Lattice instability of B-site substituted SrTiO3 crystals studied by terahertz time-domain spectroscopic ellipsometry

The B-site substituted quantum paraelectric strontium titanate, SrTiO3 (STO), crystals with the perovskite structure were studied regarding their dielectric properties in the terahertz frequency range. When Ti4+ ions at the B-site are substituted by heterovalent Nb5+ ions, free electrons are introduced and the conductivity increases. The complex dielectric constants were accurately determined by the terahertz time-domain spectroscopic ellipsometry. The dielectric spectra were analyzed by the damped harmonic oscillator model for a ferroelectric soft mode and the Drude-Smith model for free electrons. The soft mode frequency and the plasma frequency increased with the increasing Nb content. The square of the plasma frequency is proportional to the concentration of free electrons. It was found that the change in the ferroelectric soft mode frequency is approximately proportional to the concentration of free electrons. This fact suggests that the increase in the free electrons by donor-doping suppresses the ferroelectric instability.