Magnetic Phase Transitions in NdCoAsO

Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase transitions below room temperature. The crystal and magnetic structures of NdCoAsO have been determined by powder neutron diffraction, and the effects of the phase transitions on physical properties are reported. Near 69 K a ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and corresponds to the antiferromagnetic ordering, with the same propagation vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In addition, transport properties and heat capacity results are presented, and show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction results are included. Accepted for publication in Physical Review

Composition−Structure Relationships in Polar Intermetallics: Experimental and Theoretical Studies of LaNi1+xAl6-x (x = 0.44)

A new ternary aluminide, LaNi1+xAl6-x (x = 0.44), has been synthesized from La, Ni, and Al in sealed silica tubes. Its structure, determined by single-crystal X-ray diffraction, is tetragonal P4/mmm (No. 123) with Z = 1 and has the lattice parameters a = 4.200(8) and c = 8.080(8) Å. Refinement based on Fo2 yielded R1 = 0.0197 and wR2 = 0.020 [I \u3e 2σ(I)]. The compound adopts a structure type previously observed in SrAu2Ga5 and EuAu2Ga5. The atomic arrangement is closely related to the one in BaAl4 as well as in other rare-earth gallide compounds such as LaNi0.6Ga6, HoCoGa5, Ce4Ni2Ga20, Ce4Ni2Ga17, Ce4NiGa18, and Ce3Ni2Ga15. This structure exhibits a large open cavity which may be filled by a guest atom. Band structure calculations using density functional theory have been carried out to understand the stability of this new compound

Atomic Distributions in the γ-Brass Structure of the Cu−Zn System: A Structural and Theoretical Study

The crystal structures, atomic distributions, and theoretical electronic structures of five different Cu5-xZn8+x γ-brass compounds (x = −0.59(3), −0.31(3), 0.00(3), 0.44(3), and 0.79(3)) are reported with the goal of identifying chemical influences on the observed phase width. These structures have been refined by both neutron and X-ray powder diffraction to obtain accurate crystal chemical parameters. All compounds crystallize in the space group I4̄3m (No. 217) (Z = 4), and the unit cell parameters are a = 8.8565(4), 8.8612(5), 8.8664(3) , 8.8745(4), and 8.8829(7) Å, respectively, for Cu5.59Zn7.41, Cu5.31Zn7.69, Cu5.00Zn8.00, Cu4.56Zn8.44, and Cu4.21Zn8.79. The results indicate specific site substitutions on both sides of the ideal composition “Cu5Zn8”. In all cases, the 26-atom cluster building up the γ-brass structure shows a constant inner [Cu4Zn4] tetrahedral star with compositional variation occurring at the outer octahedron and cuboctahedron. First principles and semiempirical electronic structure calculations using both a COHP and Mulliken population analysis were performed to understand the observed compositional range and to address the “coloring problem” for the site preferences of Cu and Zn atoms for this series of compounds

Relations entre propriétés optiques et structures électroniques dans de nouveaux matériaux des familles Ln-M-S-X (Ln=La, Ce, M=P, Si, Ge, Sb et X=Cl, Br, I)

Ce mémoire porte sur l'étude de plusieurs familles ternaires et quaternaires contenant La et Ce et appartenant aux systèmes Ln-M-Q-X, Ln-P-Q et Ln-Sb-S-Br (Ln=La, Ce, M=Si, Ge, X=Cl, Br, I). Ces composés possèdent une chimie structurale riche et de nombreuses propriétés physiques intéressantes du fait du degré élevé de coordination des ions LnIII et de la présence d'orbitales 4f très localisées. Il est possible de modifier par effet inductif la covalence de la liaison Ce-S, et donc le gap 4f-5d, en substituant les différents éléments constituants ces matériaux.The aim of this work was the study of cerium and lanthanum compounds belonging to the Ln-M-Q-X, Ln-P-Q and Ln-Sb-S-Br (Ln=La, Ce, M=Si, Ge, X=Cl, Br, I) systems. These compounds present a rich structural chemistry and widely varying physical properties related to a high coordination number of the LnIII cations and the occurrence of very localized 4f orbitals.NANTES-BU Sciences (441092104) / SudocSudocFranceF

Transmission Electron Microscopic Study of Pyrochlore to Defect-Fluorite Transition in Rare-Earth Pyrohafnates

A structural transition in rare earth pyrohafnates, Ln2Hf2O7 (Ln=Y, La, Pr, Nd, Tb, Dy, Yb and Lu), has been identified. Neutron diffraction showed that the structure transforms from well-ordered pyrochloric to fully fluoritic through the lanthanide series from La to Lu with a corresponding increase in the position parameter x of the 48f (Fdm) oxygen site from 0.330 to 0.375. As evidenced by the selected area electron diffraction, La2Hf2O7, Pr2Hf2O7 and Nd2Hf2O7 exhibited a well-ordered pyrocholoric structure with the presence of intense superlattice spots, which became weak and diffuse (in Dy2Hf2O7 and Tb2Hf2O7) before disappearing completely as the series progressed towards the Lu end. High resolution electron microscopic studies showed the breakdown of the pyrochlore ordering in the form of antiphase domains resulting in diffused smoke-like superlattice spots in the case of Dy2Hf2O7 and Tb2Hf2O7

