Crystallographic and hyperfine parameters of PrTi(Fe,Co)₁₁ and their carbides

PrTiFe_11-xCo_x alloys (x<~3) prepared by melting and subsequent annealing have been investigated with their carbides PrTiFe_11-xCo_xC_y (y=1) synthesized by means of reaction with solid hydrocarbon at 750 K. X-ray diffraction analysis by Rietveld method has shown that both series crystallize in the tetragonal ThMn₁₂-type structure. From x-ray studies of magnetic aligned samples, the magnetic anisotropy of the carbided alloys is found to turn uniaxial. Their Curie temperature enhancement is linearly correlated to the volume expansion of the host lattice upon carbonation. The Mössbauer spectra recorded from 77 to 293 K, have been analyzed on the basis of the binomial law, related to the different Fe nearest neighbor in the assumption of Ti atoms distributed in 8i sites. The hyperfine parameter sets were assigned according to the relationship between the Wigner Seitz cell volume of each iron site and their isomer shift δ so that δ{8i}>δ{8j}>δ{8f}. It results that Co preferentially occupies the 8f site with the recurrent sequence H_HF{8i}>H_HF{8f}>H_HF{8j}. The increase with Co content of hyperfine field, is correlated to the increase of the core electron polarization field ruled by the asymmetrical filling of the 3d band by the additional 3d Co electron. Upon carbonation, both the hyperfine fields and the isomer shifts increase in agreement with the unit cell expansion and the increased Curie temperature of PrTiFe_11-xCo_xC as compared to PrTiFe_11-xCo_x.