Gotthard Saghi-Szabo

Silicate-phosphate interactions in silicate glasses and melts: I. A multinuclear (27Al,29Si,31P) MAS NMR and ab initio chemical shielding (31P) study of phosphorous speciation in silicate glasses

Abstract An investigation of the structure of sodium aluminum silicate glasses [composition (x)Na 2 O, (1-x)Al 2 O 3 , 0.9SiO 2 perturbed through the addition of 2 mol.% P 2 O 5 ] was performed using a combination of 27 Al, 29 Si, and 31 P solid state NMR spectroscopy and ab initio chemical shielding calculations (GIAO method). Speciation within these glasses was estimated using a physically consistent method for the deconvolution of inhomogeneously broadened 31 P MAS-NMR spectra by employing a combination of spinning sideband analysis with the results of ab initio shielding calculations. A minimum of 13 different phosphorus-bearing species were estimated to contribute to the spectral complexity in these glasses over the compositional range investigated. The majority of these species are oxygen-bridged to the silicate lattice. These are described as (Al,Si)Q p n species, where n corresponds to the number of bridging oxygens ( n = 1–4) and where there is variation in the Al/Si ratio of the tetrahedral nearest neighbors. In addition to the species bridging to the silicate lattice, nonlattice bridged Na-P type species, including ortho-, pyro-, and tripolyphosphates, are observed or deduced based on spectral behavior. A sharp crossover in the predominance of Na-P species relative to (Al,Si) Q p n dominant species occurs at low Al 2 O 3 content. Within the group of nonbridging (Na-P) species, increased bulk aluminum content shifts the predominance from orthophosphate toward tripolyphosphate. Within the group of bridging [(Al,Si) Q p n ] species, increasing bulk aluminum content shifts the predominance toward a greater number of oxygen bridges to the silicate lattice. To highlight the perturbative effect of 2 mol.% P 2 O 5 addition on glass structure, 29 Si MAS-NMR of P-bearing and P-absent glasses were compared. The speciation estimated from the combined 31 P MAS-NMR and ab initio shielding calculations is consistent with 29 Si MAS-NMR spectroscopic data. Analysis of 27 Al MAS-NMR spectra for P-bearing and P-absent glasses reveals extensive Al-O-P interactions evident by systematic shifts in peak maxima to lower frequencies (greater shielding). Increases in the central transition peak half widths in the P-bearing samples reflect enhanced second-order quadrupolar interactions, also consistent with an abundance of (Al,Si) Q p n species. Analysis of the 27 Al inner satellite transition sidebands reveals only a minor amount of VI Al. It is concluded that the addition of even a small amount of P 2 O 5 (2 mol.%) results in a large perturbation of the structure of silicate glasses and should manifest significant effects on the physical properties of melts derived from them.

Calculation of the 13C NMR chemical shift of ether linkages in lignin derived geopolymers:: Constraints on the preservation of lignin primary structure with diagenesis

Abstract Methodology for the calculation of 13C NMR shieldings on molecular organic fragments, representative of monomers in a type III kerogen, is presented. Geometry optimization of each molecular fragment was carried out using Density Functional Theory employing the generalized gradient approximation. NMR shieldings were calculated using the Individual Gauge for Localized Orbital Method. Convincing agreement was obtained between calculated and experimentally derived isotropic chemical shielding values over a broad frequency range. Shielding calculations employing the localized orbitals/local origin method resulted in nearly identical results. NMR chemical shift static powder patterns also exhibit excellent agreement with experimental values. These quantum mechanical calculations were applied to determine the extent of lignin primary structure preservation with diagenesis. Specifically, the calculations were used to assess whether inhomogeneous spectral broadening due to both functional group variation and local configurational variability may inhibit the detection of otherwise significant quantities of alkyl-aryl ethers in lignin derived geopolymers. Determination of the chemical-shielding tensor principle axis values reveals a strong correlation between anisotropy and asymmetry with local configuration effects such as dihedral rotation, phenyl group rotation, and bond angle variation. These results indicate that a range of 9 ppm in the isotropic chemical shift can be ascribed to local configuration. Consequently, an upper limit of 5% alkyl-aryl-linkages may go undetected using NMR spectroscopy on lignin-derived geopolymers at the liginite–sub-bituminous transition. It is concluded that the primary structure of lignin does not persist in kerogens even at relatively low thermal maturities.

Long-range order effects in Pb(Zr1/2Ti1/2)O3

Abstract Total energies were calculated for Pb(Zr1/2Ti1/2)O3 (PZT) superlattices with B site cations ordered along the [001] and [111] directions within the local density approximation. The local orbital extension of the Linearized Augmented Planewave (LAPW + LO) method was used to obtain energies for the unrelaxed structures and the ab initio Potential-Induced Breathing (PIB) method was applied to establish relaxation energies. We compare the relative stabilities for the PZT superlattices, pure PbTiO3 (PT) and PbZrO3 (PZ). B site cation ordering along the [111] direction results in a lower energy superstructure than that of the [001] ordered structure. The unrelaxed [111] ordered structure results in lower energy than the average energy of the unrelaxed separate PZ and PT phases. If we expect relaxation energies of the same magnitude accompanying ferroelectric and antiferroelectric instabilities in PZ, PT and PZT, we can anticipate simultaneous competition between various processes, such as phase separat...

FIRST-PRINCIPLES STUDY OF PIEZOELECTRICITY IN TETRAGONAL PBTIO3

Abstract The e 33 component of the piezoelectric strain tensor in tetragonal PbTiO3 was studied by an allelectron ab initio method, and was found to be in good agreement with the experimental piezoelectric stress constant of high quality twin-free single PbTiO3 crystals. Equations of the density functional theory were solved within the general gradient approximation (GGA) using the general potential linearized augmented plane-wave local orbital (LAPW + LO) method. Macroscopic polarization values necessary to calculate the piezoelectric coefficient and elements of the Born effective charge tensors were determined via finite differences using the geometric phase formulation. In this approach, bulk quantities are expressed as the property of the phase of the wavefunction in terms of a Berry connection. The e 33 component of the piezoelectric tensor, i.e. the change in polarization with strain, was extracted from results of groundstate Berrys phase calculations performed on units cells of different strains a...

Long-Range Order Effects in Ferroelectric Pb(Zr 1/2 Ti 1/2 )O 3

The local orbital extension of the Linearized Augmented Planewave (LAPW+LO) method within the local density and general gradient approximations was used to optimize internal coordinates and calculate total energies of Pb(Zri 1/2 Ti 1/2 )O 3 (PZT) superlattices with B site cations ordered along the [001] and [111] directions. Ferroelectric structures and bond-length distributions similar to those obtained from experimental data were found for all three investigated chemically ordered phases. The Ti atom sits in an off-center position of a slightly distorted TiO 6 octahedron, the structure of which is mostly independent of the chemical ordering pattern. Coupling between the Ti and Zr containing octahedra results in highly distorted ZrO 6 units. This relatively high energy part of the structure could decrease the stability of these perovskite compounds against zone-boundary rotations of the BO 6 octahedra. Polar, zone-center only distortions result in lower energy [111] ordered superstructures when compared to the [001] ordered structure. The lowest energy chemically ordered PZT is the one with B-site cations ordered along the [111] direction and has 14mm symmetry. Our total energy results and a simple statistical model predict a wide miscibility gap PZT at the morphotropic composition is therefore likely a multiphase or metastable material which may be responsible for its sensitivity to synthesis and annealing conditions.