The structure of defect centres generated by incorporation of bivalent (Ni2+, CO2+, Fe2+, Mn2+, Ba2+ Ca2+, Sr2+) and trivalent (Fe3-. Mn3+, Y3+, La3+) impurity ions in potassium dihydrogen phosphate (KDP) crystals is studied. The study is based on the results of modelling KDP structure with the incorporated bivalent (Me2+) or trivalent (Me3+) impurity ions differing considerably in values of their ionic radii. For the considered ions, three types of defect centres are revealed: (1) isolated centres formed by Ni2+ and Me3+ impurity ions; (2) impurity chains oriented at an angle of 60-75degrees to the [001] direction, formed by Me2+ ions with ionic radii approximately from 0.75Angstrom to LOA; (3) centres formed by large Me2+ ions (e.g. Ba2+) in K+ position with either the heterovalent isomorphism or K+ vacancy formation. The formation energy of Me2+. defect centres is found to be much greater than that of Me3+ ones. This fact and an analysis of deformations of KDP crystal structure imply that stronger lattice stresses are generated by Me2+ ions than by Me3+ ones. (C) 2004 Elsevier B.V. All rights reserved.