Werner Treutmann

Synthesis and structural properties of clinopyroxenes of the hedenbergite CaFe2+Si2O6 – aegirine NaFe3+Si2O6 solid-solution series

Clinopyroxenes along the solid solution hedenbergite-aegirine M2[Ca2+1-xNa+xM1{Fe2+1-xFe3+x}Si2O6 were synthesized using hydrothermal techniques at 4 kbar. Different temperatures and redox conditions were used to determine optimum synthesis conditions and the stability range of individual compositions in the T - log fO2 field. Synthesized samples were characterized using microprobe analysis, X-ray powder diffraction and Mossbauer spectroscopy at 298 K and 80 K. The structure was refined in the C 2/ c space group by means of the Rietveld method. Along the solid-solution series between hedenbergite (a = 9.8448(6) A, b = 9.0296(6) A, c = 5.2452(4) A, β = 104.813) and aegirine endmembers (a = 9.6547(6) A, b = 8.7941(8) A, c = 5.2944(4) A, β = 107.398), the changes in unit cell dimensions show significant deviations from linearity. Mean and individual M1-O distances decrease linearly from hedenbergite to aegirine; mean M2-O and T-O distances do not change significantly, whereas individual length may vary. While in hedenbergite the coordination of the M2 site is 6+2, it is 4+4 in aegirine. The Mossbauer spectra of the solid-solution endmembers display narrowly split resonance absorption lines with hyperfine parameters typical for Fe2+ (δ = 1.18 mm/s, Δ = 2.25 mm/s at 298 K) and Fe3+ (δ = 0.38 mm/s, Δ = 0.30 mm/s at 298 K). Fe occupies only the M1 site. The Fe2+ resonance absorption is somewhat broadened in the 80 K spectra of the solid solution, which is due to a distribution of quadrupole splittings.

Structure and phase transitions in Ca2CoSi2O7–Ca2ZnSi2O7 solid-solution crystals

While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca(2)Co(1 - x)Zn(x)Si(2)O(7) system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P42(1)m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P2(1)2(1)2. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T(1) tetrahedra in the 3 x 3 superstructure. In this structure, the clustering of the low-coordinated Ca(2+) ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.

Transition from the incommensurately modulated structure to the lock-in phase in Co-åkermanite

The adaptation of the incommensurate structure modulation in Ca(2)CoSi(2)O(7) (dicalcium cobalt disilicate) single crystals to decreasing temperature has been examined using in situ high-resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the commensurate lock-in phase of Co-åkermanite exhibits a pronounced hysteresis of a highly strained metastable state with a characteristic microdomain morphology. A network of domain walls surrounding single orientation domains develops out of the room-temperature tartan pattern, the domains increase in size and their alignment changes from crystallographic to random. At 100 K the phase transition becomes almost complete. In parallel, the evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close-packed configuration of overlapping octagons in the commensurate low-temperature lock-in phase. Thereby, the octagon represents the ordered distribution of low-coordinated Ca clusters within a nanodomain extending over 4 x 4 subunits, on average [Riester et al. (2000). Z. Kristallogr. 215, 102--109]. The modulation wavevector was found to change from q(1,2) = 0.295 (a* +/- b*) at 300 K to q(1,2) = 0.320 (a* +/- b*) at 100 K.

Magnetic and low-temperature structural behavior of clinopyroxene-type FeGeO3: A neutron diffraction, magnetic susceptibility, and 57Fe Mössbauer study

