Disorder and dynamics in pollucite from 133Cs and 27Al NMR

Pollucite, CsAlSi2O6, a Cs polymorph of leucite (KAlSi2O6), has been proposed for ceramic immobilization of 135Cs and 137Cs fission products. 133Cs NMR of both low- (tetragonal) and high-temperature (cubic) forms of pollucite exhibit a considerable distribution of local Cs environments. 29Si and 27Al NMR data from directly prepared pollucite show greater Al/Si disorder than either leucite, or pollucite produced by ion exchange. Little evidence for Cs motion is observed in tetragonal or cubic pollucite, and only at high temperatures (∼850°C) is any substantial dynamic behavior detected. Dynamic NMR lineshape calculations allow a determination of the frequency of Cs motion and diffusivity

Disorder and dynamics in pollucite from 133Cs and 27Al NMR

Pollucite, CsAlSi2O6, a Cs polymorph of leucite (KAlSi2O6), has been proposed for ceramic immobilization of 135Cs and 137Cs fission products. 133Cs NMR of both low- (tetragonal) and high-temperature (cubic) forms of pollucite exhibit a considerable distribution of local Cs environments. 29Si and 27Al NMR data from directly prepared pollucite show greater Al/Si disorder than either leucite, or pollucite produced by ion exchange. Little evidence for Cs motion is observed in tetragonal or cubic pollucite, and only at high temperatures (∼850°C) is any substantial dynamic behavior detected. Dynamic NMR lineshape calculations allow a determination of the frequency of Cs motion and diffusivity

Combined NMR and X-ray diffraction study of structural aspects, dynamics and charge ordering mechanism in LixVOPO4.2H2O intercalation compounds

International audienceWe carried out a detailed investigation of the local ordering and dynamics of the lithium intercalation in paramagnetic LixVOPO4.2H2O (with 0 < x ≤ 1) materials. This question was addressed using a combination of X-ray diffraction, 31 P and 7 Li MAS NMR experiments. We first studied the structure of the fully ordered end-member of the series, Li1VOPO4.2H2O, revisiting the X-ray single crystal diffraction data on the basis of the information provided by 31 P MAS NMR. We then carried out 7 Li MAS and exchange NMR experiments and 31 P MAS experiments on the polycrystalline powders obtained after partial lithium insertion in VOPO4.2H2O phases. These experiments evidenced an unexpected ageing of the material related with lithium dynamics between the VOPO4 layers and a V 4+ /V 5+ charge ordering mechanism within the layers

Electric field gradient calculations in paramagnetic compounds using the PAW approach. Application to ²³Na NMR in layered vanadium phosphates.

This article presents ab initio calculations of electric field gradient (EFG) parameters as a tool for the structural characterization of paramagnetic crystalline compounds. Previously reported ²³Na NMR parameters of vanadium + IV containing vanado-phosphate compounds were computed within density functional theory using both cluster and fully periodic approaches. Quadrupolar parameter values measured by ²³Na NMR experiments were reproduced with a level of accuracy comparable to that achievable in diamagnetic compounds and allowed the assignment of observed ²³Na NMR signals. This work demonstrates the utility of the periodic planewave pseudopotential + PAW approach for the calculation of EFG parameters in paramagnetic compounds

95 Mo Magic Angle Spinning NMR at High Field: Improved Measurements and Structural Analysis of the Quadrupole Interaction in Monomolybdates and Isopolymolybdates

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis

Crystal Structures, Physical Properties and NMR Experiments on the Ternary Rare-Earth Metal Silicide Boride Compounds RE5Si~2B8 (RE = Y, Sm, Gd, Tb, Dy, Ho).

The ternary rare-earth metal silicide borides RE5Si2B8 (RE ¼ Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to T ¼ 1850 K. The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/mbm, Z ¼ 2; unit cell parameters a ¼ 7:2616ð3Þ, c ¼ 8:2260ð3Þ (A and a ¼ 7:1830ð2Þ, c ¼ 7:9900ð3Þ (A for Sm5Si2B8 and Ho5Si2B8, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB4-like and U3Si2-like slabs of composition REB4 and RE3Si2, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y5Si2B8 is a Pauli-type paramagnet above 1.8 K, Gd5Si2B8 undergoes a weak (canted) ferromagnetic-like order at 70K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb5Si2B8 and Dy5Si2B8 order antiferromagnetically at a Ne´ el temperature of TN ¼ 45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE3+ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. 11B, 29Si and 89Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y5Si2B8 shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. 11B NMR on Y5Si2B8 indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB4

Moosh: A Numerical Swiss Army Knife for the Optics of Multilayers in Octave/Matlab

The aim of Moosh is to provide a complete set of tools to compute all the optical properties of any multilayered structure: reflection, transmission, absorption spectra, as well as gaussian beam propagation or guided modes. It can be seen as a semi-analytic (making it light and fast) solver for Maxwell’s equations in multilayers. It is written in Octave/Matlab, available on Github and based on scattering matrices, making it perfectly stable. This software is meant to be extremely easy to (re)use, and could prove useful in many research areas like photovoltaics, plasmonics and nanophotonics, as well as for educational purposes for the large number of physical phenomena it can illustrate