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

Abstract

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 Ln3+ 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 YbPS4a = 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.

Two Hitherto Unknown Lithium Holmium(III) Ortho‐Thiophosphates(V): Li9Ho2[PS4]5 and Li15Ho7[PS4]12

“…Similar, yet weaker effects were observed for Li 15 Ho 7 [PS 4 ] 12 too, but the vibrational modes originating from the isolated [PS 4 ] 3À tetrahedra could be detected in the spectrum. The vibrations observed are in accordance with literature-known [20] modes for [PS 4 ] 3À tetrahedra. The sharpest intense band at 420 cm À 1 results from ν 1 symmetric stretching modes, whereas the modes between 500 and 600 cm À 1 are caused by ν 3 asymmetric stretching vibrations.…”

“…The sulfur atoms are 281 to 289 pm away from the Ho 3 + cations (Table 3), common for many mid-size RE 3 + cations in rare-earth metal thiophosphates. [20,24] The [HoS 8 ] 13À polyhedra are decorated with four edge-attached [PS 4 ] 3À tetrahedra (Figure 2) with only slightly distorted tetrahedral shape (Table 3 and 4). The bond lengths found within these tetrahedra agree quite well with other ortho-thiophosphates (d(PÀ S) = 202-205 pm).…”

“…[6,7,9] Li 3 [PS 4 ] [10,11,12] and Na 3 [PS 4 ] [13] or even the europium(II)-containing LiEu[PS 4 ] [14] are also examples of thiophosphate-based alkali-metal cation conductors. Moreover, the lithium-containing rare-earth metal(III) ortho-thiophosphates(V) with the compositions Li 3 RE[PS 4 ] 2 (RE = La), [15] Li 9 RE 2 [PS 4 ] 5 (RE = Pr, Nd, [16] Sm, Gd, Er, Yb [17] ) [18] and Li 6 RE 3 [PS 4 ] 5 (RE = Y, Gd, Dy, Yb, [17] Lu) [19] can be treated as mixtures of Li 3 [PS 4 ] and RE[PS 4 ] [20] according to (Li 3 [PS 4 ]) x (RE[PS 4 ]) y with a ratio of integer numerators and denominators for the fraction x/y. [4] In the composition Li 3 RE[PS 4 ] 2 , so far only scarcely known, e. g. for RE = La, [15] but also predicted for RE = Y, [21] the x/y-ratio of Li 3 [PS 4 ] to RE[PS 4 ] equals 1.0, whereas for the Li 9 RE 2 [PS 4 ] 5 representatives the x/y-ratio becomes 1.5.…”

Li3La[PS4]2: The First Lithium Lanthanum Ortho‐Thiophosphate

“…The Raman modes of Li 3 La[PS 4 ] 2 (Figure 8) agree quite well with those presented for Tm[PS 4 ] and Yb[PS 4 ]. [12] The most intense modes stem from the vibrations within the [PS 4 ] 3À tetrahedra, while these with the lowest wavenumbers around 50 to 150 cm À 1 might be attributed to vibrations involving Li + and La 3 + cations, although lattice-vibration modes also occur at lowest wavenumbers making assignments not easy. The ν 2 and ν 4 modes result from the symmetric and asymmetric bending vibrations of the [PS 4 ] 3À tetrahedra (ñ = 210-300 cm À 1 ).…”

“…Alkali-metal containing lanthanoid(III) ortho-thiophosphates(V) occur with a rich variety of compositions, for example K 9 Ce-[PS 4 ] 4 , [1] K 3 Ce[PS 4 ] 2 , [2] K 6 Yb 3 [PS 4 ] 5 , [3] K 3 Nd 3 [PS 4 ] 4 , [4] Rb 3 Sm[PS 4 ] 2 , [5] Rb 3 Pr 3 [PS 4 ] 4 , [6] Cs 3 Sm[PS 4 ] 2 , [5] Cs 3 Nd[PS 4 ] 2 , [4] Cs 3 La 5 [PS 4 ] 6 [7] and Cs 3 Pr 5 [PS 4 ] 6 , [8] especially when the heavy representatives potassium, rubidium and cesium are considered. In their crystal structures the discrete [PS 4 ] 3À tetrahedra do not only provide coordination spheres with mostly eight sulfur atoms to the central Ln 3 + cations, as it is also the case for the ternary Ln[PS 4 ] phases (Ln = LaÀ Yb [9,10,11,12] except for Lu [13] with C.N. = 7), the involved heavy alkali-metal cations A + (A = KÀ Cs) compete to gain high coordination numbers (C.N.…”