  The latent heat (L) of orbital order-disorder transition in single-valent
perovskite manganite series La(1-x)R(x)MnO3 (R = Pr, Nd, Gd; x = 0.0-1.0)
decreases with the decrease in average R-site radius <r_R> following a
universal scaling law L.<r_R>^2/sigma^2 ~ exp (<r_R>), where sigma^2 is the
variance in R-site radius, and eventually reaches zero at a critical R-site
radius <r_R>_c ~ 1.180 Angstrom. Such a drop in L is due, possibly, to a
universal pattern of evolution of finer orbital domain structure with the drop
in <r_R> as well as with the increase in sigma^2 irrespective of R-site ion
type.
