Lone‐Pair Enhanced Birefringence in an Alkaline‐Earth Metal Tin(II) Phosphate BaSn 2 (PO 4 ) 2

: The first alkaline-earth metal tin(II) phosphate, BaSn2 (PO4 )2 , has been discovered, which consists of layered structures constructed from strictly alternating [SnO3 ]4- and [PO4 ]3- moieties. This compound is expected to have a large birefringence with Δn≈0.071 at 1064 nm, owing to the presence of stereochemically active lone pair metal cations.