Arthur Miller

Distribution of Cations in Spinels

The concept of octahedral site preference energy in spinels has been extended to include Madelung and short‐ranged as well as crystal field terms. A set of site preference energies is formulated which can be used to predict the ionic distribution of spinels involving the nontransition as well as the transition ions. The agreement between predicted and experimentally determined ionic distributions is surprisingly good, and a number of heretofore puzzling distributions are explained.

Determination of the valence state of copper in cubic CuMn2O4 spinel by X-ray absorption edge measurements

Abstract It cannot be determined from stoichiometry and electroneutrality considerations alone whether copper in CuMn 2 O 4 is univalent or divalent. In order to resolve this ambiguity, the energy of the X-ray K absorption edge for copper was measured in the following three compounds: CuCr 2 O 4 which is known to contain Cu 2+ ; Cu 2 O, which is known to obtain Cu + and CuMn 2 O 4 . The excitation energies measured in the solids were corrected by taking into account their electron affinities and the Coulomb potentials of copper in the solids. If it is assumed that CuMn 2 O 4 contains divalent copper, then the measurements on CuMn 2 O 4 , and CuCr 2 O 4 yield corrected values for the absorption edge which are consistent with each other within the limits of experimental error. On the other hand, if it is assumed that CuMn 2 O 4 contains univalent copper, then the measurements on CuMn 2 O 4 and Cu 2O 4 yield values which are discrepant by several times the estimated error. It is concluded that in the sample of CuMn 2 O 4 investigated, copper is predominantly divalent. New expressions for the Coulomb potentials at the ionic sites in the spinel and cuprite structures are also reported.

Optical grating coupling between low‐index fibers and high‐index film waveguides

Grating couplers between low‐refractive‐index (n ≈1.5) optical fibers and high‐refractive‐index (n ≈2.2) electro‐optic planar guides have been analyzed and demonstrated. Coupling efficiencies of 6% between single modes of a film and a fiber have been observed. Efficiencies of approximately 86% between single‐mode fibers and films using blazed backward‐wave gratings are predicted. In addition, low‐loss low‐optical‐aperture multimode fibers may be coupled to single‐mode films with efficiencies greater than 50% and angular spreads less than 0.5°.

Preparation of poled, twin-free crystals of ferroelectric bismuth tttanate, Bi4Ti3O12

Twinning involving interchange of the a- and b-axis directions in ferroelectric bismuth titanate, Bi4Ti3O12, can be eliminated by annealing the crystals through the Curie transition in an appropriate thermal gradient. The crystals can be poled at a temperature just below the Curie transition by the application of pulsed electric fields, using a suitable electrode configuration. The techniques employed permitted the application of fields up to 30 kv/cm to crystals with resistivities of the order of 103 ohm cm. The transition at the Curie point from monoclinic to tetragonal symmetry is complex, and appears to occur in at least two stages.

The growth of large single-crystal Bi4Ti3O12

In order to establish methods for the growth of large, high-quality crystals of ferroelectric bismuth titanate, Bi4Ti3O12, the Czochralski, flux evaporation, fluxed horizontal gradient freeze, and vertical gradient flux techniques were investigated. Best results were obtained with the vertical gradient flux technique. It was found with this technique that the cooling rate and the negative vertical temperature gradient at the surface of the melt were important factors in determining the size and quality of the crystals. The purity of the starting materials was found to affect the ease with which the crystals could be poled.

Depoling of single-domain bismuth titanate

Repeated c-axis switching of previously a-axis poled bismuth-titanate crystals generally results in a loss of the initial optical uniformity. Experimental results indicate that this optical degradation results from a recurrence of domains of opposite a-axis components in thin surface layers on both sides of the crystals. The cause of such depoling is not well understood, since the crystals are rather difficult to pole initially. A reasonable explanation may lie in the strain-relieving mechanism which involves strain both from c-axis domain switching and from initial crystal imperfections. This depoling is sharply reduced at low temperatures.Bismuth titanate has potential application to optically read memories and display devices. The depoling is not a serious problem for a read-only memory, for a write occasionally-read often memory, or for display and page-composer applications. However, for a pattern (and possibly for a holographic) read-write memory, the depoling may result in decreased read-out effici...

Phase Holograms in a Ferroelectric–Photoconductor Device

Some characteristics of an integrated ferroelectric-photoconductor (FE-PC) device useful for hologram storage are described. An improved phase readout technique for reconstructing holograms is analyzed. The results of a simple theory of device resolution are given.

Crystal Growth and Electro‐optic Effect of Some Double Sulfates with the Langbeinite Structure

Single crystals of several isomorphs of the mineral langbeinite, K2Mg2(SO4)3, space group T4, were prepared from aqueous solutions and from the melt. The electro‐optic powers, n03r41 of (NH4)2Mn2(SO4)3, K2Mn2(SO4)3, Rb2Mn2(SO4)3, Tl2Mn2(SO4)3 and K2Ni2(SO4)3 were found respectively to be 0.7, 8.6, 7.7, 12.4, and 4.9×10−10 cm/V. Refractive indices and dielectric constants were also measured. By analysis of the observed data it was found possible to partially resolve the over‐all electro‐optic effect into single ion contributions.

Valence and Distribution of Manganese Ions in Ferrospinels

Anomalous values of the magnetic moments observed for ferrospinels containing manganese have suggested that on octahedrally ligated spinel sites the ion pair [Fe+2Mn+3] is more stable than [Fe+3Mn+2]. Such behavior is opposed to that observed in aqueous solutions, and to what is usually expected in oxidic compounds. The magnetic measurements, however, cannot be interpreted unequivocally, since they can also be explained by invoking spin quenching, or deviations from the simple antiparallel spin-only coupling scheme such as Yafet-Kittel angular coupling.A crystallographic rather than a magnetic criterion was devised to elucidate the valence behavior in these compounds. When a sufficient fraction of the octahedrally ligated spinel sites are occupied by Mn+3, a macroscopic tetragonal distortion, due to the Jahn-Teller effect, occurs. For appropriately selected spinel systems, the compositions for which this distortion is observable at room temperature depend on whether [Fe+3Mn+2] or [Fe+2Mn+3] are the stably...

Ferroelectric-photoconductor optical storage medium utilizing bismuth titanate

A basic feasibility study has been made on a ferroelectric-photoconductor optical storage medium utilizing bismuth titanate and zinc selenide. Both the theory of device operation and experiment are described. Coincident light and electric field are used for WRITE and ERASE functions. The nondestructive READ operation is based on the birefringence properties of the bismuth titanate. Optical-electrical measurements, pattern storage and hologram storage are described. Holographic gratings as fine as 1.26μ were stored. The WRITE energy was approximately 10μJ/(mm)2. Diffraction READ efficiencies of approximately. 011% were measured. The ferroelectric photoconductor optical storage medium compares favorably with other erasible media.