F. Bourée

Contribution of powder diffraction for structure refinements of aperiodic misfit cobalt oxides

Two families of misfit cobalt oxides have been reinvestigated using neutron and synchrotron powder diffraction data and Rietveld structure refinements in the four-dimensional formalism for aperiodic structures. In the first system [Ca2CoO3][CoO2]1.62, the structure-specific features of two complex polytypic phases previously described from single-crystal diffraction data have been confirmed for the main term of the series and are described with the C2/m(1,δ,0)s0 superspace symmetry (Rp = 0.026, Rwp = 0.031, RB = 0.014). The high-resolution synchrotron diffraction experiments have been interpreted with a triclinic distortion of the structure, probably related to an intrinsic modulation of the [Ca2CoO3] sublattice in relation with Co and O split atomic sites. In the second system Ca0.82(Cu0.65Co0.35O2), a new occupation modulation of the metallic site is shown with the Fmmm(α11)0ss superspace symmetry (Rp = 0.031, Rwp = 0.039, RB = 0.027) and is interpreted in relation with the modulated interatomic distances of the square coordination of the cationic site. In both cases, recently developed tools for structure refinements of aperiodic systems using powder diffraction data (in the JANA2000 package) have proved their efficiency, in particular as specific complements to previous single-crystal structure refinements.

Crystal and magnetic structure of LaTiO 3 : Evidence for nondegenerate t 2 g orbitals

The crystal and magnetic structures of LaTiO 3 have been studied by x-ray and neutron-diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering T N = 146 K. In addition, the octahedra in LaTiO 3 exhibit an intrinsic distortion, which implies a splitting of the t 2 g levels. Our results indicate that LaTiO 3 should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering.

Crystal and magnetic structure of the uranium digermanide UGe2

Abstract The crystal structure of the uranium digermanide UGe 2 was determined and refined from single crystal X-ray diffraction data to R = 0.040, Rw = 0.052. UGe 2 crystallizes with the orthorhombic ZrGa 2 type ( Cmcm ) instead of the ZrSi 2 type ( Cmcm ) as was previously assumed on the basis of rather old and unreliable results. Our magnetic study of a polycrystalline are melted UGe 2 sample confirms the onset of ferromagnetic interactions below ca. 52 K. Neutron powder diffraction studies show that the structure is collinear ferromagnetic, with uranium magnetic moments parallel to the a -axis of the orthorhombic cell: at T = 1.4 K, the uranium magnetic moment value is M 1 = 1.42(4) μ B.

Magnetoelastic coupling in RETiO3 (RE = La, Nd, Sm, Gd, Y)

A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.

Influence of Mn-site doping upon orbital and charge ordering in the Pr0.5A0.5Mn1−xMxO3 manganites (A=Sr, Ca and M=Cr, Al)

Abstract The influence of Cr and Al-doping upon orbital and charge ordering in Pr0.5A0.5MnO3 manganites (A=Sr, Ca) has been investigated combining magnetic and transport measurements with neutron powder diffraction and electron microscropy. It is shown that Cr-doping induces or reinforces the ferromagnetic metallic state, and destroys the ordering and associated antiferromagnetism whatever is its nature, A or CE-type. On the contrary, the orbital and charge ordering and associated antiferromagnetic ground states are essentially retained after doping with diamagnetic aluminium. Moreover, the Al-doping in Pr0.5Sr0.5MnO3 expands the antiferromagnetic region and suppresses the ferromagnetism at intermediate temperatures.

Charge ordering and structural transitions in Pr0.5Sr0.41Ca0.09MnO3

Abstract The structural and magnetic transitions in Pr0.5Sr0.41Ca0.09MnO3 have been investigated by neutron diffraction and electron microscopy. Two structural transitions, Imma to I4/mcm and I4/mcm to Pmmn, are observed by decreasing the temperature. Two magnetic transitions, from a paramagnetic insulating to a ferromagnetic metallic and from a ferromagnetic metallic to an antiferromagnetic insulating states at T C =250 K and T N =180 K , respectively, are also observed. The structures of these three forms have been determined from neutron powder diffraction data. The first important result concerns the low temperature antiferromagnetic CE type and charge ordered structure, which has been refined in the Pmmn space group, without any constraint. This structure is completely long range ordered, with two Mn-sites, Mn3+ in tetragonally elongated octahedra, and Mn4+, off-centered in nearly regular octahedra. The second important point concerns the abrupt character of the structural transition from the I4/mcm to the Pmmn structure, without any appearance of incommensurability. The magnetic and transport properties of this compound are compared with those of Pr0.5Sr0.5MnO3.

