent $\rm Bi^{3+}$. Despite the apparent absence of strong-correlation effects in $\rm BaBiO_3$ standard density functional (DFT) theory yields a much too small band gap of 0.14 eV. By means of an hybrid-DFT approach combined with self-consistent GW including vertex corrections we investigate the electronic, vibrational and dielectric properties of $\rm BaBiO_3$. We show that the inclusion of strong-correlation effects increases the band gap up to 1.2 eV, shifts the oxygen breathing modes upwards by $\approx$ 2 THz and reduces the dielectric constant by a factor of 3. The overall agreement with available experimental data is significantly improved. [Preview Abstract] |
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