P. Fischer

High-resolution powder diffractometer HRPT for thermal neutrons at SINQ

Abstract Design characteristics and first experience concerning the new high-resolution powder diffractometer for thermal neutrons at the Swiss spallation neutron source SINQ are summarized. It is based on a linear position-sensitive 3 He detector with 1600 wires and angular separation of 0.1°, permitting also real-time experiments.

Geometrically frustrated magnetic structures of the heavy-fermion compound CePdAl studied by powder neutron diffraction

The heavy-fermion compound CePdAl with ZrNiAl-type crystal structure (hexagonal space group ) was investigated by powder neutron diffraction. The triangular coordination symmetry of magnetic Ce atoms on site 3f gives rise to geometrical frustration. CePdAl orders below with an incommensurate antiferromagnetic propagation vector , and a longitudinal sine-wave (LSW) modulated spin arrangement. Magnetically ordered moments at Ce(1) and Ce(3) coexist with frustrated disordered moments at Ce(2). The experimentally determined magnetic structure is in agreement with group theoretical symmetry analysis considerations, calculated by the program MODY, which confirm that for Ce(2) an ordered magnetic moment parallel to the magnetically easy c-axis is forbidden by symmetry. Further low-temperature experiments give evidence for a second magnetic phase transition in CePdAl between 0.6 and 1.3 K. Magnetic structures of CePdAl are compared with those of the isostructural compound TbNiAl, where a non-zero ordered magnetic moment for the geometrically frustrated Tb(2) atoms is allowed by symmetry.

Structural and magnetic properties of RFe4P12 (R=Pr, Nd) studied by neutron diffraction

Abstract In this work we present a neutron powder diffraction study of the ternary rare-earth phosphides PrFe 4 P 12 and NdFe 4 P 12 . High-resolution measurements of the structural temperature dependence reveal large-amplitude thermal vibrations of the rare-earth ions, consistent with speculations that the ion is weakly bound in an oversized atomic ‘cage’ formed in the skutterudite crystal structure. In addition, low temperature neutron diffraction measurements of RFe 4 P 12 (R=Pr, Nd) were performed. Whereas the proposed long-range antiferromagnetic ordering in PrFe 4 P 12 could not be confirmed, NdFe 4 P 12 orders ferromagnetically below T C =1.94 K with an ordered magnetic Nd 3+ moment of 1.61(7) μ B at 1.5 K.

Crystal and magnetic structures of ternary metal hydrides: A comprehensive review

The crystal and magnetic structures of hydrides and deuterides of binary metal compounds as determined by neutron diffraction analysis are reviewed. Emphasis is placed on the distribution of hydrogen (deuterium) atoms in the metal atom host structure. At room temperature these distributions are characterized by thermal disorder and preferential occupation of large interstices which are surrounded by atoms belonging to hydride forming elements. The metal atom arrangements are usually similar to those of the binary metal compounds, except that their lattices are generally expanded and their symmetries reduced. At low temperatures the hydrogen (deuterium) atoms tend to order, thus inducing structural phase transitions and further symmetry lowering. Efforts to rationalize observed hydrogen (deuterium) site occupancies and maximum hydrogen (deuterium) contents in terms of empirical models are reviewed. The influence of hydrogenation (deuteration) on the magnetic structures is discussed.

Evidence for an isostructural phase transition in the metastable high-temperature modification of TbPdAl

Abstract The intermetallic rare-earth aluminide TbPdAl crystallizes in the hexagonal ZrNiAl-type structure (metastable high-temperature modification, HTM) and in the orthorhombic TiNiSi-type structure (stable low-temperature modification, LTM). Based on neutron diffraction, magnetic susceptibility and electrical resistivity data for TbPdAl we provide clear evidence for a first-order isostructural phase transition (from HTM I to HTM II), which is absent in the LTM phase, and determine the structural parameters for all three phases. For TbPdAl HTM jumps in the hexagonal lattice parameters a (decrease of 1.0%) and c (increase of 1.9%) occur at T sh =106 K (heating) and T sc =91 K (cooling), and the two phases HTM I and HTM II coexist with a volume fraction ≥20% in a temperature interval of 17 K. The structural phase transition is likely to be of electronic origin.

