Session Z31: Complexed Structured Materials, Glasses and Clathrates

Electronic structure of interfaces between insulating LaAlO$_3$ and SrTiO$_3$ perovskite oxides

Since heteroepitaxial structures of perovskite oxides are potent candidates for multifunctional devices, understanding their interface properties is very important for applications, because they often show completely different properties from bulk. Recently, interfaces between the wide-band gap insulators of LaAlO$_3$ and SrTiO$_3$, were found to be insulating when hole-doped, and conducting when electron-doped \footnote{A. Ohtommo and H. Y. Hwang, Nature {\bf 427}, 423 (2004)}. First-principles calculations with the highly precise full-potential linearized augmented plane wave (FLAPW) method \footnote{Wimmer, Krakauer, Weinert, Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} for the stoichiometric compounds on [001] superlattices composed of perovskite unit cells of LaAlO$_3$ and SrTiO$_3$, show metallicity in both the hole-doped AlO$_2$/SrO and electron-doped LaO/TiO$_2$ interfaces, even with inclusion of geometry relaxation. Only by considering oxygen vacancies is the experimental result of insulating behavior in the hole-doped AlO$_2$/SrO interface obtained.   

Local atomic ordering in nanocrystalline ZrO$_{2}$ and Zr$_{x}$Ce$_{1-x}$O$_{2}$ studied with the Atomic Pair Distribution Technique.

A number of ZrO$_{2}$ and Zr$_{x}$Ce$_{1-x}$O$_{2}$ nanocrystalline samples have been prepared using different techniques, such as ball-milling (ZrO$_{2})$ and soft chemistry (ZrO$_{2}$ and Zr$_{x}$Ce$_{1-x}$O$_{2})$. The atomic-scale structure of these samples has been studied using high-energy x-ray diffraction and the atomic pair distribution function technique. For the ball-milled materials, the studies show that the parent crystalline material, monoclinic ZrO$_{2}$, evolves into a nanocrystalline phase that is locally similar to monoclinic zirconia but shows a cubic-type ordering at nanometer-range distances. The atomic-scale structure of the ZrO$_{2}$ samples obtained by soft chemistry techniques varies depending on the preparation method and the degree of structural coherence. The studies reveal that all samples show Zr-O distances typical for monoclinic-like local atomic ordering, while the Zr-Zr distances depend on the morphology of the samples. For the Zr$_{x}$Ce$_{1-x}$O$_{2}$ samples, interatomic distances typical for both monoclinic ZrO$_{2}$ and cubic CeO$_{2}$ are observed up to 5{\AA}, while the longer interatomic distances are attributed solely to a cubic-type structure. The result underlines the importance of local structural disorder and the medium-range order in stabilizing the technologically important cubic zirconia at room temperature.   

Atomic PDF study of size and structure of CdSe nanoparticles

The atomic pair distribution function (PDF) is used to address the size and structure of series of CdSe nanoparticles prepared by the method of distribution focusing [1]. Due to their limiting structure coherence conventional crystallographic methods, such as Rietveld can't be used to assess quantitative size or structural information. Total scattering techniques, such PDF have been successfully applied recently on some similar systems [2, 3]. The PDF includes both Bragg and diffuse scattering and provides quantitative information about the local structure of the materials at different length scales. We report on results of the PDF analysis of synchrotron x-ray diffraction on series of CdSe nanoparticles, data were collected using the rapid acquisition PDF (RA-PDF) technique [4]. [1] X. G. Peng, J. Wickham, A. P. Alivisatos, \textit{J. Am. Chem. Soc.} \textbf{1998}, $120$, 5343-5344 [2] B. Gilbert, F. Huang,$^{ }$H. Zhang,$^{ }$G. A. Waychunas,$^{ }$J. F. Banfield. S\textit{cience}, 305, 651-654 (2004). [3] R. B. Neder, V. I. Korsunskiy, J. Phys: Condens. Matter 17, 125-134 (2005) [4] P. J. Chupas, X. Qiu, J. C. Hanson, P. L. Lee, C. P. Grey and S. J. L. Billinge. J. Appl. Crystallogr. 36, 1342-1347 (2003).   

