M. Meven - Academia.edu (original) (raw)

Papers by M. Meven

Review of Scientific Instruments, 2016

A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been d... more A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been developed at the Maier-Leibnitz Zentrum, Garching, Germany. After reviewing existing devices, spherical neutron polarimetry has been implemented on POLI as a main experimental technique using a third-generation cryogenic polarization analysis device (Cryopad) built in cooperation between RWTH University and Institut Laue-Langevin. In this report we describe the realization and present the performance of the new Cryopad on POLI. Some improvements in the construction as well as details regarding calibrations of Cryopad and its practical use are discussed. The reliable operation of the new Cryopad on POLI is also demonstrated.

Acta Crystallographica Section A Foundations of Crystallography, 2008

Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, pl... more Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, playing an essential geochemical role in the evolution processes of pegmatites. A good knowledge of their crystal chemistry is therefore necessary to better understand the genesis of pegmatites. Paravauxite is a mineral found in hydrothermal tin veins and granite pegmatites. Its ideal chemical formula is Fe2+Al2(PO4)2(OH)2\ub78H2O. Its crystal structure was solved and refined by Baur in 1969 on the basis of single-crystal X-ray diffraction data. This structure model appears to be consistent. However, due to the technical limitations of X-ray diffraction, the refinement only provided the isotropic displacement parameters, and the positions of nine independent proton sites were assigned but not refined. This led to a poor description of (the expected) complex H-bonding scheme in the paravauxite structure. In light of this, the crystal structure of a natural paravauxite was reinvestigated usi...

An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Strontium titanate (SrTiO 3) is one of the best studied substances showing a displacive phase tra... more Strontium titanate (SrTiO 3) is one of the best studied substances showing a displacive phase transition [1]. At ambient temperature, it exhibits a cubic perovskite structure with the space group Pm3m. On cooling, a Brillouin zone-boundary softmode drives the second-order transition to an antiferrodis-tortive phase at T C = 105 K. The primary order parameter is the antiphase rotation of TiO 6-octahedra in adjacent unit cells. As a result, a paraelectric, tetragonal phase with the space group I4/mcm is formed. The tetragonal distortion c/a is slightly larger than unity and represents a secondary order parameter of the transition. Without an external perturbation none of the three possible tetragonal axes is favored over the others and the twin domains are evenly distributed. The isostuctural barium titanate (BaTiO 3) shows a completely different behavior: Here, the condensation of a Γ-point softmode leads to a ferroelectric low temperature phase. A softmode with the same polar eigenv...

Physical Review Materials, 2018

Physics and Chemistry of Minerals, 2018

The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, D... more The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, Dawson mining district, Yukon, Canada) was investigated by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction at 298 and 3 K. Its empirical formula, based on EMPA data, is: (Mg 0.871 Fe 0.127) Σ0.998 Al 2.030 (P 1.985 Ti 0.008 Si 0.007 O 4) 2 (OH) 2. The neutron diffraction experiments at room and low T proved that the H-free structural model of lazulite previously reported, on the basis of X-ray structure refinement, is correct: the building unit of the lazulite structure consists of a group of three face-sharing (Al-octahedron)+(Mg,Feoctahedron)+(Al-octahedron), connected to the adjacent one via a corner-shared OH-group and two corner-shared oxygen sites of the P-tetrahedron, to form a dense 3D-edifice. Only one crystallographically independent H site occurs in the structure of lazulite, forming a hydroxyl group with the O5 oxygen, with O5-H= 0.9997 Å at room temperature (corrected for riding motion effect). The H-bonding scheme in the structure of lazulite is now well defined: a bifurcated bonding scheme occurs with the O4 and O2 oxygen sites as acceptors. The two H-bonds are energetically different, as shown by their bonding geometry: the H-bond with the O2 site as acceptor is energetically more favorable, being O5-H•••O2 = 152.67(9)°, O5•••O2 = 3.014(1) Å and H•••O2 = 2.114(1) Å, whereas that with O4 as acceptor is energetically more costly, being O5-H•••O4 = 135.73(8)°, O5•••O4 = 3.156(1) Å and H•••O4 = 2.383(1) Å, at room temperature. No T-induced phase transition occurs within the T-range investigated. At low temperature, the O5-H•••O2 bond is virtually identical to the room-T one, whereas the effects of T on O5-H•••O4 are more pronounced, with significant differences of the O donor •••O acceptor and H•••O acceptor distances. The experimental findings of this study do not support the occurrence of HPO 4 or H 2 PO 4 units into the structure of lazulite, recently reported on the basis of infrared and Raman spectra.

