We measured magnetic susceptibility of single-crystal rare-earth-doped
RBa2Cu4O8 (where R = Y, Dy, Gd, Ho, Er, Tm, Yb) in the normal state a
long the three principal axes. The anisotropy measured fits well to ca
lculated values obtained from crystal electric field (CEF) analysis. F
rom our results we developed a general scaling relation for the CEF pa
rameters across the rare-earth series.