Heribert A. Graetsch

NMR, XRD and IR study on microcrystalline opals

Microcrystalline opal-CT and opal-C were investigated by 29Si MAS NMR and 29Si {1H} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite. Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.

Structure of strontium barium niobate SrxBa1 − xNb2O6 (SBN) in the composition range 0.32 ≤ x ≤ 0.82

The structure of strontium barium niobate crystals SrxBa1-xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x=0.32-0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate.

The thermal expansion of the zeolites MFI, AFI, DOH, DDR, and MTN in their calcined and as synthesized forms

The thermal expansion coefficients of the silica zeolites MFI, DOH, DDR, and MTN and of the alumophosphate AFI were determined using the X-ray Guinier technique. At low temperatures the as synthesized microporous materials show positive volume expansion coefficients of the order of magnitude of 10·10−6K−1 which are in the expected range for silicates or phosphates. After calcination the thermal expansion of these materials is positive below the displacive phase transitions to their high symmetry forms. The high temperature phases, however, contract with increasing temperature. Their volume expansion coefficients are negative and in the order of −10·10−6K−1.

Crystallographic and magnetic investigation on W-type-hexaferrite single crystals in the solid solution series SrZn 2-x Co x Fe 16 O 27

The saturation magnetization and the magnetocrystalline anisotropy were measured on single crystals in the solid solution series SrZn 2-x Co x Fe 16 O 27 (SrZn 2-x Co x -W) at 298 K and 6 K. Lattice constants and Curie-temperatures are also given as a function of the composition. The magnetocrystalline anisotropy changes from uniaxial to planar dependent on temperature and cobalt substitution at x \approx 0.5 for 6 K and x \approx 0.85 for 298 K. The six-fold anisotropy in the basal plane of the planar W-hexaferrites increases strongly with increasing cobalt content. Crystals with a low Co substitution (x = 0.33 and 0.67) have an anomalous discontinuity in their hard direction magnetization curves; indicating a first-order magnetization process.

Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature

Abstract Boralsilite, the only natural anhydrous ternary B2O3-Al2O3-SiO2 (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B2O3 and H2O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V = 1245.8(2) Å3. Boron replaces 14% of the Si at the Si site, and Si or Al replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase (“boron-mullite”) synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) Å3. “Boron-mullite” also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar. A possible natural analogue of “boron-mullite” is replacing the Fe-dominant analogue of werdingite in B-rich metapelites at Mount Stafford, central Australia; its composition extends from close to stoichiometric Al2SiO5 to Al2.06B0.26Si0.76O5, i.e., almost halfway to Al5BO9. Boralsilite is a minor constituent of pegmatites cutting granulite-facies rocks in the Larsemann Hills, Prydz Bay, East Antarctica, and at Almgotheii, Rogaland, Norway. Electron-microprobe analyses (including B) gave two distinct types: (1) a limited solid solution in which Si varies inversely with B over a narrow range, and (2) a more extensive solid solution containing up to 30% (Mg,Fe)2Al14B4Si4O37 (werdingite). Boralsilite in the Larsemann Hills is commonly associated with graphic tourmaline-quartz intergrowths, which could be the products of rapid growth due to oversaturation, leaving a residual melt thoroughly depleted in Fe and Mg, but not in Al and B. The combination of a B-rich source and relatively low water content, together with limited fractionation, resulted in an unusual buildup of B, but not of Li, Be, and other elements normally concentrated in pegmatites. The resulting conditions are favorable in the late stages of pegmatite crystallization for precipitation of boralsilite, werdingite, and grandidierite instead of elbaite and B minerals characteristic of the later stages in more fractionated pegmatites.

