Structure, luminescence and scintillation properties of the MgWO₄–MgMoO₄system

Abstract

The importance of luminescent tungstates and molybdates in several technological applications motivated the study of the structural, luminescence and scintillation properties of the MgWO4–MgMoO4 system. X-ray diffraction studies allowed the identification of three main types of structures in the pseudo-binary MgWO4–MgMoO4 system (sanmartinite β-MgMoO4, cuprosheelite α-MgMoO4, and wolframite MgWO4) and the refinement of the parameters of the crystal lattice. It is found that the single-phase solid solution MgMo1−xWxO4 with a β-MgMoO4 structure is created only at x<0.10, while for a higher tungsten content a mixture of different phases is formed. The x-ray luminescence spectra of a series of samples of the MgWO4–MgMoO4 system are measured at T = 8 K. The principal emission bands are assigned to the main structural phases as follows: β-MgMoO4, 520 nm; α-MgMoO4, 590 nm; MgWO4 (wolframite), 480 nm. The phase composition of the sample determines the actual shape of the observed spectra. Possible relations between the crystal structure and luminescence properties of different phases are discussed in terms of a configuration coordinate model. Of all the compounds under test, MgWO4 is found to have the best scintillation response for particle excitation (0.90 ± 0.15 that of ZnWO4 at T = 295 K). Further, the light yield also remains high with decreasing temperature, which makes this material potentially useful for cryogenic applications.

Luminescent properties of MgWO<inf>4</inf> crystals

“…1. The luminescence of MgWO 4 , excited in the region of direct exciton creation (4.1 eV), is characterized by the single intense wide-band emission peaking at 2.45 eV and with full width of half maximum 0.7 eV. The luminescence is of intrinsic origin and is usually ascribed to the radiative annihilation of excitons [6].…”

“…2, inset (a). It implies self-trapping of the excitons in MgWO 4 since the values below unity are inherent for the self-trapping. Luminescence excitation spectrum of MgWO 4 (Kh) in the fundamental absorption region is presented in Fig.…”

“…It demonstrates a broad intensive luminescence band in the blue spectral region (470 nm) that is not quenched up to the room temperatures [2]. Recently this material also attracted attention of researchers due to prospects of application as a scintillator in rare event search [3,4]. Magnesium tungstate is of particular interest for such application due to the advantageous combination of heavy and light elements in its composition [3].…”

“…Only recently magnesium tungstate crystals of ~1 cm 3 volume were obtained for the first time by the flux growth method [5]. The performance of MgWO 4 as a scintillator has been demonstrated in [3], however the photoelectron output from MgWO 4 was found to be only about 35% that of CdWO 4 . Relatively low output may not be due to the potential worse quality of the crystal but have physical background.…”

“…Magnesium tungstate has at least one feature, which distinguish it from other representatives of wolframite group (CdWO 4 and ZnWO 4 ). Absence of d states of Mg (that are initially vacant) in the lower part of the valence band may lead to the changes in the energy conversion to luminescence quanta in MgWO 4 . So, the aim of this work is to study the luminescent properties of MgWO 4 and energy transfer mechanisms to the emission centers taking into account this feature of the band structure of MgWO 4 crystals.…”