Interactions of Ga$_{n}$As$_{n}$ Clusters with CGaAs Cages: Possible Nanostructures

Extending our work on fullerene-alkali complexes$^{1}$, we now examine the interactions of small Ga$_{n}$As$_{n}$ clusters (n = 1 thru 10) with mixed CGaAs cage clusters. First, we derive the physical and chemical properties of the GaAs clusters, including their binding energies, bondlengths, ionization potentials and charge distributions. The geometries of the small gallium arsenide clusters are based on full optimizations. Electron correlation effects are included for binding energies and optimal intermolecular bondlengths. Next we focus on the physical and chemical properties of 60-atom mixed CGaAs cages. The optimization of the cages is subject to symmetry constraints. The third phase of the investigation examines the interactions of the small GaAs clusters with the CGaAs cages. For these complexes various properties, including dissociation channels and dissociation energies, are tabulated. We also explore the implications of this research for the design of nanostructures. 1. Daniel Bulnes, Nichole Moya-Leyba, Erica Velarde and Ajit Hira, " Theoretical Study of Na$_{3}$C$_{60}$ and Na$_{4}$C$_{60 }$Clusters: Pathways to Nanoscale Contacts", \textit{Bull. Am. Phys. Soc}. 50, 1475 (March 2005 \textbf{).}   

Statistical Mechanics for Linking Length Scales in Complex Solids

Linking scales is usually pursued by computational means, either by passing information from calculations at one scale to another or by constructing a composite simulation with different features treated at different length scales. Here we report on a formulation that is based on the principles of statistical mechanics, applied to a complex solid. Two new concepts, `lattice space' and `atom space', are introduced. The deformed state of a crystal is described in terms of deformations of the lattice space and atom space. The distribution function is decomposed in a Born-Oppenheimer-like fashion into slow and fast components. Phonons are integrated out to give a driving force to the lattice deformation. Interplay between the two spaces leads to the formation of defects and such phenomena as elastic deformation, thermal expansion, creep, and dislocation motion. New distribution functions are constructed by adapting the method of local integrals of motion from the theory of liquids. Simulations can be implemented at different mesoscopic length scales, tracking fluxes of point defects, impurities, dislocations, etc.   

Study of structure and thermophysical properties of molten BaGe by using electrostatic levitation technique

BaGe alloys with two compositions around their eutectic point form open framework structures called clathrate structure. However, the formation mechanism of clathrate structure has not yet been clarified due to lack of study and understanding of their liquid state structure and properties. Therefore, in order to clarify the formation mechanism of the clathrate structure, thermophysical properties (density, surface tension, and viscosity) of BaGe alloys melts around eutectic compositions were measured by using the electrostatic levitation (ESL) technique and also the structure of them was observed by using the high-energy X-ray diffraction method combined with the ESL. We found that from experimental results, the short range order based on the clathrate structure would exist even in the liquid state at the clathrate forming compositions.   

Localized probe of dielectric response of polymer free surface near the glass transition

Experimental study of length scales in glassy systems can give new insights into the glassy dynamics. Finite size effects are studied in a polymer (PVAc) utilizing UHV non-contact atomic force microscopy (NCAFM) methods. Localized dielectric susceptibility is probed using the electrostatic interaction of a metal-coated AFM tip with a thick polymer film on a metal substrate. The depth probed below the free surface of the polymer is as small as 10 nm. A lockin amplifier is utilized to measure response from the surface in the second harmonic to extract the Tan(delta) (C’’/C’) response over frequencies of 0.1 - 100 Hz. Preliminary results for the smallest probed depths show an increase in the frequency of the alpha relaxation peak in Tan(delta) as compared with macroscopic measurements. Results for various probed depths will be discussed.   

