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: τ 1 -Ce 7 Zn 21 (Zn 1-x Si x) 2 (unique type; 0.45 ≤ x ≤ 0.99), τ 2 -Ce(Si 1-x Zn x) 2 (AlB 2 -type; 0.36 ≤ x ≤ 0.73), τ 5 -CeZn(Zn 1-x Si x) 2 (CeNiSi 2 -type; 0.68 ≤ x ≤ 0.76), τ 6 -CeZn 2 (Si 1-x Zn x) 2 (ThCr 2 Si 2 -type; 0.25 ≤ x ≤ 0.30) and τ 7 -Ce 37 Zn 48 Si 15 (structure unknown). Whereas τ 1, τ 2 and τ 5 are stable at 600 and 800°C, the phases τ 6, τ 7 are unstable at 800°C. Atom site distribution in the crystal structures of τ 5, τ 6 and {La,Ce} 7 Zn 21 (Zn 1-x Ge x) 2 have been elucidated from X-ray intensity refinements on single crystals. The small amounts of the stabilizing tetrel element in {La,Ce} 7 Zn 21 [Zn 1-x Si(Ge) x ] 2 suggest a hypothetical binary phase "{La,Ce} 7 Zn 23". The stabilizing effect of Ge in Ce 7 Zn 23-x Ge 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 Ce 7 Zn 23-x Ge 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 τ 5 and τ 6 both form in degenerate ternary peritectic reactions: L + CeSi 2,β-Ce 2 Zn 17 ζ τ 5 at 865 ± 5°C and L + τ 5,CeZn 11 ζ τ 6 at 695 ± 5°C, respectively. Magnetic susceptibility, specific heat and resistivity measurements of τ 5 -CeZn(Zn 1-x Si x) 2 revealed Kondo lattice behavior with ferromagnetic ordering below T C = 4.4 K, whereas susceptibility and specific heat studies of τ 6 -CeZn 2 (Zn 0.28 Si 0.72) 2 revealed Curie-Weiss paramagnetic behaviour down to 3 K. The effective paramagnetic moments of Ce obtained from Curie-Weiss fits of τ 5 (2.50μ B) and τ 6 (2.34μ B) reveal a ground state close to trivalent Ce.