Orbital order-disorder transition in La(1-x)Nd(x)MnO(3) (x=0.0-1.0) and La(1-x-y)Nd(x)Sr(y)MnO(3) (x=0.1; y=0.05,0.1)

The nature of orbital order-disorder transition has been studied in the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(x=0.0\char21{}1.0)$ series which covers the entire range between two end points \char22{} $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{Nd}\mathrm{Mn}{\mathrm{O}}_{3}$ \char22{} as well as in ${\mathrm{La}}_{0.85}{\mathrm{Nd}}_{0.1}{\mathrm{Sr}}_{0.05}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.8}{\mathrm{Nd}}_{0.1}{\mathrm{Sr}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$. It has been observed that the first-order nature of the transition gives way to higher order with the increase in ``$x$'' in the case of pure manganites. The latent heat $(L)$ associated with the transition, first, drops with a steeper slope within $x=0.0\char21{}0.3$ and, then, gradually over a range $0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.9$. This drop could, possibly, be due to evolution of finer orbital domain structure with ``$x$.'' In the case of Sr-doped samples, the transition appears to be of higher-order nature even for a doping level $5\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$. In both cases, of course, the transition temperature ${T}_{\mathrm{JT}}$ rises systematically with the drop in average $A$-site radius $⟨{r}_{A}⟩$ or rise in average $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Mn}$ bond bending angle $⟨{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\varphi}⟩$ while no apparent correlation could be observed with doping induced disorder ${\ensuremath{\sigma}}^{2}$. The cooperative nature of the orbital order, therefore, appears to be robust.