An ab initio calculation of 17O and 29Si NMR parameters for SiO2 polymorphs

Ab initio molecular orbital calculations (Hartree–Fock, HF and density functional theories, DFTs) have been carried out for SiO₂ polymorphs coesite, low cristobalite, and -quartz, in order to investigate the reliability of this method for predicting ²⁹Si and ¹⁷O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the ²⁹Si and ¹⁷O chemical shifts (δ_i^Si and δ_i^O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δ_i^Si and δ_i^O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δ_i^Si values agree well (within ±1 ppm) with experimental data. The calculated ¹⁷O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δ_i^O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting ²⁹Si and ¹⁷O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method.     

29Si and 19F MAS NMR spectra of isolated 29Si(19F)2 and 29Si(19F)3 spin systems: experiments and simulations

We consider ²⁹Si and ¹⁹F MAS NMR spectra of isolated ²⁹Si(¹⁹F)₂ and ²⁹Si(¹⁹F)₃ spin systems in two organosilicon compounds of the type RR’SiF₂ and RSiF₃(R,R′=organic ligands). Experimental spectra are analysed by means of numerical simulations. It is found that the SiF₃ group in RSiF₃ is reorienting rapidly around the molecular Si–C bond direction in the solid state. The two ¹⁹F shielding tensors in RR’SiF₂ have strongly differing orientations relative to the two Si–F bond directions in the molecule. Possibilities and limitations of straightforward MAS NMR approaches for the full characterisation of ²⁹Si(¹⁹F)₂ and ²⁹Si(¹⁹F)₃ spin systems and other dipolar coupled two and three-spin systems are discussed.     

Characterisation of sol-gel prepared (HfO2)x(SiO2)1-x (x=0.1, 0.2 and 0.4) by 1H, 13C, 17 O and 29Si MAS NMR, FTIR and TGA

The HfO2-SiO2 system is attracting interest as a possible new dielectric material in semiconductor devices. Knowledge of the location of hafnium within the silica network and the effect hafnium has on the structure will be central to the successful use of this material system in this application. Here, sol-gel techniques have been used to manufacture (HfO2)x(SiO2)1-x samples (x=0.1, 0.2 and 0.4, each heat treated at 250, 500 and 750 degrees C) and these have been characterised by magic angle spinning (MAS) NMR (1H, 13C, 17 O, 29Si), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. 29Si MAS NMR showed that increasing the hafnia content decreases the connectivity of the silicate network, i.e. increases the range of differently connected SiO4 (Qn) units with more having increased numbers of non-bridging oxygens (i.e. lower n). FTIR and 17 O MAS NMR showed unequivocally that the x=0.4 sample phase-separated at higher temperatures, while in the x=0.1 sample the hafnium was homogeneously mixed into the SiO2 phase without any phase separation.     

NMR of 29Si and 25Mg In Mg2SiO4 with dynamic polarization technique

Nuclear magnetic resonance of ²⁹Si and ²⁵Mg in natural abundance has been observed in Cr³⁺ doped Mg₂SiO₄ after cooling the nuclear spin system by dynamic nuclear polarization. The quadrupole coupling tensors of ²⁵Mg at the 4a and 4c positions have been determined.     

Sensitivity enhancement in (13)C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing (1)H T(1) relaxation

We discuss a simple approach to enhance sensitivity for (13)C high-resolution solid-state NMR for proteins in microcrystals by reducing (1)H T(1) relaxation times with paramagnetic relaxation reagents. It was shown that (1)H T(1) values can be reduced from 0.4-0.8s to 60-70 ms for ubiquitin and lysozyme in D(2)O in the presence of 10 mM Cu(II)Na(2)EDTA without substantial degradation of the resolution in (13)C CPMAS spectra. Faster signal accumulation using the shorter (1)H T(1) attained by paramagnetic doping provided sensitivity enhancements of 1.4-2.9 for these proteins, reducing the experimental time for a given signal-to-noise ratio by a factor of 2.0-8.4. This approach presented here is likely to be applicable to various other proteins in order to enhance sensitivity in (13)C high-resolution solid-state NMR spectroscopy.     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Spin relaxation in the amorphous spin glass Al2Mn3Si3O12

Neutron scattering measurements on the amorphous spin glass Al₂Mn₃Si₃O₁₂ have been made using the time-of-flight method. The scattering lawS(Q, ) reveals a quasielastic line with temperature andQ-dependent linewidth and an elastic line with constant intensity between 15 and 294K. The linewidth of the quasielastic scattering diminishes with decreasing temperature following an Arrhenius law at least down to 15K. Deviations from this exponential form are strictly correlated with an increase of the elastic intensity below 15K. We favour the opinion that this effect is caused by the instrumental resolution rather than by the onset of spin glass freezing.     

Determination of 27Al-29Si connectivities in zeolites with 2D 27Al-->29Si RAPT-CPMG-HETCOR NMR

The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr-Purcell-Meiboom-Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of approximately 4, which enabled acquisition of a high quality two-dimensional 27Al-29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.     

Dielectric relaxation and ac conductivity study of Ba(Sr1/3Nb2/3)O3

Single phase perovskite Ba(Sr_1/3Nb_2/3)O₃ ceramics was prepared using the columbite precursor method. X-ray diffraction (XRD) technique is used to verify the single phase formation. It stabilizes in hexagonal phase with lattice constants a=12.1243 Å and c=15.3747 Å. Impedance analysis shows distributed relaxation time. Single semicircular arc observed in complex impedance plot confirms that only semiconducting grains are contributing to the polarization. The scaling behavior of both Z″ and M″ infers that the dynamical processes are temperature independent. Comparison of the impedance and electrical modulus data shows that the bulk response has contributions from both localized, i.e. defect relaxation, and non-localized conduction, i.e. ionic or electronic conductivity. The ac conductivity of the ceramics at higher temperatures indicates NTCR (negative temperature coefficient of resistance) behavior like semiconductors. The ac conduction activation energies are estimated from Arrhenius plots and conduction is largely due to hopping process.     