Crystal and magnetic structures of U2Ni2Sn investigated by neutron diffraction and 119Sn Mössbauer spectroscopy

Abstract The U 2 Ni 2 Sn stannide has been studied by means of neutron powder diffraction and Mossbauer spectroscopy. This compound crystallizes in the tetragonal P4/mbm space group in an ordered version of the U 3 Si 2 -type structure and orders antiferromagnetically below T N = 25(1) K. At T = 1.5 K the magnetic structure is collinear with uranium magnetic moment (1.05(5)μ B ) perpendicular to the tetragonal c -axis. 119 Sn Mossbauer spectroscopy at 4.2 K shows that tin atoms see a zero hyperfine field, in agreement with the proposed antiferromagnetic order.

Structural defects in LaMnO3 phase studied by neutron diffraction

Abstract The crystal structure of air-annealed lanthanum manganite of three different La/Mn ratios (0.91:1, 1:1 and 1:0.9) was refined based on neutron powder diffraction data. Rhombohedral (space group R 3 c ) and orthorhombic (space group Pnma) structure models were taken from the literature available as starting points for the crystal structure analysis. In the course of the Rietveld refinement, several models of different site occupancy were tested. Selection of the most probable models has been based on the reliability factors obtained and on the results of sample density measurements. This selection appeared to be successful for the sample of nominal composition La0.91MnO2.961 (La/Mn=0.91:1) quenched from 1270 K. The resulting distribution model, i.e. [La0.922Mn0.013]MnO3, is characterised by an excess of 0.013 Mn-ions/formula located on the La-site. The type of structural defects existing in samples with nominal compositions of LaMnO3.250 (La/Mn=1:1) quenched from 830 K, and LaMn0.9O3.062 (La/Mn=1:0.9) quenched from 1070 K, both characterised by a supplementary cationic deficiency accompanied by a fraction of excess oxygen, remain undetermined due to large uncertainties in the precise location of the excess oxygen.

Magnetic structure of CoFe2O4

Abstract Up to now, the cubic spinel CoFe 2 O 4 was assumed to have a collinear ferrimagnetic structure; however a High Field Mossbauer Study (HFMS) recently evidenced for canting angles at both A and B iron sites. In order to have a coherent view on the CoFe 2 O 4 magnetic structure, we performed high field magnetic measurements and neutron diffraction experiments on the same powdered sample, already used in the HFMS. Neutron diffraction shows unambiguously a long range ferrimagnetic collinear structure ( T = 300 and 5 K). All the experimental results are coherent in a model of non-uniform canting (local order).

Crystal chemistry of the G-phases in the systems Ti–{Fe, Co, Ni}–Al with a novel filled variant of the Th6Mn23-type

Abstract The crystal structures of the compounds Ti6M7Al17 with M=Fe,Co,Ni have been investigated by X-ray powder and single crystal, neutron powder and electron diffraction. These compounds crystallize with a new filled variant of the Mg6Cu16Si7-type (space group Fm 3 m ). A close structural relation is encountered for the series of crystal structures: Th6Mn16Mn7?, Mg6Cu16Si7?, Sc6Sc16Ir7Ir, Zr6Zn16Zn7Si, Ti6Al16M7Al, and Ti6Ni16Si7Si. Electrical resistivity of the Fe, Co-based compounds is typical metallic and temperature dependence follows the Bloch Gruneisen relation with a Debye temperature of about 300 K. No hydrogen was found to be absorbed in the Fe-, and Co-based compounds at room temperature under a H2 pressure of 5 MPa.