The system thorium-palladium-boron: A DFT study on the stability and properties of Th2Pd15B5

• Published in: Journal of alloys and compounds Vol. 811; p. 151578
• Main Authors: Steiner, Soner, Rogl, Gerda, Flandorfer, Hans, Noel, Henri, Gonçalves, Antonio Pereira, Giester, Gerald, Rogl, Peter F.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 30.11.2019; Elsevier BV; Elsevier
• Subjects: Actinide alloys and compounds; Borides; Boron; Chemical Sciences; Crystal structure; Elasticity; Electron probes; Electronic band structure; High resolution electron microscopy; Palladium; Palladium compounds; Phase diagram Th-Pd-B; Phase equilibria; Single crystals; Solid solutions; Solubility; Ternary alloys; Ternary systems; Thermochemistry; Thorium; X ray powder diffraction; Elasticity; Actinide alloys and compounds; Thermochemistry; Electronic band structure; Phase diagram Th-Pd-B; Crystal structure
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2019.07.290

Phase relations in the Th-Pd-B system were determined via electron microprobe and X-ray powder diffraction analyses of ∼20 ternary alloys. Phase equilibria are dominated by the two high-melting thorium borides ThB4, Th1-yB6 and one ternary compound, namely τ1-Th2Pd14+xB5, with a structure deriving from the Sc4Ni29B10-type. Th1-yB6 is the binary starting point of a continuous solid solution up to ThPd0.53B4.28 closely corresponding to the superconducting phase Th1-yPdxB6-2x (x ≤ 0.65; 0 ≤ y ≤ 0.22 at x = 0; TC = 21 K), which was described by Zandbergen et al. in 1995 for x = 0.65 from high resolution electron microscopy. All thorium-rich palladium compounds from 0 to 75 at.% Pd form stable two-phase equilibria with ThB4 without any significant mutual solid solubilities. The solubility of Th in βB has been determined for the first time from a single crystal X-ray study of ThB99: Th atoms occupy sites A1 (occ. = 13%), D (occ. = 11%), and a rather small amount of 2.9% resides in site 18f (Dd-hole). To shed light on the physical properties of the ternary compound, DFT (density functional theory) calculations were performed for two idealized representatives of the τ1-phase, namely for Th2Pd14B5 and Th2Pd15B4. The calculations cover ionic relaxation, electron density of states, band structure and Fermi surfaces, elastic constants (computed via the linear response method) and the phonon density of states. Furthermore the thermodynamic stability (energy of formation) as well as the phonon contribution of the thermodynamic properties, such as the specific heat have been calculated clearly indicating Th2Pd15B4 as the composition thermodynamically more stable than Th2Pd14B5. At the Fermi energy the Pd-d contribution is dominant for both structures, but the number of states is higher for Th2Pd15B4 with DOS(EF) = 13 states/eV/f.u. than for Th2Pd14B5 with DOS(EF) = 8.5 states/eV/f.u. The corresponding Sommerfeld constants, γe = DOS(EF)kBπ2/3 = 30.55 mJ/molK2 for Th2Pd15B4 and 19.98 mJ/molK2 for Th2Pd14B5, clearly indicate metallic behavior of the τ1-phase. In addition, mechanical properties were measured for Th2Pd15B5 and compared with calculated values and data from the literature. Th2Pd15B5 on account of its high content of Pd is only slightly harder than the rather soft and almost ductile binary Pd-borides. [Display omitted] •Novel phase diagram Th-Pd-B; isothermal section at 950 °C.•Superconducting ThPd0.53B4.28 endpoint of ss-Th1-yPdxB6-2x (x ≤ 0.65; 0 ≤ y ≤ 0.22atx = 0).•Crystal structure of ThB99 (beta boron) determined from single crystal X-ray study.•Structure of τ1-phase is disordered between Th2Pd14B5 and Th2Pd15B4.•Full DFT characterization of physico-chemical behavior of the τ1-phase.