Photochromism and polaronic photocharge localization in diluted KTa1−xNbxO3

Ultraviolet (UV)-light-induced optical absorption in the near infrared (NIR) region was observed in diluted KTa1−xNbxO3 single crystals (x=0,0.004,0.007,0.012,0.07) at low temperatures. Illumination by wideband light (3.10–4.13eV, 300–400nm) is accompanied by the appearance of a broad NIR absorption band with the position of the maxima varying in the 0.69–0.8eV (1.54–1.79μ, T=1.3K) region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure KTaO3, as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIR absorption bands are tied to interband optical transitions of pair Nb4+ electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair Nb4+ polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic dielectric effects and possible phase transitions, which have been recognized recently in incipient ferroelectrics at low temperatures.

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