A cubic and an axial Fe 2+ centre, Fe 2+ Ta and Fe 2+ Ta -V O , are found in reduced Fe-doped KTaO 3 crystals in conjunction with their trivalent partners. These four defects are characterized by optically detected magnetic resonance (ODMR) via the magnetic circular dichroism (MCD) of the absorption. The g-values are determined by and interpreted using crystal-field terms. For all centres, the optical absorption bands are identified by the tagged-MCD method. The main MCD bands of the divalent ions are attributed to the well known 5 T 2 → 5 E crystal-field transition in the 10 000 cm −1 region. Bands at higher energies are interpreted as intervalence transfer transitions to conduction band-like states.

In photorefractive Pb 5 Ge 3 O 11 and (Pb 0.98 Ba 0.02 ) 5 Ge 3 O 11 crystals, Pb 2+ host lattice ions can be recharged by visible light to Pb 3+ . The magnetic and structural properties of this hole-trapping centre, metastable at low temperatures, are characterized by means of electron paramagnetic resonance. The axial gparameters are found to be g ⊥ = 2.0005(4) and g = g ⊥ + 0.0064(3) and the 207 Pb hyperfine interaction constants are A ⊥ = 34.77(5) and A = 34.64(5) GHz. This information is correlated with optical properties derived from optically detected magnetic resonance via the magnetic circular dichroism of the absorption. In this way, broad photochromic absorption bands extending from the band edge to 1.9 eV can be attributed to the Pb 3+ centre. Several observations give evidence that the Pb 3+ centre is involved in the holographic recharging effects observed in lead germanate.

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