Abstract The clinopyroxene-type compound FeGeO3 was synthesized using ceramic sintering techniques at 1273 K in evacuated silica tubes and investigated by powder neutron diffraction between 4 and 300 K, X-ray diffraction, SQUID magnetometry, and 57Fe Mössbauer spectroscopy. The title compound shows space group C2/c symmetry (high pressure, HP-topology) between 4 and 900 K. No structural phase transition is present within this temperature interval, whereas lattice parameters show discontinuities around 50 and 15 K, which are due to magnetic phase transitions and the associated magneto-elastic coupling of the lattice. The magnetic susceptibility data show two maxima in their temperature dependence, one at ~47 K, the second around 12 K (depending on the external field), indicative of two magnetic transitions in the title compound. Neutron data shows that for T < 45 K, FeGeO3 orders magnetically, having a simple collinear structure, with space group C2/c, and with the spins aligned parallel to the crystallographic b-axis, both on M1 and M2. The coupling within the M1/M2 band is ferromagnetic, whereas between them it is antiferromagnetic. As the bulk magnetic measurements in the paramagnetic state revealed a dominating ferromagnetic coupling, the intra-chain interactions dominate the inter-chain interaction. At 12 K, additional magnetic reflections appear, revealing a second magnetic phase transition. Spins are rotated away from the b-axis toward the a-c plane. The coupling within the M1 chain is still ferromagnetic and antiferromagnetic between the M1 chains. However, spins on M1 and M2, are no longer collinear. The moment on the M2 site is rotated further away from the b-axis than on M1.

Single crystal Mössbauer spectroscopy on the three principal sections of a synthetic fayalite sample in the antiferromagnetic state

The present work reports Mössbauer investigations for several temperatures below TN on fayalite single crystal sections cut perpendicularly to the crystallographic a and b-axis (Pnma). The previously detected correspondence between the c-component of the magnetic moment on M1 from neutron diffraction and our own Mössbauer measurements published elsewhere are confirmed for the other principal sections to a large extent. Small humps in the angular dependence of two components of the internal magnetic field H(0) on T below T=23 K are in good agreement with magnetometric and calorimetric data published elsewhere; a reinterpretation of single reflection neutron data has been possible by our results. Moreover, the axes of the electric field gradient (efg) are oriented within the crystallographic axes for the M1-site at low temperatures.The violation of symmetry on the M2 position as a result of our previous investigations could be confirmed for the section ⊥ a, but not with respect to b. A possible explanation in terms of saturation effects of large line intensities at the expense of the small ones is given in the context.

Multi-Walled Carbon Nanotubes without and with Metal Filling

Electron microscope studies are reported of MWCNTs without and with metallic encapsulations prepared by pyrolysis of organo-metallic precursors such as iron(II)phthalocyanine and ferrocene/anthracene mixtures. For straight and well-ordered MWCNTs, we obtained clear evidence of a scroll type structure with uniform chirality. Conical growth implicated the opening of the tube walls by termination of the graphene layers at the wall surfaces. Evidence is provided of the participation of iron carbide as an intermediate phase during graphite formation. Coercivities of the nanowire material up to 2550 Oe at 5 K, and constant saturation magnetization were measured.

Temperature-dependent crystal structure refinement and 57Fe Mössbauer spectroscopy of Cu2Fe2Ge4O13.

The germanate compound Cu2Fe2Ge4O13, dicopper diiron germanate, was synthesized by solid-state reaction at 1403 K and ambient pressure. There is no change of space-group symmetry between 10 and 900 K. Between 40 K and room temperature the a lattice parameter shows a negative thermal expansion which can be connected to a decreasing Cu-Cu interatomic distance. Above room temperature all the lattice parameters are positively correlated with temperature. Among the structural parameters several alterations with temperature occur, which are most prominent for the distorted Fe3+ octahedral site. Besides an increase of the average bond length and of the interatomic Fe-Fe distances, distortional parameters also increase with temperature, while the average Cu-O bond length remains almost constant between 100 and 900 K, as do the average Ge-O distances. 57Fe Mössbauer spectroscopy was used to detect long-range magnetic ordering in Cu2Fe2Ge4O13. While around 100 K, which is the temperature at which a broad maximum is observed in the magnetic susceptibility, no magnetic ordering was detected in the Mössbauer spectrum, below 40 K a narrow split sextet is developed which is indicative of a three-dimensional magnetic ordering of the sample.