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Caesium iodide is one of the more extensively studied scintillators. Here we present X-ray luminescence spectra, scintillation light output and decay curves as function of temperature, from room temperature down to below 10 K. Features of the observed intrinsic luminescence are explained in terms of radiative recombination of on-and off-centre STE. A model permitting interpretation of the dynamics of luminescence changes in CsI with temperature is suggested. This model includes adiabatic potential energy surfaces (APES) associated with singlet and triplet states of self-trapped excitons (STE) and explains the variation of the luminescence spectra with temperature as a result of re-distribution in the population between on-and off-centre STE. The temperature dependence of the scintillation light yield is discussed in the framework of the Onsager mechanism.

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.

The absorption spectra of the [NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 crystals grown from a solution in ethyl alcohol and also of solid solutions of [NH 2 (C 2 H 5 ) 2 ] 2 Zn 0.5 Cu 0.5 Cl 4 in the visible spectral region have been investigated in the vicinity of thermochromic phase transitions. It is shown that the phenomenon of thermochromism in these crystalline compounds is primarily related to the replacement of the square-planar geometry of the coordination surrounding of the Cu 2+ ion by a tetrahedral one.Introduction. Thermochromic crystals have attracted the attention of researchers in connection with their possible use as sensitive materials of sensor technology and also in recording optical information. The compounds of the group A 2 MeCl 4 and AMeCl 3 (Me = Cu, Co) [1-5] with an alkylammonium cation, to which the crystals of diethylaminetetrachlorocuprate -[NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 (DEACC) -are related, constitute a new class of thermochromic materials. According to [1], these crystals at T t = 323 K undergo a thermochromic phase transition of the first kind accompanied by a sharp change in the color of a sample from green to yellow in the regime of heating. The hydrogen bonds that influence the degree of deformation of the metal-haloid polyhedra play an important role in the phenomena of thermochromism, which, in turn, is manifested in the change in the absorption spectra.At room temperature, the [NH 2 (C 2 H 5 ) 2 ] 2 CuCl 4 crystals grown from a solution in ethyl alcohol (from now on, DEACC-I) belongs to the space group P2 1 ⁄ n. The parameters of the elementary cell are: a = 7.362 A°, b = 15.025 A°, c = 45.193 A°, β = 89.94 o , and Z = 12. One asymmetric structural group includes three tetrahedrally and nine octahedrally coordinated ions [CuCl 4 ] 2− in an abnormally large cell. All DEA + cations participate in the formation of the two-dimensional network of strong hydrogen bonds N-H⋅⋅⋅Cl parallel to the plane (bc). The high-temperature phase is also monoclinic with the space group P2 1 ⁄ c (a = 25.055 A°, b = 10.531 A°, c = 15.455 A°, β = 100.63 o , and Z = 8) [1].The temperature of the thermochromic phase transition in these crystals is greatly determined by the conditions of their growing. For the crystals grown from an aqueous solution (from now on, DEACC-II) it was equal to T t = 311 K (in the regime of heating) [2,4].In view of the thermochromic character of the indicated phase transition, absorption spectroscopy has turned out to be an effective tool for studying its nature [4]. The studies of DEACC-II crystals have shown that a phase transition is associated with changes in the coordination surrounding of the copper ion. Here, just as in the case of DEACC-I, the high-temperature phase is characterized by a distorted tetrahedral position of the surrounding of the copper ion, whereas the low-temperature one is characterized by the co-existence of the anions of the plane quadratic and tetrahedral forms (each surrounding of the copper ion yields two absorption bands). A sharp change in the config...

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