ABSORPTION AND LUMINESCENCE SPECTROSCOPY OF ZN2SIO4 WILLEMITE CRYSTALS DOPED WITH CO2+

The polarized absorption spectra of Zn2SiO4 willemite crystals doped w ith Co2+ consist of three band systems in the near-infrared and visibl e spectral region centered at 3800, 7000, and 17000 cm(-1), respective ly. In the tetrahedral approximation they are assigned to the d --> d transitions (4)A(2) --> T-4(2), T-4(1)(F-4), and T-4(1)(P-4), respecti vely. The crystal field parameter 10Dq is 4000 cm(-1), and the Racah p arameters B and C are 740 and 3330 cm(-1), respectively. From the fine structure in the origin region of the (4)A(2) --> T-4(2) absorption b and it follows that the (4)A(2) ground-state splittings of Co2+ occupy ing the two crystallographically inequivalent Zn2+ sites of C-1 symmet ry are 13 and 15 cm(-1), respectively. Only a weak emission originatin g at 15335 cm(-1) is observed upon (4)A(2) --> T-4(1)(4P) photoexcitat ion. It is assigned to the (2)E --> (4)A(2) sharp-line luminescence of Co2+ located in a minority site. No luminescence is observed from reg ularly incorporated Co2+ ions.