Deviations from stoichiometry occurring during the crystal growth of lithium niobate have been studied by 93Nb NMR and by shifts in the ferroelectric transition temperature. The high‐temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by these techniques together with DTA and crystal‐growth experiments. The congruently melting composition is located at 48.6 mole% Li2O and the solid‐solution region at room temperature extends from 48% to 50% Li2O. The implications of growth at noncongruently melting compositions on the compositional uniformity of lithium niobate are discussed in detail.

Nonstoichiometry and Crystal Growth of Lithium Niobate

Laser‐induced optical index damage in LiNbO3 and other pyroelectrics of ABO3 structure is discussed in the light of recent results of stoichiometry investigations. The damage is attributed to the presence of B‐excess stacking‐fault defects. The dc photovoltaic effect in LiNbO3 as well as the detailed spatial distribution of refractive‐index variation is explained in terms of photoinduced variation of the macroscopic spontaneous polarization density. This variation results in a polarization charge‐density distribution, and an approximately compensatory space‐charge distribution. The index damage results from the polarization‐based electrooptic effect.

Optical Index Damage in LiNbO3 and Other Pyroelectric Insulators

The temperature characteristics of elastic, piezoelectric, dielectric, and optical constants of Ba2NaNb5O15 have been measured from room temperature up to the Curie point. The shear elastic compliance related to the tetragonal principal axes (TS66) is found to become large near the transition point from tetragonal to orthorhombic phase. A phenomenological approach is tried, in which the free energy is expanded in power series of the polarization and strain, and it becomes clear that this tetragonal‐orthorhombic transition is little dependent of the behavior of spontaneous polarization but that elastic instability is the main cause of this phase transition. The elastic, piezoelectric, dielectric, and optical constants in the orthorhombic phase are derived from the constants in the tetragonal phase by the biased effect of the spontaneous shear strain to the elastic higher‐order terms in the free energy.

Elastic Anomaly of Ba2NaNb5O15

Structural and electrical properties of Na2Pb2Eu2W2Ti4X4O30 (X = Nb, Ta) ferroelectric ceramics

This work is primarily concerned with the measurements of some of the properties of crystals of barium sodium niobate (BSN) which are significantly related to the degree of alignment of the ferroelectric domains. We point out that many of the previous measurements of the low‐frequency half‐wave voltage (HWV) might have been in error because of imperfect alignment (poling). The variations observed in these HWV measurements can be related, in principle, to a connection between the low‐frequency resistivity in different regions of the crystal and the degree of poling in those regions. For instances in which we observed variations in the free HWV we could demonstrate that such effects were not related to stoichiometric (compositional) variations within crystals, or from one crystal to another. In fact stoichiometric variations appeared to play no role in determining the outcome of our measurements of the low‐ and high‐frequency electro‐optic (EO) coefficients and dielectric constants. We present values for th...

Measurements of Second‐Harmonic Generation and the Variations in the Free and Clamped Values of the Dielectric Constants and Electro‐Optic Coefficients in Barium Sodium Niobate

Accurate lattice parameters (at 25 and 48°C), density, thermal expansion coefficients, the indexed powder pattern, and optical data are given for Ba(4+x)Na(2−2x)Nb10O30, where x = 0.13±0.013, grown by pulling from a nominally stoichiometric melt.

R. L. Barns

Papers

Barium sodium niobate (Ba(4+x)Na(2−2x)Nb10O30): crystallographic data and thermal expansion coefficients