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Redetermination of the crystal structures of rare-earth trirhodium diboride RERh₃B₂ (RE = Pr, Nd and Sm) from single-crystal X-ray data.

ABSTRACT: The crystal structures of the rare-earth (RE) trirhodium diborides praseo-dymium trirhodium diboride, PrRh₃B₂, neodymium trirhodium diboride, NdRh₃B₂, and samarium trirhodium diboride, SmRh₃B₂, were refined on the basis of single-crystal X-ray diffraction data. The crystal chemistry of RERh₃B₂ (RE: Pr, Nd, and Sm) compounds has previously been analyzed mainly on the basis of powder samples [Ku et al. (1980 ▸). Solid State Commun. 35, 91-96], and no structural investigation by single-crystal X-ray diffraction has been reported so far. The crystal structures of the three hexa-gonal RERh₃B₂ compounds are isotypic with that of CeRh₃B₂; RE, Rh and B sites are situated on special positions with site symmetry 6/mmm (Wyckoff position 1a), mmm (3g) and m2 (2c), respectively. In comparison with the previous powder X-ray study of hexa-gonal RERh₃B₂, the present redetermination against single-crystal X-ray data has allowed for the modeling of all atoms with anisotropic displacement parameters (ADPs). The ADPs of the Rh atom in each of the structures result in an elongated displacement ellipsoid in the direction of the stacking of the Rh kagomé-type layer. The features of obtained ADPs of atoms are discussed in relation to RERh₃B₂-type and analogous structures.

SUBMITTER: Tokuda M

PROVIDER: S-EPMC8739198 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Redetermination of the crystal structure of R 5Si4 (R = Pr, Nd) from single-crystal X-ray diffraction data.

Project description:The crystal structures of praseodymium silicide (5/4), Pr5Si4, and neodymium silicide (5/4), Nd5Si4, were redetermined using high-quality single-crystal X-ray diffraction data. The previous structure reports of Pr5Si4 were only based on powder X-ray diffraction data [Smith et al. (1967 ?). Acta Cryst. 22 940-943; Yang et al. (2002b ?). J. Alloys Compd. 339, 189-194; Yang et al., (2003 ?). J. Alloys Compd. 263, 146-153]. On the other hand, the structure of Nd5Si4 has been determined from powder data [neutron; Cadogan et al., (2002 ?). J. Phys. Condens. Matter, 14, 7191-7200] and X-ray [Smith et al. (1967 ?). Acta Cryst. 22 940-943; Yang et al. (2002b ?). J. Alloys Compd. 339, 189-194; Yang et al., (2003 ?). J. Alloys Compd. 263, 146-153] and single-crystal data with isotropic atomic displacement parameters [Roger et al., (2006 ?). J. Alloys Compd. 415, 73-84]. In addition, the anisotropic atomic displacement parameters for all atomic sites have been determined for the first time. These compounds are confirmed to have the tetra-gonal Zr5Si4-type structure (space group: P41212), as reported previously (Smith et al., 1967 ?). The structure is built up by distorted body-centered cubes consisting of Pr(Nd) atoms, which are linked to each other by edge-sharing to form a three-dimensional framework. This framework delimits zigzag channels in which the silicon dimers are situated.

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Evolution of two bulk-superconducting phases in Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, Sm) by external hydrostatic pressure effect.

Project description:Polycrystalline samples of Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transition temperatures (Tc) within the range of 2.1-2.7 K under ambient pressure, the estimated superconducting volume fraction was small, which indicates non-bulk nature of superconductivity in those samples under ambient pressure. A dramatic increase in shielding fraction, which indicates the emergence of the bulk superconductivity was achieved by applying external hydrostatic pressures. We found that two phases, low-P phases with Tc?=?2.5-2.8 K and high-P phases with Tc?=?10.0-10.8 K, were induced by the pressure effect for samples with RE?=?La, Ce, Pr, and Nd. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions, which was explained by the correlation between external and chemical pressure effects. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for understanding the effects of both external and chemical pressures to the emergence of bulk superconductivity and pairing mechanisms in BiCh2-based superconductors.

