Synchrotron-based high-pressure X-ray diffraction measurements indicated that compressibility, a fundamental materials property, could have a size-specific minimum value. The bulk modulus of nanocrystalline titania exhibited a maximum at a particle size of 15nm. This could be explained by dislocation behavior because very high dislocation contents could be achieved when shear stress induced within nanoparticles countered the repulsion between dislocations. As the particle size decreased, compression increasingly generated dislocation networks (hardened by overlap of strain fields) that shielded intervening regions from external pressure. However, when particles became too small to sustain high dislocation concentrations, elastic stiffening declined. The compressibility exhibited a minimum at intermediate sizes.

Size-Dependent Elasticity of Nanocrystalline Titania. Chen, B., Zhang, H., Dunphy-Guzman, K.A., Spagnoli, D., Kruger, M.B., Muthu, D.V.S., Kunz, M., Fakra, S., Hu, J.Z., Guo, Q.Z., Banfield, J.F.: Physical Review B, 2009, 79[12], 125406