Magnetic ground states of the tetragonal U2Fe2Sn-type structure using an anisotropic RKKY exchange modeling

In the light of numerous magnetic data obtained on the ternary compounds R2T2X (R=U, Ce; T=Ni, Rh, Pd and X=Sn, In), which crystallize in the tetragonal U2Fe2Sn-type structure (P4/mbm space group) but exhibit various magnetic structures, we demonstrate the efficiency of a Ruderman-Kittel-Kasuya-Yosida model based on a non-spherical Fermi surface to account for the magnetic interactions involved in these systems. Considering as magnetic ground states the basis vectors of the irreducible representations associated to P4/mbm space group, (4h) Wyckoff position [crystallographic site for Ce or U] and..

Magnetic ordering in the ternary germanide Ce2Ni3Ge5 as studied by neutron powder diffraction

International audienceNeutronpowderdiffraction investigations on Ce2Ni3Ge5 have been carried out. This compound adopts the U2Co3Si5-type structure (Ibam space group) with only one crystallographic site available for Ce atoms. Magnetisation measurements indicate that Ce2Ni3Ge5 undergoes two antiferromagnetic transitions at low temperature, at TN1=4.8(2) K and TN2=4.2(2) K, respectively. From our neutrondiffraction experiments, only one transition is evidenced, below TN1, Ce2Ni3Ge5 is a collinear antiferromagnet with magnetic moments parallel to the a-axis and equal to 0.4(1) µB/Ce at T = 1.4 K

Magnetic structures of the M2TbF6 (M=Li, K, Rb) fluorides: A complex behavior resulting from frustration

Neutron powder diffraction has been performed on Li2TbF6, K2TbF6 and Rb2TbF6 fluoroterbates. Incommensurate long-range magnetic order is observed below TN=2.02, 1.60 and 2.07 K. The square-modulating of the magnetic structures can be correlated with the geometric frustration induced by the pseudo-hexagonal packing of the [TbF6]2− chains in these hexafluorides. This frustration and the magnetic interactions are discussed on the basis of experimental data and topological considerations..

Magnetic structures of some strongly correlated electron Ce-Ni-Ge systems

Extending our study on the characterisation of the magnetic properties of the ternary germanides based on cerium and nickel, we have performed magnetisation measurements and neutron powder diffraction on CeNiGe3. The antiferromagnetic structure of this compound is dependent on temperature: (i) for T between TN=5.9(2) and 5.0(2) K, the magnetic peaks are associated with the k 1=(1 0 0) propagation vector; (ii) below 5.0(2) K, the two propagation vectors k 1=(1 0 0) and k 2=(0 0.409(1) 1/2) coexist, but the intensities of the magnetic peaks corresponding to k 1 decrease with temperature below 4.1(2) K. We discuss this complex magnetic behaviour in relation to the magnetic structure reported for Ce3Ni2Ge7

Determination and Rietveld refinement of the crystal structure of Li0.50Ni0.25TiO(PO4) from powder x-ray and neutron diffraction

The structure of the oxyphosphate Li0.50Ni0.25TiO(PO4) has been determined from conventional X-ray and neutron powder diffraction data. The parameters of the monoclinic cell (space group P21/c, Z=4), obtained from X-ray results, are: a=6.3954(6) Å, b=7.2599(6) Å, c=7.3700(5) Å, and β=90.266(6)°; those resulting from neutron study are: a=6.3906(7) Å, b=7.2568(7) Å, c=7.3673(9) Å, and β=90.234(7)°. Refinement by the Rietveld method using whole profile, leads to satisfactory reliability factors: cRwp=0.128, cRp=0.100, and RB=0.038 for X-ray and cRwp=0.110, cRp=0.120, and RB=0.060 for neutrons. The structure of Li0.50Ni0.25TiO(PO4) can be described as a TiOPO4 framework constituted by chains of tilted corner-sharing TiO6 octahedra running parallel to the c axis and cross linked by phosphate tetrahedra. In this framework, there are octahedral cavities occupied by Li and Ni atoms: Li occupies the totality of the 2a sites and Ni occupies statistically half of the 2b sites. Ti atoms are displaced from the center of octahedra units in alternating long (2.242 Å) and short (1.711 Å) Ti-O bonds along chains

Nuclear and magnetic structures and magnetic properties of synthetic brochantite, Cu₄(OH)₆SO₄

