  The thermodynamically stable long-range orbital order in bulk LaMnO3 becomes
metastable at nanoscale around a critical particle size d_C~20 nm. The orbital
order-disorder transition switches from reversible to irreversible at d_C while
the resistance in the orbital ordered state decays by 2-4% over a time scale of
~3000s. At well below d_C, of course, a stable orbital disordered phase
emerges. The orthorhombic distortion of the underlying crystallographic
structure (space group Pbnm) decreases systematically with the decrease in
particle size and at far below d_C (e.g., at ~10 nm), the structure becomes
cubic (space group Pm-3m). Using the crystallographic and electrical resistance
data, a phase diagram has been constructed showing the evolution of different
orbital phases as a function of particle size across ~10 nm to bulk for
stoichiometric LaMnO3.
