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.
A multitude of ternary compounds based on the orthothiophosphate anion [PS4]3− has been reported over the last decades, and they continue to receive significant attention owing to their varied materials properties, even more than 125 years after their discovery , , , , , , , , , , . The first compound, SbPS4, was reported by Glatzel in 1891 , followed 2 years later by a comprehensive study on “Normale Sulfophosphate”, describing their properties and syntheses . However, the crystal structure of SbPS4 remained unknown until 2006, when Malliakas and Kanatzidis reported that this compound naturally forms single walled nanotubes with an outer diameter of 13 Å and an overall hexagonal close packing of the tubes .
The trivalent orthothiophosphates show a large structural variety which is realized by linking different types of polyhedra. Recent reports on superionic conductivities found in the alkali metal orthothiophosphates has established these compounds as promising solid electrolytes for all-solid-state batteries (e.g. β-Li3PS4 , Na3PS4 ). Additionally, they are studied as materials for next-generation digital electronics due to antiferromagnetic semiconducting properties in some layered compounds (e.g. CrPS4 ). Interest in this family of compounds increased further when first principles and experimental investigations showed their potential for applications in nonlinear optics, in particular for mid infrared coherent light generation .
Lanthanide orthothiophosphates LnPS4 (Ln=lanthanide)  are, with few exceptions, isostructural to the archetypical SmPS4  that features three-dimensional connectivity and crystallizes in the tetragonal space group I41/acd (no. 142). They all contain slightly disordered, discrete [PS4]3− tetrahedra, and the two crystallographically different Ln3+ cations are eight-fold coordinated by sulphur atoms, [LnS8]13−. The two candidates that fall out of line are LuPS4  and ScPS4 . Both exceptions are layered compounds with van der Waals gaps between the LnPS4 sheets. Although ScPS4 is reported to crystallize in the triclinic space group P1̅ (no. 2) and LuPS4 crystallizes in the monoclinic space group P21/n (no. 14), their structures are similar: The cations Sc3+ and Lu3+ are seven-fold coordinated by sulphur atoms, [LnS7]11−, and contain isolated [PS4]3− tetrahedra. Furthermore, LuPS4 is the only diamagnetic member of the lanthanide series. The only missing member is “EuPS4”, probably due to the fact that the divalent compound Eu2P2S6 is more stable and, therefore, formed preferentially during the synthetic attempts , .
During our investigations on lanthanide orthothiophosphates we noticed that TmPS4 and YbPS4 have not been synthesized as single crystals , even though their lattice parameters were reported from X-ray powder diffraction experiments several times , , , . For TmPS4, Huang et al.  arrived at lattice parameters of a=10.5962(2), c=18.881(1) Å, and Wibbelmann et al.  described a brownish color (contrary to the pale-yellow color of the compound presented here). For YbPS4, different lattice parameters were reported in the literature, namely a=10.571(1), c=18.817(2) Å in a study by Le Rolland et al. , which are significantly smaller than the powder lattice parameters a=10.664(2), c=18.989(4) Å reported by Palkina et al. , . Further, Le Rolland et al. described the color of YbPS4 to be black, while we will show that the color is reddish brown .
In this publication, we report a new synthesis protocol for TmPS4 and YbPS4, by which we were able to obtain single crystals suitable for structure elucidation. TmPS4 and YbPS4 were further characterized by Raman spectroscopy supported by quantum chemical calculations, evidencing the presence of the [PS4]3− unit. Further, we demonstrate an exceptionally high thermal stability of both compounds as determined by differential thermal analysis (DTA).
2 Results and discussion
2.1 Syntheses of TmPS4 and YbPS4
TmPS4 and YbPS4 were synthesized in a solid state reaction from the respective rare earth metals, P4S10 and sulphur at elevated temperatures. Both compounds form crystals roughly 100 μm in size. TmPS4 was obtained as pale-yellow, transparent, columnar crystals (Fig. 1) on the surface of the Tm metal, contrary to Wibbelmann and Brockner’s description of the color being brownish . Single crystals of YbPS4 appear dark red in color if large (ca. 100 μm) and well-shaped, reddish brown yet transparent if thin (Fig. 1). We did not observe the black color that was described previously by Le Rolland et al. .
