19F isotropic chemical shifts for alkali, alkaline earth and rare earth of column 3 basic fluorides are measured and the corresponding isotropic chemical shieldings are calculated using the GIPAW method. When using the PBE exchange–correlation functional for the treatment of the cationic localized empty orbitals of Ca2+, Sc3+ (3d) and La3+ (4f), a correction is needed to accurately calculate 19F chemical shieldings. We show that the correlation between experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows us to predict 19F NMR spectra of crystalline compounds with a relatively good accuracy. In addition, we experimentally determine the quadrupolar parameters of 25Mg in MgF2 and calculate the electric field gradients of 25Mg in MgF2 and 139La in LaF3 using both PAW and LAPW methods. The orientation of the EFG components in the crystallographic frame, provided by DFT calculations, is analysed in terms of electron densities. It is shown that consideration of the quadrupolar charge deformation is essential for the analysis of slightly distorted environments or highly irregular polyhedra.

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NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations

Tm3+/Yb3+ doped CaF2, SrF2, and cubic phase NaYF4 nanoparticles dispersed as colloids in water (H2O and D2O) or saline solutions have been directly prepared by a one-step hydrothermal technique, using citrate anions as capping agents, without the need for any postsynthesis reaction. The size monodispersed nanoparticles are directly dispersed in water. Comparison of the upconversion emissions at 800 nm (Tm3+ ions) among the CaF2, SrF2, and NaYF4 hosts indicates that the SrF2 host leads to the highest emission intensity, 2 orders of magnitude higher than the NaYF4 one. Alkali ions (Na+ or K+) counter cations of the citrate salts used as precursors can enter the fluoride host crystals as charge compensators and strongly influence the spectroscopic properties of the lanthanide ions. The Tm3+/Yb3+ doped SrF2 nanoparticles dispersed in a 0.4 g/L concentration solution show detectable upconversion with laser excitation intensities as low as 1 W/cm2.

Water (H2O and D2O) dispersible NIR-to-NIR upconverting Yb3+/Tm3+ doped MF2 (M = Ca, Sr) colloids: Influence of the host crystal

Neutron diffraction measurements have been performed on a single crystal of 7Li2O to investigate the time-averaged positions of the nuclei at seven temperatures between 293 and 1603 K. At temperatures up to 873 K the observed Bragg peak intensities can be well fitted using the regular antifluorite structure, including harmonic and anharmonic temperature factors to describe the thermal vibrations. At higher temperatures an improved fit to the experimental data is obtained by allowing a fraction of cations to leave their regular lattice sites. This is indicative of a transition to a 'fast-ion' phase, analogous to that observed in the fluorite-structured compounds at elevated temperatures. The model of the time-averaged disordered structure which best fits the data consists of interstitial lithium ions occupying mean positions very close to the midpoint between two nearest-neighbour Li+ ions. The fraction of Li+ ions occupying these sites increases rapidly above 1200 K, reaching approximately 16% at 1603 K. There is some evidence that the presence of the interstitial cations produces slight relaxations of the two nearest-neighbour Li+ ions in (111) directions, away from the interstitial.

https://iopscience.iop.org/article/10.1088/0953-8984/3/26/001/pdf

Investigation of thermally induced Li+ ion disorder in Li2O using neutron diffraction

A comprehensive set of interatomic potential parameters for modeling natural apatite crystals is presented. These potentials build on those previously used in research on apatites with new potentials fitted empirically to crystal structures and their properties using the GULP program. We demonstrate that the new potentials produce good models for the different compounds used for fitting, as well as for several natural apatites. Also presented are predicted enthalpies of mixing of strontium and calcium apatites and predicted cation site preferences in strontium calcium fluorapatite.

Interatomic potential models for natural apatite crystals: Incorporating strontium and the lanthanides

19F NMR isotropic chemical shift (δiso) calculations are performed in crystallized compounds using the GIAO method with the B3LYP hybrid functional at DFT level. Clusters centered on the studied fluorine atoms mimic the crystalline structures. The 6-311+G(d) basis set is chosen for the central fluorine atom, and the LanL2DZ basis set for the others. The metal atoms are described by the 3-21G(2d) basis set or, when not available, by the CRENBL basis set with the corresponding ECP, and augmented with 2d polarization functions when existing. First, for high-symmetry systems (MF, MF2, and MF3 compounds), a systematization of the cluster building up from coordination spheres is proposed, generalized to fluoroperovskites and fluoroaluminates KAlF4 and RbAlF4. When applied to rather low symmetry systems such as barium fluorometalates BaMgF4, BaZnF4, and Ba2ZnF6, the definition of the coordination spheres is far from easy. Then, for structures built up from a MF6 octahedron network, we may define different “start...

Cluster Models and ab Initio Calculations of 19F NMR Isotropic Chemical Shifts for Inorganic Fluorides

Measurements have been made, at wavelengths in the range 0.3-6.2 A, of hhl reflections of SrF2 on the single-crystal diffractometer (SXD) at the ISIS Spallation Neutron Source. After application of a variable-wavelength extinction correction to the derived I F,,k,I values, a refinement of the anharmonicity parameter /3F of the fluorine atoms was carried out, yielding a value of-4.19 (30) x 10-19 J ~-3.

J. B. Forsyth

Papers

A time‐of‐flight neutron diffraction study of anharmonic thermal vibrations in SrF2, at the spallation neutron source ISIS