K. Nassau

The growth of large SbSI crystals: Control of needle morphology

Abstract SbSI crystals have been grown by many techniques, all of which yield long thin needles because of an inherent growth rate anisotropy greater than 50:1. A modified flux technique using excess SbI 3 as the solvent was devised to circumvent this difficulty. Nucleation is initiated in a large temperature gradient in a narrow extension of the growth vessel. The main part of the growth vessel is essentially isothermal. As the temperature is lowered a few crystals grow from the nucleation region into the main part of the growth vessel. In the absence of further nucleation these crystals are forced to grow in thickness, yielding crystals up to 1 cm in diameter.

Synthetic emerald: The confusing history and the current technologies

Abstract Two techniques, flux and hydrothermal, have been used to grow emerald crystals, and various solvents have been employed. The lithium molybdate flux technique originated with Hautefeuille and Perrey in 1888 and was independently brought to feasibility by Nacken and at I.G.-Farben. This technique has proved to be commercially viable in the hands of Chatham and Gilson, who both perfected their processes independently of the Nacken and I.G.-Farben work, as well as of each other. Hydrothermal work culminated in the success of Flanigen and others at Linde, who found two acid mineralizers which gave satisfactory emerald growth; this process has not been commercially viable. Other crystal growth approaches of less significance are also discussed. Much confusion has existed because several flux techniques have in the past been misidentified as hydrothermal (there is, in fact, no evidence that even Nacken ever grew hydrothermal emerald) and some techniques claimed to produce crystals probably only yielded emerald glass, an inferior product. This account is based on original research, on an extensive literature search, as well as on many personal interviews. It is intended to clarify the confusing record as well as to present what is known about the current technology of gem emerald synthesis.

The growth of crystals from the boiling solution

Abstract A novel technique is described in which crystals have been grown from solutions at their boiling point. The technique permits accurate control of solvent loss, permits seeding, and avoids undesirable nucleation at the liquid-gas interface. Various iodates have been grown from boiling water and from boiling concentrated nitric acid.

A model for the Fe2+-Fe4+ equilibrium in flux-grown yttrium iron garnet

Abstract Wood and Remeika found that the addition of Ca 2+ and Si 4+ during the flux growth of YIG produced optical absorption changes which were interpreted in terms of the formation of Fe 4+ and Fe 2+ respectively. By considering an equilibrium to exist between Fe 2+ and Fe 4+ during growth, the optical data can be used to obtain the equilibrium constant and values of the Fe 2+ and Fe 4+ concentrations in the crystal. The minimum value of the optical absorption should be independent of the growth conditions apart from the Ca or Si addition necessary for locating the position of the minimum absorption. An extension of this model to include all the species expected to be present in the crystal is found not to be applicable to the data, probably due to lack of equilibrium. An outline of this extension is also presented as a guide for the analysis of other flux systems.

The growth and properties of single crystal cupric molybdate

Abstract CuMoO4 is shown to decompose slowly close to the peritectic melting point of 800°C. The decomposition was studied by differential thermal analysis and thermogravimetric analysis in various atmospheres. No decomposition takes place, in air or oxygen, below the peritectic but loss of MoO3 and oxygen occurs above. Single crystals of CuMoO4 were grown from the melt. The structure is reported and an indexed powder diffraction pattern of triclinic CuMoO4, space group P 1 , is presented. Among properties described are the optical transmission characteristics.

Crystal growth of ferroelastic K2Cd2(SO4)3 and the K2SO4-CdSO4 phase diagram

Crystals of congruently melting K2Cd2(SO4)3 (having the langbeinite structure with a ferroelastic transition temperature of 156°C) were grown by the Bridgman and Czochralski techniques. The former yielded colorless crystals when using oxygen under pressure; the latter yielded tan crystals of slightly smaller unit cell volume and are assumed to be oxygen deficient. The ferroelastic transition was studied by thermal expansion measurements. Reexamination of the phase diagram showed the existence of a previously unreported phase K6Cd(SO4)4 which is stable only between 520°C and the melting point of about 890°C.

