Thermal expansion of compounds Rn+1Co3n+5B2n (R=Y and Gd; n=1, 2, 3, and ∞)

Thermal expansions of the lattice parameters a and c of the system in the title have been measured by x-ray diffraction technique to investigate the nature of magnetic interactions as well as chemical bond in these systems. Our data reveals that a spontaneous magnetostriction caused by the Co–Co intrasublattice interaction in YCo4B (n=1) occurs in the c plane unlike YCo5 whose magnetostriction occurs only along the c axis. It is also found that the positive spontaneous magnetostriction of GdCo4B in the c plane can be divided into three contributions due to the Co–Co, Co–Gd, and Gd–Gd interactions in the crystal. Thermal expansion coefficients in the c plane in the paramagnetic temperature region strongly decrease with increasing n in the Gd system. This implies a significant covalent bonding in the R–B layer in the present compounds. These results are discussed on the basis of the results of energy band calculation together with the previous magnetic data.