Cu4(OH)6SO4 (1) and Cu4(OD)6SO4 (2) were obtained by hydrothermal syntheses from copper sulfate and sodium hydroxide in H2O and D2O, respectively. They crystallize in the monoclinic system, space group P2(1)/a (14), a = 13.1206(5), b = 9.8551(3), c = 6.0295(2) Angstroms, beta = 103.432(3) degrees, V = 758.3(1) Angstroms(3), Z = 4 and a = 13.1187(5), b = 9.8552(3), c = 6.0293(2) Angstroms, beta = 103.410(3) degrees, V = 758.3(1) Angstroms(3), Z = 4, respectively. They are iso-structural to the mineral brochantite and consist of double chains of edge-sharing copper octahedra that are connected to one another by corners to form corrugated planes along bc; these planes are in-turn bridged by the unprecedented mu7-sulfate tetrahedra to give a 3D-structure. All the hydrogen atoms were precisely located from refinement of the neutron powder diffraction data of the deuterated sample. Magnetic susceptibility data reveal a low-dimensional behavior at high temperature and the presence of both ferromagnetic and antiferromagnetic super-exchanges resulting in a 3D long-range antiferromagnetic ordering at 7.5 K accompanied by a small canting of the moments. The transition is confirmed by a lambda-peak in the specific heat. The magnetic structure at 1.4 K shows the moments are oriented perpendicular to the corrugated planes with alternation along +/-a for neighboring chains within the double chains. The enhanced incoherent scattering at low-angle suggests the existence of short-range ferromagnetic clusters

Crystal structure and spectroscopic properties of a new oxyarsenate Li0.5Ni0.25TiOAsO4

The new oxyarsenate Li0.5Ni0.25TiOAsO4 has been synthesized and studied by a combination of X-ray powder diffraction, neutrons powder diffraction and vibrational spectroscopy. Li0.5Ni0.25TiOAsO4 crystallizes in the monoclinic P21/c space group with the unit cell parameters..

New process of preparation, X-ray characterisation, structure and vibrational studies of a solid solution LiTiOAs₁₋<i>_x</i>P<i>_x</i>O₄ (0 ≤ x ≤ 1)

LiTiOAs1−xPxO4 (0 ≤ x ≤ 1) compounds have been prepared using solutions of Li, Ti, As and P elements as starting products. Selected compositions have been investigated by powder X-ray or neutrons diffraction analysis, Raman and infrared spectroscopy. The structure of LiTiOAs1−xPxO4 (x=0, 0.5 and 1) samples determined by Rietveld analysis is orthorhombic with Pnma space group. It is formed by a 3D network of TiO6 octahedra and XO4 (X=As1−xPx) tetrahedra where octahedral cavities are occupied by lithium atoms. TiO6 octahedra are linked together by corners and form infinite chains along a-axis. Ti atoms are displaced from the centre of octahedral units in alternating short (1.700–1.709 Å) and long (2.301–2.275 Å) Ti–O bonds. Raman and infrared studies confirm the existence of Ti–O–Ti chains. Thermal stability of LiTiOAsO4 has been reported

New process of preparation, X-ray characterisation, structure and vibrational studies of a solid solution LiTiOAs₁₋<i>_x</i>P<i>_x</i>O₄ (0 ≤ x ≤ 1)

LiTiOAs1−xPxO4 (0 ≤ x ≤ 1) compounds have been prepared using solutions of Li, Ti, As and P elements as starting products. Selected compositions have been investigated by powder X-ray or neutrons diffraction analysis, Raman and infrared spectroscopy. The structure of LiTiOAs1−xPxO4 (x=0, 0.5 and 1) samples determined by Rietveld analysis is orthorhombic with Pnma space group. It is formed by a 3D network of TiO6 octahedra and XO4 (X=As1−xPx) tetrahedra where octahedral cavities are occupied by lithium atoms. TiO6 octahedra are linked together by corners and form infinite chains along a-axis. Ti atoms are displaced from the centre of octahedral units in alternating short (1.700–1.709 Å) and long (2.301–2.275 Å) Ti–O bonds. Raman and infrared studies confirm the existence of Ti–O–Ti chains. Thermal stability of LiTiOAsO4 has been reported

Magnetic behaviour of the MTbF6 fluoroterbates (M=Cd, Ca, Sr, (α/β)-Ba)

Neutron powder diffraction has been performed on the MTbF6 fluorides (M=Cd, Ca, Sr, (α/β)-Ba). Four of these fluorides (Cd, Ca, Sr, β-Ba) are built of a (pseudo-) tetragonal packing of [TbF6]2− chains and only differs by the chains relative orientations. Thus this series represents a valuable opportunity to evaluate the Tb4+-Tb4+ magnetic interactions. All the compounds displayed antiferromagnetic order (TN=2.70 K (Cd), 2.15 K (Ca), 2.60 K (Sr), 2.10 K (β-Ba)), except for the α form of BaTbF6. The crystal structure of this latter fluoroterbate has also been investigated by means of high-resolution neutron powder diffraction. From Neutron Powder Diffraction data, CdTbF6 and β-BaTbF6 magnetic structures were determined, together with the metamagnetic behaviour of β-BaTbF6 as a function of an external magnetic field. A tentative phase diagram is then given for β-BaTbF6. Advantage was taken of the polymorphism of the BaTbF6 fluoroterbate to analyse, on the basis of topological parameters such as bond distances and angles, the magnetic behaviour of its α and β forms. It was shown that superexchange interactions are present in β-BaTbF6, and that these interactions may also rule the magnetic behaviour of the other MTbF6 (M=Ca, Sr, Cd) tetravalent terbium fluorides