Review of Scientific Instruments, 2016

We report the design of a radio-frequency induction-heated rod casting furnace that permits the p... more We report the design of a radio-frequency induction-heated rod casting furnace that permits the preparation of polycrystalline ingots of intermetallic compounds under ultra-high vacuum compatible conditions. The central part of the system is a bespoke water-cooled Hukin crucible supporting a casting mold. Depending on the choice of mold, typical rods have a diameter between 6 mm and 10 mm and a length up to 90 mm, suitable for single-crystal growth by means of float-zoning. The setup is all-metal sealed and may be baked out. We find that the resulting ultra-high vacuum represents an important precondition for processing compounds with high vapor pressures under a high-purity argon atmosphere up to 3 bar. Using the rod casting furnace, we succeeded to prepare large high-quality single crystals of two half-Heusler compounds, namely the itinerant antiferromagnet CuMnSb and the half-metallic ferromagnet NiMnSb.

Physical Review B, 2016

The magnetic ground state of the Eu 2+ moments in a series of Eu(Fe 1−x Co x) 2 As 2 single cryst... more The magnetic ground state of the Eu 2+ moments in a series of Eu(Fe 1−x Co x) 2 As 2 single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe 1−x Co x) 2 As 2 is established. The ground-state magnetic structure of the Eu 2+ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic c direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around x = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu 2+ moments, which is mediated by the conduction d electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu 2+ and Fe 2+ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe 1−x Co x) 2 As 2 single crystals with 0.10 x 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.

Acta crystallographica Section B, Structural science, crystal engineering and materials, 2016

In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectr... more In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectric coupling. For a symmetry-consistent theoretical description of this multiferroic phase, precise knowledge of its crystal structure is a prerequisite. Here we report the results of single-crystal neutron diffraction on Ca2CoSi2O7 at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space group P21212 with a 3 × 3 × 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) Å] compared with the high-temperature normal state [tetragonal space group P\overline {4}2_{1}m, a = b ≃ 7.86, c ≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7 at 10 K are presented and compared with the literature X-ray diffraction results at 130 and 170 K (low-temperature commensurate phase), as well as at ∼ 500 K (high-temperature normal phase).

Acta Crystallographica Section A Foundations and Advances, 2014

In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffrac... more In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffraction. Letovicite with a chemical composition (NH4)3H(SO4)2 belongs to a large family of M3(H,D)(XO4)2 compounds, where M = K+, Rb+, (NH4)+, Cs+, Tl+ and X = Se6+ and S6+. This compound crystallizes in the monoclinic space group A2/a with a = 9.789(7) Å, b = 5.6815(9) Å, c = 14.703(2) Å and β = 103.03(4)0at 300K. At 2.3K, the lattice parameters are a = 9.687(20) Å, b = 5.648(13) Å, c = 14.613(9) Å and β = 103.23(14)0. Data at 2.3K were measured up to (sinθ/λ) = 0.807Å-1 with the single crystal neutron diffractormeter HEiDi at the FRM-II, Germany. H/D shows a dynamic disorder at high temperature, which can be related to very high proton conductivity. In letovicite, two types of disorder related with hydrogen atoms are reported [1]. Although letovicite shows various phase transitions owing to the proton ordering at low temperature, K3H(SO4)2, without the possibility of an orientational diso...

