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.