Negative pressure effects in strontium titanate nanoparticles

Size effects on the lattice parameter and optical gap energy Eg are investigated in SrTiO3 nanoparticles in the range of the average size D for 12–40 nm. An anomalous lattice expansion is found in SrTiO3 nanoparticles, i.e., the lattice parameter a increases from 3.909 Å at image to 3.920 Å at image, which is in contrast with the lattice contraction in metal nanocrystals. We have ascribed the lattice expansion to the negative pressure effects due to the enhanced surface-defect dipole in the smaller particles. Such a model also explains change of the Eg-value with particle size.