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Project description:Six different rare-earth oxyapatites, including Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2, were synthesized using solution-based processes followed by cold pressing and sinter-ing. The crystal structures of the synthesized oxyapatites were determined from powder X-ray diffraction (P-XRD) and their chemistries verified with electron probe microanalysis (EPMA). All the oxyapatites were isostructural within the hexa-gonal space group P63/m and showed similar unit-cell parameters. The isolated [SiO4]4- tetra-hedra in each crystal are linked by the cations at the 4f and 6h sites occupied by RE 3+ and Ca2+ in Ca2 RE 8(SiO4)6O2 or La3+ and Na+ in NaLa9(SiO4)6O2. The lattice parameters, cell volumes, and densities of the synthesized oxyapatites fit well to the trendlines calculated from literature values.

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Project description:The relationship between the compositions of the Earth and chondritic meteorites is at the center of many important debates. A basic assumption in most models for the Earth's composition is that the refractory elements are present in chondritic proportions relative to each other. This assumption is now challenged by recent (142)Nd/(144)Nd ratio studies suggesting that the bulk silicate Earth (BSE) might have an Sm/Nd ratio 6% higher than chondrites (i.e., the BSE is superchondritic). This has led to the proposal that the present-day (143)Nd/(144)Nd ratio of BSE is similar to that of some deep mantle plumes rather than chondrites. Our reexamination of the long-lived (147)Sm-(143)Nd isotope systematics of the depleted mantle and the continental crust shows that the BSE, reconstructed using the depleted mantle and continental crust, has (143)Nd/(144)Nd and Sm/Nd ratios close to chondritic values. The small difference in the ratio of (142)Nd/(144)Nd between ordinary chondrites and the Earth must be due to a process different from mantle-crust differentiation, such as incomplete mixing of distinct nucleosynthetic components in the solar nebula.

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Anomalous Dome-like Superconductivity in RE2(Cu1-xNix)5As3O2 (RE = La, Pr, Nd).

Project description:A significant manifestation of interplay of superconductivity and charge density wave, spin density wave, or magnetism is a dome-like superconducting critical temperature (Tc) in cuprate, iron-based, and heavy Fermion superconductors. Pesudogap, quantum critical point, and strange metals emerge in different doping ranges. Exploring dome-like Tc in new superconductors is of interest to detect emergent effects. Here we report the superconductivity in a new layered Cu-based compound RE2Cu5As3O2 (RE = La, Pr, Nd), in which the Tc exhibits dome-like variation with a maximum Tc of 2.5, 1.2, and 1.0 K with substitution of Cu by large amount of Ni ions. Simultaneously, the structural parameters like As-As bond length and c/a ratio exhibit unusual variations as the Ni-doping level goes through the optimal value. The robustness of superconductivity, up to 60% of Ni doping, reveals the unexpected impurity effect on inducing and enhancing superconductivity in these novel layered materials.

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Project description:RhPb2 (rhodium dilead) is a superconductor crystallizing in the CuAl2 structure type (space group I4/mcm). The Rh and Pb atoms are located at the 4a (site symmetry 422) and 8h (m.2m) sites, respectively. The crystal structure is composed of [RhPb8] anti-prisms, which share their square faces along the c axis and the edges in the direction perpendicular to the c axis. We have succeeded in growing single crystals of RhPb2 and have re-determined the crystal structure on basis of single-crystal X-ray diffraction data. In comparison with the previous structure studies using powder X-ray diffraction data [Wallbaum (1943 ▸). Z. Metallkd. 35, 218-221; Havinga et al. (1972 ▸). J. Less-Common Met. 27, 169-186], the current structure analysis of RhPb2 leads to more precise unit-cell parameters and fractional coordinates, together with anisotropic displacement parameters for the two atoms. In addition and likewise different from the previous studies, we have found a slight deficiency of Rh in RhPb2, leading to a refined formula of Rh0.950 (9)Pb2.

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