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abstracts
Pyroxene compounds are a common form of natural minerals and have been studied as such for a long time. More recently the research on quasi-one-dimensional magnetic and multiferroïc materials has renewed the interest in pyroxenes of the stoichiometry AMX2O6 (A = alkali metal, M = transition metal, X = Si or Ge), since the magnetic M3+ ions form chains. Chemical substitution on the A and M sites can change the magnetic coupling along these chains making this system a rich field for the exploration of new phases of interesting magnetic properties [1]. In this work we report the discovery of a new phase in the Li-Mn-Ge-O system. A HP-HT solid state reaction was performed on a mixture of nominal stoichiometry LiMnGe2O6 during 1 h at a temperature of 8500C and a pressure of 3 GPa in a belt press. Powder X-ray diffraction yielded a diffractogram that could not be indexed by known phases of this system. An electron diffraction study in a transmission electron microscope was conducted in order to identify any unknown phases. In the case of structures that promise interesting properties a more targeted synthesis can then be undertaken. For the purpose of this work, we studied one of several unknown phases in the powder in more detail. From standard selected area electron diffraction the unit cell was determined to be triclinic with cell parameters a = 2.51 nm, b = 1.30 nm, c = 1.30 nm, α = 96.00, β = 98.80 and γ = 80.80. No comparable unit cell could be found in the databases neither in this system nor with different A, M or X ions. Intensities were recorded by in-zone axis precession electron diffraction and by electron diffraction tomography. Combining the data from both methods yielded the first model of the structure which we will present here.
abstracts
Compounds belonging to the Pyroxene family are well known as rock-forming minerals, and have thus drawn substantial interest by mineralogists. In this family of general chemical formula AM(Si, Ge)2O6, A is usually an alkali metal monovalent cation or a divalent alkaline earth cation, and B may be a trivalent or divalent transition metal cation. Among pyroxene compounds, the monoclinic clinopyroxenes are characterized by isolated one-dimensional chains of MO6 octahedra linked by edge-sharing. Due to this specific arrangement, clinopyroxene compounds where M is a magnetic transition metal cation have attracted considerable attention in recent years. Investigations revealed that these compounds present a rich diversity of intriguing low-dimensional magnetic properties. The existence and possible interplay of low dimensionality and magnetic frustration results in multiferroic and/or magneto- electric (ME) properties. We have undertaken the study of the CaCo1-xMnxGe2O6 (0
abstracts
It is well known that ferroelectricity and magnetic order are considered as hardly compatible in perovskite compounds. In this respect, PbVO3 is quite interesting: it is isostructural to ferroelectric PbTiO3 (P4mm) and contains V4+ spin ½ cations [1]. However, previous studies have failed to observe magnetic order in PbVO3, which was attributed either to a 2D magnetic behaviour or to magnetic frustration on a square lattice [2]. We present here the study of the substitution of V4+ by Fe3+ or Ti4+ cations, aiming at a better understanding of the relations between structural, magnetic and electric properties. PbVO3 single crystals and powders of substituted compounds were prepared at 6GPa , 9500C in belt and Conac type systems. A single crystal diffraction experiment confirmed the proposed structure for PbVO3, evidencing merohedral twinning related to ferroelectric domains also observed by SEM. For the Ti substitution, a complete Pb(V4+ 1-x Ti4+ x)O3 solid solution is observed for x = 0 to 1, while the Fe substitution stops at x=0.5 due to heterovalent cation replacement, the formula being Pb(V4+ 1-2x V5+ x Fe3+ x)O3. The cation oxidation states were checked by XANES (FAME-ESRF). The structures were studied by joint refinements of NPD (D1B-ILL) and XPD (Laboratory and ID31-ESRF) data, yielding the coordinations of V and M cations and the spontaneous polarization using a point charge model. A decrease of tetragonality, domain size along the c-axis and polarization is observed with increasing substitution. For the Ti series, the magnetic behaviour progressively changes from 2D to Curie-Weiss with increasing x. In the case of Fe, a broad peak in magnetic susceptibility is observed, at temperatures increasing from 12K to 30K for x going from 0.1 to 0.5. This was checked as due to a spin glass behaviour. No sign of magnetic order was observed by NPD. At the same time, a broad, frequency dependent anomaly of the dielectric constant is observed, reminiscent of a relaxor behaviour.
