Evidence of short-range antiferroelectric local states in lanthanum-and titanium-modified Sr 0.3 Ba 0.7 Nb 2 O 6 ferroelectric ceramics

BaTiO 3  (BTO)/SrTiO3 (STO), and BaTiO3/BaZrO3 (BZO) artificial superlattices were fabricated by the molecular beam epitaxy process and their dielectric properties and refractive indices were measured. Superlattices formed on Nb-doped STO substrates showed large leakage currents in comparison with those on pure STO substrates. This indicated that Nb-doped substrates were not suitable for bottom electrodes in dielectric measurements. Dielectric properties were therefore measured for films on pure STO substrates using planar interdigital electrodes. Superlattice films were so thin (32 nm) that fine planer electrodes were necessary to reduce the penetration of electric flux into the substrate. Interdigital electrodes with the interdigital width of 5 μm were formed by electron beam lithography. Dielectric permittivity of superlattices was determined from admittance data using an electromagnetic field analysis. It was found that the dielectric permittivities of [(BTO)10/(STO)10]4 and [(BTO)10/(BZO)10]4 were εr=720 000 and εr=330 000, respectively. A large Debye type dielectric dispersion was observed in the [(BTO)10/(BZO)10]4 superlattice. The refractive index of the superlattices was measured by spectroscopic ellipsometry. It was found that the refractive index of the superlattices changed with their structures and those of [(BTO)10/(STO)10]4 and [(BTO)10/(BZO)10]4 superlattices were larger than other superlattices. This indicated that the superlattice structure affected not only on the ionic polarization but also the electronic structure or chemical bonding nature of the superlattices.

Artificial BaTiO3(BTO)/SrTiO3(STO) superlattices on STO substrates were fabricated by the molecular beam epitaxy process. The capacitance and the complex admittance of superlattices with interdigital electrode structures were measured at 1–110 MHz. The dielectric properties of the superlattices were evaluated by the electromagnetic analysis. It was found that the dielectric permittivity changed with the structure of superlattices and the highest permittivity of 33 000 was obtained for [(BTO)10/(STO)10]4 superlattice from 1 to 110 MHz.

The piezoelectric properties and the crystallographic nature of (1-x)(Na,Bi)TiO3–xBaTiO3 (NBT–BT) thin films around the morphotropic phase boundary (MPB) composition (x=0.05–0.10) were studied. NBT–BT thin films were grown epitaxially on Pt(100)/MgO(100) substrates by RF magnetron sputtering and exhibited highly (001)-oriented single-phase perovskite with a tetragonal structure. A maximum piezoelectric coefficient e31 of -14.4 C/m2 was obtained at the x = 0.07 composition of the tetragonal side near MPB (x = 0.06). These results indicate that NBT–BT thin films around the MPB are a promising lead-free replacement for Pb(Zr,Ti)O3 (PZT) based applications.

Large transverse piezoelectricity has been demonstrated in lead-free epitaxial (Na,Bi)TiO 3 -BaTiO 3 (NBT-BT) thin films grown on MgO(110) substrates. Through the internal strain caused by the difference in thermal expansion between NBT-BT and MgO, the crystal structure of the films was distorted to orthorhombic lattice, which does not form in bulk NBT-BT. The films showed a planar anisotropic nature where the effective transverse piezoelectricity along the orthorhombic b-axis was much larger than that along the orthorhombic a-axis. For the NBT-BT film with 9% BaTiO 3 , transverse piezoelectric coefficient d à 31 along the orthorhombic b-axis reached as high as À221 pC/N.