A ¹¹⁹Sn Mössbauer-spectroscopic characterization of the diamagnetic birefringence material Sn₂B₅O₉Cl

1 Introduction

¹¹⁹Sn Mössbauer spectroscopy is an excellent tool for the study of tin oxidation states and for analyses of the local electronic structures and the chemical bonding peculiarities of solids. The important experimental parameters are the isomer shift, relating variations of the electron density at the tin nuclei and the electric quadrupole splitting, monitoring the deformation of the electron density. The most prominent examples are the tin oxides Sn^IIO (SnO₄ square pyramids and hence a pronounced stereochemical lone-pair activity) and Sn^IVO₂ (slightly distorted SnO₆ octahedra). The lower valence of the tin atom and the anisotropy of the coordination sphere of SnO lead to an isomer shift around 2.7 mm s⁻¹ and substantial quadrupole splitting of 1.4 mm s⁻¹. SnO₂ samples show an isomer shift around 0 mm s⁻¹, similar to that of the usually used Ca^119mSnO₃ or Ba^119mSnO₃ sources, and the only slight distortion of the octahedra induces a much weaker quadrupole splitting of 0.5 mm s⁻¹. These values have repeatedly been reported for diverse SnO and SnO₂ samples [1], [2], [3], [4].

Thus, divalent and tetravalent tin in solids can safely be distinguished on the basis of ¹¹⁹Sn Mössbauer-spectroscopic data. Nevertheless, also within a family of Sn^II and Sn^IV compounds, small variations of the local electron density and the tin coordination environment can be monitored. This has competently been summarized for a large number of tin chalcogenides by Lippens [2] and for a huge number of organotin compounds by Zuckerman [5].

Especially the Sn^II compounds show peculiar behavior. To give an example, while SnO with its SnO₄ square pyramids shows pronounced quadrupole splitting, SnTe with the rocksalt structure shows no quadrupole splitting at all [6], [7], [8]. The gradual change of the quadrupole splitting parameter from SnO to SnTe and the particular isomer shift of the oxide have been studied by ¹¹⁹Sn Mössbauer spectroscopy by Lefebvre et al. [8] and were discussed on the basis of DFT calculations.

The quadrupole splitting parameters (ΔE_Q) of Sn^II compounds cover a much broader range than observed for the SnO₄ square pyramids in SnO. Our recent studies on the arene-stabilized monocation [Sn^II{N(SiMe₃)(Ar)}]⁺ [Al(OC₄F₉)₄]⁻ [9] revealed ΔE_Q=3.69 mm s⁻¹. Many organotin carboxylates [5] show even values >4 mm s⁻¹.

We have now probed the resolution of ¹¹⁹Sn Mössbauer spectroscopy with respect to Sn^II borates. β-SnB₄O₇ [10] contains one crystallographically independent tin site which shows an isomer shift of 4.14 mm s⁻¹. The SnO₁₀ polyhedron shows a distortion, accounting for the lone pair effect. This leads to a weak quadrupole splitting parameter of 0.78 mm s⁻¹. Herein we report on the ¹¹⁹Sn Mössbauer-spectroscopic characterization of the recently reported borate chloride Sn₂B₅O₉Cl [11], which contains two different SnO₇Cl₂ polyhedra, favorable to enhance the optical performance via cation substitution [12].

2 Experimental

3 Crystal chemistry

Sn₂B₅O₉Cl [11] crystallizes with the non-centrosymmetric Ca₂B₅O₉Br-type structure [17], space group Pnn2. The borate substructure consists of the fundamental building unit (FBU) [B₅O₁₂]⁹⁻, a condensation of three BO₄ tetrahedra and two BO₃ triangles. For a detailed crystal chemical discussion of the many isotypic compounds we refer to the relevant original work [11], [17], [18], [19]. Herein we redraw only to the coordination of the tin cations that is relevant for the interpretation of the ¹¹⁹Sn Mössbauer spectra.

The Sn₂B₅O₉Cl structure contains two crystallographically independent tin sites which both have site symmetry 1. The coordination environment of both tin sites is shown in Fig. 1. The distances to the seven nearest oxygen neighbors of each tin cation cover a broad range: 227.7–309.2 pm for Sn1 and 226.9–337.3 pm for Sn2. Three of the seven oxygen neighbors at each tin site show distinctly longer Sn–O distances. These bonds are drawn in violet color in Fig. 1. This coordination asymmetry is in line with the lone-pair activity and approximately corresponds with the cutouts presented in Fig. 2 resulting from the electron localization function (ELF) by the first principles calculations (for details see [11]). We draw back to this asymmetry when discussing the quadruple splitting parameters (vide infra).

