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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, = 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 is put into context with the structural and magnetic properties of other pyroxenes – among them magnetoelectric and multiferroic pyroxene-type compounds.
Zeitschrift für Kristallographie. 02/2013; 228(3):140-150.
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Günther J. Redhammer,
Anatoliy Senyshyn,
Martin Meven,
Georg Roth,
Sebastian Prinz,
Astrid Pachler,
Gerold Tippelt, Clemens Pietzonka,
Werner Treutmann,
Markus Hoelzel,
Björn Pedersen,
Georg Amthauer
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ABSTRACT: 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 2.5K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between
2.5K and room temperature, while the GeO4 tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33K, which marks the
onset of low-dimensional magnetic ordering. Below 12K 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.
KeywordsClinopyroxene–NaFeGe2O6
–Aegirine NaFeSi2O6
–Multiferroic compounds–Neutron diffraction–Magnetic ordering–Helical spin structure
Physics and Chemistry of Minerals 04/2012; 38(2):139-157. · 1.73 Impact Factor
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ABSTRACT: CoGeO3 was synthesized at 1,273 and 1,448K using ceramic sintering techniques in the monoclinic and orthorhombic modification,
respectively. The two compounds were analysed by magnetic susceptibility measurements and neutron diffraction in order to
study magnetic ordering and spin structures at low temperature. The monoclinic form of CoGeO3 has C2/c symmetry and orders magnetically below 36K with a small negative paramagnetic Curie temperature θ
P=−4.6 (2)K. The magnetic structure can be described with k=(1, 0, 0) in the magnetic space group C2′/c′ having a ferromagnetic spin arrangement within the chains of M1 sites, but a dominating antiferromagnetic coupling between
the chains. At the M1 sites the magnetic spins are aligned within the a–c plane forming an angle of 120° with the +a-axis and they are not parallel to the spins at M2. Here spins are also ferromagnetically coupled within, but antiferromagnetically
coupled between the M1/M2 site bands. The orthorhombic phase of CoGeO3 displays Pbca symmetry and transforms to an antiferromagnetically ordered state [θ
P=−18.6(2)K] below 33K. The magnetic spin structure can be described with k=(0, 0, 0) in space group Pbca′ and it is similar to the one of the C2/c phase except that it is non-collinear in nature, i.e. there are components of the magnetic moment along all three crystallographic
axes. Small magneto-elastic coupling is observed in the orthorhombic phase.
KeywordsCoGeO3-Pyroxene-Nuclear structure-Magnetic structure determination-Neutron diffraction-Magnetic susceptibility
Physics and Chemistry of Minerals 04/2012; 37(5):311-332. · 1.73 Impact Factor
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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 Fe-57 Mossbauer 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 similar to 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 anti ferromagnetic. 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.
American Mineralogist 04/2012; 97(4):694-706. · 2.17 Impact Factor
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ABSTRACT: The compounds CaFeSi2O6 (hedenbergite), CaNiGe2O6, CaCoGe2O6 and CaMnGe2O6 have been synthesized by hydrothermal or ceramic sintering techniques and were Subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/c, Z = 4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite MI chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a-c plane and form an angle of 43 degrees with the crystallographic a-axis. Magnetic ordering in CaFeSi2O6 causes significant discontinuities in lattice parameters, Fe-O bond lengths and interatomic Fe-Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe2O6 and CaNiGe2O6 show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM Coupling within and an AFM coupling of spins between the MI chains. The moments lie within the a-c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi2O6, (+40.2 K) to CaCoGe2O6 (+20.1 K) and CaNiGe2O6 (-13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the MI chains. CaMnGe2O6 finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a-c plane. (C) 2008 Elsevier Inc. All rights reserved.
Journal of Solid State Chemistry 11/2008; 181(11):3163-3176. · 2.16 Impact Factor
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Journal of Solid State Chemistry. 182(9):2374-2384.
