Georg Roth

QUANTITATIVE PHASE ANALYSIS OF BENTONITES BY THE RIETVELD METHOD

Thirty six bentonite samples from 16 different locations were examined in order to demonstrate the applicability of a new Rietveld description approach for quantitative phase analysis. X-ray diffraction patterns of the bulk material were obtained and analyzed by the Rietveld method. The samples contain up to ten different minerals, with dioctahedral smectite as the major component. A model for turbostratic disorder of smectites was formulated inside a structure-description file of the Rietveld program BGMN. The quality of the refinements was checked using an internal standard mineral (10.0 or 20.0 wt.% corundum) and by cross-checking results with X-ray fluorescence (XRF) data. The corundum content was reproduced with only small deviations from the nominal values. A comparison of the chemical composition obtained by XRF and the composition as re-calculated from quantitative Rietveld results shows a satisfactory agreement, although X-ray amorphous components such as volcanic glasses were not considered. As a result of this study, the Rietveld method combined with the new structure model for turbostratic disorder has proven to be a suitable method for routine quantitative analysis of bentonites with smectites as the dominant clay minerals.

Description of X-ray powder pattern of turbostratically disordered layer structures with a Rietveld compatible approach

Abstract We address the problem of the quantitative description of X-ray powder pattern of turbostratically disordered layer compounds. The Debye formula is used, which allows the aperiodic description of any arrangement of atoms. With the extension of Yang and Frindt (1996) for the ideal turbostratic case, these calculations are used to generate reference data that are subsequently treated by the Rietveld method. We are able to show that the case of uncorrelated turbostratic disorder can be modelled equally well in a periodic supercell approach with a single layer in the supercell that is suitable for the Rietveld technique. A brief introduction of this new model was given as an oral contribution at EUROCLAY 2003 (Ufer et al., 2003). The fundamental principles are described in this article because of its complexity. The applicability of this approach to real systems is demonstrated for smectite and corundum mixtures.

Structural variation and crystal chemistry of LiMe3+Si2O6 clinopyroxenes Me3+ = Al, Ga, Cr, V, Fe, Sc and In

Abstract A total of 32 synthetic end-member and solid-solution compounds of the LiM3+Si2O6 (Li = M2 site, M3+ = M1 site = Al, Ga, V, Fe, Sc and In) clinopyroxene series have been investigated by single-crystal X-ray diffraction. Except LiCrSi2O6, all compounds show C2/c symmetry at 295 K. LiCrSi2O6 has space group P21/c but transforms to the high temperature C2/c structure at 335 K. The variations of structural parameters in the LiMe3+Si2O6 clinopyroxenes are dominated by the Me3+ site. The average M1—O bond length is linearly correlated with the ionic radius of the M1 cation. Octahedra reflect the increasing size of the M1 cations by steadily increasing bond and edge lengths, the variations however are not uniform. With increasing size of the M1 cation, octahedra deviate from ideal octahedral geometry. Octahedral edges, shared with other structural units, are distinctly hampered in expansion with increasing size of the M1 cation. The increasing size of the M1 octahedral chain is compensated by changing the kinking of the tetrahedral chain and by alterations of bond and edge lengths as well as the bond angles within the tetrahedron. Three different mechanisms of adapting of the structural building units with increasing M3+ cationic radius can be identified: (i) expansion of the tetrahedral chain by stretching (ii) transition form “O” to “S” chain conformation after full expansion and (iii) finally a limit of expansion in a direction. We stress that cations larger than In3+ cannot be substituted at the M1 site because of too large geometrical differences between octahedral and tetrahedral chains.

Structural changes upon the temperature dependent C2/c → P21/c phase transition in LiMe3+Si2O6 clinopyroxenes, Me = Cr, Ga, fe, V, Sc and In

Abstract Within the Li-clinopyroxene series LiMe3+Si2O6, a temperature induced C2/c → P21/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al3+ and the pure In3+ compound down to 80 K. Within the LiSc1–xInxSi2O6 solid solution se ries, the transition temperature rapidly decreases with increasing In3+ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi2O6, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q2 (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc3+ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi2O6, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P21/c, decrease of the coordination number of Li+ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi2O6 is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P21/c phase transition.

Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298 < T < 100 K).

The alkali-metal clinopyroxene NaTi(3+)Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100 K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti(3+)-Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti-Ti singlet pairs in the low-temperature phase. Mean Ti-O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti-O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti-O(apex) bond lengths, but also with respect to the four Ti-O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P1; low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P1; to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na-O bond lengths decrease with decreasing temperature and the variations in individual Na-O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c-->P2(1)/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site.

Spinnability and Characteristics of Polyvinylidene Fluoride (PVDF)-based Bicomponent Fibers with a Carbon Nanotube (CNT) Modified Polypropylene Core for Piezoelectric Applications

This research explains the melt spinning of bicomponent fibers, consisting of a conductive polypropylene (PP) core and a piezoelectric sheath (polyvinylidene fluoride). Previously analyzed piezoelectric capabilities of polyvinylidene fluoride (PVDF) are to be exploited in sensor filaments. The PP compound contains a 10 wt % carbon nanotubes (CNTs) and 2 wt % sodium stearate (NaSt). The sodium stearate is added to lower the viscosity of the melt. The compound constitutes the fiber core that is conductive due to a percolation CNT network. The PVDF sheath’s piezoelectric effect is based on the formation of an all-trans conformation β phase, caused by draw-winding of the fibers. The core and sheath materials, as well as the bicomponent fibers, are characterized through different analytical methods. These include wide-angle X-ray diffraction (WAXD) to analyze crucial parameters for the development of a crystalline β phase. The distribution of CNTs in the polymer matrix, which affects the conductivity of the core, was investigated by transmission electron microscopy (TEM). Thermal characterization is carried out by conventional differential scanning calorimetry (DSC). Optical microscopy is used to determine the fibers’ diameter regularity (core and sheath). The materials’ viscosity is determined by rheometry. Eventually, an LCR tester is used to determine the core’s specific resistance.

