Neutron diffraction study, magnetic properties and thermal stability of YMn2D6 synthesized under high deuterium pressure

A new phase YMn₂D₆ was synthesized by submitting YMn₂ to 1.7 kbar deuterium pressure at 473 K. According to X-ray and neutron powder diffraction experiments, YMn₂D₆ crystallizes in the space group with at 300 K. The Y and half of the Mn atoms occupy statistically the 8c site whereas the other Mn atoms are located in 4a site and surrounded by 6 D atoms (24 e). This corresponds to a K₂PtCl₆-type structure with a partially disordered substructure which can be written as [YMn]MnH₆. No ordered magnetic moment is observed in the NPD patterns and the magnetization measurements display a paramagnetic behavior. The study of the thermal stability by Differential Scanning Calorimetry and XRD experiments indicates that this phase decomposes in YD₂ and Mn at 625 K, and is more stable than YMn₂H_4.5.     

Single-crystal growth, crystal and electronic structure of NaCoO2

Single crystals of NaCoO₂ have been successfully synthesized for the first time by a flux method at 1323 K. A single-crystal X-ray diffraction study confirmed the trigonal space group and the lattice parameters , . The crystal structure has been refined to the conventional values R=1.9% and wR=2.1% for 309 independent observed reflections. The electron density distribution of NaCoO₂ has been studied by the maximum entropy method (MEM) using single-crystal X-ray diffraction data obtained at 298 K. From the results of the MEM analysis, the strong covalent bonding was clearly observed between Co and O atoms, while no bonding was observed around Na atoms. We also calculated the electron density of NaCoO₂ by first principles calculations. The electron density obtained experimentally is in good agreement with the theoretical one.     

Twinned tetragonal structure and equation of state of NaTh2F9

The crystal structure and stability of NaTh₂F₉ have been studied using thermal analysis, powder X-ray diffraction at atmospheric conditions, and single-crystal X-ray diffraction at high pressure. Sodium dithorium fluoride is stable at least up to 5.0 GPa at room temperature and to 954 K at ambient pressure. In contrast to earlier investigations, which have reported the structure to be cubic (, Z=4), we observe a tetragonal distortion of the lattice. The actual crystal structure (, Z=4) is twinned and composed of corner-sharing distorted ThF₉ tricapped trigonal prisms and distorted NaF₆ octahedra. The twinning element is a three-fold axis from cubic symmetry. The ThF₉ polyhedra are rigid and it is the volume changes of the octahedra around the Na atoms that have the major contribution to the bulk compressibility. The zero-pressure bulk modulus B₀ and the unit-cell volume at ambient pressure V₀ are equal to 99(6) GPa and 663.1(1.0) Å³, respectively, with the fixed first pressure derivative of the bulk modulus B′=4.00. An inspection of the known crystalline phases in the system NaF-ThF₄ reveals that their bulk moduli increase with the increasing ThF₄ content.     

Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

A Raman spectroscopic study of the phase transition of BaZr(PO4)2: Evidence for a trigonal structure of the high-temperature polymorph

We have studied the structural evolution of monoclinic BaZr(PO₄)₂ during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO₄ vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a site, the P and two O atoms at a C_3v(3m), and six O atoms at a C_s(m) site in the D_3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either , , or .     

Hybrid iodoperfluoroalkane-ferrocene supramolecular arrays: the shortest contacts iodine forms with nitrogen atoms and unsaturated moieties

α,ω-Diiodoperfluoralkanes self-assemble in solution and in the solid phase with (S)-(−)-N,N-dimethyl-1-ferrocenylethylamine. The formed adducts have been characterized by DSC, IR, Raman, and NMR techniques. Single crystal X-ray analysis of one of the obtained adducts shows that perfluorinated and organometallic modules alternate giving rise to infinite one-dimensional (1D) chains. The interacting modules are connected through strong n-type halogen bondings, involving nitrogen and iodine atoms, and weak π-type halogen bondings, involving Cp rings and iodine atoms. Both interactions are the shortest reported to now in their respective classes. Theoretical calculations have been performed to quantify the observed π-type halogen bonding. For the first time a metallocene-perfluorocarbon (PFC) hybrid material is described.     

Low-temperature and high-pressure structural behaviour of NaBi(MoO4)2—an X-ray diffraction study

NaBi(MoO₄)₂ has been characterized by single-crystal and powder X-ray diffraction in the temperature and pressure ranges 13-297 K and 0-25 GPa, respectively. The domain structure developing below proves that NaBi(MoO₄)₂ undergoes a ferroelastic phase transition associated with tetragonal I4₁/a to monoclinic I2/a symmetry change. The character of the unit cell evolution as a function of temperature indicates a continuous transition with the spontaneous strain as an order parameter. The structural distortion, due to small displacements of Bi³⁺ and Na⁺ ions, develops slowly. Therefore the overall changes, as measured in single-crystal diffraction at 110 and 13 K, appear to be subtle. High-pressure powder X-ray diffraction shows that the elastic behaviour is anisotropic, the linear compressibility along the a- and c-axes of the tetragonal unit cell being β_a=2.75(10)×10^-3 and , respectively. The cell contraction, stronger along the c-axis, causes the distances between the MoO₄ layers to be shortened. Consequently, the cation migration in the channels formed by MoO₄ tetrahedra becomes hindered, and any symmetry lowering phase transition is not observed up to 25 GPa. The zero-pressure bulk modulus is , and its pressure derivative .     

