Crystal structure and nonlinear optical properties of KSnOPO4 and their comparison with KTiOPO4

Crystals of KTiOPO4 (potassium titanyl phosphate, KTP) and its tin analogue KSnOPO4 (potassium stannyl phosphate, KSP), were grown and charac- terized for their structural and nonlinear optical properties. The crystal structure of KTiOPO4 [Mr = 197.97, orthorhombic, space group Pna21(C90, a= 12-819(3), b=6"399(1), c= 10.584 (2) A, V= 868"1A3] was refined to an R factor of 2.5% from room-temperature single-crystal X-ray data. Using the refined coordinates, a model for the centro- symmetric prototype phase with suggested space group Pnan (D6h)is constructed. From this model, the magnitude and direction of the spontaneous polarization developed in the room-temperature crystal structure are calculated. The crystal structure of KSnOPO4 [Mr= 268"76, orthorhombic, space group Pna21, a=13"145(3), b=6"526(2), c = 10.738 (3)A, V = 921-1 /~3] was determined at room temperature by single-crystal X-ray diffraction and is shown to be isostructural with KTiOPO4. Certain differences in the details of the crystal structures of KSP and KTP are, however, very pronounced. In particular, in KSP, the oxygen framework approxi- mates very closely to a centrosymmetric arrangement and the Sn coordination octahedra are much less distorted than their TiO6 counterparts so that anomalously shortened Sn--O bonds equivalent to the very short (1.716 and 1.733A) Ti--O bonds in KTP do not appear. The principles of bond polariz- ability theory and the structural data are used to correlate the structural differences between KTP and KSP with the fact that the second-harmonic gener- ation output for KSP is only 2% of that shown by an equivalent KTP sample. The results demonstrate further proof that the short Ti--O bonds in KTP are the important structural features leading to the high optical nonlinearity of this material.