R.A. Laudise

Stability of superconducting YBa2Cu3O7 in the presence of water

The new high transition temperature superconductors such as YBa2Cu3O7 in powdered form have been observed to react vigorously at room temperature with water producing a nonsuperconducting cuprate with copper in oxidation state +2, O2, CuO, and Ba(OH)2. Similar reactions will probably occur with nearly equal rapidity in all cuprate superconductors. We have used room‐temperature and liquid N2 resistance measurements to study the stability of smaller surface area specimens (thin sheets). We have found that such specimens react slowly with humid air at room temperature and rapidly in air at 85 °C and 85% rel. humidity, the standard conditions used for accelerated humidity testing. The resultant end products are nonsuperconducting phases formed in from hours to days, depending on conditions. Furthermore, we have found that, while a plastic encapsulant considerably slowed such reactions, it did not eliminate them. The implications for both scientific experiments to characterize high transition temperature super...

The crystal structure of the high-temperature polymorph of α–hexathienyl (α–6T/HT)

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported. 1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult. 2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Huckel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET) 2 Cu(NCS) 2 .

Crystal growth of high temperature superconductors — Problems, successes, opportunities

Abstract The chemistry of oxide superconductors, especially those of the YBa 2 Cu 3 O x family, is discussed with respect to their crystal growth. YBa 2 Cu 3 O x melts incongruently, loses oxygen at elevated temperatures and reacts with water. Present knowledge of the appropriate phase diagrams is reviewed. Major important methods of crystal growth are summarized and their present applicability to YBa 2 Cu 3 O x is discussed. In addition, recent results on the melt growth of bulk crystals from melt compositions near the ternary eutectic and on the growth of thin oriented, partly epitaxial films by vapor growth using thermal and e-beam vapors and MBE are described.

Czochralski growth and characterization of GaSb

Abstract Single crystal GaSb is important as a substrate for long wavelength ( ⩾ 1.5 μm) detectors and lasers. Procedures for the reproducible synthesis of GaSb, and Czochralski growth of single crystals with varied stoichiometry are discussed. The distribution of dopants including Te, Se, Si and Ge was studied and the dependence of carrier concentration on dopant concentration (determined by atomic absorption analysis) is reported. In the Te concentration range studied, 1.5 × 10 17 -3 × 10 19 cm -3 , the carrier concentration is always substantially less than the Te concentration ranging from ≈ 0.25 at low Te to ≈ 0.14 at high Te. Room temperature electron mobilities > 3000 cm 2 V -1 s -1 are obtained at low carrier concentrations. The interface shape and its change with fraction frozen is shown to affect the impurity distribution. Normal freeze behavior is qualitatively observed for Te partition as a function of fraction of melt frozen. Equations for the effect on impurity distribution of the changing Ga/Sb ratio in the solid as freezing proceeds are developed and compared with experimental results. It is shown that the most important correction to the distribution constant is based on interface shape changes and on and off facet growth as freezing proceeds.

Hydrothermal Crystal Growth

Publisher Summary This chapter describes hydrothermal crystal growth. In discussing hydrothermal crystal growth, it stresses experimental results and techniques rather than the theoretical aspects of crystal growth. This may be done for two reasons: (1) the experimental technique of hydrothermal crystal growth differs greatly from other methods, and to provide a practical guide for use of the method, and to encourage its adoption, emphasis on experimental details and results seems justified, and (2) in spite of some progress in recent years, crystal growth theory is still rather crude. The theory of crystal growth from solutions is especially difficult to relate to experimental results and, in its present form, is of little quantitative use to the practical crystal grower. Many of the disadvantages of the hydrothermal technique are those that arise because of the extreme pressures required. They include direct pressure hazards, the need for rather unusual apparatus, and the inability to directly observe the crystallization while in progress. The problems then are principally those of technique, and the authors hope that what has been presented here will facilitate the adoption of the techniques by others.

Phase relations, solubility and growth of potassium titanyl phosphate, KTP

Abstract Potassium titanyl phosphate is an unusually good nonlinear optical material whose use is constrained because of the difficulty of growing crystals. Promising new hydrothermal conditions for growth have been discovered. The yield of the reaction: KH 2 PO 4 +TiO 2 → KTiOPO 4 +H 2 O is used to deduce the hydrothermal phase diagram KPO 3 -TiO 2 -H 2 O between 500 and 600°C at 10 kpsi in the presence of the mineralizer, 1m K 2 HPO 4 . It is shown that KTiOPO 4 , “KTP”, is congruently saturating above 600°C and crystallizes as the sole solid in an excess of KPO 3 below 600°C. Solubility is appreciable above 600°C. Phase equilibria and solubility results are used to find growth conditions (2m K 2 HPO 4 , 70% fill, 375–425°C) with rates of KTP crystallization from a new mil/day to ∼27 mil/day (∼0.68 mm/day). Good quality crystal growth takes place at the lower rates. These results hold promise of practical growth at mild conditions where low carbon steel, comparatively inexpensive autoclaves can be used.