Fig. 1:

The coordination environment of the tin atoms in Sn₂B₅O₉Cl. Tin, oxygen and chlorine atoms are drawn as light grey, red and green circles, respectively. Atom designations and bond distances are given. For details see text.

Fig. 2:

Cutouts of the tin coordination spheres in Sn₂B₅O₉Cl. The lone pairs around the Sn1 and Sn2 ions in Sn₂B₅O₉Cl are visualized by the electron localization function (ELF).

The pronounced lone-pair character is also expressed in the lattice parameters. The A₂B₅O₉Cl chloride series exist for A=Ca, Sr, Ba, Eu, Pb [19] and Sn. Of all chlorides, Sn₂B₅O₉Cl has the largest c lattice parameters, even larger than that of Pb₂B₅O₉Cl. This is consistent with the longer Sn–O distances emphasized in Fig. 1. Their resultant, and consequently the lone pairs, point in c direction.

The local electron density at the tin nuclei is the second important parameter. In Table 1 we present bond valence sums for Sn₂B₅O₉Cl calculated with the bond length/bond strength (ΣV) [20], [21] and Chardi concepts (ΣQ) [22], [23]. The computed charges show good agreement with the ideal model; however, both calculations consistently show a distinctly lower charge for the Sn1 cations.

4 Magnetic properties

The temperature dependence of the magnetic susceptibility, determined at an applied external field of 10 kOe is presented in Fig. 3. The susceptibility is negative and almost independent of temperature above ca. 150 K, indicating diamagnetism. The upturn in the low temperature regime results from tiny amounts of paramagnetic impurities (Curie tail). The room temperature value of −80×10⁻⁶ emu mol⁻¹ is in the same order of magnitude as the sum of the diamagnetic increments of (in units of 10⁻⁶ emu mol⁻¹) 2×−20 for Sn²⁺, 5×−0.2 for B³⁺, 9×−12 for O²⁻ and −23.4 for Cl⁻, resulting in a value of −172.4×10⁻⁶ emu mol⁻¹ [24]. The smaller experimental value is attributed to (i) the by-product SnO₂ which is a weaker diamagnet (the sum of the diamagnetic increments is only −44×10⁻⁶ emu mol⁻¹) and (ii) minor paramagnetic impurities.

Fig. 3:

Temperature dependence of the magnetic susceptibility of the Sn₂B₅O₉Cl sample measured at 10 kOe. The red line serves as a guideline to the eye.

5 ¹¹⁹Sn Mössbauer spectroscopy

The experimental and simulated ¹¹⁹Sn Mössbauer spectrum of the Sn₂B₅O₉Cl sample is presented in Fig. 4 along with a transmission integral fit. The corresponding fitting parameters are summarized in Table 2. The total counting time of 4 days led to a well-resolved spectrum and all parameters could be refined independently without any constraint. The spectrum shows two main signals. The one at −0.020 mm s⁻¹ corresponds to the by-product SnO₂, which occured in the products of all synthesis attempts [11]. The weak quadrupole splitting parameter of this sub-signal results from the slightly distorted octahedral coordination in rutile-type SnO₂. Both, the isomer shift and the quadrupole splitting parameter are in good agreement with literature data [2], [4], [25]. The slightly broadened line can be attributed to the amorphous SnO₂ content, as also observed for SnO₂ nanoparticles [14], [26].

Fig. 4:

Experimental (data points) and simulated (colored lines) ¹¹⁹Sn Mössbauer spectrum of a Sn₂B₅O₉Cl sample at room temperature. The sub-signal emphasized with a blue line corresponds to the by-product SnO₂.

The second sub-signal occurs at a higher isomer shift. This spectral contribution is much broader than the SnO₂ signal and was well reproduced with a superposition of two quadrupole split signals (with fixed 1:1 contribution) at isomer shifts of 3.887(8) and 4.137(7) mm s⁻¹, clearly indicating divalent tin. These isomer shifts lie in between the values for SnO (2.7 mm s⁻¹) and SnCl₂ (4.1 mm s⁻¹) [2]. In ¹¹⁹Sn Mössbauer spectroscopy, an increase of the isomer shift goes along with an increasing s electron density at the tin nuclei [2]. Thus, on the basis of the bond valence calculations (vide ultra), we can clearly assign the tin sub-spectra to the individual tin sites. The Sn1 atoms with the lower calculated charge has the higher isomer shift. In view of the low site symmetry and the similar distortions, both tin sites show very similar quadrupole splitting parameters.