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Redhammer G.J.,University of Salzburg | Roth G.,RWTH Aachen | Senyshyn A.,Forschungneutronenquelle Heinz Maier Leibnitz FRM II | Tippelt G.,University of Salzburg | Pietzonka C.,University of Marburg
Zeitschrift fur Kristallographie | Year: 2013

CaFeGe2O6, the germanium-analogue to the mineral Hedenbergite, has been synthesized at 1273 K in evacuated SiO 2-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-clinopyrox-enes. The present data are put into context with the structural and magnetic properties of other pyroxenes - among them magnetoelectric and multiferroic pyroxene-type compounds. © by Oldenbourg Wissenschaftsverlag, München. Source

Redhammer G.J.,University of Salzburg | Senyshyn A.,Forschungneutronenquelle Heinz Maier Leibnitz FRM II | Senyshyn A.,TU Darmstadt | Meven M.,Forschungneutronenquelle Heinz Maier Leibnitz FRM II | And 10 more authors.
Physics and Chemistry of Minerals | Year: 2011

The compound NaFeGe2O6 was grown synthetically as polycrystalline powder and as large single crystals suitable for X-ray and neutron-diffraction experiments to clarify the low temperature evolution of secondary structural parameters and to determine the low temperature magnetic spins structure. NaFeGe2O6 is isotypic to the clinopyroxene-type compound aegirine and adopts the typical HT-C2/c clinopyroxene structure down to 2.5 K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between 2.5 K and room temperature, while the GeO4 tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33 K, which marks the onset of low-dimensional magnetic ordering. Below 12 K NaFeGe2O6 transforms to an incommensurately modulated magnetic spin state, with k = [0.323, 1.0, 0.080] and a helical order of spins within the M1-chains of FeO6 octahedra. This is determined by neutron-diffraction experiments on a single crystal. Comparison of NaFeGe2O6 with NaFeSi2O6 is given and it is shown that the magnetic ordering in the latter compound, aegirine, also is complex and is best described by two different spin states, a commensurate one with C2′/c′ symmetry and an incommensurate one, best being described by a spin density wave, oriented within the (1 0 1) plane. © 2010 Springer-Verlag. Source

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