2.2 Structures of TmPS4 and YbPS4
The crystal structures of TmPS4 and YbPS4 were investigated by single-crystal X-ray diffraction. Both compounds crystallize in the tetragonal space group I41/acd (no. 142) with sixteen formula units per unit cell, and are isostructural to SmPS4  (cf.Tables 1, S1, S2, Supporting Information available online). The structure contains isolated [PS4]3− tetrahedra that are slightly distorted (Fig. 2a) and two crystallographically independent Ln3+ cations (Wyckoff positions 8a and 8b). The Ln3+ cations are eight-fold coordinated by sulphur atoms (trigonal dodecahedral [LnS8]13− polyhedra) and each cation is coordinated by four [PS4]3− tetrahedra (Fig. 2b).
|Crystal color||Pale-yellow transparent||Reddish brown transparent|
|Diffraction method||Single crystal||Single crystal|
|Space group||I41/acd (no. 142)||I41/acd (no. 142)|
|Cell volume, Å3||2120.2(6)||2106.2(6)|
|Cell content Z||16||16|
|2θ range, °||6.49≤2θ≤58.38||6.96≤2θ≤58.46|
|R1/wR2 (Fo2>4 σ(Fo2))||0.0258/0.0526||0.0246/0.0551|
|Res. electron dens. (max/min), e Å−3||1.32/−1.37||1.24/−1.88|
For atomic coordinates and equivalent isotropic displacement factors please refer to Tables S1 and S2 (Supporting Information).
The lattice parameters of TmPS4 are a=10.598(2), c=18.877(4) Å with V=2120.2(6) Å3 and agree well with the ones reported by Huang et al. (a=10.5962(2), c=18.881(1) Å)  which were determined by powder X-ray diffraction. We determined the lattice parameters of YbPS4 to be a=10.577(2), c=18.827(4) Å with V=2106.2(7) Å3. They are consistent with the powder lattice parameters reported by Le Rolland et al. (a=10.571(1), c=18.817(2) Å) , but significantly smaller than the powder lattice parameters reported by Palkina et al. (a=10.664(2), c=18.989(4) Å) , . Based on their 2004 study , Schleid et al. rationalized in Ref.  this apparent inconsistency by the assumption that the compound reported as “YbPS4” may in fact have been YPS4, which would match the expected trends in the structural parameters within the lanthanide orthothiophosphate series.
In TmPS4, the Tm–S bond lengths of the [TmS8] polyhedra are 2.767(2)–2.994(2) Å and the P–S bond lengths of the slightly distorted [PS4]3− tetrahedra are 2.027(1)–2.033(2) Å with S–P–S angles of 106.23(7)–116.7(1)°. In YbPS4, the Yb−S bond lengths of the [YbS8] polyhedra amount to 2.753(1)–2.840(1) Å and the P–S bond lengths of the slightly distorted [PS4]3− tetrahedra vary between 2.0265(2) and 2.0326(2) Å with S–P–S angles between 106.15(1) and 116.75(1)°. These bond lengths and angles agree well with those of the isostructural LnPS4 compounds (Ln=La–Nd, Sm, Gd–Er) .
To gain further insights into the electronic structure, we calculated the band structures based on density functional theory (DFT). Figure 3 exemplarily shows the electronic band structure of YbPS4, confirming its semiconducting properties (cf. Fig. S1 for TmPS4; Supporting Information). The band gap is indirect with the valence band maximum found at the Z point and the conduction band minimum located near the Σ1 point. The indirect band gap was calculated with the dedicated meta-GGA potential SCAN for higher accuracy  resulting in 2.11 eV, which is in good agreement with the red color of YbPS4.
