Thermal reduction of KTiOPO4 single crystals

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Abstract
KTiOPO4 (KTP) samples heated in vacuum at temperatures in the 400–650 °C range show a broad optical absorption similar to that observed by H2 reduction. The position of the maximum depends on the reduction degree. The transparency of the sample is recovered by heating in air at high temperature (600 °C). Vacuum annealing leads to the reduction of Ti4+ to Ti3+ and also to the reduction of Fe, W, and Rh impurities present in the samples, as inferred from electron paramagnetic resonance measurements. X‐ray irradiation at 15 K induces an optical absorption peaking at 585 nm. This absorption has been ascribed to the superposition of the 2T2→2E transition of Ti3+ in the two different Ti sites of the lattice. It is suggested that the absorption observed in vacuum‐reduced samples consists of the overlapping of the latter contribution and a charge transfer band of Fe‐Ti pairs. A loss of K and P ions has been observed after the annealing of KTP in vacuum at 800 °C forming a surface layer that scatters light. Moreover, the K and P loss decreases the optical absorption induced at lower temperature (T<650 °C).