¹¹⁹Sn Mössbauer spectroscopy of solvothermally synthesized fluorides ASnF₃ (A = Na, K, Rb, Cs)

1 Introduction

The complex fluorides ASnF₃ (A = Na, K, Rb, Cs) have repeatedly been studied with respect to their synthesis conditions, structures, and properties. Samples can be prepared by a careful aqueous route AF + SnF₂ → ASnF₃ [1, 2] or via different solvothermal routes [3, 4]. The main synthetic difficulty is to maintain tin in the oxidation state + 2.

The ASnF₃ fluorides show poor fluorine conductivity [1]. They can be used as fluorine precursors, e.g. KSnF₃ for the synthesis of highly fluorine-doped SnO₂ nanoparticles [5]. An interesting technical application concerns the use of KSnF₃ and CsSnF₃ as fluxing agent for soldering metal components [6, 7].

The common structural motif of the ASnF₃ fluorides is a distorted trigonal-pyramidal or square-pyramidal fluorine coordination of the tin atoms, enforced by the lone pair (Fig. 1). The arrangement of these pyramids depends on the size of the alkali cation. NaSnF₃ [3] and CsSnF₃ [4] contain isolated SnF₃^– species and two, respectively, one crystallographic tin sites. The monomeric units condense via common corners in the structures of KSnF₃ [1] and RbSnF₃ [4]. Both structures contain infinite chains of SnF_2/2F₂^– pyramids and four crystallographically independent sites.

The pronounced lone-pair character leads to a distinctly asymmetric electron density distribution around each tin atom, which can easily be monitored by ¹¹⁹Sn Mössbauer spectroscopy. Numerical data and some noisy ¹¹⁹Sn spectra of ASnF₃ fluorides prepared by the aqueous route were published [8–10] in the course of systematic studies of divalent tin complexes. In the meantime correct structural data were reported for the rubidium and cesium compounds [4] which allow the simulation of X-ray powder data for evaluation of the sample purity. Herein we report on the ¹¹⁹Sn Mössbauer spectroscopic data of the ASnF₃ fluorides prepared through the solvothermal route.

Fig. 1:

Fluorine coordination of the tin atoms in NaSnF₃ [3], KSnF₃ [1], RbSnF₃ [4], and CsSnF₃ [4]. Atom designations and Sn–F bond lengths (in units of pm) are given. Only the dinuclear units of the different infinite chains of corner-sharing distorted square pyramids for KSnF₃ and RbSnF₃ are shown.

2 Experimental

3 Results and discussion

The experimental and simulated ¹¹⁹Sn Mössbauer spectra of the ASnF₃ fluorides are presented in Fig. 2 along with transmission integral fits. The corresponding fitting parameters are listed in Table 1. All samples showed well resolved spectra with two spectral components. The minor contributions (5–9 %) around an isomer shift of 0 mm s^–1 correspond to Sn(IV) [12] decomposition/hydrolyses phases. These contributions were included as a simple Lorentzian into the fits.

Fig. 2:

Experimental and simulated ¹¹⁹Sn Mössbauer spectra of solvothermally prepared samples of ASnF₃ (A = Na, K, Rb, Cs).

Although the structures of NaSnF₃, KSnF₃ and RbSnF₃ contain two, respectively four crystallographically independent tin sites (Fig. 1), all spectra could be well reproduced with a single quadrupole split signal. The single signals could not be resolved. This indicates almost identical electronic situation of the tin atoms within the chains of corner-sharing SnF_2/2F₂^– square pyramids, the experimental spectra showing the envelope of the individual sub-spectra. This is expressed in a slightly increased line width parameter. CsSnF₃ with only one tin site shows the lowest value of experimental line widths.

Within the combined error limits, the isomer shift values and the quadrupole splitting parameters are almost equal. This is also in line with bond valence sum calculations [4]. The small trend in isomer shifts reported in [8] (where the origin and purity of the samples were not stated) is not observed for the solvothermally synthesized samples. The presently studied materials were pure on the level of X-ray powder diffraction.