H. J. Levinstein

A comprehensive study of trivalent tungstates and molybdates of the type L2(MO4)3

Some 66 compounds of rare earth and other trivalent ions of the types L2(WO4)3 and L2(MoO4)3 as well as some mixed compounds such as YbSc(WO4)3 have been studied by means of X-ray powder diffraction, differential thermal analysis, and thermogravimetric analysis. At least six types of structures are found, with several compounds per structure ; three of these have been previously described, and the structure reported for one of these typified by Eu2(WO4)3. The limits of occurrence of these structures appear to depend primarily on the temperature and on the ionic radius of the trivalent ions. Two types of compounds, exemplified by Sc2(WO4)3 and Gd2(MoO4)3, have been studied by single crystal X-ray diffraction and the unit cell determined. The former is orthorhombic, probable space group Pnca and the latter is tetragonal, probably P4212 or P421m. Lattice parameters and indexed powder diffraction patterns are given for three series of compounds. Many of these compounds have been grown as single crystals by Czochralski pulling from the melt. About half of the compounds show phase transitions and many form hydrates hygroscopically. Also included are some optical data, transmission curves for Sc2(WO4)3 and Gd2(MoO4)3, and diffuse reflectance curves for Nd2(WO4)3 and Nd2(MoO4)3.

Growth of High‐Quality Garnet Thin Films from Supercooled Melts

A new dipping technique is described for producing uniform‐thickness low‐defect‐density magnetic garnet films suitable for bubble‐domain devices. The technique is based on the observation that melts having the appropriate flux and garnet composition can be supercooled over a large temperature range and will remain supercooled, permitting the growth at constant temperature of epitaxial magnetic garnet films.

Ferroelectric lithium niobate. 1. Growth, domain structure, dislocations and etching

Abstract Lithium niobate (LiNbO 3 ) is a material of considerable interest in view of its unusual ferroelectric, optical, acoustic and piezoelectric properties. The growth of large single crystals by the Czochralski and Bridgman-Stockbarger techniques is described. Angles between crystallographic planes and a stereographic projection are given. The crystals show the presence of ferroelectric domains, the polar axis being the c -axis (hexagonal setting). Polishing, etching, microscopic, X-ray topographic and other techniques were used to examine the domains, dislocations and other imperfections in some detail. The preparation of single domain material and poling experiments will be discussed in paper 2 of this series.

THE NONLINEAR OPTICAL PROPERTIES OF Ba2NaNb5O15

The useful nonlinear coefficients of Ba2NaNb5O15 are approximately twice those of LiNbO3. This material is a filled (A sites) structure composition and at room temperature does not have the serious problem of optically induced refractive index inhomogeneities (``optical damage). Also, the material has unique and reproducible phasematch temperatures. The electro‐optic half‐wave voltage of this material in the optimum direction is 1570 V.

Ferroelectric lithium niobate. 2. Preparation of single domain crystals

Abstract Lithium niobate is shown to be an unusual ferroelectric material with the very high Curie temperature of 1210°C. Three techniques are presented for the preparation of single domain LiNbO 3 crystals. The first uses the presence of MoO 3 or WO 3 in the melt. The other two techniques require the application of an electric field either during or after growth. Poling is complete at 1200°C with the application of as little as 0·2 V cm for a period of 10 min. Other physical properties given include dielectric constant, resistivity, thermal expansion, and optical data. LiNbO 3 and LiTaO 3 differ from each other in several important respects.

CURIE TEMPERATURE, BIREFRINGENCE, AND PHASE‐MATCHING TEMPERATURE VARIATIONS IN LiNbO3 AS A FUNCTION OF MELT STOICHIOMETRY

The Curie temperature, birefringence, and phase‐matching temperature of LiNbO3 are shown to vary with the stoichiometry of the melt from which the crystals are pulled. The range of melt stoichiometry, indexed by the Li/Nb mole ratio, was varied from 1.20 to 0.80. This Li/Nb variation changes the Curie temperature, refractive index for the extraordinary ray, and the phase‐matching temperature by 120°C,.03, and 320°C respectively. The relevance of these changes are discussed in terms of the susceptibility of LiNbO3 to index inhomogeneities.

Ferroelectric lithium niobate. 5. Polycrystal X-ray diffraction study between 24° and 1200°C

Abstract The atomic arrangement of LiNbO 3 remains essentially unchanged from 24° to 1200°C, with the z -coordinate of oxygen ( z (O)) increasing from 0.0647 at 24°C to 0.0702 at 1200°C. It is proposed that at the Curie temperature (1210°C), z (O) becomes 1 12 and y (O) becomes 1 3 (from 0·3446 at 24°C). The niobium position then acquires the symmetry of an inversion center, and the space group becomes R 3 instead of R 3 c . Movement of the Li and Nb point charges, from nonpolar R 3 to ferroelectric R 3 c , is in agreement with the sign of the ferroelectric polarization experimentally determined. The thermal vibrations at 1210°C are large, and at 1253°C the crystal melts. The linear thermal expansion coefficients are α a = 16·7 × 10 −6 ° C −1 between 24° and 800°C, α c ≈ 2 × 10 −6 ° C −1 between 24° and 600°C; the volume expansion coefficient is β = 36·5 × 10 −6 ° C −1 between 24° and 1000°C.

A NEW AND STABLE NONLINEAR OPTICAL MATERIAL

The nonlinear coefficient d31 of K.6Li.4NbO3 is equal to d31 of LiNbO3. Also, the serious problem of optically induced refractive‐index inhomogeneities which has been observed in many nonlinear materials, notably LiNbO3, is not observed in this new material. The electro‐optic half‐wave voltage of this new material is 930 V.

Some characteristics of niobates having “filled” tetragonal tungsten bronze-like structures

Abstract Alkali metal and mixed alkali metal-alkaline earth niobates having “filled” tetragonal tungsten bronze-like structures are of particular interest for electro-optic and nonlinear optic applications. The characteristics of a number of the more useful materials and particularly Ba 2 NaNb 5 O 15 , Sr 2 NaNb 5 O 15 , and K 3 Li 2 Nb 5 O 15 are considered in some detail here.

CONTINUOUS OPTICAL PARAMETRIC OSCILLATION IN Ba2NaNb5O15

A tunable, optical parametric oscillator using a continuous pump is reported. The threshold was measured to be 45 mW of multimode power at .532 μ. The efficiency was found to be 1% with 300 mW of pump power.