# Mössbauer Spectroscopy on Antimony Borosulfates Reveals Weak Coordination Behavior

## TL;DR

This paper uses Mössbauer spectroscopy to study antimony borosulfates and discovers unique coordination behavior and new crystal structures.

## Contribution

The study reveals record-low Mössbauer shifts in antimony borosulfates and identifies a new structure type with unique crystal chemistry.

## Key findings

- 121Sb Mössbauer spectra show unprecedented negative isomer shifts of nearly −22 mm·s−1.
- A new one-dimensional borosulfate structure with B─O─B bridges was discovered and crystallizes in space group Pnma.
- Sb(NH4)[B(SO4)2]4 exhibits a non-linear SHG response comparable to KDP.

## Abstract

With MIIIMI[B(SO4)2]4 (MIII = Bi3+, Sb3+, Lu3+; MI = H3O+, NO2
+, Li+, Na+, K+, Rb+, Cs+), we recently described the first modular system within borosulfate chemistry comprising a three‐dimensional anion. Herein, we shed light on the respective series of antimony compounds SbX[B(SO4)2]4 (X = Li+, Na+, K+, Rb+, Cs+, Ag+, Tl+, NO+, NH4
+). While maintaining the same anionic topology, the compounds crystallize in the space groups I
4¯ (no. 82), P
4¯ (no. 81), and C2 (no. 5) and are strongly influenced by the lone pair of antimony as well as the size of the monovalent cations. In the course of this investigation SbX[B4O2(SO4)6] (X = Li+, Na+) were discovered. This borosulfate with a one‐dimensional anion comprising B─O─B bridges crystallizes in the space group Pnma (no. 62) and features a new structure type. 121Sb Mössbauer spectra revealed negative isomer shifts of almost −22 mm·s−1 not observed before and hinting towards a very weak coordination behavior of the borosulfate anion. The spectra are confirmed by DFT calculations. Furthermore, single crystal X‐ray diffraction, infrared spectroscopy, thermal analysis, and temperature programmed X‐ray diffraction experiments were carried out.

The weakly coordinated trivalent antimony atoms show record‐breaking low chemical Mössbauer shifts, and in combination with several monovalent cations including silver, nitrosonium, thallium cations a rich crystal chemistry of closely related but different crystal structures. Sb(NH4)[B(SO4)2]4 showed a non‐linear SHG response comparable to that of KDP. Finally, the thermal decomposition beyond 180 °C is discussed.