Dieter Mateika

Mixed-perovskite substrates for high-Tc superconductors

Abstract Mixed perovskites of high quality have been grown from multicomponent melts using the Czochralski method. The formation of mixed crystals is accomplished by coupled substitution. Rare-earth aluminium perovskites are used as host lattices. The cations of the host lattice are partially replaced by the guest ions Sr 2+ , Ca 2+ , Ta 5+ and Nb 5+ . The distribution coefficients of the cations are adjusted to unity by appropriate choice of the melt compositions. Single crystals up to 30 mm in diameter and 80 mm in length have been obtained. Depending on the composition the crystallographic structure of the mixed perovskites is cubic, tetragonal or orthorhombic. The cubic crystals are twin free. The lattice constants can be varied in the range 0.378 ≤ a 0 ≤ 0.388 nm. The dielectric constants (13 ≤ ϵ ≤ 21) and the dielectric losses (tan δ ≤ 1 × 10 -3 are reasonably low. The thermal expansion coefficient α of the cubic perovskites is about 1 × 10 -5 ΔC -1 in the temperature range between 100 and 900ΔC.

Controlled lattice constant mismatch by compositional changes in liquid phase epitaxially grown single crystal films of rare earth yttrium iron gallium garnets on gadolinium gallium garnet substrates

Abstract Liquid phase epitaxy (LPE) by dipping substrates into supercooled fluxed solution provides a convenient way of changing the melt and film composition. The apparatus, method and composition of the melts that we use will be described in detail. The composition of the layer is essentially Y 3 GaFe 4 O 12 , which has a smaller lattice constant than the Czochralski-grown gadolinium gallium garnet substrate. By substitutions such as gadolinium, samarium or lanthanum for yttrium the lattice constant mismatch can be controlled. The effect of these substitutions on film properties such as cracks, stresses and magnetic domain pattern as well as the interaction of substrate defects such as dislocations with the epitaxially-grown film are discussed.

Growth of mullite single crystals (2Al2O3 · SiO2) by the Czochralski method

Abstract Single crystals of mullite up to 2 cm in diameter and 6 cm long were grown by the Czochralski method. The process was carried out in a nitrogen atmosphere under normal atmospheric pressure with precise programmed control of rf heating. The vertical pull rate leading to the best results was 1.5 mm/h with a seed rotation of 10 rpm. All runs were made with growth along the c -axis ([001]). Monocrystallinity was checked by optical and X-ray methods. The initial composition of Al 2 O 3 and SiO 2 was varied between 71.8 wt% Al 2 O 3 (3:2 mullite) and 77.3 wt% Al 2 O 3 (2:1 mullite).

Lattice parameters and distribution coefficients as function of Ca, Mg and Zr concentrations in czochralski grown rare earth gallium garnets

Abstract Solid solutions of rare earth gallium garnets with calcium, magnesium and zirconium as guest ions have been grown using the Czochralski technique. Single crystals up to 82 mm in diameter and 120 mm in length were obtained. The best crystals have dislocation densities below 5 cm -2 and inclusion densities below 5 cm -3 . The lattice parameters are varied by the coupled substitution of Ga by Mg and Zr, and Gd by Ca in the range from 12.290 to 12.620 A. The distribution coefficients of the guest ions depend on their concentrations in the melt and on the chemical composition of the host garnet.

Growth and investigation of {Gd3−xCax}[Ga2−y−zZryGdz] (Ga3)O12 garnets

Solid solutions of single crystals corresponding to the formula {Gd3−xCax} [Ga2−y−zZryGdz] (Ga3)O12 have been grown with coupled substitution of calcium and zirconium for gadolinium and gallium at dodecahedral and octahedral sites, respectively, using the Czochralski technique. Single crystals 20 mm in diameter and 100 mm in length have been obtained, having dislocation and inclusion densities below 5 per cm2. The lattice parameter, for single crystals grown from melt compositions in which 0.1 ⩽ x = y ⩽ 0.8, increases linearly from (12.3820 ± 4) x 10-4 A to (12.5272 ± 4) x 10-4 A. The distribution coefficients of calcium and zirconium increase in this concentration range from below to above unity and are approximately unity for a melt composition in which x = y = 0.45.

