Lanthanon-tin-carbon systems. The cubic phases Ln₃Sn and Ln₃SnC

An examination of the binary system Ln₃Sn (Ln La, Ce, Pr, Sm, Ho, Yb) shows evidence for a cubic phase for Ce and Pr. X-ray and Mössbauer spectra of these phases suggest that they are not ordered. The value of the function r_ν.Ln-r_4f, where r_ν.Ln is the radius of the lanthanon derived from the volume of the unit cell and r_4f the radius of maximum density of the 4f shell, can be correlated with the existence of cubic phases in only two of these systems. The reaction of the cubic binary phases or eutectic alloys of Ln₃Sn composition with carbon has been studied between 950° and >2500°. Single-phase carbides Ln₃SnC_x(x < 0·68) are shown to exist. These carbides have been studied by mild hydrolysis techniques, metallography, X-ray and Mössbauer spectroscopy, magnetic susceptibility, and their general chemical behaviour noted. Evidence is presented for the non-participation of Sn in the bonding scheme, mainly from the fact that its chemical isomer shift is independent of the carbon content and remains unchanged with respect to the shift in the Ln₃Sn phase. The very slow rate of attack by water vapour to produce methane indicates that the LnC bond is very covalent. These facts are discussed in terms of the limiting range of carbon composition found for these materials, their possible structure, and the type of bonding that might exist.