Morphotropic phase boundaries, which separate two competing phases of different chemical composition, are the crucial ingredient for lead-based piezoelectrics. Here, the authors show that similar enhanced properties and analogous thermotropic phase boundaries can occur in simple, lead-free ferroelectrics.

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Thermotropic phase boundaries in classic ferroelectrics

The frequency dependence of the electrical properties of the Bi12TiO20 single crystal was investigated by impedance spectroscopy The data were presented in the Nyquist diagram form, from which electrical resistivity was determined Values ranged from 193×105 and 107×103 Ω cm in the temperature range of 400–700 °C The electrical conductivity followed the Arrhenius law, with an activation energy of 099 eV The dielectric behavior was investigated from room temperature to 700 °C Permittivity was calculated based on the relaxation frequency, using an alternative approach based on the variation of the imaginary impedance component as a function of reciprocal angular frequency The permittivity values obtained by means of these two methods showed good agreement up to 600 °C The frequency dependence of real and imaginary permittivities from room temperature to 700 °C was also investigated

Electric and dielectric properties of Bi12TiO20 single crystals

We relate the linear electro‐optic coefficients to the quadratic polarization‐optic and the elasto‐optic tensors by analyzing the dielectric phenomena driven by the spontaneous and induced polarization as well as the mechanical effects given by the deformation of the crystals. We determine the temperature dependence and the wavelength dispersion of the electro‐optic tensor in all the ferroelectric phases from a smaller set of data: the quadratic polarization‐optic tensor of the cubic phase, the dielectric permittivity, and the spontaneous polarization. Under the assumption that the elasto‐optic tensor at constant polarization is constant, the elasto‐optic coefficients at constant electric field in the ferroelectric phases are derived. Numerical evaluations of these coefficients for two perovskite crystals KNbO3 and BaTiO3 are presented. Modifications of the electro‐optic properties in doped crystals, mainly due to a change of the dielectric permittivity, are discussed as well.

Temperature dependence and dispersion of electro-optic and elasto-optic effect in perovskite crystals

Photocatalytic activity of bismuth germanate Bi12GeO20 powders

Recently determined complete sets of materials parameters describing the dielectric, elastic, piezoelectric, elasto-optic, and electro-optic properties of BaTiO3 and KNbO3 crystals at room temperature are used to calculate the effective electro-optic coefficients and dielectric constants required for describing photorefractive phenomena. We show a substantial deviation of the new values from the electro-optic coefficients for homogeneously applied electric fields that were used previously in describing the photorefractive effects. We derive angular dependences of the effective electro-optic coefficients and the effective dielectric constants relevant for grating recording in both crystals and verify them experimentally.

Electro-optic and dielectric properties of photorefractive BaTiO 3 and KNbO 3

Single crystals of B12 TiO20 are grown from solutions containing an excess of Bi2O3 by the TSSG (“top-seeded solution growth”) technique. Doping with iron, manganese, aluminium, and chromium is performed by adding the oxides to the melt. Measurements of dielectric constant, refractive indices, optical activity, absorption, light-induced absorption, and dark conductivity are performed.



Bi12TiO20-Einkristalle werden aus Losungen mit einem Uberschus an Bi2O3 unter Verwendung der TSSG (“top-seeded solution growth”)-Technik gezuchtet. Dotierung mit Eisen, Mangan, Aluminium und Chrom erfolgt durch Beimischung der entsprechenden Oxide zur Schmelze. Messungen der Dielektrizitatskonstanten, der Brechungsindizes, der optischen Aktivitat, der Absorption, der lichtinduzierten Absorption sowie der Dunkelleitfahigkeit werden durchgefuhrt.

Growth and characterization of undoped and doped Bi12 TiO20 crystals

The refractive indices of photorefractive bismuth titanate, barium-calcium titanate, bismuth germanium oxide, and lead germanate are measured in the range from 450 to 700 nm using the method of minimum deviation. The results are fitted to the Sellmeier equation and the parameters are given.

Refractive indices of photorefractive bismuth titanate, barium-calcium titanate, bismuth germanium oxide, and lead germanate

Refractive Indices of Single Domain BaTiO3 for Different Wavelengths and Temperatures

The refractive indices no and ne, the linear electrooptic coefficient r33, and the relative (low frequency) permittivities e3 and e2 = e2 of nominally pure KTN crystals with tetragonal symmetry (4 mm) are measured at different temperatures. In the case of the optical properties (no, ne, and r33) the light wavelength is varied, too.

F. Mersch

Papers

Growth and characterization of undoped and doped Bi12 TiO20 crystals

Refractive indices of photorefractive bismuth titanate, barium-calcium titanate, bismuth germanium oxide, and lead germanate

Refractive Indices of Single Domain BaTiO3 for Different Wavelengths and Temperatures

Refractive Indices, Permittivities, and Linear Electrooptic Coefficients of Tetragonal Potassium Tantalate-Niobate Crystals

Refractive Indices of Sr0.61Ba0.39Nb2O6 Single Crystals