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We demonstrate a new modulation for a high-temperature polymorph of BaSrFe4O8 via synchrotron powder X-ray diffraction and transmission electron microscopy (TEM) experiments. Although the powder X-ray diffraction pattern was successfully analyzed using a structure model of space group Pnna , an unusually large isotropic atomic displacement parameter ( U iso ) was observed for the O1 atom. A split atom model that was applied to the O1 site and the adjacent Ba/Sr site, both of which are on the two-fold axis along the c-axis, indicated that positional disorder occurred in these atoms along the c-axis. The electron diffraction pattern and high-resolution TEM observations combined with the structure refinements revealed an incommensurate structure with a modulation vector q = 0.4270 ( 1 ) a * and a displacement vector perpendicular to the a-axis, which gave rise to the large U iso value of the O1 atom. The modulation observed in this study may be a signature of structural instability, which has commonly been discussed in the stuffed tridymite-type oxides.

Temperature-induced phase transitions for stuffed tridymite A Ga 2 O 4 ( A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO 3 and Ga 2 O 3 led to the formation of γ-SrGa 2 O 4 ( P 2 1 /n ) below 1200 °C, and transform to β-SrGa 2 O 4 ( P 2 1 /c ) when heated at 1200 °C. A similar phenomenon was found for CaGa 2 O 4 , and the temperature boundary between α-CaGa 2 O 4 ( Pna 2 1 ) and the high temperature polymorph β-CaGa 2 O 4 ( P 2 1 /c ) was about 1350 °C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr 2+ /Ca 2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa 2 O 4 and α-CaGa 2 O 4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.

The partial substitution of Al by Fe atoms in SrAl2O4 allowed to stabilize four stuffed tridymite derivative structures SrAl1.5Fe0.5O4. The different phases have been characterized by TEM and XRPD techniques. Two are isotypes of those observed for the undoped oxides, namely the hexagonal phase with 3 A× 3 A×C (with A and C being the tridymite unit cell parameters) and space group P63 and the monoclinic one with A× 3 A×C, β≈93° and space group P21, with a synthesis temperature lower than the one required for SrAl2O4. By annealing, two original phases, denoted O1 and O2, are obtained; they are metrically similar (3A× 3 A×C and β≈90°) and only differ by their space groups. The TEM study showed that the transitions between the different phases follow topotactic mechanisms, through the formation of twinning boundaries, antiphase boundaries and planar defects. The annealed sample exhibits a mesomorph state and a reversible transition from this semi-ordered state to a crystalline phase. This dynamic transition takes place over a very wide temperature range from 620 to 1120 °C. The reversibility of the transition has been studied by DSC measurements. The crystallization energy of the orthorhombic phases is of the order of 10 J/g, at T≈622 °C as T decreases. The variation of the peak height observed as the annealing temperature increases is explained by the complex microstructures, which create an ill-defined energy barrier. Structural models related to the stuffed tridymite derivative structures are proposed for the new forms of the ferri-aluminate.

Lithium zinc silicate glasses of composition (mol%): 17.5Li2O–(72−x)SiO2–xZnO−5.1Na2O−1.3P2O5−4.1B2O3, 5.5⩽x⩽17.7, were prepared by conventional melt-quenched technique and converted to glass-ceramic by controlled crystallization process. 29Si and 31P MAS-NMR was used to characterize the structure of both glass and glass-ceramic samples. Despite the complex glass composition, Q2, Q3 and Q4 sites are identified from 29Si MAS-NMR, which relative intensities are found to vary with the ZnO content, indicating a network depolymerization by ZnO. Moreover, well separated Q3 and Q4 resonances for low ZnO content indicates the occurrence of phase separation. From 31P MAS-NMR, it is seen that phosphorus is mainly present in the form of ortho-(Q0) and pyro-phosphate (Q1) structural units and variation of ZnO content did not have much effect on these resonances, which provides an additional evidence for phase separation in the glass. On conversion to glass-ceramics, lithium disilicate (Li2Si2O5), lithium zinc ortho-silicate (Li3Zn0.5SiO4), tridymite (SiO2) and cristobalite (SiO2) were identified as major silicate crystalline phases. Using 29Si MAS-NMR, quantification of these silicate crystalline phases is carried out and correlated with the ZnO content in the glass-ceramics samples. In addition, 31P spectra unambiguously revealed the presence of crystalline Li3PO4 and (Na,Li)3PO4 in the glass-ceramics.

The crystal structure of SrAl{sub 2}O{sub 4} at 1073K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was hexagonal (space group P6{sub 3}, Z=6) with a=0.89260(3)nm, c=0.84985(2)nm and V=0.58639(3)nm{sup 3}. Final reliability indices were R{sub wp}=7.87%, R{sub p}=5.87% and R{sub B}=4.19%. The [AlO{sub 4}] tetrahedra are linked to form trigonally distorted rings and they are joined in layers. These layers are stacked with a two-layer repeat and connected by the tetrahedral apices. All of the Sr atoms occupy the centers of the rings when viewed along the c-axis. The structure is described as a stuffed derivative of tridymite.

The effect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural effect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS diffractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identification of the fraction of the different silica polymorphs like quartz, tridymite, and cristobalite. SAXS measurements, using the classical method based on Porod's law, yielded the total surface area of the systems. The calculated areas were compared with the surface areas determined by the nitrogen adsorption technique using the analytical method of Brunauer–Emmett–Teller. The results are explained in terms of sizes of the alkali ions and cell volume of the different crystalline phases.

