Energy transfer in solid solutions Zn x Mg 1x WO 4 D. Spassky a,b,⇑ , S. Omelkov a , H. Mägi a , V. Mikhailin b,c , A. Vasil’ev b , N. Krutyak c , I. Tupitsyna d , A. Dubovik d , A. Yakubovskaya d , A. Belsky e a Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia b Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia Federation c Physics Department, Moscow State University, Leninskie Gory 119991, Russia Federation d Institute for Scintillation Materials, NAS of Ukraine, Kharkiv 61001, Ukraine e Institute of Light and Matter, CNRS, University Lyon1, Villeurbanne 69622, France article info Article history: Available online xxxx Keywords: Zinc tungstate Magnesium tungstate Solid solutions Luminescence Energy transfer Scintillating bolometers abstract It is shown that the light output of Zn x Mg 1x WO 4 solid solutions has a maximum at x = 0.5 under X-ray excitation. Excitation spectra of exciton emission under vacuum ultraviolet excitation also show the increase of the probability of exciton creation by the geminate e–h pairs for the intermediate values of x. Numerical simulation of the relaxation of hot electrons and holes demonstrates that the observed effects are due to the decrease of the mean distance between thermalized charge carriers. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction An enhancement of scintillation light yield can be achieved by the transition from pure compounds to solid solutions of inorganic insulators. This effect has been shown recently for several com- pounds, e.g. (Lu x Y 1x )AlO 3 :Ce [1],Y 3 (Al x Ga 1x ) 5 O 12 :Ce [2], (Lu x Gd 1 x ) 2 SiO 5 :Ce [3], (Lu x Sc 1x )BO 3 :Ce [4], (Lu x Gd 1x ) 3 (Al y Ga 1y ) 5 O 12 :Ce [5], BaBr 2x I x :Eu [6], (Lu x Y 1x )BO 3 :Eu [7], CsBrI [8], Lu 3 Al 5x Sc x O 12 [9]. One of the possible explanations of the observed enhancement is the decrease of the thermalization length of hot charge carriers at the stage of energy relaxation after the absorption of excitation quanta [1]. The decrease of thermalization length is supposed to be due to non-uniform distribution of substituted ions with the for- mation of clusters constructed from one of the component or due to the modulation of the bottom of conduction band and of the top of valence band by electronic states of randomly distributed substituted ions [1]. In both cases the increase of the efficiency of recombination and energy transfer to the emission centers can be achieved. Most of the solutions mentioned above are doped with rare earth ions. Rare-earth ions were often chosen to achieve high light yield and fast response. However the solid solutions with the dopant emission are not optimal for the study of the effect of thermalization length modification. The effect may be masked by the following effects, which also influence the efficiency of dopant emission in the solid solutions: (I) Location of the dopant energy levels within the forbidden gap depends on the concentration of one component x. For instance in Gd 3 (Al x Ga 1x ) 5 O 12 :Ce the lowest 5d state of Ce 3+ moves very close to the bottom of conduction band or even into the conduction band, when x value approaches 0 [10,11]. In this case the emission of Ce 3+ ion is quenched due to the thermal ionization of the emission center. (II) Dopants are sometimes incorporated inhomogeneously into solid solutions due to the mismatch in the ionic radii of dopant and substituted cations. For instance in case of (Lu x Y 1x )AlO 3 :Ce cerium ions (ionic radius of Ce 3+ = 1.143 Å) occupy rather Y 3+ (ionic radius = 1.02 Å) sites rather than Lu 3+ (ionic radius = 0.98 Å) sites. (III) Phase com- position might vary with the ratio of components of a solid solu- tion. For instance the second phase is found in (Lu x Y 1x )BO 3 :Eu for x > 0.5, and its presence substantially modifies the energy transfer processes [7]. In order to study modification of energy transfer processes in solid solutions exclusively as a function of thermalization length of charge carriers we have to avoid the influence of all mentioned effects and choose solid solutions with the following properties: (I) Emission centers should be of intrinsic origin, e.g. emission of self- trapped excitons (STE). In case of the STE emission the process of energy transfer to the emission centers implies the efficiency of exciton creation from separated electrons and holes. (II) The par- entage of the states responsible for the intrinsic emission should not be affected by the changing composition of the solid solution. (III) Solid solutions with a single phase should be selected. Solid solutions of Zn x Mg 1x WO 4 meet all of these requirements. ZnWO 4 0925-3467/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.12.039 ⇑ Corresponding author at: Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia. Tel.: +7 495 939 3169; fax: +7 495 939 2991. E-mail address: deris2002@mail.ru (D. Spassky). Optical Materials xxx (2014) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Please cite this article in press as: D. Spassky et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2013.12.039