We report on the luminescence of doped and undoped PbMoO., and PbW04 single crystals; a red emission is found for the first time for PbMoO+ The degree of polarization of both the emissions of PbW04 and of the shorter-wavelength emission of PbMo04 is measured. The shorter-wavelength emissions are assigned to the 3Tl-'A, transition in the tetrahedral MOO 4'-(WO4*-) group. It is assumed that the 3 TV level is split due to spin-orbit coupling. The longer-wavelength emissions are assigned to a transition in a molybdate (tungstate) group lacking an oxygen ion, i.e., a Mo03(W03) group.

The luminescence properties of calcium molybdate are reported. Two emission bands are observed, one in the green and another in the orange spectral region. The shorter wavelength emission band is assigned to the ,T1 + lA1 transition in the tetrahedral molybdate group. The longer wavelength band is ascribed to a similar transition in a molybdate group lacking an oxygen ion, i.e. a MOO, group. Similar observations are made in the case of calcium tungstate. Es werden die Lumineszenzeigenschaften von Kalziummolybdat mitgeteilt. Zwei Emissionsbanden werden beobachtet, eine im grunen, die andere im orangen Spektralbereich. Die Emissionsbande mit der kurzeren Wellenllnge wird dem 3T, -lA,-Ubergang in der tetraedrisfhen Molybdatgruppe zugeordnet. Die Bande mit der liingeren Wellenlange wird einem ahnlichen Tfbergang in einer Molybdatgruppe, in der ein Sauerstoffion fehlt, d. h. einer Moo,-Gruppe, zugeschrieben. Ahnliche Beobachtungen werden fur den Fall des Kalziumwolframats gemacht. ExperimentalSamples of doped and undoped CaMoO, were prepared by firing intimate mixtures of CaCO,, MOO,, Na,MoO,, and Y,O, for 4 h a t 850 "C in air, followed by milling and firing for 4 h a t 1000 "C in air. The compositions of the starting mixtures for the Ya n d Na-doped samples were CaCO, + (1 + 6) MOO, + 8 Y,O, (6 = 0.0025) and CaCO, + (1 + 6) MOO, + 6 Na,MoO, (6 = 0.0025), respectively. Samples of doped l) P.O. Box 80000, 3508 TA Utrecht, The Netherlands. 330J. A. GROENINK, C. HAKFOORT, and G. BLASE and undoped CaWO, were prepared by firing intimate mixtures of CaCO,, WO,, Na,CO,, and Y,O, for 12 h a t 950 "C in air, followed by milling and firing for 12 h at 1000 "C in air. Starting mixtures for the Yand Na-doped samples were CaCO, +

The ionic and electronic conductivities of Czochralski-grown single crystals of PbMo04 and PbWO, are reported and discussed. Nominally pure crystals, as well as crystals with various aliovalent dopants, were used. From the electrical measurements it is concluded that oxygen vacancies are responsible for the ionic conductivities in both compounds. At high temperatures electronic conductivity predominates. Measurements of the oxygen partial pressure dependence of the electronic conductivity reveal that hole conduction prevails in PbWO4, while mixed conduction by holes and electrons is present in PbMoO,.

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