Universal scaling of latent heat of orbital order-disorder transition with average R-site ion size in perovskite RMnO3 systems

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.