research papers Acta Cryst. (2008). B64, 287–290 doi:10.1107/S0108768108009063 287 Acta Crystallographica Section B Structural Science ISSN 0108-7681 Coherence length of the KY(WO 4 ) 2 single crystal A. Shalimov, a M. T. Borowiec, a * T. Zayarnyuk, a M. C. Pujol, b M. Aguilo ´ b and F. Dı ´az b a Institute of Physics, Polish Academy of Sciences, Al. Lotniko ´w 32/46, Pl 02-668, Warsaw, Poland, and b Fı ´sica i Cristallografia de Materials (FiCMA), Universitat Rovira i Virgili (URV), Campus Sescelades c/Marcellı ´ Domingo, s/n, E-43007-Tarragona, Spain Correspondence e-mail: borow@ifpan.edu.pl # 2008 International Union of Crystallography Printed in Singapore – all rights reserved Potassium yttrium tungstate is a monoclinic crystal with space group C2/c. The specific X-ray investigations presented in this article determine the coherence scattering length for this crystal. The scattering profiles were obtained using a Philips MRD X-ray diffractometer. The Williamson–Hall method was used to determine the coherence scattering length and internal stresses. Received 20 December 2007 Accepted 3 April 2008 1. Introduction Diode-laser-pumped solid-state laser devices have a variety of applications owing to the versatility of the process. A number of crystalline hosts (more than 280) have been developed with rare-earth ions, transition metals and actinides (Kaminiski & Kaminskii, 1996; Kaminskii, 2003). There is little real appli- cation in the laser world for parameters such as overall effi- ciency, average power, thermal lensing, optical and mechanical stability. Potassium yttrium tungstate [KY(WO 4 ) 2 , hereinafter KYW] single crystals offer an unusual wealth of properties, such as a large cross section, when they are doped with lanthanides and some of these physical properties exhibit an interesting anisotropy. KYW biaxial crystals are also called self-active lasers, since the rare-earth ion laser emission and the stimulated Raman scattering (SRS) occur inside the same crystal (Griebner et al., 2005; Grabtchikov et al. , 2002; Han et al., 2002; Metrat et al., 1997; Gallucci et al., 1998). The physical characteristics of the majority of crystals depend on their structural properties and defect structures. The development of crystal growth technology will require further structural investigation. The measurements of coherence length in a relatively large single crystal of KYW is of great importance because it can be used as a reference point for a subsequent study involving the determination of the sizes of nanocrystals (Borowiec et al., 2006). The grain size of nanocrystals of a given material cannot be measured accurately using the X-ray method unless the coherence length of the bulk material is known. For nearly perfect crystals, such as Si or GaAs, the length of coherence scattering mainly depends on the spectral purity of the radiation (Holy et al. , 1999). Considering that, the ratio of the wavelength dispersion to the radiation wavelength, Á\, is ca 10 5 –10 6 for a triple crystal diffractometer, and the coherence scattering length for nearly perfect crystals reaches tens of microns and defines the upper limit for the determi- nation of coherence block size. 2. Samples and experiment This paper is concerned with the most relevant structural phase of the potassium rare-earth tungstate (KREW)