Session T21: Focus Session: Lattice Dynamics and Surface Chemistry

Phonon dynamics near high temperature phase transition in Na$_{1/2}$Bi$_{1/2}$TiO$_{3}$

In this report, we present recent inelastic neutron scattering results on the disordered perovskite system Na$_{1/2}$Bi$_{1/2}$TiO$_{3}$(NBT). NBT exhibits the relaxor ferroelectric behavior (strong frequency dispersion of the dielectric constant) between 850K and 600K. X-ray and neutron diffraction has shown that the structural transition occurs at Tc$\approx $820K corresponds to the in-plane tilting of oxygen octahedral associated with the softening of a zone boundary acoustic mode. Inelastic neutron scattering was measured in the (002) and (220) Brillouin zones, both above and below the high temperature transition. Transverse acoustic and transverse optic phonon modes were mapped out in these two Brillouin zones. The key observations of the study are: 1) the zone boundary soft mode behavior of both TA and TO modes in (002) zone, 2) the critical TA-TO coupling anomaly around q$=$0.15 (r.l.u.). The latter phenomenon has been well studied in other perovskite systems such as KTaO$_{3}$ where a pronounced kink is observed in the dispersion curve of the TA and TO branches at a critical q value. Our results on NBT suggest an anti-crossing type coupling of the TA and TO branches in the dispersion curves.   

Investigation of relaxations and central peaks in the Raman spectra of NBT

Raman spectroscopic measurements of sodium bismuth titanate (Na$_{\mathrm{0.5}}$Bi$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$ or NBT) have been carried out from 80 K to 1000 K using an Ar$^{\mathrm{+\thinspace }}$ion laser at 514.5 nm, with a particular emphasis on its two transitions. Full spectral deconvolution has been performed to examine the temperature evolution of the ``central features'' and low frequency phonons below 100 cm$^{\mathrm{-1}}$. The central intensity profile is found to be composed of two well-defined Lorenzian peaks, one narrow and the other broad. The temperature dependence of the two central peaks reveals the presence of fluctuations/relaxations in both M and R-point rotations of the oxygen octahedra coupled to the cation displacements, the latter giving rise to polar nano-domains (PND's) and the relaxor behavior. These fluctuations/relaxations are shown to not follow the Bose thermal occupancy factor, similar to central peaks in glasses.   

Dynamical Properties of PbTiO3 Under Pressure, Stress and Strain

Ferroelectric perovskites have been in the focus of attention for many years owing to their remarkable properties and variety of applications. Of notable importance is the manner in which external stimuli alter the properties and dynamics of such materials. Here we take advantage of first principles based molecular dynamics simulations to probe the dynamics of PbTiO$_{3}$ at finite temperature and under the application of pressure, stress and strain. Our simulations show that the complex dielectric response and soft-mode dynamics of PbTiO$_{3}$ can be tuned by the application of pressure, stress and strain over a range of values available in a laboratory setting. This tunability can lead to the use of PbTiO$_{3}$ and other polar perovskite oxides in novel applications.   

Phonon dispersion relation in PbTiO$_3$

The phonon dispersion relations for cubic PbTiO$_3$ ($T_c=$763 K) have been determined along the high symmetry directions at $T=$793 K using inelastic neutron scattering. A set of the TO branches drops significantly toward the zone center. This is quite different from the soft mode anomaly in the Pb-based relaxors, named as the waterfall phenomenon. The zone-center TO mode energy softens with decreasing temperature from 1173 to 793 K. The TA branch along [$\xi,\xi,\xi$] shows significant softening around $\xi=$0.25 and 0.5. These two anomalies persist up to 1173 K and are weakly temperature dependent. Moreover, the TA branches along [1,0,0] and [1,1,0] soften in the entire $q$ range as the temperature approaches $T_c$. Although the phonon softening occurs simultaneously, the softening of the zone center TO mode plays an important role in the single phase transition. The phonon dispersion relations for cubic and tetragonal PbTiO$_3$ are discussed in connection with BaTiO$_3$, KTaO$_3$, Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$, and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.   

Static and dynamic properties of PbTiO$_3$ at finite temperatures

The ABO$_3$-type perovskite crystals are key to several important technological applications. To mention a few, electro-optics, waveguides, laser frequency doubling and high capacity computer memory cells. In this work, we develop a route to first-principles parametrization of effective Hamiltonian for ferroelectric ferovskites [1] which allows an accurate description of both static and dynamic properties of such materials. We use this method to examine softening of the transverse optical mode in both paraelectric and ferroelectric phases of PbTiO$_3$. The computed static and dynamic properties are in good agreement with the available theoretical and experimental data. Our study also predicts a crossover between a displacive to an order-disorder transition near the Curie point.\\[4pt] [1] W.~Zhong, D.~Vanderbilt, and K.~M.~Rabe, Phys. Rev. B {\bf 52}, 6301 (1995).   