Dr. A. V. L. Verneuil: The man and the method

Abstract This account is the first detailed biography of Professor A. V. L. Verneuil (1856–1913) to appear in the 58 years since the death of the well-known discoverer of the flame-fusion process for the synthesis of ruby and sapphire. Many facts, never previously published have been brought together with rarely seen photographs to give fresh insight into the history of an important technology and an interesting scientist. Until recently only two techniques have been of significance in the synthesis of ruby: growth from the flux and growth from the melt. The forerunner of the flux growth was the multiphase Al 2 O 3 · BaF 2 · KOH · K 2 Cr 2 O 7 growth system worked out by Professor E. Fremy, at first with C. Feil, and later with Verneuil. This was the first practical synthesis of clear ruby crystals, but it was found impossible to obtain crystals larger than a few millimeters in size. While this work was proceeding Verneuil was shown some “Geneva rubies” in 1886 and this initiated his interest in the melt-growth concept. Within a few years he revealed his process in the sealed notes of 1891 and 1892, and in 1902–1904 he published the full details of the flame fusion process, the “Verneuil” process. The scope of Verneuils professional work is indicated by some 70 publications, several prizes, and extensive consulting and teaching over a 33 year period. Yet when all else has been said, Verneuil will always be remembered as “the Father of Synthetic Ruby”.

New sources of quartz nutrient for the hydrothermal growth of quartz

New sources of nutrient to replace Brazilian α-quartz for hydrothermal crystallization are examined. Quartz from seven geographic regions in North America was used as nutrient in crystallization experiments. Differences in acoustic Q and strain were observed depending upon the geographic region. However, at least one locality in each region provided quartz whose properties were satisfactory for use as nutrient for the preparation of quartz suitable for piezoelectric applications. Surprisingly both vein and appropriately chosen pegmatitic quartz can be used for nutrient. High purity sand can be used as nutrient provided process conditions are altered so as to compensate for its effective lower surface area. Non-α-quartz nutrients such as silica glass and silica gel produce initially higher supersaturation and fast growth rates leading to poor quality growth. However, alterations in process conditions can be used to reduce initial growth rates and prepare reasonable quality crystals. Thus it is shown that there are a number of viable North American quartz nutrient sources.

“Reconstructed” or “Geneva” ruby

Abstract An early form of synthetic ruby of unknown technology was investigated by chemical analysis and by macroscopic and microscopic examination. Based on this a flame-fusion technique using two torches, crystal rotation, and a complex three-step seeding and growth procedure is deduced. Both from the low iron content, and from the result of melting experiments using natural ruby, it is deduced that purified alumina was used as the feed material. Since this early “Geneva” synthetic ruby (it was later also designated “reconstructed” ruby) was manufactured as early as 1885, the process predates the Verneuil flame-fusion technique.

Vapor growth of II–VI compounds and the identification of donors and acceptors

By a series of improvements in the growth and doping technique it has been possible to prepare single crystals of II–VI compounds such as CdS, CdSe, and ZnSe in a rapid, reproducible and impurity controlled manner. This has resulted in the identification of the chemical nature of the shallow donors and acceptors for the first time. A sublimation process was used to provide thin strain-free platelets for laser stimulated fluorescence studies in the edge emission region. Essential features of the technique include the use of high carrier gas flow and rapid growth (∼ 1 hr for CdS) necessary to prevent excessive impurity exchange with the apparatus, and a dilution series technique in which small amounts of impurity are added and successive growth runs are performed without further addition. In this way the very narrow and low concentration region can be reached between the amount always present in starting material (0.1 ppm Na, 1 ppm Cl, etc.) and the amount that produces excessive line broadening and makes the chemical identification impossible (< 20 ppm Li, < 200 ppm Cl, etc.). By this technique the chemical species involved in the edge luminescence (excitons bound at donors and acceptors and sharp line pair spectra) have been identified for the first time. In CdS only Li and Na were found to give shallow acceptors; shallow donors include F, Cl, Br, I, Ga, In, and interstitial Li and Na.