@article {625,
	title = {The amblygonite (LiAlPO4F)-montebrasite (LiAlPO4OH) solid solution: A combined powder and single-crystal neutron diffraction and solid-state Li-6 MAS, CP MAS, and REDOR NMR study},
	journal = {American Mineralogist},
	volume = {88},
	number = {1},
	year = {2003},
	note = {ISI Document Delivery No.: 635DVTimes Cited: 10Cited Reference Count: 29},
	month = {Jan},
	pages = {195-210},
	type = {Article},
	abstract = {The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO4(F, OH). These compounds are ideal for studying F <{\textendash}{\textendash}> OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state Li-6 MAS, CP MAS, and REDOR NMR techniques to study the effect of F <{\textendash}{\textendash}> OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U-3 eigenvalue, which lies between the c and c* directions. U-eq values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder. At the montebrasite end of the series, there are five short Li-phi (phi = O, OH, F) distances and one very long Li-O4 bond. With increasing F content, the Li-O4 distance decreases and the Li-O3f distance increases, such that at the amblygonite end, the coordination is 4 + 2. The disorder in the Li site is obviously caused by the substitution of F for OH. The driving force is the loss of the hydrogen bond to 04, which causes the Li-O4 bond to strengthen and improves the bond valence to O4. The results show that the H atom position is imbedded within the distorted octahedral oxygen coordination of the Li atom. To represent the disorder better, we used a split Li site model. The results show that Li1 occupancy increases and the Li2 occupancy decreases with increasing F content, and that the Li1-Li2 distance is longer for intermediate compositions than for the end-members. The Li-6 MAS NMR experiments provide important structural information complementary to the neutron diffraction results. The spectra of samples in the amblygonite-montebrasite series show two well-resolved peaks, confirming the presence of Li disorder over two distinct sites, and highly resolved Li-6 MAS NMR spectra are obtained at the very high magnetic field strength of 18.8 T. The peaks at -0.3 ppm and -0.9 ppm were unambiguously assigned to the Li2(OH) and Li1(F) sites found in the neutron diffraction structures. The isotropic chemical shifts are consistent with the coordinations of these Li sites found in the neutron diffraction structures. The relative intensities of the two peaks across the series of samples reflect the varying F/(F + OH). In addition to confirming the assignments of the peaks, it is possible to measure H-Li2 and F-Li1 internuclear distances by Li-6 {H-1} and Li-6 {F-19} CP and REDOR NMR that are consistent with the corresponding distances from the neutron diffraction structures. The Li-6 {H-1} and Li-6 {F-19} CP and REDOR results indicate that the Li disorder is random throughout the crystals rather than over large domains, a conclusion that cannot be made from diffraction experiments. Variable temperature Li-6 MAS NMR spectra confirm that the disorder is static and there is no dynamic exchange involving F, OH, or Li. Each Li ion has access to only one of the two observed sites as determined by the presence of either OH or F in its immediate environment and there is no possibility of a dynamic exchange.},
	keywords = {CROSS-POLARIZATION, DISTANCE DETERMINATIONS, FLUORINE},
	isbn = {0003-004X},
	url = {<Go to ISI>://000180383600023},
	author = {Groat, L. A. and Chakoumakos, B. C. and Brouwer, D. H. and Hoffman, C. M. and Fyfe, C. A. and Morell, H. and Schultz, A. J.}
}
@article {382,
	title = {Combined solid state NMR and X-ray diffraction investigation of the local structure of the five-coordinate silicon in fluoride-containing as-synthesized STF zeolite},
	journal = {Journal of the American Chemical Society},
	volume = {124},
	number = {26},
	year = {2002},
	note = {ISI Document Delivery No.: 567YYTimes Cited: 41Cited Reference Count: 48},
	month = {Jul},
	pages = {7770-7778},
	type = {Article},
	abstract = {The local structure of the [SiO4/2F](-) unit in fluoride-containing as-synthesized STF zeolite has been experimentally determined by a combination of solid-state NMR and microcrystal X-ray diffraction to be very close to trigonal bipyramidal. Because the fluoride ions are disordered over two sites, the resulting local structure of the [SiO4/2F](-) unit from a conventional XRD refinement is an average between tetrahedral SiO4/2 and five-coordinate [SiO4/2F](-), giving an apparent F-Si distance longer than expected. The correct F-Si distance was determined by slow spinning MAS and fast spinning F-19/Si-29 CP and REDOR solid-state NMR experiments and found to be between 1.72 and 1.79 Angstrom. In light of this, the X-ray structure was re-refined, including the disorder at Si3. The resulting local structure of the [SiO4/2F]- unit was very close to trigonal bipyramidal with a F-Si distance of 1.744 (6) Angstrom, in agreement with the NMR results and the prediction of Density Functional Theory calculations. In addition, further evidence for the existence of a covalent F-Si bond is provided by a F-19{\textendash}>Si-29 refocused INEPT experiment. The resonance for the five-coordinate species at -147.5 ppm in the Si-29 Spectrum is a doublet due to the F-19/Si-29 J-coupling of 165 Hz. The peaks in this doublet have remarkably different effective chemical shift anisotropies due to the interplay of the CSA, dipolar coupling, and J-coupling tensors. The distortions from tetrahedral geometry of the neighboring silicon atoms to the five-coordinate Si3 atom are manifested in increased delta(aniso) values. This information, along with F-Si distances measured by F-19{\textendash}>Si-29 CP experiments, makes it possible to assign half of the Si-29 resonances to unique tetrahedral sites. As well as determining the local geometry of the [SiO4/2F](-) unit, the work presented here demonstrates the complementarity of the solid-state NMR and X-ray diffraction techniques and the advantages of using them together.},
	keywords = {5-COORDINATE SILICON, C-13 NMR, CONTAINING OCTADECASIL, CROSS-POLARIZATION, DISTANCE DETERMINATIONS, LOCATION, MOLECULAR-SIEVES, P-31 NMR, POLARIZATION TRANSFER, PROTON-ENHANCED NMR},
	isbn = {0002-7863},
	url = {<Go to ISI>://000176516400032},
	author = {Fyfe, C. A. and Brouwer, D. H. and Lewis, A. R. and Villaescusa, L. A. and Morris, R. E.}
}
