Lanthanide orthothiophosphates revisited: single-crystal X-ray, Raman, and DFT studies of TmPS4 and YbPS4

The crystal structures of the lanthanide orthothiophosphates LnPS4 (Ln=lanthanide) have been extensively investigated in the past. Up to now, however, single crystals of two members of this series – TmPS4 and YbPS4 – have not been available. Here, we report a modified synthesis protocol for TmPS4 and YbPS4 yielding single crystals suitable for X-ray diffraction. Both compounds crystallize in the tetragonal space group I41/acd (no. 142) with 16 formula units per unit cell and adopt the SmPS4 parent structure, like most reported lanthanide orthothiophosphates. The structures contain isolated [PS4]3− tetrahedra and two crystallographically independent Ln 3+ cations, which form trigonal-dodecahedral [LnS8]13− polyhedra. The lattice parameters for TmPS4 are a = 10.598(2), c = 18.877(4) Å with V = 2120.2(6) Å3, and for YbPS4 a = 10.577(2), c = 18.827(4) Å with V = 2106.2(7) Å3. The DFT-calculated electronic band structures indicate semiconducting behavior and reveal indirect band gaps of 2.1–2.2 eV, consistent with the reddish brown color of YbPS4, but underestimating the band gap of pale-yellow TmPS4. The Raman spectra are dominated by [PS4]3− vibrations as confirmed by DFT-calculated phonon spectra. DTA measurements reveal remarkably high thermal stability compared to other known orthothiophosphate compounds.