Lattice-constant-adaptable crystallographics: II. Czochralski growth from multicomponent melts of homogeneous mixed-garnet crystals

Abstract Homogeneous mixed crystals of rare-earth garnets containing Ca 2+ , Mg 2+ , Zr 4+ , Hf 4+ and Ge 4+ as guest ions have been grown from multicomponent melts. Garnet compositions that grow with distribution coefficients k eff (cat) of unity or nearly unity have been found. These crystals could be classified into mixed-crystal systems and groups. Crystal-structural parameters are used for the graphic representation of these mixed-garnet compositional series. The distribution coefficients of the cations are adjusted to unity by appropriate choice of the melt compositions. The lattice constants of the mixed garnets can be varied in the range from 1.240 up to 1.286 nm. Single crystals have been grown with dimensions between 18 and 100 mm in diameter and 50 and 120 mm in length. The crystal quality is comparable with that found in unsubstituted garnets.

Czochralski growth of barium hexaaluminate single crystals

Abstract The temperatures of the phase transformations in the Al2O3-BaO system were determined in the concentration range between 100 and 65 mole% Al2O3. The corresponding part of the T-x phase diagram has been constructed. A method to grow single crystals of barium hexaaluminate up to 90 mm in length was developed using the results of the phase diagram. The hexagonal prismatically-shaped crystals are colourless and optically transparent. The crystal quality was examined by polarization microscopy, X-ray topography and etching.

Ein isothermes schmeltztropfen-verfahren zur herstellung von grossen, sehr reinen und homogenen blei-zinn-tellurd-einkristallen aus der dampfphase

Abstract Pb 1− x Sn x Te single crystals with x between 0 and 1 have been grown by an isothermal melt-drop-method from the vapour phase. A melt drop which is hanging on the interface is continuously fed by condensation from the vapour phase. The size of the crystals is 60 mm in length and 9 mm in diameter. The crystals are very homogeneous and have fewer dislocations than crystals grown by the Stockbarger-Bridgman method. The electrical properties of the grown crystals are better than those of crystals grown by other methods.

Lattice-constant-adaptable crystallographics: I. Heteroepitaxial growth of semiconductors and other specific compounds on lattice-constant-adapted garnets, spinels and perovskites; a systematic geometrical (not experimental) evaluation

Abstract Crystals of garnets, spinels and perovskites have the interesting property that various ions can be substituted on different lattice sites. This property implies that the lattice constants of these materials are adaptable. From a geometrical treatment of heteroepitaxial growth on such materials having a (large) non-adapted lattice constant it is found that two lattice-matching relations have to be fulfilled: (1) an axial-matching relation which determines the value of the lattice constant of the substrate material to be grown and (2) a cross-sectional relation which determines the appropriate growth plane to be cut from that crystal. A large variety of cubic compounds like Si, GaAs, Ge, InP and In 2 O 3 , and also non-cubic compounds like hexagonal LiNbO 3 , can be lattice-matched to prescribed samples of these crystalline groups. Circumstances of in-plane rotation of the crystallographic plane of the epilayer, as compared to the plane of the substrate, are discussed. In this approach to heteroepitaxial growth, crystallographic planes as well as interplanar spacings play an essential role in the basic framework.

Coupled substitution of gallium by magnesium and zirconium in single crystals of gadolinium gallium garnet

Single crystals of gadolinium gallium garnet in which the gallium ions on the octahedral sites were partially substituted by coupled substitution of magnesium and zirconium have been grown using the Czochralski technique. Single crystals of 36 mm in diameter and 100 mm in length corresponding to the formula Gd3Ga5-x-yMgxZryO12 have been obtained from melt compositions in which 0.1 ⩽ x = y ⩽ 0.7. The dislocation and inclusion densities of the single crystals are below 5 cm-2. The lattice parameters increase linearly from 12.382 A for gadolinium gallium garnet to 12.489 A for x = y ≈ 0.54. The distribution coefficient increases in this concentration range from Keff = 0.58 to Keff = 0.89. Boules grown from melt compositions in which x = y ⩾ 0.7 appear cloudy through precipitation of Gd2Zr2O7 as a second phase.