Properties of BaTiO3 confined in nanoporous Vycor and artificial opal silica

Using the sol-gel technique, BaTiO3 was embedded into nanoporous Vycor and artificial vitreous opal silica for the first time. About 50 vol% of the pores was filled. In case of the Vycor glass (pore diameter 4-6 nm) only amorphous phase was revealed by XRD, IR reflectivity and Raman spectra. After additional gradual annealing, no crystallization was achieved. Chemical reaction with the SiO2 skeleton started at ~1000 K. The room-temperature IR and Raman spectra clearly show characteristic vibrational modes of the ferroelectrically distorted TiO6 octahedra without any long-range order. In case of the opal matrix (densely packed silica spheres, pore diameter up to ~50 nm), crystallization of the ferroelectric BaTiO3 appeared in coexistence with the amorphous phase, but the penetration depth of the crystalline BaTiO3 was limited. From the apparent temperature independence of the effective wide-frequency dielectric response due to the essentially temperature independent effective soft mode stiffened to ~100 cm-1, we can deduce that no macroscopic percolation of the crystalline BaTiO3 has appeared in our opal matrix. Nevertheless, Raman spectra bring evidence of a diffuse ferroelectric phase transition in the opal-BaTiO3 composite.