Spatio-temporal imaging of polarization fluctuations near the glass transition

Mesoscopic scale spatio-temporal fluctuations are mapped using novel non-contact atomic force microscopy (NCAFM)-based methods in polymer (PVAc) films near the glass transition. Utilizing the localized electrostatic interaction of a NCAFM conducting tip, we measure polarization fluctuations within depths as small as 10nm. Applying a sinusoidal bias voltage between the AFM tip and sample results in a response in the AFM cantilever frequency that is offset by surface potential due to thermally induced local polarizations. The measured fluctuations agree quantitatively with predictions. Measurements of these spontaneous fluctuations along a single space-dimension as a function of time yield space-time maps of the dynamics. Spatial and temporal correlation functions were studied as a function of temperature. Results show the expected slowing of the dynamics with decreasing temperature, and hints of spatially varying dynamics. Space-time maps will be compared with recent model simulations which show foam-like space-time trajectories.   

Raman scattering and modulated-DSC experiments on Potassium Germanate glasses*

We have synthesized titled glasses in the 0 $<$ x $<$ 0.16 range by traditional melt-quenching, and have examined them in Raman scattering and modulated-DSC (MDSC) experiments. Raman lineshapes observed in the present work are quite similar to those reported by Henderson and Wang $^{1}$. Preliminary MDSC experiments reveal glass transition temperatures, T$_{g}$(x), starting from a value of 570\r{ }C at x = 0, to decrease to 508\r{ }C near x = 0.06, and to increase thereafter almost linearly to 552\r{ }C as x increases to 0.15. On the other hand, the non-reversing enthalpy associated with T$_{g}$ provides evidence of a global minimum in the 0.08 $<$ x $<$ 0.10 range, the reversibility window$^{2}$. These results are consistent with glasses at x $<$ 0.08 as \textit{Stressed-Rigid}, those at x $>$ 0.10 as \textit{Floppy,} while those in the reversibility window as representing the \textit{Intermediate Phase}$^{2}$. The space filling nature of the \textit{Intermediate Phase} is, independently, corroborated by trends in molar volumes which show a broad global minimum in the 9-11{\%} range. Identification of the three elastic phases provides a physical basis to understand the origin of the Germanate anomaly, and the electrical conductivity threshold when glasses become mechanically floppy. *Supported by NSF grant DMR 04-56472. $^{1}$ G.S.Henderson and H.M.Wang, Eur. J. Mineral. \underline {14}, 733 (2002). $^{2}$ P.Boolchand, G.Lucovsky, J.C. Phillips and M.F.Thorpe, Phil. Mag \underline {85},3823 (2005).   

Damped elastic relaxation seen in glassy colloidal suspensions

We study concentrated colloidal suspensions, a model system which has a glass transition when the particle concentration is high. We use an optical confocal microscope to view the motion of these colloidal particles in three dimensions. We add small magnetic particles to locally ``poke'' the colloidal samples. We find a yield force (below which the magnetic particles are unable to move through the sample), which grows as the glass transition is approached. When a force is applied below this yield force, the sample deforms elastically, but with relaxation seen after the force is removed. This relaxation seems to behave in a nonexponential fashion. We characterize this elastic response and relaxation and study how both change as the glass transition is approached.   

Cohesion in clathrates I; covalencies and ionicity

We present the roles of endohedral atoms in the cohesion of group 14 clathrates I. Taking a view that the clathrate I consists of a sheaf of one-dimensional connections of Na@Si$_{24}$ cages interleaved in three perpendicular directions, we calculate the electronic structures with an \textit{ab initio} method and find that 30\% of the 3s$^1$ charge of each endohedral sodium atom transfers to the frame and the remaining charge forms a bonding state between the endohedral atoms: the roles of cohesion is the covalent bonds between the endohedral atoms in the cages and between the frame atoms, together with the ionic bond between the host and the endohedral atoms.   

