Helmut Klapper

Characterization of flux-grown KTiOPO4 (KTP) crystals by X-ray topography

Abstract Single crystals of KTP have been grown from the flux of composition K 6 P 4 O 13 by spontaneous crystallization and by using seed crystals in single-zone and three-zone furnaces. The crystals are clear, transparent and are up to 25×15×7 mm in size. X-ray topographic study shows that the crystals are of good quality, with very low dislocation densities.

X-ray diffraction study of KTP (KTiOPO4) crystals under a static electric field

The X-ray rocking curves recorded with an electric field along the polar b axis show a strong enhancement of the 040 reflexion intensity whereas the h0l reflexions do not show any intensity change. For an electric field normal to the polar axis no intensity change, 040 or in h0l reflections, occurs.

Growth twinning in quartz-homeotypic gallium orthophosphate crystals

Abstract Crystals of quartz-homeotypic gallium orthophosphate have been grown by the slow heating method, by a combination of slow heating and solvent evaporation, by a two chamber growth technique and by the vertical temperature-gradient method. Growth twinning according to the Dauphine twin law, the Brazil twin law and a “combined” twin law have been found and characterized by means of etching, optical and X-ray topographical methods. Dauphine (or electrical) twinning occurs as growth sector twinning, originating from the first nucleated seed and is often recognizable by morphological features (re-entrant edges, interpenetration of rhombohedra). Brazil (or optical) twinning is polysynthetic, and is correlated with other growth defects.

X-Ray Topography of Organic Crystals

X-ray topography is a powerful non-destructive method for the direct observation of defects in nearly perfect crystals. Since organic molecular crystals usually contain ‘light’ atoms only, they exhibit low X-ray absorption and scattering power. Thus ‘thick’ crystal plates (up to 3 mm for CuKα radiation) can be studied by transmission topography, and defects appear by broad kinematical contrast. The image widths of dislocation lines, however, are large, usually > 20 µm, leading to limited spatial resolution and to the requirement of high-quality crystals.

Defect Generation and Interaction during Crystal Growth

This chapter provides an overview of the most important defect types and their origins during bulk crystal growth from solutions and melts. Thermodynamic and kinetic principles are considered as the driving forces of defect generation and incorporation. Results of modeling and practical in situ control are presented. The overview starts with zero-dimensional defect types (i.e., native and extrinsic point defects), summarizing their generation and incorporation mechanisms as well as segregation phenomena. One-dimensional imperfections and dislocations, as well as their generation, propagation, and interaction during growth from solutions and melts, are treated next. Special focus is directed on high-temperature dislocation dynamics producing collective arrangements, such as cell structures and bunches. Then, second-phase precipitation and inclusion trapping are discussed. The importance of in situ stoichiometry is emphasized. Finally, two special defects are treated—faceting and twinning. The formation possibilities growth in twins by nucleation, inclusions, and faceting, as well as postgrowth ones by phase transitions and ferroelastic switching, are described.

X-RAY TOPOGRAPHIC CHARACTERIZATION OF DEFECTS IN ORGANIC CRYSTALS

X-ray topography is a powerful non-destructive method for the direct observation of defects in nearly perfect crystals. Organic molecular crystals, consisting of ‘light’ atoms, exhibit low X-ray absorption and scattering power. Thus ‘thick’ crystal plates (up to 3mm for CuKα radiation) can be studied, and defects appear by broad kinematical contrast. The ‘image widths’ of dislocation lines are usually > 20 μ, leading to limited spatial resolution of the method. Typical defects observed in nearly perfect organic crystals are inclusions, growth striations, faulted growth-sector boundaries, grown-in and glide dislocations. The determination of Burgers vectors of dislocations is discussed and demonstrated. Topographs of benzil, benzophenone, salol and 2,3-dimethylnaphthalene crystals grown from solution, from undercooled melt, by the Bridgman method or by the Czochralski technique are presented.

Dynamic proton disorder and the II–I structural phase transition in (NH4)3H(SO4)2

X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction methods were used to investigate triammonium hydrogen disulfate (NH(4))(3)H(SO(4))(2) (TAHS) in the temperature range between 293 and 493 K. The temperature-dependent X-ray powder diffraction measurements show a clear hysteresis of the I <-->II phase transition of TAHS with transition temperatures of T(up) = 412.9 (1) K on heating and of T(down) = 402.6 (1) K on cooling. From the existence of hysteresis and from the jump-like changes of the lattice parameters, the I <--> II phase transition of TAHS is considered to be first order. With DSC/TG measurements we confirmed that there is only one phase transition between 293 and 493 K. Through careful investigation on single crystals of TAHS using neutron diffraction, the correct space group (C2/c) of room-temperature TAHS-II phase was confirmed. Crystal structure analysis by single-crystal neutron diffraction showed a strongly elongated displacement ellipsoid of the proton which lies in the middle of the (SO(4))H(SO(4)) dimer with \bar 1 local symmetry. The protons of the NH(4) groups also show strongly enlarged anisotropic mean-square displacements. These findings are interpreted in terms of a characteristic proton disorder in the TAHS-II phase.

High-temperature phase transitions and domain structures of KLiSO4: studied by polarisation-optics, X-ray topography and liquid–crystal surface decoration

Abstract The transitions between the room temperature phase III (space group P63) and the two high-temperature phases II (Pcmn) and I (P63/mmc) of KLiSO4 and the domain structures generated by them were investigated by high-temperature polarisation optics (birefringence) and room-temperature X-ray topography, optical activity and nematic–liquid–crystal (NLC) surface decoration. The transition from the polar hexagonal phase III into the centrosymmetric orthorhombic phase II at 708 K leads, due to the loss of the trigonal axis and the radial temperature gradient of the optical heating chamber used, to a roughly hexagonal arrangement of three sets of thin orthorhombic {110} lamelleae with angles of 60° (120°) between them. The associated twin law “reflection m{110}orth” corresponds to the frequent growth twin m{101̅0}hex of phase III. The domains are easily ferroelastically switched. Upon further heating above 949 K into phase I (P63/mmc) all domains vanish. Upon cooling back into phase II the three domain states related by 60°(120°) reflections m{110}orth re-appear, however (due to the higher thermal agitation at 949 K) with a completely different domain structure consisting of many small, irregularly arranged {110}orth domains. Particular attention is paid to the domain structure of the hexagonal room temperature phase III generated during the re-transition from the orthorhombic phase II. Curiously, from the expected three twin laws inversion 1̅, rotation 2⊥[001]hex and reflection m{101̅0}hex only the latter, which corresponds to the frequent growth twinning, has been found. Finally a short treatise of the structural relations of the KLiSO4 high-temperature polymorphs is given.