Abstract
At temperatures below 50 K magnesium chloride crystals simultaneously doped with Ti 2+ and V 2+ show broad-band luminescence from V 2+ ions and sharp-line luminescence from Ti 2+ ions and mixed Ti 2+V 2+ clusters in the near infrared. In addition to the known Ti 2+ and V 2+ absorption bands a double excitation of a Ti 2+ and a neighbouring V 2+ ion was observed in the green spectral range. Using tunable laser excitation in the range of 19 250–19 400 cm -1, exchange-coupled Ti 2+−V 2+ dimers, Ti 2+−V 2+−V 2+ trimer chains and Ti 2+Ti 2+V 2+ triangular trimers were studied selectively. From energy differences of luminescence lines the following parameters for antiferromagnetic exchange interactions were derived (using the bilinear Heisenberg operator H ̂ ex = -2J AB S A ·S A ): 2J TiV = -67 cm -1 in Ti 2+−V 2+ dimers, 2 J VV = -41 cm -1 in Ti 2+−V 2+−V 2+ chains and 2 J TiTi = -101 cm -1 in Ti 2+Ti 2+V 2+ triangles. Ti 2+−Ti 2+ dimers in MgCl 2 crystals containing only Ti 2+ ions can be excited selectively at 24 040 cm -1. For these species an exchange parameter of 2 J TiTi = -109 cm -1 was found.
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