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Acta Cryst. (2014). A70, C53
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In the 1970s, a large number of metal iodates compounds were extensively investigated for their nonlinear optics (NLO) properties as well as for their ferroelectric, piezoelectric, and pyroelectric properties. Interest in these compounds resumed in the early 2000s. We have shown that metal iodates are particularly interesting for for quadratic NLO in mid IR, as they possess a large domain of transparency from the visible region to the beginning of the far IR region (12.5 μm), thus covering the three atmospheric transparency windows. The synthesis of metal iodates has so far been mainly investigated by solution chemistry, under hydrothermal conditions or by the flux method. The solid state synthesis of these compounds at high pressure has never been explored. To date, only the structural evolution of α-LiIO3 with pressure has been studied by X-ray powder diffraction [1]. It was shown that, at room temperature, α-LiIO3 is stable up to 75 GPa; only compression of the lattice parameters with pressure was observed. In this work, we present a new phase of silver iodate obtained at high pressure from α-AgIO3 and characterized by X-ray powder diffraction. The α-AgIO3 to β-AgIO3 transition was characterized by differential thermal analysis (DTA) at high pressure [2-3]. The thermal behaviors of α-AgIO3 and β-AgIO3 were studied by differential scanning calorimetry (DSC) at ambient pressure and in situ temperature-dependent X-ray powder diffraction. Structural studies of these two phases were carried out to understand the formation of β-AgIO3.

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Acta Cryst. (2014). A70, C370
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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.

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Acta Cryst. (2014). A70, C373
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Magnetically frustrated materials have been the subject of many studies over the last decades. In search for a 3-dimensional quantum spin liquid, where quantum-mechanical fluctuations prevent magnetic order, different phases of stoichiometry Ba3NiSb2O9 have recently [1] been synthesized some of them at high pressure. Two of these phases are hexagonal. The hexagonal phases (space groups P63/mmc and P63mc, respectively) have different structures but cell parameters that differ by less than 1%. Similar phases have been obtained with Cu [2] or Co [3]. These phases are well distinguished by powder X-ray diffraction when they appear in sufficient quantity in a newly synthesized powder. When these phases are present only in minor quantities, which is a common situation when synthesizing new materials, only transmission electron microscopy can give structural information on a very local scale. However, the accuracy of unit cell parameter determination by electron diffraction (usually 1% or worse) and the identical extinction conditions for the 2 space groups don't permit to distinguish between the two phases. Convergent beam electron diffraction could show the difference between the centrosymmetric and non-centrosymmetric space groups provided a suitably oriented particle can be found. In this work we propose a different method of distinguishing structures in such complicated cases by actually solving the structure. Sufficient in-zone axis precession electron diffraction and/or electron diffraction tomography data can be obtained from any crystal regardless of its orientation. In the subsequent structure solution we have tested both space groups. The quality (or absence thereof) of the structure solutions obtained clearly makes it possible to distinguish between the two hexagonal structures.

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Acta Cryst. (2014). A70, C389
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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

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Acta Cryst. (2014). A70, C978
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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.
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