Our contribution adresses important features of the emerging compound class of silicate-analogous borosulfates, i.e. their rich crystal chemistry, their exciting optical properties and of course their syntheses – the chemistry behind. Silicate-analogous materials comprise tetrahedral anionic basic building units lacking an inversion centre enhancing the chance of non-centrosymmetric surroundings of metal ions promoting excellent optical properties. Since the very first characterization of crystalline borosulfates in 2012 over sixty members have been found. Therein, the reaction of boric and sulfuric acid yields supertetrahedral BX4 (X=SO4) moieties giving rise to a rich crystal chemistry from non-condensed [B(SO4)4]5– anions via band (see Fig.) and layer structures to anionic frameworks [B(SO4)2]– – which can be understood by principles well known from silicates (see Fig.). The selective synthesis of borosulfates can be challenging but we meanwhile found some basic principles helping to selectively synthesize new compounds as phase-pure samples. Great impact is ascribed to the nature of the boron source, the metal (salt) employed and the amount of oleum added. On one hand, borosulfates feature a low coordination strength which is beneficial for the luminescence and UV-Vis properties of compounds containing lanthanide and transition metal ions, such as Ce3+ (see Fig.), Eu3+, Tb3+ or Co2+ and Ni2+. On the other hand, borosulfates frequently adopt non-centrosysmmetric structures yielding optical properties like SHG (second harmonic generation) which – in combination with large band-gaps – makes them highly promising materials for frequency doubling in the high energy regime. Also ionic conductivity was observed recently.

Silicate-analogous borosulfates featuring promising luminescence and frequency-doubling (SHG) properties based on a rich crystal chemistry

ACS Spring Meeting 2021

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