High-pressure synthesis and crystal structures of new ternary caesium magnesium hydrides, CsMgH3, Cs4Mg3H10 and Cs2MgH4

Abstract The title compounds were synthesized by heating stoichiometric mixtures of the binary hydrides (deuterides) to 820 K in a multi-anvil press at 30 kbar pressure. X-ray and neutron powder diffraction on the deuterides at ambient conditions indicates rhombohedral symmetry for CsMgH 3 (space group R-3m , a =6.2521(6), c =22.382(3) A, BaRuO 3 type structure), orthorhombic symmetry for Cs 4 Mg 3 H 10 ( Cmca , a =6.2569(9), b =14.743(2), c =13.791(2) A, Cs 4 Mg 3 F 10 type structure), and tetragonal symmetry for Cs 2 MgH 4 ( I4/mmm , a =4.3264(5), c =14.760(2) A, K 2 NiF 4 type structure). The compounds contain octahedral [MgH 6 ] 4− anions and have considerably better volume efficiencies for hydrogen storage than those containing tetrahedral [MgH 4 ] 2− anions that are synthesised under hydrogen gas pressure.

Neutron diffraction at SINQ

Abstract The high resolution powder diffractometer for thermal neutrons, HRPT, with 1600 detectors (PSD) started operation in 1999 at the SINQ target station [1]. For 20, = 120° and a sample diameter of 10 mm already, resolutions down to δ/d < 0.001 (d = lattice spacing) were reached. As an example, Figure 1 shows a precise determination of deuterium positions for the metal deuteride Cs,MgD, [2]. Figure 2 illustrates the detection of rather small structural distortions in the case of HOB, at low temperatures [3].

Low-temperature antiferromagnetic moments at the 4a site in Ce3Pd20Ge6

We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce3Pd20Ge6, which exhibits ferroquadrupolar ordering at TQ1≈1.2 K and antiferromagnetic ordering at TN2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R3Pd20X6 (X = Si, Ge; space group Fmm), it is common that at the Neel temperature TN1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k1 = [1,1,1] and at TN2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k2 = [0,0,1]. In Ce3Pd20Ge6 the Neel temperature TN1 is replaced by TQ1 and at the 8c site a possible ordered cerium moment is found to be reduced to an undetectable small value down to the lowest measured temperature T = 0.05 K. In contrast, at the 4a site the cerium moments order below TN2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k2 = [0,0,1] with a saturation value µ2(4a) = (1.1±0.1) µB/Ce that is reasonably large for the crystal field ground-state quartet Γ8. Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at TQ1.

Three-dimensional antiferromagnetic ordering in the light rare-earth high-Tc superconductor NdBa2Cu3O6.86

Powder neutron diffraction investigations performed in the temperature range from 20 mK to 300K prove that the magnetic Nd moments of the superconductor NdBa2Cu3O6.86 withTc=88 K order three-dimensionally (3D) antiferromagnetic belowTN=(551±10) mK. As in similar Gd and Dy compounds the corresponding wave vector isk=[1/2, 1/2, 1/2]. In approximate agreement with crystal field calculations the ordered magnetic moment of Nd amounts at saturation to (1.14±0.06) μB and at 25 mK the magnetic moments are oriented parallel [0,0,1]. The transition to the magnetically ordered state corresponds to the Landau type critical exponent β≈0.5, in contrast to the predominant 2D character of such heavy rare-earth systems. The crystal structure of NdBa2Cu3O6.86 is orthorhombic similar to the one of YBa2Cu3O7−x.

Successive magnetic ordering of the Tb sublattices in Tb3Pd20Si6

The chemical and magnetic structures of the intermetallic compound were studied by means of bulk magnetic measurements and neutron diffraction. The crystal structure of was confirmed to be isotypic with that of , space group , No 225, with Tb being located at the two crystallographic sites 4a and 8c. The compound undergoes two successive second-order magnetic phase transitions at the Neel temperatures and . Below , the 8c sites order in an antiferromagnetic sequence with a magnetic propagation vector , leaving the 4a sites disordered down to , where additional antiferromagnetic ordering of the 4a sites sets in according to .