Evolution of orbital phases with particle size in nanoscale stoichiometric LaMnO3

TL;DR

This study investigates how reducing particle size in LaMnO3 affects its orbital order, revealing a transition from stable to metastable phases around 20 nm and structural changes from orthorhombic to cubic.

Contribution

It provides a detailed phase diagram showing the evolution of orbital phases with particle size in nanoscale LaMnO3, highlighting the size-dependent transition from ordered to disordered states.

Findings

Orbital order becomes metastable below ~20 nm.

Resistance decay indicates irreversible transition at critical size.

Structural change from orthorhombic to cubic at ~10 nm.

Abstract

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.