Hydrothermal synthesis of aluminum orthophosphate

Abstract We have investigated extensions and modifications of previous AlPO 4 growth techniques, designed to overcome the difficulties posed by the negative temperature coefficient of solubility. On the assumption that solubility could be improved even at higher temperatures provided pressure was kept high, we investigated crystal growth in the range 150–325°C at 1000–10,000 psi. The preparation of nutrient material of large enough particle size that it would not be convected in the hydrothermal fluid and become included in the crystal posed a problem. Successful techniques for the recrystallization of small particle AlPO 4 by slowly raising the temperature of a hydrothermal vessel are described. Processes for growth by slowly increasing the temperature of a saturated solution and for growth in a negative temperature gradient where the large particle nutrient is contained in a basket in the upper cooler region of the autoclave are described. The fact that growth in a positive temperature gradient is possible has been established and growth on seeds where nutrient was contained in the hotter bottom region of the autoclave and seeds were held in the upper cooler region was carried out. The processes have been used to prepare crystals grown on seeds at rates up to 0.41 mm/day (16 mil/day).

Dislocation-free and low-dislocation quartz prepared by hydrothermal crystallization

Abstract A reproducible technique for the hydrothermal crystallization of low dislocation-density and dislocation-free α-quartz on (0001) and (01 1 1) seeds is described. Dislocations are shown to propagate from pre-existing dislocations in seeds and from particulate inclusions (lithium sodium iron silicate) grown into the crystal. To prepare dislocation-free material in high yields dislocation free seeds which are carefully etched to remove damage and growth in noble-metal-lined autoclaves to reduce iron containing inclusions are necessary. Etching studies showed that shallow etch pits are probably associated with ordinary dislocations while etch pipes are associated with dislocations found under growth conditions were iron decoration is probable. Growth rate had no obvious effect on perfection over a growth rate range of 2–4X. Problems associated with a unique Burgers vector determination are outlined. The relationship of dislocations and strain to cracking previously observed in growth on (01 1 1) seeds is discussed.

The growth of high acoustic Q quartz at high growth rates

Abstract A technique for the hydrothermal growth of high acoustic Q (>106) quartz at rates above 100 mil/day (2.5 mm/day) is reported. This technique hinges upon the fact that the solid solubility in quartz of the proton which causes loss is decreased as the growth temperature increases. Typical conditions for high Q-high rate growth are: 374 °C, crystallization temperature; 23°, temperature difference (ΔT) between dissolving and growth zones; 88% fill; 40000 psi (2759 bar) and 103 mil/day growth rate on a surface within 5° of a basal (0001) plane. The Q is 1.4 × 106. The concept of effective distribution constant developed to describe impurity segregation in melt growth is shown to be applicable to solution growth with appropriate modifications. A new quantity, the effective equilibrium constant for the segregation of (M+3) ions and (H+) in quartz is shown at constant temperature to depend on rate (where rate changes are brought about by fill and ΔT changes) in a manner analogous to the dependence of the effective distribution constant in melt growth. It is suggested that the concept of effective equilibrium constant can be extended to polycomponent growth in general.

Phase equilibria of Y3Al5O12, hydrothermal growth of Gd3Ga5O12 and hydrothermal epitaxy of magnetic garnets☆

Abstract Nonmagnetic garnets are important as laser hosts and as substrates for thin magnetic garnet films for bubble memory and related studies. Hydrothermal growth presents the advantages of perfection and easy scale-up for these materials. The complete Y 2 O 3 -Al 2 O 3 -H 2 O phase diagram was determined at conditions previously used for YAG growth in (CO 3 ) = and (OH) - . Y 3 Al 5 O 12 was found to be congruently saturating, while YA1O 3 was never the primary phase. These results are correlated with growth studies and used to deduce conditions for Gd 3 Ga 5 O 12 growth. Gd 3 Ga 5 O 12 was found to be incongruently saturating and grew easily from (OH) - solutions containing more than 1 M (molar) excess Ga 2 O 3 . Optimum conditions for growth and some data on perfection are reported. Techniques for the hydrothermal epitaxial growth of magnetic garnets on Gd 3 Ga 5 O 12 were studied and compared: (1) growth in a thermal gradient from Gd 3 Ga 5 O 12 , (2) reaction of the oxides and hydroxides, and (3) hydrothermal tipping. Macroscopically good films were prepared by all these methods and the conditions required for each are reported. The advantages and disadvantages of the techniques are reported and the quality of the films produced is discussed.