It was recalled that a strong near-infrared luminescence band, comprising over 70 lines, in the cubic material had been attributed to an M-center. The luminescence was identified, as being an internal W2 transition, on the basis of high-resolution spectroscopic and doping experiments. The W occupied a cation site, and the crystal field was sufficiently strong for a low-spin configuration. The rich line pattern was attributed mainly to the interaction with one resonant phonon mode at 15.5meV, with A1 symmetry, and 2 gap modes at 29.8 and 32.9meV. The W2+ formed a deep acceptor which was 2.548eV above the valence band. A transient shallow acceptor state (W+,hVB), with a binding energy of 0.215eV, was identified.

R.Heitz, P.Thurian, A.Hoffmann, I.Broser: Materials Science Forum, 1992, 83-87, 1247-52