Isolated spin 3/2 plaquettes in Na3RuO4

The crystal structure of Na₃RuO₄ determined by powder neutron diffraction is reported. The structure consists of isolated tetramers of edge sharing RuO₆ octahedra in the ab plane, creating isolated four-member plaquettes of Ru atoms comprised of two equilateral triangles sharing an edge. Magnetic susceptibility measurements reveal an antiferromagnetic transition at ∼29 K, with . Neutron diffraction data indicate the onset of three-dimensional magnetic ordering at 29 K.     

Syntheses and structures of Li3ScF6 and high pressure LiScF4−, luminescence properties of LiScF4, a new phase in the system LiF-ScF3

Colorless single crystals of Li₃ScF₆ have been prepared by reacting the binary components LiF and ScF₃ at 820 °C for 16 h in argon atmosphere. This complex fluoride is the only stable phase in the system LiF-ScF₃ under ambient pressure. According to a structure refinement based on single crystal X-ray diffraction data it crystallizes in the centrosymmetric space group with and . The new structure of Li₃ScF₆ is a filled variant of the Na₂GeF₆ type structure and can be described in terms of a hexagonal close packing of fluorine in which 2/3 of the octahedral holes are occupied by Sc and Li.High pressure/high temperature studies of the system LiF-ScF₃ show that the new phase LiScF₄ is formed at around 5.5 GPa and 575 °C. According to Rietveld refinements of powder X-ray diffraction data LiScF₄ adopts the Scheelite type structure (space group I4₁/a) with and . A sample of LiScF₄ doped with 1% Er exhibits an intense luminescence in the far IR region.     

Growth of high quality and large-sized Rb0.3MoO3 single crystals by molten salt electrolysis method

High quality and large-sized Rb_0.3MoO₃ single crystals were synthesized by molten salt electrolysis method. X-ray diffraction (XRD) patterns and rocking curves, as well as the white beam Laue diffraction of X-ray images show the crystals grown by this method have high quality. The lattice constants evaluated from XRD patterns are a₀=1.87 nm, b₀=0.75 nm, c₀=1.00 nm, β=118.83^°. The in situ selected area electron diffraction (SAED) patterns along the , and zone axes at room temperature indicate that the Rb_0.3MoO₃ crystal possess perfect C-centered symmetry. Temperature dependence of the resistivity shows this compound undergoes a metal to semiconductor transition at 183 K.     

Magnetic study of two isotypic manganese chloro-sulfides: MnSbS2Cl and the new compound MnBiS2Cl

Among the MnPnQ₂X compounds (Pn: pnictide; Q: chalcogen; X: halogen), two isotypic chloro-sulfides, MnSbS₂Cl and MnBiS₂Cl, have been studied. MnBiS₂Cl is a new compound synthesized by solid state reaction at 500 °C. It is orthorhombic, space group Pnma, with a=9.502(2), b=3.8802(8), , , Z=4. Its X-ray single crystal study shows (001) waved layers of MnS₄Cl₂ octahedra, opposite edge-sharing along b, and corner-sharing along a. Similar magnetic susceptibilities for both compounds have been recorded, indicating high spin Mn²⁺ with anti-ferromagnetic exchange. Correlatively, specific heat versus temperature shows a magnetic transition at for the Sb-bearing compound, and a two-steps magnetic transition at 28 and 32 K for the Bi isotype. The magnetic structure of MnSbS₂Cl has been determined by neutron diffraction, revealing a magnetic ordering at 1.5 K with an incommensurate wave-vector along b (k=[0, 0.3838, 0]). Two modulation models, sinusoidal and helicoidal, give quite equivalent reliability factors (R_mag=0.0573 and 0.0586, respectively).     

