| 1. |
- Pani, Marcella, et al.
(author)
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Four ternary silicides in the La-Ni-Si system : from polyanionic layers to frameworks
- 2022
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In: CrystEngComm. - : Royal Society of Chemistry (RSC). - 1466-8033. ; 24:47, s. 8219-8228
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Journal article (peer-reviewed)abstract
- The central part of the La–Ni–Si system has been investigated at 800 °C by means of single crystal X-ray diffraction, microscopy and analytical microprobe techniques. The result led to the identification of four new metal-rich silicides: LaNi2Si (Rm, a = 4.0263(3) Å, c = 15.066(2) Å, Z = 3), La2Ni3Si2 (P21/c, a = 6.8789(7) Å, b = 6.2167(3) Å, c = 12.214(1) Å, β = 90.92(1), Z = 4), La3Ni3Si2 (Pnma, a = 7.501(2) Å, b = 14.316(4) Å, c = 6.149(2) Å, Z = 4), La6Ni7Si4 (Pbcm, a = 6.066(1) Å, b = 7.488(1) Å, c = 29.682(5) Å, Z = 4). LaNi2Si belongs to the SrCu2Ga structure type, which is not represented among silicides, while La2Ni3Si2 crystallizes in its own structure type. Both compounds feature layered polyanionic motifs consisting of Ni and Si, which are separated by La. Instead, La6Ni7Si4 and La3Ni3Si2 are characterized by polyanionic networks. The former compound belongs to the Pr6Ni7Si4 structure type, with only two other representatives (Ce and Nd); the latter has been observed only with Rh and Ir. The two structures reveal close structural relationships having multiple identical slabs. Tight-binding electronic structure calculations by linear muffin-tin-orbital methods were performed for LaNi2Si, La2Ni3Si2 and La3Ni3Si2 to gain insights into their structure–bonding relationships. Their band structures suggest a metallic character for all compounds. The overall crystal orbital Hamilton populations are dominated by polar Ni–Si bonds, though homoatomic Ni–Ni and La–Ni(Si) bond contributions are not negligible. The variety of bonding patterns may serve as a logical explanation for the number of discovered compounds in this system as well as for the diversity of the observed structures.
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| 2. |
- Provino, Alessia, et al.
(author)
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Solubility limits, magnetic and magnetocaloric properties of MoB-type GdCoxNi1−x (0.47 ≤ x ≤ 0.72)
- 2023
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In: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 948
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Journal article (peer-reviewed)abstract
- The binary GdNi adopting the orthorhombic CrB-type (oS8, Cmcm) dissolves up to 21 at% Co (GdCo0.42Ni0.58), but when the Co content reaches 23.7 at% the crystal structure of the GdCoxNi1−x phase changes from the orthorhombic CrB-type to the tetragonal MoB-type (tI16, I41/amd). The latter remains stable between 23.7 and 36.0 at% Co (0.47 ≤ xCo ≤ 0.72), and there is a two-phase region compositional gap of ≈ 2.5 at% Co between the two structures. One of the two 8e sites available in the MoB-type structure is filled by Gd atoms, while Co and Ni atoms are randomly occupying the other 8e site. The lattice parameter a increases from 3.9485(1) Å to 3.9591(1) Å, while c decreases from 21.3286(1) Å to 21.2404(1) Å as xCo increases, both following a quadratic trend. As a result, the unit cell volume increases from 332.53(2) Å3 to 332.93(2) Å3 for, respectively, the Co-poor (GdCo0.47Ni0.53) and the Co-rich (GdCo0.72Ni0.28) sides of the solid solution, also following a quadratic trend with a negative deviation of the Zen’s law. The title compound becomes ferromagnetic with TC increasing linearly from 150 K for GdCo0.50Ni0.50 to 171 K for GdCo0.70Ni0.30; linear interpolation leads to TC = 127 K and 174 K for the two ends of the solid solution with xCo = 0.47 and 0.72, respectively. The magnetic susceptibility follows the Curie-Weiss law, indicating a trivalent state of Gd ions, with an effective magnetic moment slightly increasing with the Co content. The magnetocaloric effect, calculated in terms of the isothermal magnetic entropy change, decreases slightly as xCo increases, remaining large in the range 127–174 K.
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| 3. |
- Provino, Alessia, et al.
(author)
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Stability, Crystal Chemistry, and Magnetism of U2+xN21-xB6 and Nb3-yNi20+yB6 and the Role of Uranium in the Formation of the Quaternary U2-zNbzNi21B6 and U delta Nb3-delta Ni20B6 Systems
- 2019
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In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 58:22, s. 15045-15059
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Journal article (peer-reviewed)abstract
- We investigated the U-Ni-B and Nb-Ni-B systems to search for possible new heavy fermion compounds and superconducting materials. The formation, crystal chemistry, and physical properties of U2Ni21B6 and Nb3-yNi20+yB6 [ternary derivatives of the cubic Cr23C6-type (cF116, Fm3m)] have been studied; the formation of the hypothetical U3Ni20B6 and Nb2Ni21B6 has been disproved. U2Ni21B6 [a = 10.6701(2) angstrom] crystallizes in the ordered W2Cr21C6-type, whereas Nb3-yNi20+yB6 [a = 10.5842(1) angstrom] adopts the Mg3Ni20B6-type. Ni in U2Ni21B6 can be substituted by U, leading to the solid solution U2-xNi21+yB6 (0 <= x <= 0.3); oppositely, Nb in Nb3Ni20B6 is partially replaced by Ni, forming the solution Nb3-yNi20+yB6 (0 <= y <= 0.5), none of them reaching the limit corresponding to the hypothetically ordered U3Ni20B6 and Nb2Ni21B6. These results prompted us to investigate quaternary compounds U2-zNbzNi21B6 and U6Nb3-delta Ni20B6: strong competition in the occupancy of the 4a and 8c sites by U, Nb, and Ni atoms has been observed, with the 4a site occupied by U/Ni atoms only and the 8c site filled by U/Nb atoms only. U2Ni21B6, U2.3Ni20.7B6, and Nb3Ni20B6 are Pauli paramagnets. Interestingly, Nb2.5Ni20.5B6 shows ferromagnetism with T-c approximate to 11 K; the Curie-Weiss fit gives an effective magnetic moment of 2.78 mu(B)/Ni, suggesting that all Ni atoms in the formula unit contribute to the total magnetic moment. The M(H) data at 2 K further corroborate the ferromagnetic behavior with a saturation moment of 10 mu(B)/fu (approximate to 0.49 mu(B)/Ni). The magnetic moment of Ni at the 4a site induces a moment in all of the Ni atoms of the whole unit cell (32f and 48h sites), with all atoms ordering ferromagnetically at 11 K. Density functional theory (DFT) shows that the formation of U2Ni21B6 and Nb3Ni20B6 is energetically preferred. The various electronic states generating ferromagnetism on Nb2.5Ni20.5B6 and Pauli paramagnetism on U2Ni21B6 and Nb3Ni20B6 have been identified.
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