Structural investigation of strontium titanate nanoparticles and the core-shell model
Résumé
Nanoparticles of strontium titanate (SrTiO3) have been synthesized in the 15 nm to 1 μm size range and studied using a combination of dielectric, Raman, x-ray, and neutron measurements. When diminishing the grain size, a strong reduction of dielectric permittivity, an enhancement of (normally forbidden) Raman polar modes and a progressive decoupling (or nonlinear coupling) between the antiferrodistortive (AFD) order parameter and the spontaneous strain, is observed. A qualitative explanation of all these effects could be achieved using the Petzelt core-shell model of SrTiO3 nanoparticles, with a core constituted of nonferroelectric AFD phase and a shell with frozen polarization. Depending on the route of synthesis, a strong increase of the AFD ferroelastic critical temperature Tc is observed. This behavior cannot be explained by considering only pure size or external strain effects, and a more complicated mechanism involving defects induced by the synthesis process should probably be considered. Interestingly, those are able to strongly affect the core grain structure, modifying thereby the macroscopic physical properties of SrTiO3 nanoparticle-based-materials.