  The nature of the cooperative Jahn-Teller (JT) transition accompanied by
orbital order-disorder in Nd1-xCaxMnO3 has been studied by high temperature
(300 - 1200K) synchrotron and laboratory X-ray powder diffraction in the low
doping region (0<x<0.1). For very low doping a large temperature range of phase
coexistence associated with a first-order transition has been observed,
resembling a martensitic transformation. The transition appears continuous
because of a gradual evolution of the volume fractions of the phases over a
broad temperature interval. The first-order nature of the transition and the
phase coexistence is suppressed with increasing of doping.
