Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation

Structure factors for strontium/chromium- (Sr/Cr) and barium- (Ba) doped potassium titanyl phosphate (KTiOPO4, KTP) were measured with focused synchrotron X-radiation [0.75000 (9) A] using a fast avalanche photodiode counter. Space group Pna21, Z = 8, a = 12.786 (2), b = 6.3927 (8), c = 10.5585 (9) A, T = 293 (1) K, R = 0.028 (SrCrKTP); a = 12.851 (6), b = 6.418 (3), c = 10.620 (5) A, T = 120 (1) K, R = 0.031 (BaKTP). The refinement of the dopant positions showed that Ba2+ is positioned in the larger of the two K cavities of KTP, while the smaller Sr2+ ion is located in both. Split positions are found for the strontium dopant in both cavities and they are located in the positive c direction from the potassium cation. The chromium dopant has two different oxidation states, namely +III and +VI; in both states the dopant is located inside the TiO6 octahedra. The two structures show slightly less distorted TiO6 octahedra than pure KTP.