变生锆石的29Si MAS NMR与红外光谱研究

关于锆石变生过程中SiO₄四面体行为的信息对于了解变生态结构及变生过程机理有重要意义。虽然有人推测,在变生过程中,伴随着ZrO₈多面体上氧空位的产生可能同时发生SiO₄四面体的聚合^[7],但迄今并无直接的实验证据证明这一假设。本文的²⁹Si MASNMR研究表明,在锆石变生的最初阶段,Si的局部结构环境没有明显的变化,但在中等变生程度的样品中,探测到Q³和Q⁴Si环境,这是一个有力的证据,支持随着变生程度增加SiO₄四面体发生逐步聚合的假设。红外光谱的结果也显示,变生锆石中可能有无定形的氧化硅畴存在。     

The 28Si(3He,pp)29Si reaction induced by 33 MeV polarized 3He

Differential cross sections and analysing powers were measured for the ²⁸Si(³He, pp)²⁹Si reaction using a 33 MeV polarized ³He beam. The sequential breakup model successfully described the data as a function of both the emission angle and relative proton energy of the ²He cluster for several states in ²⁹Si. An unambiguous j-dependence of the analysing powers was observed and used to asign J^π values for some final states. A statistical analysis of the breakup continuum revealed that a large fraction of such events originate from a formation of the ³¹S compound system.     

Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material

A polycrystalline vanadium doped lead free dielectric material of Bi(Zn_2/3V_1/3)O₃ (BZV) has been prepared using a standard high-temperature solid state reaction technique. Its temperature and frequency dependent capacitive, conductive and resistive characteristics are outlined though experimental investigation. The formation of single phase compound of BZV material with orthorhombic crystal symmetry is identified through X-ray diffraction data analysis, and the homogeneous distribution of grains are realized through scanning electron micrograph. The acquaintance of frequency–temperature dependent electrical parameters with the obtained micrograph provides the experimental evidence of contributions of grain as well as grain boundary in its capacitive and resistive characteristics. The negative temperature coefficient of resistance behaviour of the material is revealed from impedance characteristic, and non-Debye type relaxation has been realized from the Nyquist plot. The charge carriers of this electronic compound have both long & short range order that has been validated from the complex modulus and impedance analysis. The prepared electronic material substantiate some important dielectric features which props up the material as promising component for electronic devices.     

Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

We present an experimentally feasible triple-resonance NMR method that establishes the correlation among three different nuclei, avoiding the difficulty to directly explore the weak coupling between two NMR nuclei, such as (29)Si and (27)Al. Using this method, we are able to give an unambiguous assignment to the various peaks in (29)Si CP NMR spectrum of MCM-22 zeolite and discriminate (29)Si signals from SiOHAl and SiOH groups. In addition, in combination with (1)H/(27)Al double-resonance technique, the (1)H/(27)Al/(29)Si triple-resonance experiment suggests the presence of two different kinds of Br?nsted acid sites in H-MCM-22 zeolite.     

Temperature-dependent 29Si NMR resonance shifts in lightly- and heavily-doped Si:P

Careful NMR measurements on a very lightly-doped reference silicon sample provide a convenient highly precise and accurate secondary chemical shift reference standard for ²⁹Si MAS-NMR applicable over a wide temperature range. The linear temperature-dependence of the ²⁹Si chemical shift measured in this sample is used to refine an earlier presentation of the paramagnetic (high-frequency) ²⁹Si resonance shifts in heavily-doped n-type silicon samples near the metal–nonmetal transition. The data show systematic decreases of the local magnetic fields with increasing temperature in the range 100–470 K for all samples in the carrier concentration range from 2×10¹⁸ cm⁻³ to 8×10¹⁹ cm⁻³. This trend is qualitatively similar to that previously observed for the two-orders of magnitude larger ³¹P impurity NMR resonance shifts in the same temperature and concentration ranges. The ²⁹Si and ³¹P resonance shifts are not related by a simple scaling factor, however, indicating that impurity and host nuclei are affected by different subsets of partially-localized extrinsic electrons at all temperatures.     

Spatially inhomogeneous development of antiferromagnetism in URu2Si2: evidence from 29Si NMR under pressure

From 29Si NMR study, we present evidence for spatially inhomogeneous development of antiferromagnetic (AF) ordering below T(o) = 17.5 K in URu2Si2. In the pressure range between 3.0 and 8.3 kbar, we have observed the 29Si NMR lines arising from the AF region as well as the previously observed 29Si NMR line which correspond to the nonmagnetic region in the sample. The AF volume fraction is enhanced by applied pressure, whereas the magnitude of internal field at the Si site remains constant (910 Oe) up to 8.3 kbar. In the AF region, the ordered moment is about an order of magnitude larger than 0.03 mu(B)/U.     

Proton NMR T 1 Studies in Methylammonium TrichlorO Stannate(II) (CH 3 NH 3 SnCl 3 )

Proton spin lattice relaxation time ( T 1 ) measurements have been carried out in methylammonium trichloro stannate(II) (CH 3 NH 3 SnCl 3 ) as a function of temperature in the range 317-5 K at a Larmor frequency of 10 MHz. The temperature dependence of T 1 shows a phase transition around 220 K and four T 1 minima (294 K, 62 K, 32 K and 12 K). The results are discussed in terms of proton dynamics, namely, uncorrelated reorientation of NH 3 and CH 3 groups at high temperatures and tunnelling of NH 3 and CH 3 protons at low temperatures.