274 J. Appl. Cryst. (1992). 25, 274-280 X-ray Diffraction Study of KTP (KTiOPO4) Crystals under a Static Electric Field BY M. T. SEBASTIAN AND H. KLAPPER* lnstitut fflr Kristallographie der R WTH, W-5100 Aachen, Germany AND R. J. BOLT RIM Laboratorium Voor Vaste Stof Chemie, Faculteit der Natuurwetenschappen, Katholieke Universiteit, Toernoiveld 6525 ED, Nijmegen, The Netherlands (Received 4 July 1991; accepted 24 October 1991) Abstract X-ray diffraction studies are made on ion-conducting potassium titanyl phosphate (KTP) crystals with in situ DC electric field along different crystallographic directions. The X-ray rocking curves recorded with an electric field along the polar b axis (which is the direction of ion conduction) show a strong enhance- ment of the 040 reflection intensity (reflecting planes normal to the b axis) whereas the hOl reflections (reflecting planes parallel to the polar axis) do not show any intensity change. For an electric field nor- mal to the polar axis no intensity change, either in 040 or in hOl reflections, occurs. This observation is supplemented by X-ray topography. The 040 X-ray topographs recorded with in situ electric field along b exhibit strong extinction contrast in the form of stria- tions parallel to the polar (ion-conduction) axis. The 040 intensity increase and the striation contrast are attributed to lattice deformation by the space-charge polarization due to the movement of the K ÷ ions under the influence of the electric field. 1. Introduction Potassium titanyl phosphate (KTP) has several unique properties such as large non-linear optical coefficients, wide acceptance angles, phase-matching properties and high optical-damage threshold, making this material attractive for frequency- doubling and parametric devices (Zumsteg, Bierlein & Gier, 1976; Liu, Drafall, Dentz & Belt, 1982; Bierlein & Arweiler, 1986; Bierlein & Vanherzeele, 1989). Its large electro-optic R coefficients and low dielectric constant make it very useful for various electro-optic applications such as modulators and Q switches (Bierlein & Arweiler, 1986). The figure of merit of a KTP electro-optic wave-guide modulator is reported * Present address: Mineralogisches lnstitut, Universit~it Bonn, W-5300 Bonn 1, Germany. 0021-8898/92/020274-07503.00 to be twice that of any other inorganic material. This indicates that KTP is a promising material for integrated-optic applications (Bierlein, Ferreti, Brixner & Asu, 1987). KTP belongs to the point group m2m and is ferro- electric with a Curie temperature of 1209 K. Domains have been observed indirectly by piezoelectric, pyro- electric and electro-optic tests both in flux-grown (Loiacono & Stolzenberger, 1988; Voronka, Gvozdover & Vanoskii, 1987) and hydrothermally grown crystals (Bierlein & Ahmed, 1987). The presence of domains increases the electro-optic switching voltages and decreases the efficiency of non-linear optical conversion (Bierlein & Vanherzeele, 1989). Single-domain crystals can be achieved by polarizing at about 775 K (Bierlein & Ahmed, 1987). The crystal structure of KTP consists of a three- dimensional network of TiO 6 octahedra and PO4 tetrahedra (Tordjman, Masse & Guitel, 1974). The K ions are highly coordinated by eight or nine O atoms. There are structural channels parallel to the polar b axis (Fig. 1), through which K ions can easily move under the influence of an electric field by a vacancy mechanism (Bierlein & Vanherzeele, 1989). The ionic DC conductivity is four orders of magnitude higher Fig. 1. Crystal structure of KTP showing the b-axis channels in the framework of PO4 tetrahedra and TiO 6 octahedra. Projection along polar axis b. Open circles: K ions. C(~ 1992 International Union of Crystallography