Bonding in the Doubly Disordered Ba1-xSnxCl1+yF1-ySolid Solution

Abstract

New materials were prepared in the SnF2/BaCl2 system by precipitation, and in the SnF2/BaCl2/BaF2 system by direct reactions at high temperature in dry conditions. Stoichiometric BaSn2Cl2F4 and BaSnClF3⋅0.8H2O, and a wide Ba1−x Sn x Cl1+y F1−y solid solution were prepared for the first time. Elemental analysis, X-ray diffraction and 119Sn Mossbauer spectroscopy were used for the characterization and study of bonding in the new materials. Mossbauer spectroscopy was shown to be an excellent method for probing both the type of bonding at tin(II) (ionic or covalent) and the bond strength at the tin sublattice. Tin(II) is covalently bonded in the stoichiometric phases and ionic (Sn2+ stannous ion) in the precipitated Ba1−x Sn x Cl1+y F1−y solid solution. The case of Ba1−x Sn x Cl1+y F1−y prepared in dry conditions is more complex. At negative y values (Cl: F 1) and more particularly at low x (poor in tin), all the tin(II) is in the ionic form. Furthermore, at high x and high y, the tin(II) sublattice strength decreases so drastically that the tin recoil free fraction at ambient temperature is nearly zero. The bonding type and tin sublattice strength can be explained in terms of preference of covalent bonding with F and when tin clustering occurs, whereas an excess Cl around Sn(II) forms ionic bonding and tin rattling due to ionic size mismatch.