On phase equilibria and crystal structures in the systems Ce–Pd–B and Yb–Pd–B. Physical properties of R2Pd13.6B5 (R=Yb, Lu)

Abstract Phase equilibria and crystal structures of ternary compounds were determined in the systems Ce–Pd–B and Yb–Pd–B at 850 °C in the concentration ranges up to 45 and 33 at% of Ce and Yb, respectively, employing X-ray single crystal and powder diffraction. Phase relations in the Ce–Pd–B system at 850 °C are governed by formation of extended homogeneity fields, τ 2 -CePd 8 B 2− x (0.10 x τ 3 -Ce 3 Pd 25− x B 8− y (1.06 x y 3 B x (0 x 3 . Crystallographic parameters for the new structure type τ 2 -CePd 8 B 2− x (space group C 2/ c , a =1.78104(4) nm, b =1.03723(3) nm, c =1.16314(3), β =118.515(1)° for x =0.46) were established from X-ray single crystal diffraction. The crystal structures of τ 2 -CePd 8 B 2− x and τ 3 -Ce 3 Pd 25− x B 3− y are connected in a crystallographic group–subgroup relationship. Due to the lack of suitable single crystals, the novel structure of τ 1 -Ce 6 Pd 47− x B 6 ( x =0.2, C 2 /m space group, a =1.03594(2) nm, b =1.80782(3) nm, c =1.01997(2) nm, β =108.321(1)°) was determined from Rietveld refinement of X-ray powder diffraction data applying the structural model obtained from single crystals of homologous La 6 Pd 47− x B 6 ( x =0.19) (X-ray single crystal diffraction, new structure type, space group C 2 /m , a =1.03988(2) nm, b =1.81941(5) nm, c =1.02418(2) nm, β =108.168(1)°). The Yb–Pd–B system is characterized by one ternary compound, τ 1 -Yb 2 Pd 14 B 5 , forming equilibria with extended solution YbPd 3 B x , YbB 6 , Pd 5 B 2 and Pd 3 B. The crystal structures of both Yb 2 Pd 14 B 5 and isotypic Lu 2 Pd 14 B 5 were determined from X-ray Rietveld refinements and found to be closely related to the Y 2 Pd 14 B 5 -type ( I 4 1 / amd ). The crystal structure of binary Yb 5 Pd 2− x (Mn 5 C 2 -type) was confirmed from X-ray single crystal data and a slight defect on the Pd site ( x =0.06) was established. The three structures τ 1 -Ce 6 Pd 47− x B 6 , τ 2 -CePd 8 B 2− x and τ 3 -Ce 3 Pd 25− x B 8− y are related and can be considered as the packings of fragments observed in Nd 2 Fe 14 B structure with different stacking of common structural blocks. Physical properties for Yb 2 Pd 13.6 B 5 (temperature dependent specific heat, electrical resistivity and magnetization) yielded a predominantly Yb-4 f 13 electronic configuration, presumably related with a magnetic instability below 2 K. Kondo interaction and crystalline electric field effects control the paramagnetic temperature domain.