The system Ce–Zn–Si for <33.3 at.% Ce: phase relations, crystal structures and physical properties

Abstract

Phase equilibria of the system Ce–Zn–Si have been determined for the isothermal section at 600 °C for <33.3 at.% Ce by XRPD and EPMA. This partial section is characterized by the formation of five ternary compounds with homogeneity regions at constant Ce-content and partial substitution of Zn/Si: τ₁-Ce₇Zn₂₁(Zn_1−xSi_x)₂ (unique type; 0.45 ≤ x ≤ 0.99), τ₂-Ce(Si_1−xZn_x)₂ (AlB₂-type; 0.36 ≤ x ≤ 0.73), τ₅-CeZn(Zn_1−xSi_x)₂ (CeNiSi₂-type; 0.68 ≤ x ≤ 0.76), τ₆-CeZn₂(Si_1−xZn_x)₂ (ThCr₂Si₂-type; 0.25 ≤ x ≤ 0.30) and τ₇-Ce₃₇Zn₄₈Si₁₅ (structure unknown). Whereas τ₁, τ₂ and τ₅ are stable at 600 and 800 °C, the phases τ₆, τ₇ are unstable at 800 °C. Atom site distribution in the crystal structures of τ₅, τ₆ and {La,Ce}₇Zn₂₁(Zn_1−xGe_x)₂ have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce}₇Zn₂₁[Zn_1−xSi(Ge)_x]₂ suggest a hypothetical binary phase “{La,Ce}₇Zn₂₃”. The stabilizing effect of Ge in Ce₇Zn_23−xGe_x has been elucidated from density functional theory (DFT) calculations discussing the electronic structure in terms of the density of states (DOS) and defining enthalpies of formation for Ce₇Zn_23−xGe_x (x = 0, 0.5, 2) as well as for several neighbouring binary Ce–Zn phases. A Schultz–Scheil diagram for the solidification behaviour in the (Zn,Si)-rich part of the diagram was constructed from DTA measurements in closed silica crucibles along with partial isothermal sections determined in the temperature range from 400 to 900 °C. The phases τ₅ and τ₆ both form in degenerate ternary peritectic reactions: L + CeSi₂,β-Ce₂Zn₁₇ ⇔ τ₅ at 865 ± 5 °C and L + τ₅,CeZn₁₁ ⇔ τ₆ at 695 ± 5 °C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ₅-CeZn(Zn_1−xSi_x)₂ revealed Kondo lattice behavior with ferromagnetic ordering below T_C = 4.4 K, whereas susceptibility and specific heat studies of τ₆-CeZn₂(Zn_0.28Si_0.72)₂ revealed Curie–Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie–Weiss fits of τ₅ (2.50μ_B) and τ₆ (2.34μ_B) reveal a ground state close to trivalent Ce.