THALLIUM (I) ORTHOFLUORBERILLATE González-Silgo, C.a, Ruiz-Pérez, C., Martínez-Sarrin, M. L.b, Mestres L., Solans, X.c, aDpto. Fisica Fundamental y Experimental, Universidad de La Laguna. Spain, bDpto. Quimica Inorgnica, cDepto. Cristallogafia, Mineralogia y Deposits Minerals Universitat de Barcelona. Spain.
(NH4)2BeF4 and Tl2BeF4, with similar ion radii from Rb+, are isoestructural in Pnam space group. The first shows undergoes two phase transition at lower temperature and crystal structure of Tl2BeF4 isnt reported, but it isnt knowed phase transition. Otherwise, Thallium oxisalts, some of them show structural instabilities, has been lately studied and may be inferred two types of cation with 11 and 9 surrounding O atoms and the importance of the shortest bonds. The structural determination is important to access the role of the atoms placed in large cavities like a particular result of the general rule pointed out I.D.Brown (1992). Tl2BeF4 was obtained via reaction BeF2+ +H2F2+Tl2CO3=Tl2BeF4+CO2+H2O. Mixed crystal were obtaining mixing aqueous solution of (NH4)2BeF4 and Tl2BeF4 thermostating at 30C. We have determined the structure of Tl2BeF4 and Tl1.84(NH4)0.16BeF4: orthorhombic, Pnam, Z=4, a=7.679(2), 7.689(2), b=10.383(3), 10.398(3), and c=5.871(2), 5.886(2). We have found differences between the crystals at room temperature: 1st) In Tl2BeF4, a BeF4 group deformation mainly along b direction and one Be-F bond is near to parallel a axis. 2nd) The arrangement of 11-coordinated cation is similar in both compounds, but the linkage of 9-coorditated cation indicates this (NH4)+ ion plays the most important role in (NH4)2BeF4 phase transition. 3rd) Bond-Valence model predicts structural instabilities in (NH4)2BeF4 while Tl2BeF4 is more stable.
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