β-Y2Si2O7, a new thortveitite-type compound, determined at 100 and 280 K

A new form of Y2Si2O7 (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc2Si2O7, and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y3+ cation occupies a distorted octahedral site, with Y—O bond lengths in the range 2.239 (2)–2.309 (2) A. The SiO4 tetrahedron is remarkably regular, with Si—O bond lengths in the range 1.619 (2)–1.630 (2) A. The bridging O atom of the Si2O7 pyrosilicate group shows a large anisotropic displacement perpendicular to the Si—O bond. Changes in lattice and structural parameters upon cooling are small with, however, a distinct decrease of the anisotropic displacement of the briding O atom. Structure solution and refinement in the non-centrosymmetric space group C2 are possible but do not yield a significantly different structure model. The Si—O—Si bond angle of the isolated Si2O7 groups is 179.2 (1)° at 280 K in C2 and 180° per symmetry in C2/m. The C2/m structure model is favoured.

Hagendorfite (Na,Ca)MnFe2(PO4)3 from type locality Hagendorf (Bavaria, Germany) : crystal structure determination and 57Fe Mössbauer spectroscopy

The crystal structure of hagendorfite (Na,Ca)MnFe 2 (PO 4 ) 3 from type locality has been determined and it was found to be isostructural with alluaudite. It accepts space group symmetry C2/c both at room temperature and at 100 K. At room temperature the lattice parameters are a = 11.9721(9) A, b = 12.5988(8) A, c = 6.5029(5) A, β = 114.841(8)° with Z = 4. For reasons of comparison the structure of an alluaudite sensu strictu (s.s.) from Buranga (Rwanda) was re-determined. The specific arrangement of M(1) and M(2) octahedral sites and of P(1) and P(2) tetrahedral sites gives rise to two different channels aligned along the crystallographic c-axis, containing the A(1) and A(2)’ sites. In both compounds the A(1) site is fully occupied and shows a mixed occupation of Na + , Ca 2+ and Mn 2+ (hagendorfite) and Ca 2+ and Na + (alluaudite s.s.). The A(2)’ is fully occupied by Na + in hagendorfite and partly filled by 0.14 Na + atoms per formula unit in Buranga alluaudite. The structural topology of hagendorfite is described in detail and structural parameters are compared to alluaudite s.s. and to other, mostly synthetic compounds also crystallizing in the alluaudite structure type. 57 Fe Mossbauer spectroscopy shows that iron is exclusively in the trivalent state in alluaudite s.s. while in hagendorfite about 2/3 of the total iron are in the divalent state. The ferrous iron resonance absorption contribution appears to be broad and can only be refined with four different Fe 2+ subcomponents, which are all ascribed to the M(2) site. The four different spectroscopic signals presumably arise from different next nearest neighbour occupations of adjacent M(2) and A(1) sites.

Lithium and sodium yttrium ortho­silicate oxy­apatite, LiY9(SiO4)6O2 and NaY9(SiO4)6O2, at both 100 K and near room temperature

Lithium yttrium orthosilicate oxyapatite [lithium nonayttrium hexakis(silicate) dioxide], LiY(9)(SiO(4))(6)O(2), crystallizes in the centrosymmetric space group P6(3)/m at both 295 and 100 K. The structure closely resembles those of fluorine apatite and sodium yttrium orthosilicate oxyapatite [sodium nonayttrium hexakis(silicate) dioxide], NaY(9)(SiO(4))(6)O(2), which was also investigated, at 270 and 100 K, in this study. There are two different crystallographic sites for the Y(3+) ion, which are coordinated by seven and nine O atoms. One-fourth of the nine-coordinated site is occupied by Li or Na atoms, thus maintaining charge balance. The Si atom occupies a tetrahedral site. The two compounds show no symmetry change between room temperature and 100 K, and the alterations in structural parameters are small.

Crystal and magnetic spin structure of Germanium-Hedenbergite, CaFeGe2O6, and a comparison with other magnetic/magnetoelectric/multiferroic pyroxenes

Abstract CaFeGe2O6, the germanium-analogue to the mineral Hedenbergite, has been synthesized at 1273 K in evacuated SiO2-glass-tubes. Powder neutron diffraction data collected between 4 K and 300 K were used to evaluate the magnetic spin as well as the nuclear crystal structure and its T-evolution. CaFeGe2O6 is monoclinic, C2/c, a = 10.1778(5) Å, b = 9.0545(4) Å, c = 5.4319(3) Å, β = 104.263(3)°, Z = 4 at room temperature. No change of symmetry was observed down to 4 K. Below 43 K, additional magnetic Bragg reflections appear, which can be indexed on the basis of a commensurate magnetic propagation vector k [1, 0, 0]. The successful description of the magnetic spin structure reveals a ferromagnetic spin coupling within the Fe2+O6 M1 chains, while the coupling between the chains is antiferromagnetic. Spins are oriented collinearly within the a–c plane and form an angle of ∼60° with the crystallographic a-axis. The magnetic moment at 4 K amounts to about 4.4 μB. The observed magnetic structure is similar to that of other Ca-clinopyroxenes. The present data are put into context with the structural and magnetic properties of other pyroxenes – among them magnetoelectric and multiferroic pyroxene-type compounds.