The electronic band-structure and band-gap dependence on the d character of A 2+ cation in AWO 4 wolframitetype oxides is investigated for different compounds (A = Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled d levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled d electron shells, give rise to direct and wide band gaps, Mn, with a half-filled d shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO 4 , ZnWO 4 , and CdWO 4 blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO 4 red-shifts at −22 meV/GPa. Finally, in MnWO 4 , absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO 6 complexes and how it affects their band-gap closure.

Zinc molybdate (ZnMoO 4 ) single crystals were grown for the first time by the Czochralski method and their luminescence was measured under X ray excitation in the temperature range 85-400 K. Properties of ZnMoO 4 crystal as cryogenic low temperature scintillator were checked for the first time. Radioactive contamination of the ZnMoO 4 crystal was estimated as ≤ 0.3 mBq/kg ( 228 Th) and 8 mBq/kg ( 226 Ra). Thanks to the simultaneous measurement of the scintillation light and the phonon signal, the α particles can be discriminated from the γ/β interactions, making this compound extremely promising for the search of neutrinoless Double Beta Decay of 100 Mo. We also report on the ability to discriminate the α-induced background without the light measurement, thanks to a different shape of the thermal signal that characterizes γ/β and α particle interactions.

The radioactive contamination of ZnWO 4 crystal scintillators has been measured deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN in Italy with a total exposure 3197 kg × h. Monte Carlo simulation, time-amplitude and pulse-shape analyses of the data have been applied to estimate the radioactive contamination of the ZnWO 4 samples. One of the ZnWO 4 crystals has also been tested by ultra-low background γ spectrometry. The radioactive contaminations of the ZnWO 4 samples do not exceed 0.002 -0.8 mBq/kg (depending on the radionuclide), the total α activity is in the range: 0.2 − 2 mBq/kg. Particular radioactivity, β active 65 Zn and α active 180 W, has been detected. The effect of the re-crystallization on the radiopurity of the ZnWO 4 crystal has been studied. The radioactive contamination of samples of the ceramic details of the set-ups used in the crystals growth has been checked by low background γ spectrometry. A project scheme on further improvement of the radiopurity level of the ZnWO 4 crystal scintillators is briefly addressed.

GPa in MgWO 4 . The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO 4 . We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low-and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO 4 , MnWO 4 , FeWO 4 , CoWO 4 , NiWO 4 , ZnWO 4 , and CdWO 4 .