Acta Crystallographica Section A Foundations of Crystallography, 2013

Review of Scientific Instruments, 2016

A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been d... more A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been developed at the Maier-Leibnitz Zentrum, Garching, Germany. After reviewing existing devices, spherical neutron polarimetry has been implemented on POLI as a main experimental technique using a third-generation cryogenic polarization analysis device (Cryopad) built in cooperation between RWTH University and Institut Laue-Langevin. In this report we describe the realization and present the performance of the new Cryopad on POLI. Some improvements in the construction as well as details regarding calibrations of Cryopad and its practical use are discussed. The reliable operation of the new Cryopad on POLI is also demonstrated.

Acta Crystallographica Section A Foundations of Crystallography, 2008

Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, pl... more Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, playing an essential geochemical role in the evolution processes of pegmatites. A good knowledge of their crystal chemistry is therefore necessary to better understand the genesis of pegmatites. Paravauxite is a mineral found in hydrothermal tin veins and granite pegmatites. Its ideal chemical formula is Fe2+Al2(PO4)2(OH)2\ub78H2O. Its crystal structure was solved and refined by Baur in 1969 on the basis of single-crystal X-ray diffraction data. This structure model appears to be consistent. However, due to the technical limitations of X-ray diffraction, the refinement only provided the isotropic displacement parameters, and the positions of nine independent proton sites were assigned but not refined. This led to a poor description of (the expected) complex H-bonding scheme in the paravauxite structure. In light of this, the crystal structure of a natural paravauxite was reinvestigated usi...

An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Strontium titanate (SrTiO 3) is one of the best studied substances showing a displacive phase tra... more Strontium titanate (SrTiO 3) is one of the best studied substances showing a displacive phase transition [1]. At ambient temperature, it exhibits a cubic perovskite structure with the space group Pm3m. On cooling, a Brillouin zone-boundary softmode drives the second-order transition to an antiferrodis-tortive phase at T C = 105 K. The primary order parameter is the antiphase rotation of TiO 6-octahedra in adjacent unit cells. As a result, a paraelectric, tetragonal phase with the space group I4/mcm is formed. The tetragonal distortion c/a is slightly larger than unity and represents a secondary order parameter of the transition. Without an external perturbation none of the three possible tetragonal axes is favored over the others and the twin domains are evenly distributed. The isostuctural barium titanate (BaTiO 3) shows a completely different behavior: Here, the condensation of a Γ-point softmode leads to a ferroelectric low temperature phase. A softmode with the same polar eigenv...

Physical Review Materials, 2018

Physics and Chemistry of Minerals, 2018

The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, D... more The crystal structure and crystal chemistry of a lazulite from Crosscut Creek (Kulan Camp area, Dawson mining district, Yukon, Canada) was investigated by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction at 298 and 3 K. Its empirical formula, based on EMPA data, is: (Mg 0.871 Fe 0.127) Σ0.998 Al 2.030 (P 1.985 Ti 0.008 Si 0.007 O 4) 2 (OH) 2. The neutron diffraction experiments at room and low T proved that the H-free structural model of lazulite previously reported, on the basis of X-ray structure refinement, is correct: the building unit of the lazulite structure consists of a group of three face-sharing (Al-octahedron)+(Mg,Feoctahedron)+(Al-octahedron), connected to the adjacent one via a corner-shared OH-group and two corner-shared oxygen sites of the P-tetrahedron, to form a dense 3D-edifice. Only one crystallographically independent H site occurs in the structure of lazulite, forming a hydroxyl group with the O5 oxygen, with O5-H= 0.9997 Å at room temperature (corrected for riding motion effect). The H-bonding scheme in the structure of lazulite is now well defined: a bifurcated bonding scheme occurs with the O4 and O2 oxygen sites as acceptors. The two H-bonds are energetically different, as shown by their bonding geometry: the H-bond with the O2 site as acceptor is energetically more favorable, being O5-H•••O2 = 152.67(9)°, O5•••O2 = 3.014(1) Å and H•••O2 = 2.114(1) Å, whereas that with O4 as acceptor is energetically more costly, being O5-H•••O4 = 135.73(8)°, O5•••O4 = 3.156(1) Å and H•••O4 = 2.383(1) Å, at room temperature. No T-induced phase transition occurs within the T-range investigated. At low temperature, the O5-H•••O2 bond is virtually identical to the room-T one, whereas the effects of T on O5-H•••O4 are more pronounced, with significant differences of the O donor •••O acceptor and H•••O acceptor distances. The experimental findings of this study do not support the occurrence of HPO 4 or H 2 PO 4 units into the structure of lazulite, recently reported on the basis of infrared and Raman spectra.