Raman Spectroscopy Study of Phase Transition in Layered Ferroelectric PbK$_{2}$LiNb$_{5}$O$_{15}$

PKLN is a novel material with lattice structure resembling that of tetragonal tungsten bronze. Below 640 K it assumes ferroelectric-ferroelastic orthorhombic phase of Pba2 space group. At high temperature the material is known to possess paraelectric properties characterized by tetragonal P4/mbm structure with one-dimensional electric conductivity. In order to clarify the mechanism of the transition between these two symmetries, we carried out a detailed exploration of temperature dependencies of Raman scattering spectra in five scattering geometries in the broad temperature range between 800 and 300 K, completely covering the region where the phase transformation occurs. Our data indicate that lowering the temperature, pre-transitional phenomena in the form of soft behavior of peaks start at least at 700 K that is well above the transition temperature. While most of the peaks soften towards 640 K, some of them soften towards 680K. Below both of these temperatures many peaks demonstrate splitting. The observed phenomena reveal presence either of an intermediate phase or long-living relaxations in the pretransitional temperature range.   

Lattice dynamics of Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) from first principles

Insulating bismuth pyrochlores with mixed cations randomly distributed on the B site, Bi$_2MM'$O$_7$, have been of interest primarily for their dielectric properties. As a way of helping to elucidate the effects of cation disorder from that of the highly polarizable Bi$^{3+}$ lone pair cation, systems like Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) have beed studied. Far from being simple model systems, these single B-site cation materials have been show to display surprisingly complex and local structural distortions. While Bi$_2$Sn$_2$O$_7$ and Bi$_2$Hf$_2$O$_7$ undergo three and four different phases (where the ground state structure has 352 atoms), Bi$_2$Ti$_2$O$_7$ does not show any coherent structural distortions but rather the Bi$_2$O' simply becomes disordered. In this talk we will present a comparative first-principles study of the lattice instabilities throughout the BZ of the cubic prototype structures of Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) . We then use the eigenvectors of the identified unstable force constants to perform a systematic search over all possible subgroup structure, performing full structural relaxations thereby constructing a picture of the energy landscape. Finally we studied the effect of biaxial strain along [100], [110], and [111].   

NO$_x$ Binding and Dissociation: Enhanced Ferroelectric Surface Chemistry by Catalytic Monolayers

NO$_x$ molecules are regulated air pollutants produced during automotive combustion. As part of an effort to design viable catalysts for NO$_x$ decomposition operating at higher temperatures that would allow for improved fuel efficiency, we examine NO$_x$ chemistry on ferroelectric perovskite surfaces. Changing the direction of ferroelectric polarization can modify surface electronic properties and may lead to switchable surface chemistry. Here, we describe our recent work on potentially enhanced surface chemistry using catalytic RuO$_2$ monolayers on perovskite ferroelectric substrates. In addition to thermodynamic stabilization of the RuO$_2$ layer, we present results on the polarization-dependent binding of NO, O$_2$, N$_2$, and atomic O and N. We present results showing that one key problem with current catalysts, involving the difficulty of releasing dissociation products (especially oxygen), can be ameliorated by this method.   

Optical detection of adsorbed CO$_{2}$ and other gases on ferroelectric surfaces using second harmonic generation (SHG)

Due to their polar surfaces, ferroelectrics may provide an ideal way to detect and collect gas molecules, useful for applications such as gas sensing and pollution mitigation. Since ferroelectric materials have a high reliability (at least 10$^{9}$ switching cycles) these sensors could be used for prolonged periods of time without failure. Second harmonic generation (SHG) allows us to determine the spatial orientation of surface adsorbates and to monitor in realtime the kinetics of adsorption/desorption. In preliminary experiments we see a variation of SHG signal from the surface of PbZrTiO$_{3}$ (PZT) (100 nm film 20\% Zr, 80\% Ti) when dosed with 1 atm of N$_{2}$ or CO$_{2}$. There is a 21\% increase in signal when dosed with N$_{2}$ with respect to signal in vacuum and there is a 19.9\% increase in signal when dosed with CO$_{2}$ with respect to signal in vacuum. Further studies will be performed to determine the orientation of these molecules on the surface of this device. Experiments will also be performed while polarizing the device with an external electric field to determine the effect of polarization on adsorption/desorption of molecules.   

AFM Analysis of Photcatalyzed Deposition of Silver Particles on Perovskite Surfaces

Photocatalyzed deposition of silver from a silver-nitrate solution onto well-defined perovskite surfaces was investigated using an atomic force microscope (AFM). The different materials were grown in a RF-off-axis sputter deposition chamber. Grown films have atomically flat surfaces with unit cell high step edges. Different particle accumulation structures were encountered, and the distribution of particles was analyzed. The photocatalyzed deposition of silver is a suitable proxy reaction for water splitting, and development of a technique that will allow precise determination of the catalytic ability of surfaces and specific sites on those surfaces is a priority in our group's efforts to develop new ferroelectric photocatalysts.