On phase equilibria and crystal structures in the systems Ce–Pd–B and Yb–Pd–B. Physical properties of R2Pd 13.6B 5 ( 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 8B 2− x (0.10< x<0.48); τ 3-Ce 3Pd 25− x B 8− y (1.06< x<1.87; 2.20< y<0.05), and CePd 3B x (0< x<0.65) the latter arising from binary CePd 3. Crystallographic parameters for the new structure type τ 2-CePd 8B 2− x (space group C2/ 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 8B 2− x and τ 3-Ce 3Pd 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 6Pd 47− x B 6 ( x=0.2, C2 /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 6Pd 47− x B 6 ( x=0.19) (X-ray single crystal diffraction, new structure type, space group C2 /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 2Pd 14B 5, forming equilibria with extended solution YbPd 3B x , YbB 6, Pd 5B 2 and Pd 3B. The crystal structures of both Yb 2Pd 14B 5 and isotypic Lu 2Pd 14B 5 were determined from X-ray Rietveld refinements and found to be closely related to the Y 2Pd 14B 5-type ( I4 1/ amd). The crystal structure of binary Yb 5Pd 2− x (Mn 5C 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 6Pd 47− x B 6, τ 2-CePd 8B 2− x and τ 3-Ce 3Pd 25− x B 8− y are related and can be considered as the packings of fragments observed in Nd 2Fe 14B structure with different stacking of common structural blocks. Physical properties for Yb 2Pd 13.6B 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.