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

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

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

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

Na 2 [(VO) 2 (HPO 4 ) 2 C 2 O 4 ]·2H 2 O: Crystal Structure Determination from Combined Powder Diffraction and Solid-State NMR

International audienc

Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)4AlSi3O8, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by 2H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm–1. Static 2H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (<>mgrs) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to ldquofreezing inrdquo of the ammonium ion. This observation rules out the formation of a preferred N–H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N–H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale

Lithium vanadyl oxalatophosphite: Influence of the water content on the crystal structures and the dehydration scheme

International audienceTwo new lithium vanadyl oxalatophosphites have been synthesized by hydrothermal treatment. The respective formula are Li2(VOHPO3)2C2O4 6H2O (1) and Li2(VOHPO3)2C2O4 4H2O (2). The structures of the compounds have been determined by single crystal X-ray diffraction. Compound 1 crystallizes in triclinic symmetry in space group P-1, a=6.3592(2) Å, b=8.0789(3) Å, c=9.1692(3) Å, α=64.390(2), β=87.277(2)°, γ=67.624(2) and, compound 2 in monoclinic symmetry, space group P21/a, a=6.3555(2) Å b=12.6368(7) Å c=9.0242(4) Å β=105.167(3)°. The vanadium phosphite framework consists of infinite chains of corner-sharing vanadium octahedra and hydrogenophosphite tetrahedra. The oxalate groups ensure the connection between the chains. The lithium ions and the water molecules are located between the anionic [(VO)2(HPO3)2C2O4]2− layers. Thermal behavior of both compounds was carefully studied by combining thermogravimetric analyses and thermal dependant X-ray diffraction in order to study the thermal stability of the layered oxalatophosphites and to see the influence of the decomposition of the carbon-based anions into the final lithium vanadyl phosphate. Various intermediate phases were evidenced and for both compounds the final product was LiVOPO4

Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)4AlSi3O8, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by 2H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm–1. Static 2H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (mgrs) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to ldquofreezing inrdquo of the ammonium ion. This observation rules out the formation of a preferred N–H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N–H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale

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