New Light on an Old Story: The Crystal Structure of Boron Tetrathiophosphate Revisited

The crystal structure of boron tetrathiophosphate BPS ₄ was reinvestigated using single-crystal X-ray diffraction. The structure shows unidimensional chains similar to SiS ₂ as structural motifs. BPS ₄ crystallizes in the orthorhombic space group Ibam (no. 72), with a = 5.6173(3), b = 8.9929(4), c = 5.2433(3) Å and V = 264.87(2) Å ³ and is closely related to the orthorhombic high-temperature modification of AlPS ₄ (ht-AlPS ₄ ). The SiS ₂ -like chains are all oriented parallel to the c axis, which explains the needle-like morphology and that the crystals easily cleave to thinner needles under mechanical stress. While ht-AlPS ₄ shows an ordered Al-P sublattice, the B-P sublattice in BPS ₄ is substantially disordered. The thermal properties of BPS ₄ were investigated by DTA measurements and a detailed analysis of the Raman spectrum assisted by quantum mechanical calculations is presented. The crystallographic disorder in BPS ₄ is the result of inter-chain disorder, whereas local intra-chain ordering of B and P can be concluded from Raman spectroscopy.