A New Superstructure for the BaAl4-Structure Type: An Experimental and Theoretical Study of La2NiAl7

A new ternary aluminide, La2NiAl7, has been synthesized from the elements in sealed silica tubes. The crystal structure of La2NiAl7 was determined by single-crystal X-ray diffraction and crystallizes in the tetragonal space group I4mm (No. 123) with Z = 4 and lattice parameters a = 6.221(3) and c = 21.403(2) Å. Refinement based on F02 yielded R1 = 0.0313 and wR2 = 0.0392 [I > 2σ(I)]. It is a new structure type, which belongs to the family of BaAl4-related structures. The La2NiAl7 structure corresponds to a 3D superstructure (√2a × √2b × 2c) of the BaAl4-structure type, whereas La3Al11 is only a 1D superstructure (3a × b × c). Tight-binding band structure calculations using the density functional theory have been carried out to understand the structural stability of La2NiAl7 as well as La3Al11.Reprinted (adapted) with permission from Chem. Mater., 2005, 17 (14), pp 3661–3667. Copyright 2005 American Chemical Society.</p

A New Superstructure for the BaAl4-Structure Type: An Experimental and Theoretical Study of La2NiAl7.

A new ternary aluminide, La2NiAl7, has been synthesized from the elements in sealed silica tubes. The crystal structure of La2NiAl7 was determined by single-crystal X-ray diffraction and crystallizes in the tetragonal space group I4mm (No. 123) with Z = 4 and lattice parameters a = 6.221(3) and c = 21.403(2) Å. Refinement based on F02 yielded R1 = 0.0313 and wR2 = 0.0392 [I > 2σ(I)]. It is a new structure type, which belongs to the family of BaAl4-related structures. The La2NiAl7 structure corresponds to a 3D superstructure (√2a × √2b × 2c) of the BaAl4-structure type, whereas La3Al11 is only a 1D superstructure (3a × b × c). Tight-binding band structure calculations using the density functional theory have been carried out to understand the structural stability of La2NiAl7 as well as La3Al11.Reprinted (adapted) with permission from Chem. Mater., 2005, 17 (14), pp 3661–3667. Copyright 2005 American Chemical Society.</p

Composition−Structure Relationships in Polar Intermetallics: Experimental and Theoretical Studies of LaNi1+xAl6-x (x = 0.44)

A new ternary aluminide, LaNi1+xAl6-x (x = 0.44), has been synthesized from La, Ni, and Al in sealed silica tubes. Its structure, determined by single-crystal X-ray diffraction, is tetragonal P4/mmm (No. 123) with Z = 1 and has the lattice parameters a = 4.200(8) and c = 8.080(8) Å. Refinement based on Fo2 yielded R1 = 0.0197 and wR2 = 0.020 [I > 2σ(I)]. The compound adopts a structure type previously observed in SrAu2Ga5 and EuAu2Ga5. The atomic arrangement is closely related to the one in BaAl4 as well as in other rare-earth gallide compounds such as LaNi0.6Ga6, HoCoGa5, Ce4Ni2Ga20, Ce4Ni2Ga17, Ce4NiGa18, and Ce3Ni2Ga15. This structure exhibits a large open cavity which may be filled by a guest atom. Band structure calculations using density functional theory have been carried out to understand the stability of this new compound.Reprinted (adapted) with permission from Inorg. Chem., 2004, 43 (15), pp 4604–4609. Copyright 2004 American Chemical Society.</p

Crystal Structure of Sr\u3csub\u3e0.4\u3c/sub\u3eCe\u3csub\u3e0.4\u3c/sub\u3eTiO\u3csub\u3e3\u3c/sub\u3e Ceramics

A cerium-doped SrTiO3 compound with the composition Sr0.4Ce0.4TiO3 has been produced by conventional solid-state processing. The structure of this compound was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is C2/c, corresponding to an a-b-b- tilt system. Octahedra are tilted by ~5° about the pseudo-cubic a-axis and ~1.5° about the pseudo-cubic b- and c-axes

Structure of Compounds in the Sr1–3x/2CexTiO3 Homologous Series

Four compositions in the Sr1–3x/2CexTiO3 homologous series, corresponding to x = 0.1333, 0.1667, 0.25, and 0.4, have been produced by conventional solid-state processing. The structure of these compounds was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is R3̅c, corresponding to an a−a−a− tilt system, except for the composition x = 0.4, for which an even more complex superstructure is observed. The degree of tilt increases with increasing x