Analysis of the magnetic properties ofR2Co17(R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er)

High-field isotherms at low and high temperatures and other magnetic properties of ${\mathit{R}}_2$ ${\mathrm{Co}}_{17}$ (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er) measured on single crystals are analyzed on the basis of the single-ion model. The fitted crystalline electric-field parameter ${\mathit{A}}_{20}$ for the Pr ion is several times smaller in absolute value than those for all the other rare-earth ions. The result is similar to that observed in the R${\mathrm{Co}}_5$ series, and it is suggested that the Pr ion in ${\mathrm{Pr}}_2$ ${\mathrm{Co}}_{17}$, as in ${\mathrm{PrCo}}_5$, is valence fluctuated.