Review of Scientific Instruments, 2016

We report the design of a radio-frequency induction-heated rod casting furnace that permits the p... more We report the design of a radio-frequency induction-heated rod casting furnace that permits the preparation of polycrystalline ingots of intermetallic compounds under ultra-high vacuum compatible conditions. The central part of the system is a bespoke water-cooled Hukin crucible supporting a casting mold. Depending on the choice of mold, typical rods have a diameter between 6 mm and 10 mm and a length up to 90 mm, suitable for single-crystal growth by means of float-zoning. The setup is all-metal sealed and may be baked out. We find that the resulting ultra-high vacuum represents an important precondition for processing compounds with high vapor pressures under a high-purity argon atmosphere up to 3 bar. Using the rod casting furnace, we succeeded to prepare large high-quality single crystals of two half-Heusler compounds, namely the itinerant antiferromagnet CuMnSb and the half-metallic ferromagnet NiMnSb.

Physical Review B, 2016

The magnetic ground state of the Eu 2+ moments in a series of Eu(Fe 1−x Co x) 2 As 2 single cryst... more The magnetic ground state of the Eu 2+ moments in a series of Eu(Fe 1−x Co x) 2 As 2 single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe 1−x Co x) 2 As 2 is established. The ground-state magnetic structure of the Eu 2+ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic c direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around x = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu 2+ moments, which is mediated by the conduction d electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu 2+ and Fe 2+ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe 1−x Co x) 2 As 2 single crystals with 0.10 x 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.

Acta crystallographica Section B, Structural science, crystal engineering and materials, 2016

In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectr... more In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectric coupling. For a symmetry-consistent theoretical description of this multiferroic phase, precise knowledge of its crystal structure is a prerequisite. Here we report the results of single-crystal neutron diffraction on Ca2CoSi2O7 at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space group P21212 with a 3 × 3 × 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) Å] compared with the high-temperature normal state [tetragonal space group P\overline {4}2_{1}m, a = b ≃ 7.86, c ≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7 at 10 K are presented and compared with the literature X-ray diffraction results at 130 and 170 K (low-temperature commensurate phase), as well as at ∼ 500 K (high-temperature normal phase).

Acta Crystallographica Section A Foundations and Advances, 2014

In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffrac... more In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffraction. Letovicite with a chemical composition (NH4)3H(SO4)2 belongs to a large family of M3(H,D)(XO4)2 compounds, where M = K+, Rb+, (NH4)+, Cs+, Tl+ and X = Se6+ and S6+. This compound crystallizes in the monoclinic space group A2/a with a = 9.789(7) Å, b = 5.6815(9) Å, c = 14.703(2) Å and β = 103.03(4)0at 300K. At 2.3K, the lattice parameters are a = 9.687(20) Å, b = 5.648(13) Å, c = 14.613(9) Å and β = 103.23(14)0. Data at 2.3K were measured up to (sinθ/λ) = 0.807Å-1 with the single crystal neutron diffractormeter HEiDi at the FRM-II, Germany. H/D shows a dynamic disorder at high temperature, which can be related to very high proton conductivity. In letovicite, two types of disorder related with hydrogen atoms are reported [1]. Although letovicite shows various phase transitions owing to the proton ordering at low temperature, K3H(SO4)2, without the possibility of an orientational diso...

Acta Crystallographica Section A Foundations of Crystallography, 2013