2.3 Raman spectroscopy
Raman spectroscopy was performed on single crystals of TmPS4 and YbPS4 to investigate the local bonding situation. The Raman spectra of the isostructural compounds are almost identical (cf.Fig. 4) and are dominated by the vibrations of the [PS4]3− anion which would have Td symmetry in the “free gas phase”, but this symmetry is reduced to C1 due to slight distortions of the tetrahedral anion in the crystal structure. The strongest bands around 430 cm−1 stem from vibrations that derive from the ν1 symmetric stretching vibrations of the Td symmetric [PS4]3− anion, while the less pronounced ν3 asymmetric stretching vibrations are split into several bands around 600 cm−1. The signals in the 200−300 cm−1 region can be attributed to the ν2 symmetric and ν4 asymmetric bending modes of tetrahedral [PS4]3−. These characteristic vibrations and splittings are found in all isostructural LnPS4 compounds (Ln=La–Nd, Sm, Gd–Er) , , , .
To further interpret the Raman spectra, we simulated the density of phonon modes (IR- and Raman-active modes) of TmPS4 and YbPS4 by DFT, which are both found essentially identical (cf. Fig. S2; Supporting Information). Raman-active modes for LnPS4 in space group I41/acd (no. 142) have the irreducible representations ГRaman=7 A1g+9 B1g+9 B2g+19 Eg and are marked in Fig. 4. The theoretical peak positions are slightly shifted to lower wave numbers but correspond well with the experimental Raman spectra. The visualized theoretical vibrational modes confirm the above assigned [PS4]3− vibrations.
2.4 Thermal behavior
The thermal behavior of TmPS4 and YbPS4 was studied by differential thermal analysis (DTA) measurements. TmPS4 and YbPS4 melt congruently. Upon heating we observed only one endothermic peak for both lanthanides, namely, at T=1363 K for TmPS4 and at 1305 K for YbPS4. Upon cooling, TmPS4 solidifies at 1353 K while YbPS4 was slightly subcooled to 1278 K. Comparison with the thermal behavior of other orthothiophosphates studied previously suggests that TmPS4 and YbPS4 have a particularly high thermal stability: For example, K2AuPS4 and Tl2AuPS4 melt congruently already around 660–690 K , BPS4 melts congruently at 780 K , and Ag3PS4 melts incongruently at 800 K . High melting points are also found in ht-AlPS4 (1070 K)  and Cu3PS4 (1240 K) . Note that melting points of other lanthanide family members LnPS4 are unknown.
The orthothiophosphates TmPS4 and YbPS4 were synthesized as single crystals of pale-yellow and reddish brown color, respectively. Single-crystal X-ray structure analyses have confirmed the two compounds to be isostructural and to adopt the well-known SmPS4 structure type featuring two crystallographically independent Ln3+ cations that are each coordinated by four slightly distorted [PS4]3− tetrahedra. Both compounds are semiconductors with indirect band gaps calculated by DFT to be 2.1–2.2 eV, which explains the dark red color of YbPS4, but fails to interpret the pale-yellow color of TmPS4. The Raman spectra are dominated by the vibrations of the [PS4]3− anion, as confirmed with phonon calculations. TmPS4 and YbPS4 show a remarkably high thermal stability with melting points above T=1300 K.
4 Experimental section
All preparations and manipulations were carried out in a drybox under argon atmosphere. For the solid state reactions 4 Ln (Tm; Yb)+P4S10+6 S→4 LnPS4 stoichiometric proportions of Tm or Yb (Johnson Matthey, pieces, distilled dendritic, 99.99%), P4S10 (Acros, 98+%) and S (Alfa Aesar, 99.5%) were intimately mixed and heated in vacuum-sealed uncoated quartz tubes (length: 150 mm, inside diameter: 9 mm). TmPS4 and YbPS4 are both moisture sensitive and must be stored and handled under inert conditions. In order to prevent damaging of the sealed quartz tube by the initially strong exothermic reaction, the starting material was placed into a small quartz container (length: 80 mm, outer diameter: 8 mm) prior to transferring it to the quartz tube. This crucible was fused with the outer ampoule via melting before the entire ampoule was sealed under vacuum and water cooling.
