The Chemistry Of Two-Component Fluoride Crystalline Optical Media For Heavy, Fast, Radiation Hard Scintillators
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Fig. 1. shows 55 metals which yield fluorine containing compounds. 27 elements satisfy most fully the above formulated conditions. They are shown in grey. Other 7 metals (darker colouring) partially satisfy these requirements. A total of 351 binary systems can be formed on the basis of the 27 metal fluorides, and a total of 561 - of 34. Up to now, phase diagrams for 300 systems have been obtained. We performed most comprehensive studies of the first group of the systems that comprises 27 members, 85% of the phase diagrams have been obtained by our team. In such a brief report is impossible to provide even the most general conclusions made as a result of long-term studies of the high-temperature chemistry of these systems, which serves as the basis for preparation of multicomponent fluoride materials. The main 18 types of the phase diagrams that describe phase formation in most systems, are shown in Fig. 2. More detailed data on the phase diagrams are reported in reviews [1 - 4]. Here we shall consider briefly possibilities of preparation of two-component fluoride optical media which can be used for elaboration of heavy scintillators. Three types of phases are formed in the the MFm - RFn systems: - binary pMFmxqRFncompounds, isovalent MixM'xFm solid solutions and heterovalent M 1_ xRxFm(lx)+nx solid solutions. Our estimations of the number of two-component phases of each type are as follows. In the studied systems about 80 binary compounds are in equilibrium with the melt (provided that they melt congruently and exhibit no polymorphism), as well as about 50 isovalent solid solutions and 300 heterovalent ones. However, not all of these 430 phases are adequate for high energy physics (HEPh). At least ten parameters are needed for a comprehensive assessment of a scintillator for HEPh. Here we can make the choice proceeding from such a universal and conservative (i.e., independent of uncontrolled conditions) parameter as density. Let us agree that for HEPh the lower threshold of crystal density should be 5.5 - 6.5 g/cm . From the most popular scintillators only densities of BaF 2 and
NaI:Tl are below this threshold. This constraint reduces the number of binary chemical compounds by a factor of 1/2, and to a large extent - the number of solid solutions of both types. A detailed analysis of the constraints for twocomponent crystals in their densities for each system is made in [5, 6]. Results of studies of phase diagrams of MFm - RFn systems and the phases formed in them suggest the following main conclusions, which are of immediate interest for search for dense optical media: - nonstoichiometric phases are predominant among products of chemical interactions of 34 fluorides (Li, Na, K, Rb, Cs; Mg, Ca, Sr, Ba, Zn, Cd, Pb; Sc, Y, La and 13 lanthanides, In, Bi; Zr, Hf, Th, U) at high temperatures. Isovalent solid solutions and binary compounds are the minority; - nonstoichiometric phases crystallize
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