On the mechanism of impurity influence on growth kinetics and surface morphology of KDP crystals-I: defect centres formed by bivalent and trivalent impurity ions incorporated in KDP structure-theoretical study

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 60-75° to the [0 0 1] direction, formed by Me2+ ions with ionic radii approximately from 0.75Å to 1.0 Å; (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.