Ag2CuMnO4: A new silver copper oxide with delafossite structure

The use of hydrothermal methods has allowed the synthesis of a new silver copper mixed oxide, Ag₂CuMnO₄, the first example of a quaternary oxide containing both elements. It crystallizes with the delafossite 3R structure, thus being the first delafossite to contain both Ag and Cu. Synthesis conditions affect the final particle size (30-500 nm). Powder X-ray diffraction Rietveld refinement indicates a trigonal structure and cell parameters and , where Cu and Mn are disordered within the octahedral B positions in the plane and linearly coordinated Ag occupies de A position between layers. X-ray absorption near edge spectroscopy (XANES) for copper and manganese, and XPS for silver evidence +2, +4, and +1 oxidation states. The microstructure consists of layered particles that may form large twins showing 5 nm nanodomains. Finally, magnetic measurements reveal the existence of ferromagnetic coupling yielding in-plane moments that align antiferromagnetically at lower temperatures. The singularity of the new phase resides on the fact that is an example of a bidimensional arrangement of silver and copper in an oxide that also shows clear bidimensionality in its physical properties. That is of special relevance to the field of high T_c superconducting oxides, while the ferromagnetic coupling in a bidimensional system deserves itself special attention.     

From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6

A new ternary phase, Mn₄Ir_7−xMn_xGe₆ (0?x?1.3), was studied by X-ray and neutron powder diffraction and SQUID magnetometry. The crystal structure is cubic, of the U₄Re₇Si₆ type, space group , Z=2, with the lattice parameter at 295 K. Within the limited range of homogeneity small variations of the composition yield dramatic changes of the magnetic structure. For x=0 long-range antiferromagnetic order is formed below the transition temperature 228 K, with large magnetic moments on Mn, 4.11(9) μ_B at 10 K, in a magnetic unit cell , c_M=2a_C. In contrast, for x=1.3 spin glass behavior is observed below 90 K. The Mn atoms form an ideal cubic framework, on which geometric frustration of competing nearest and next nearest neighbor antiferromagnetic interactions is suggested to explain the composition sensitive magnetic properties. A TiNiSi-type phase, IrMnGe, is found in samples of 1:1:1 composition quenched from the melt.     

Crystal structure, phase transition and anisotropic thermal expansion of barium zirconium diorthophosphate, BaZr(PO4)2

The crystal structure of BaZr(PO₄)₂ at 298 K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was monoclinic (space group C2/m, Z=2) with , , , β=93.086(1)° and . Final reliability indices were R_wp=8.21%, R_p=5.64% and R_B=2.92%. The atom arrangement is similar to that of yavapaiite (KFe(SO₄)₂), however, these crystal structures differ distinctly in the coordination numbers of barium and potassium atoms; the former is tenfold coordinated, whereas the latter is sixfold coordinated. The powder specimens were also examined by high-temperature XRD and DTA to reveal the occurrence of a phase transition from monoclinic to orthorhombic at 732 K during heating. Upon cooling the reverse transition occurred at 710 K. The monoclinic crystal expanded almost one-dimensionally along [503] during the heating process. The orthorhombic phase also showed a tendency to expand one-dimensionally along the c-axis above 732 K.     

Phase polymorphism of [Zn(DMSO)6](ClO4)2 studied by differential scanning calorimetry

Four solid phases of [Zn(DMSO)₆](ClO₄)₂ have been detected by differential scanning calorimetry (DSC). Specifically, the phase transitions were detected between: metastable phase KII ↔ supercooled phase K0 at , stable phase KIb ↔ stable phase KIa at , stable phase KIa ↔ stable phase K0 at . At T_m2 = 389 K crystals partially and at T_m1 = 465 K completely melts. From the entropy change values it was concluded that the phases: K0 and K0′ are the orientationally dynamically disordered phases, so called ODDIC crystals, and phases KIa, KIb and metastable KII are dynamically ordered but with some degree of positional disorder.     

The crystal and magnetic structure of the magnetocaloric compound FeMnP0.5Si0.5

The crystal and magnetic structure of the magnetocaloric compound FeMnP_0.5Si_0.5 has been studied by means of neutron and X-ray powder diffraction. Single phase samples of nominal composition FeMnP_0.5Si_0.5 have been prepared by the drop synthesis method. The compound crystallizes in the Fe₂P-type structure () with the magnetic moments aligned along the a-axis. It is found that the Fe atoms are mainly situated in the tetrahedral 3g site while the Mn atoms prefer the pyramidal 3f position. The material is ferromagnetic (T_C=382 K) and at 296 K the total magnetic moment is . It is shown that the magnetic moment in the 3f site is larger () than in the 3g site ().     

On the symmetry and crystal structures of Ba2LaIrO6

Accurate profile analysis of X-ray diffraction data was carried out to settle recent dispute on the symmetry and crystal structures of the double perovskite Ba₂LaIrO₆. Even through careful comparison of the full-width at half-maximum values, we found no evidence for Ba₂LaIrO₆ adopting either monoclinic (I2/m) or mixed rhombohedral and monoclinic (I2/m) structures at room temperature, becoming triclinic at below about 200 K. The correct space group is just at temperatures between 82 and 653 K. Furthermore, the phase transition does occur in Ba₂LaIrO₆, but the transition temperature is found to be much higher than the reported value.