The new compound KMgPO{sub 4} has been synthesized, and its structure has been determined at room-temperature ({alpha} form): space groups is P2{sub 1}/c (No. 14) with cell parameters a = 8.549(2) {angstrom}, b = 5.078(1) {angstrom}, c = 18.996(2) {angstrom}, {beta} = 91.68(1){degree}, and Z = 8. Phase transitions occur at 362 and 422 C, {beta}-KMgPO{sub 4} and {gamma}-KMgPO{sub 4} are orthorhombic, respectively Pna2{sub 1} (No. 33) and Pnma (No. 62). The {beta} and {gamma} forms are related to the tridymite type, with a skeleton made of MgO{sub 4} and PO{sub 4} tetrahedra, but in {alpha} form, half of the magnesium atoms are penta-coordinated.

Single crystals of monophosphate TlBePO 4 , grown by the hydrothermal process, belong to the orthorhombic space group Pna 2 1 . Unit cell contains four formulae ( Z = 4) and crystal parameters are found to be a = 9.286(3) A, b = 8.090(3) A, c = 4.837(1) A. Rows of vertices-connected PO 4 and BeO 4 tetrahedra are laid out in planes, forming six-membered rings in which bulky thallium atoms find their place. Although clearly related to the stuffed tridymite structural type, the oxygen framework shows an unusual distortion, as in the case of monoarsenate TlBeAsO 4 . Stereochemical activity of the partly hybridized thallium (I) 6s 2 pair, located after calculation of the local electrostatic field, appears as the cause of this peculiarity.

The calcination behavior of coprecipitated iron-aluminum mixed phosphates having the composition Al 1- x Fe x PO 4 , where x = 0.0, 0.25, 0.5, 0.75, and 1.0, is investigated by powder X-ray diffraction and 57 Fe Mossbauer spectroscopy. It is observed that solid solution formation between FePO 4 and AlPO 4 , which are not isostructural, takes place by incorporation of Fe 3+ ions into the AlPO 4 structure for both Al 0.75 Fe 0.25 PO 4 and Al 0.5 Fe 0.5 PO 4 . Due to this solid solution formation, the phase transformation of AlPO 4 from tridymite to the orthorhombic phase takes place at lower temperatures. It is also observed that on solid solution formation the decomposition of FePO 4 to Fe 3 PO 7 and α-Fe 2 O 3 is suppressed. The values of the isomeric shift and quadrupole splitting observed for Fe 3+ ions in the tridymite phase are different from those of the trigonal phase of FePO 4 . Mossbauer spectra of Al 0.75 Fe 0.25 PO 4 in the orthorhombic form show an unresolved quadrupole doublet with δ = 0.26 and Δ Eq = 0.25 mm/sec. In the Al 0.25 Fe 0.75 PO 4 sample, incorporation of AlPO 4 into the FePO 4 lattice takes place on calcination at ≅ 1075 K. The Mossbauer parameters for these samples did not show any change compared to those of the trigonal FePO 4 .

The crystal structures of the compounds BaMSiO4 (M = Co, Mg, Zn) have been refined by using single-crystal X-ray data (M = Co) and powder neutron data (M = Mg, Zn). The three compounds are isostructural Ba-stuffed derivatives of tridymite (SiO2) and crystallize with a ( 3 × A, C ) superstructure of hexagonal (A, C ) kalsilite (KAlSiO4). A clear correlation exists between the degree of collapse of the fully ordered tetrahedral framework and the size difference between the M and Si atoms.

The crystallization behavior of amorphous aluminum phosphate (AlPO4) has been studied by X-ray diffraction as a function of the treatment temperature (500–1150°C) in the presence of small amounts of titania (TiO2) anchored onto its surface. The compound, which like silica can exist in several possible crystalline modifications depending on the treatment temperature, is found to grow in the presence of TiO2 exclusively in a single phase in the entire range of temperatures studied. The phase has been shown to have the tridymite structure belonging to the orthorhombic system with cell parameters a = 9.638 ± 0.0019, b = 8.664 ± 0.0017, and c = 18.280 ± 0036A.

High-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB2O4 (A=Ca2+, Sr2+ and Ba2+ and B=Al3+ and Ga3+). These oxides all adopt a stuffed tridymite type structure, the precise nature of which depends on both the A- and B-type cations. Bond valence calculations reveal a range of values for the various A-type cations, in all cases at least one site is significantly underbonded. Conversely the tetrahedral B-type sites invariably exhibit unexceptional bond valencies. Attempts to dope the oxides with various lanthanides to the 1% level invariably resulted in some segregation into alternate phases located at the grain boundaries. The identity of the impurity phases is presented and the importance of bond valencies in understanding this segregation is highlighted.

Twinning on the unit cell level of the idealized cristobalite structure, using a mirror plane as the twin and composition plane, provides a simple relationship between 14 tetrahedral frameworks. Of these, 9 are found among the aluminosilicates with examples ranging from (stuffed) silicas to zeolites and include the framework types of nepheline hydrate I, zeolite LiA(BW), gismondine, phillipsite, merlinoite, tridymite, paracelsian, and monoclinic CaAl2Si2O8. Similar twinning relates the frameworks of natrolite, thomsonite, and edingtonite.

Crystal data and phase transitions of KLiWO4 and KLiMoO4 were investigated using a high-temperature X-ray powder diffractometer. Phase transitions were observed at 350°C for KLiWO4 and at 360°C for KLiMoO4. It was found that the room temperature phases were isostructural with stuffed derivatives of tridymite and the high-temperature phases were isostructural with stuffed derivatives of cristobalite.