Metastable adaptive orthorhombic martensite in zirconia nanoparticles

Reported here are observations of isolated orthorhombic (o) ZrO2 nanoparticles (NPs) in the Pbca space group under atmospheric conditions. These NPs are composed of o domains separated by internal semi‐coherent boundaries. They are identified to transform from lamellar‐twinned tetragonal (t) NPs. The constraining effect of the twin boundaries impedes a direct martensitic transformation to the monoclinic (m) phase but favours a transformation to the o phase. The internal boundaries are considered to evolve from the lamellar‐twinning boundaries, playing an important role in the stabilization of the o structure under atmospheric conditions. The observed o structure should be an adaptive martensite transformed from the t phase, different from the general consideration of the o phase as an intermediate stage. A new lattice correspondence (LC) relationship of (011)o||(100)t and [100]o||[001]t is determined for the t‐to‐o transformation. A possible transformation path is proposed to be t (space group P42/nmc) to o (Pbcm or Pbc21, named oA) to o (Pbca, named oB), and the LC relationship is identified to be (100)t||(100)oA||(011)oB and [001]t||[010]oA||[100]oB.