Photochromic centers and impurities in nominally pure KTaO3 and K1-xLixTaO3.

Abstract

Investigations of photochromic centers and impurities in nominally pure single crystals of ${\mathrm{KTaO}}_{3}$ and ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_{3}$ (KTL) have been carried out by the ESR method. In all investigated KTL samples (x=0.016, 0.02, 0.03) the axial symmetry spectrum of ${\mathrm{Fe}}^{3+}$ substituted for ${\mathrm{K}}^{+}$ [${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K)], cubic symmetry spectrum of ${\mathrm{Fe}}^{3+}$ substituted for ${\mathrm{K}}^{+}$ [${\mathrm{Fe}}_{\mathit{c}}^{3+}$(K)], and that of ${\mathrm{Ni}}^{3+}$ substituted for ${\mathrm{Ta}}^{5+}$ [${\mathrm{Ni}}_{\mathit{c}}^{3+}$(Ta)] have been observed at T\ensuremath{\le}60 K. After light illumination with 365\ensuremath{\le}\ensuremath{\lambda}\ensuremath{\le}436 nm, several ESR spectra appeared, each one with the characteristic temperature of its annealing in the region 25\ensuremath{\le}T\ensuremath{\le}120 K. In KTL samples simultaneously the ${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K) spectrum disappeared and intensities of ${\mathrm{Fe}}_{\mathit{c}}^{3+}$(K) and ${\mathrm{Ni}}_{\mathit{c}}^{3+}$(Ta) spectra decreased. In nominally pure ${\mathrm{KTaO}}_{3}$ the spectra ${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K) and ${\mathrm{Fe}}_{\mathrm{ax}}^{+}$(K), observed before illumination, disappeared after it. The gradual restoration of these aforementioned spectra in both ${\mathrm{KTaO}}_{3}$ and KTL was observed at T>25 K when some of the photoinduced spectra disappeared. One of the photoinduced spectra was shown to be that of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ in both ${\mathrm{KTaO}}_{3}$ and KTL. A scheme of local electronic levels of impurities in the band gap of nominally pure ${\mathrm{KTaO}}_{3}$ and KTL was proposed. This scheme and especially the shallow ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ level in it made it possible to explain some peculiarities of photosensitive phenomena observed in nominally pure ${\mathrm{KTaO}}_{3}$ and KTL in recent years.