Co-templating ionothermal synthesis and structure characterization of two new 2D layered aluminophosphates

For the first time, the co-templating ionothermal methodology was used in the preparation of layered aluminophosphate materials. With the addition of either 1,2-ethylenediamine or 1,6-hexanediamine to the ionic liquid 1-ethyl-3-methyl imidazolium chloride, two new 2D layered aluminophosphates RUB-A1 [Al(3)P(4)O(16)][NH(3)CH(2)CH(2)NH(3)](0.5)[C(6)N(2)H(11)](2) and RUB-A2 [Al(3)P(4)O(16)][NH(3)(CH(2))(6)NH(3)][NH(3)(CH(2))(6)NH(2)](0.5)[C(6)N(2)H(11)](0.5)[H(2)O] have been synthesized ionothermally by co-templating. The structure of RUB-A1 has been determined from single-crystal X-ray diffraction data using direct methods, while the structure of RUB-A2 has been solved ab initio from powder X-ray diffraction data with limited resolution using direct-space methods. Both of these two compounds have a 2D layered structure consisting of macroanionic sheets of composition [Al(3)P(4)O(16)](3-) stacked in an AAAA sequence. The inorganic layers are built up from alternatively vertex-sharing [AlO(4)]- and [PO(3)(=O)]-tetrahedral units forming a 4.6.8 and a 4.6.12 network for RUB-A1 and RUB-A2, respectively. The layer topology of RUB-A1 is closely related to the previously known 4.6.8-layer topology but with a different sequence of phosphoryl group orientation. Combining the results of structure analysis with the NMR, chemical analysis and TG-DTA experiments, we show that both the ionic liquid cation and the protonated diamines are located in the interlayer space and together direct the formation of these two structures.

Characterization of the high-temperature modifications of incommensurate tridymite L3-T o (MX-1) from 25 to 250 degrees C

Abstract Incommensurate tridymite L3-TO(MX-1) shows a cascade of five phase transitions at 65, HO, 150, 200, and 380 ℃ upon heating. The X-ray diffraction patterns were investigated with a Buerger precession camera revealing a sequence of four incommensurate phases in the range from room temperature to 200 ℃. The phases formed between 65 and HO ℃ and between HO and 150 ℃ are new modifications of tridymite. At 65 ℃ the monoclinic tridymite L3-T0(MX-1) phase undergoes a first-order transformation to an orthorhombic phase. The incommensurate structural modulation of the room-temperature phase with the wavevector qx = 0.663 a* -0.498 c* flips to q2 = 0.042 a* -0.388 c*. Simultaneously, a commensurate modulation with tripled b lattice parameter is formed. The wavelengths of both modulations do not depend appreciably on the temperature. The incommensurate modulation discontinuously disappears near 110 ℃ whereas the commensurate modulation along the b axis becomes non-integral with a temperature-dependent wavelength varying between 115 and 100 A. At 150 Å the symmetry is reduced to monoclinic again with 7 = 90.4°. Between 150 and 200 ℃ the monoclinic angle gradually decreases to 90° and the wavelength of the modulation from about 90 to 65 Å. At higher temperatures, the satellite reflections fade into weak streaks and the normal orthorhombic high-temperature modification of tridymite is formed. The phase transitions are reversible upon cooling except for the first transformation that is partly irreversible for single crystals and reversible but incomplete for pulverized material at room temperature.

Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)

The incommensurately modulated crystal structures of Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN28) and Ce(0.02)Ca(0.25)Ba(0.72)Nb(2)O(6) (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,-AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,-aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO(6) octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.

Hexagonal high-temperature form of aluminium phosphate tridymite from X-ray powder data

Similar to silica tridymite, AlPO4 tridymite shows a sequence of displacive phase transitions resulting in a dynamically disordered hexagonal high-temperature modification. Rietveld refinement reveals that the thermal motions of the tetrahedra can be described either by strongly anisotropic displacement parameters for oxygen or by split O atoms. Due to the ordered distribution of aluminium and phosphorus over alternating tetrahedra, the space group symmetry of high-temperature AlPO4 tridymite is reduced with respect to SiO2 tridymite from P6(3)/mmc to P6(3)mc.

Two forms of aluminium phosphate tridymite from X‐ray powder data

Monoclinic and triclinic (pseudo-orthorhombic) AlPO4 tridymites have been refined from X-ray powder diffraction data using the silica analogues as starting models. The framework structures of both forms of tridymite are made up of six-membered rings of tetrahedra which differ in the distortion patterns of the ring shapes. Ordered occupation of alternate tetrahedra by Al and P leads to a doubling of the a lattice parameter for monoclinic AlPO4 tridymite (space group Pc) and loss of the C-centring with respect to the isotypic silica tridymite (space group Cc). Triclinic AlPO4 tridymite was refined in the same space group (F1) as the SiO2 analogue.