The pure

0.005

0.01

0.2 and 0.5 wt% Cr

2

O

3

-doped strontium titanate single crystals were prepared using melt method in a reduction atmosphere (N

2

). The transmission spectra at room temperature and the fluorescence spectra above 6.5 K were measured under various annealing conditions for thess crystals. The oxidization-heating-induced absorption and the effects of annealing and concentration on the luminescence from the crystals were studied in detail.The pure as-grown SrTiO

3

crystal is dark in colour because of the absorption of oxygen vacancy

and no fluorescence occurs from it. After annealing in air atmosphere

most of oxygen vacancies are removed from the crystal and it turns a yellow colour. The yellow sample has a weak sharp line emission near 794 nm which is attributed to the ppm Cr in the crystal.The lightly doped as-grown SrTiO

3

crystals (0.005 and 0.01 wt%) have three colours: yellow

dark green and dark

in which the yellow sample has an intense emission in near infrared region consisting of sharp line at 794 nm and its sidebands and being ascribed to the vibronic transition

2

E-

4

A

2

of Cr

3+

ion. The dark green and dark ones have no fluorescence to be seen at any temperature

and after annealing in air atmosphere above 660℃ they can turn yellow or red (dark red) colour determined by whether the time of annealing is short or long and the temperature of annealing is low or high. The red sample has a weaker fluorescence than the yellow one. It is mainly due to the oxidization from Gr

3+

to Cr

4+

ion. The oxidization-heating-induced absorption peak appears near 530 nm and is attributed to the absorption of Gr

4+

ions in the crystal.The heavily doped as-grown samples (0.2 and 0.5 wt%) have dark red and dark colours and after annealing above 660℃ they turn dark. The dark red samples have a different emission from the lightly doped samples in which the sharp line emission of Gr

3+

ion is much weakened and a new emission spectrum occurs in the longer wavelength region which is attributed to the Gr

3+

-pair emission. No fluorescence occurs from the dark samples

which is mainly due to the intense absorption of Gr

4+

ions.