Observations of the absorption, infra-red emission, and excitation spectraof Cr in BaTiO3

Photoluminescence and optical absorption in Cr-doped BaTiO3 were observed f or the first time and studied within the 300-800 nm spectral region and ove r a 7-300 K temperature range. 'Nominally pure' BaTiO3 crystals, with Cr im purities detected by means of EPR, and intentionally Cr-doped BaTiO3 crysta ls grown by the top-seeded solution method were stud-led. Cr doping results in additional absorption over the whole visible region, increasing towards the band-edge area (at approximate to 400 nm), and wide absorption bands c entred at about 470 nm, 575 Mn, and 610 nm. At 74 K, exposure to optical ex citation with lambda(exc) = 400 nm results in luminescence in the near-infr a-red region consisting of four sharp lines,A(1) (781.65 nm, 12795 cm(-1)), A(2) (776.4 nm, 12 879 cm(-1)), A(3) (768.8 nm, 13 005 cm(-1)), A(4) (766. 8 nm, 13 039 cm(-1)), and several additional weak emission lines. The lumin escence excitation spectrum for each A line consists of two complex bands c entred at 630 nm (15 873 cm(-1)) and at 400 nm (25 000 cm(-1)) for T = 74 K . With decreasing temperature, all of the sharp lines, such as the R-1,R-2 zero-phonon lines of Cr3+, SrTiO3, shift to lower energies, which is opposi te to,the behaviour of such lines for ionic crystals. The thermal shift for the A(2) line is the largest approximate to 0.12 cm(-1) K-1. Taking into c onsideration EPR data, temperature transformations, and the lifetime of the sharp emission lines (approximate to 1 ms at 70 K), We argue that the A(3) and A(4) lines are R-1.2 lines, i.e. originating from zero-phonon E-2 --> (4)A(2) transitions of single Cr3+ ions replacing Ti4+ ions. The nature of the A(1,2) emission lines is not quite clear, but can be considered to orig inate from Cr3+ ions exchange coupled with other, unknown defects (includin g Cr3+ exchange-coupled pairs) or with E-2 --> (4)A(2)-type transitions of Cr3+ centres perturbed by nearest-site hole polarons, O- or OH-.