Abstract. Relaxation times (T1) and lineshapes were examined as a function of temperature through the c~-// transition for z9si in a single crystal of amethyst, and for 29Si and 170 in cristobalite powders. For single crystal quartz, the three z9Si peaks observed at room temperature, representing each of the three differently oriented SiO, tetrahedra in the unit cell, coalesce with increasing temperature such that at the c~-//transition only one peak is observed. 29Si Tl's decrease with increasing temperature up to the transition, above which they remain constant. Although these results are not uniquely interpretable, hopping between the Dauphin6 twin related configurations, el and ez, may be the fluctuations responsible for both effects. This exchange becomes observable up to 150~ below the transition, and persists above the transition, resulting in//-quartz being a time and space average of el and e2. 298i Tl's for isotopically enriched powdered cristobalite show much the same behavior as observed for quartz. In addition, 170 Tl's decrease slowly up to the e-// transition at which point there is an abrupt 1.5 order of magnitude drop. Fitting of static powder ~70 spectra for cristobalite gives an asymmetry parameter (t/) of 0.125 at room T, which decreases to <0.040 at the transition temperature. The electric field gradient (EFG) and chemical shift anisotropy (CSA), however, remain the same, suggesting that the decrease in t/is caused by a dynamical rotation of the tetrahedra below the transition. Thus, the mechanisms of the e-// phase transitions in quartz and cristobalite are similar: there appears to be some fluctuation of the tetrahedra between twin-related orientations below the transition temperature, and the//-phase is characterized by a dynamical average of the twin domains on a unit cell scale.

Abstract.Cryolite is a mixed-cation perovskite (Na2(NaA1)F6) which undergoes a monoclinic to orthorhombic displacive phase transition at ~ 550 ~ C. Chiolite (NasA13F14) is associated with cryolite in natural deposits, and consists of sheets of corner sharing [A1F6] octahedra interlayered with edge-sharing [NaF6] octahedra. Multi-nuclear NMR line shape and relaxation time (T1) studies were performed on cryolite and chiolite in order to gain a better understanding of the atomic motions associated with the phase transition in cryolite, and Na diffusion in cryolite and chiolite. 27A1, 23Na, and 19F static NMR spectra and Tl's in cryolite suggest that oscillatory motions of the [A1F6] octahedra among four micro-twin and anti-phase domains in e-cryolite begin at least 150 ~ C below the transition temperature and persist above it. Variable temperature 23Na MAS NMR further indicates diffusional exchange at a rate of at least 13 kHz between the Na sites by the time the transition temperature is reached. 27A1 and 23Na Tl'S show the same behavior with increasing temperature, indicating the same relaxation mechanisms are responsible for both. The first order nature of the cryolite transition is apparent as a jump in the 23Na and 27A1 Tl's. Above the transition temperature, the TI'S decrease slightly indicating that the motions responsible for the drop in T 1 are still present above the transition, further supporting the dynamic nature of the high temperature phase of cryolite. Chiolite 23Na static spectra decrease in linewidth with increasing temperature, indicating increased Na diffusion, which is interpreted as occurring within the [NaF6] sheets in the chiolite structure, but not between the two different Na sites. 27A1 and 23Na Tl's show similar behavior as in cryolite, but there is no discontinuity due to a phase transition. 19F TI's are constant from room temperature to 150 ~ C indicating no oscillatory motion of the [A1F6] octahedra in chiolite.

The sodium dizirconium tris(phosphate) structural family ([NZP]) includes compounds that may be represented by the general formula M′M′′ 1-3 A 2 (PO 4 ) 3 . The ability of KZr 2 (PO 4 ) 3 , a member of the [NZP] structural family, to accommodate U(IV) on the octahedrally coordinated A site has been demonstrated for compounds in the series KZr 2-x U x (PO 4 ) 3 (0 e x e 0.20). KU 2 (PO 4 ) 3 , the end member of the series, was found to adopt a monoclinic structure with 9-fold coordination of U(IV) that does not belong to the [NZP] family. The compounds were prepared from sol-gel derived precursors in an argon environment. X-ray microanalyses indicated that the precursor powders had reacted fully to produce compounds of the expected stoichiometries. Rietveld refined X-ray powder diffraction data of KZr 2-x U x (PO 4 ) 3 (0 e x e 0.20) confirmed a rhombohedral (R3 hc) structure and suggested random occupation of the A site by U/Zr. The presence of U(IV) was established by comparison of the UV/vis spectra of KZr 2-x U x (PO 4 ) 3 (0 e x e 0.20) with those of other U(IV) phosphates including KU 2 (PO 4 ) 3 . The Rietveld refined data show an increase in the volume of the R3 hc cell with increasing values of x. The structure of KU 2 (PO 4 ) 3 , determined from Rietveld refinement of powder X-ray diffraction data, is monoclinic (space group C2/c, Z ) 4) with unit cell parameters a ) 17.4705(4) Å, b ) 6.75408(13) Å, c ) 8.02522(17) Å, β ) 102.0189(17)°, and V ) 926.196-(34) Å 3 . The final indicators of the quality of the Rietveld refinement were R wp ) 14.07%, R e ) 10.02%, and R F ) 3.78%.

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The formation of zircon (ZrSiO 4 ) via sintering of milled SiO 2 and ZrO 2 powders was studied, and the effects of slurry vs dry milling, sintering time, and particle size on zircon yield were examined. It was found that very high zircon yields could be obtained via slurry milling, cold pressing, and sintering of the oxide precursors. The controlling factor in determining zircon yield was found to be the particle size of the SiO 2 and ZrO 2 powders. Zircon yield as a function of sintering time was examined, and found to be similar to previous studies in which sol-gel precursors seeded with zircon were used. SEM studies reveal a homogeneous product with particle sizes on the order of 1-5 µm. It was found that complete reaction to zircon can be achieved from a once-through milling, pressing, and sintering process of SiO 2 -ZrO 2 powders.