Stabilities of earlier row atoms in group 1 and 2 for encapsulation in silicon clathrates I

Group 14 clathrates consisting of Si, Ge, or Sn atoms have been synthesized only when they contain some specific endohedral atoms. They are Na, K, Rb, or Cs atoms in group 1, Sr or Ba atoms in group 2, and Cl, Br, or I atoms in group 17.[ K.A.Kovnir, et al. Russian Chem. Rev.73,923(2004).] No experiment has been reported on the encapsulation of the earlier row atoms than the atoms mentioned above. We predict the stability of the encapsulation of these atoms using ab initio methods. We evaluate the stability of the guest atom in double caged M2@Si42H36 cluster in the clathrate I, where M is group 1, 2, or 17 atom. The encapsulation of two lithium atoms into one cage and the other is vacant is more stable than the separate encapsulation of lithium atoms into each cage by 0.436eV. The Be2 is the same case and by 0.0859 eV. For sodium case, the separate encapsulation into each cage is more stable by 1.473eV. The strong dimer formation prevents the stabilization of the cage structure. We also evaluate the stabilities in the crystalline states.   

Electronic structures and displacements of endohedral sodium atoms in silicon clathrate II

Since group 14 clathrates encapsulate specific atoms in their cages, the atoms give rise to rattling motion in the cages leading to low thermal conductivity. The clathrate II consists of Si$_{28}$ and Si$_{20}$ cages and the Si$_{28}$ cage is the largest among the cages in the clathrates. We investigate the displacements and the charge densities between the endohedral sodium atoms in the clathrate II using ab initio methods. First we present the displacements of the guest sodium atoms in double caged Na$_{2}$@Si$_{50}$H$_{44}$ cluster, which is a piece of clathrate II being hydrogenated in order to terminate the dangling bonds. Although in a single Si$_{28}$H$_{28}$ cage the endohedral sodium atom is the most the most stable at the center of the cage, the sodium atoms in each of the double cage cluster displace about 0.06 nm away from each center of the cages to shorten the distance of the sodium atoms. The displacements are attributed to the formation of covalent bond between the sodium atoms and the ionic bonding between the sodium atoms and the cage silicon atoms. Secondly we present the bonding charge densities between the sodium atoms in the clathrate II.   

Stabilities of Al or Ga atoms for encapsulation in silicon clathrates I

Clathrates are cagelike compounds and encapsulate endohedral atoms inside the cages of the host network. On the one hand, the clathrates with host atoms silicon, germanium, or tin have been synthesized only when group 1 alkali metal atoms or group 2 alkaline earth metal atoms coexist as electron donors or group 17 halogen atoms coexist as electron acceptors. On the other hand, the group 13 atoms such as Al, Ga, In atoms are substituted for the host frame atoms. There has been no explanation of the mechanism of the solution of these atoms into the frame or insoluble into the cages. We investigate this through calculating the enthalpies of solution of these atoms into silicon clathrate I by use of an \textit{ab initio} method. We discuss the energies of solution of group 13 atoms into the cages with those into frame structure.   

Vacancy defect formation energies in Ba8@Ge46 clathrates

The clathrates I Ba8@Ge46 have been found experimentally to have vacancy defects in the framework and to form Ba8Ge43. This has been explained by the Zintl-Klemn empirical concept: the inclusion of the barium atoms leads to increase the valence electrons in the framework of the clathrates and the introduction of vacancy defects relaxes the hypervalence of the electrons. We clarify the process of the defect formation by analyzing the electric structure with the first-principle method. We calculate formation energy of a single vacancy in the clathrates in which we use Ceperley-Alder with LDA exchange correlation functional for the pseudopotential of barium atom.[J. Junquera et al. Phys.Rev.B 67,155327(2003)] The energies are 0.09eV for 6c, 0.39eV for 24k, and 0.74eV for 16i sites in the clathrate Ba8@Ge45, although 1.15eV for 6c, 1.31eV for 24k, and 1.70eV for 16i sites in the clathrate Ba8@Si45. The small energy of the 6c site is due to the small binding energy of Ge clathrate. We will present the reason why vacancy defects are introduced at the 6c sites.