4.1.1 Synthesis of TmPS4
The stoichiometric mixture was heated up (100 K h−1) to T=1073 K and kept at that temperature for 2 days. Afterwards the ampoule was cooled down (25 K h−1) and annealed first for 2 days at 973 K and then for another 2 days at 773 K. Subsequently, the furnace was switched off and the ampoule was removed from the furnace at room temperature. The quartz tube contained pale-yellow transparent crystals on the surface of the Tm metal. Caution: Heating above 1123 K led to an explosion of the evacuated quartz ampoule.
4.1.2 Synthesis of YbPS4
A stoichiometric mixture of Yb, S, and P4S10 was sealed in an evacuated quartz tube, heated up (100 K h−1) to T=1173 K and kept at that temperature for 1 h. Afterwards the ampoule was cooled down (25 K h−1) and annealed first at 1073 K and then at 673 K for 2 days. The furnace was then switched off and the ampoule was removed from the furnace at room temperature. Dark red and well-shaped crystals of YbPS4 (note: thin plates or grinded powders look reddish brown) were obtained at the bottom of the ampoule.
4.2 EDX analyses
Several single crystals were mounted on carbon tape and measured with a Tescan SEM (Vega TS 5130 MM) equipped with a SDD (silicon drift detector, Oxford) confirming the composition of the single crystals (TmPS4: 16(1) at.% Tm, 17(1) at.% P, 67(1) at.% S; YbPS4: 17(2) at.% Yb, 18(1) at.% P, 65(2) at.% S).
4.3 X-ray diffraction
Powder X-ray diffraction patterns of TmPS4 and YbPS4 were measured in sealed glass capillaries using a STOE StadiP diffractometer working with Ge-monochromated MoKα radiation in Debye-Scherrer geometry. TmPS4 and YbPS4 single crystals suitable for X-ray diffraction were picked under a microscope in dried petroleum and mounted into sealed glass capillaries. The data collection was performed with a STOE IPDS II working with graphite-monochromated MoKα radiation. For the integration of the reflection intensities and the calculation of the reciprocal lattice planes, the STOE X-Area 1.56 software was used. The structure was solved with Direct Methods and refined by least-squares fitting using the program Shelxtl .
Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CSD-1972308 (TmPS4) and CSD-1972309 (YbPS4) (Fax: +44-1223-336-033; E-Mail: firstname.lastname@example.org, http://www.ccdc.cam.ac.uk).
4.4 Raman spectroscopy
Single crystals were selected under a microscope in dried petroleum, mounted into glass capillaries and sealed under inert atmosphere. The Raman spectra were recorded with a Jobin Yvon Typ V 010 labram single grating spectrometer, equipped with a double super razor edge filter and a Peltier cooled CCD camera. The resolution of the spectrometer (grating 1800 L·mm−1) is 1 cm−1.
4.5 DTA measurements
DTA measurements were performed under inert atmosphere in sealed quartz containers with home-built equipment described in Ref. . After thermal analysis, the products were again characterized by XRD and SEM-EDX analysis.
The electronic band structure calculations were carried out with the Vienna Ab initio Simulation Package (VASP) , ,  based on density functional theory using projector-augmented-wave (PAW) potentials ,  for core and valence electron separation. Exchange and correlation contributions were treated with the generalized gradient approximation as parametrized by Perdew, Burke, and Ernzerhof (GGA-PBE) . Pseudopotentials for Tm and Yb were chosen that treat the 4f electrons as part of the atomic core. The Monkhorst-Pack  k-point mesh used for Brillouin zone integration was energetically converged for all calculations while the energy-cutoff was set to 500 eV. The convergence criterion of the electronic structure calculations was set to 10−7 eV. For higher accuracy, the compounds’ band structures were calculated with the SCAN meta-GGA potential . The density of phonon states were derived using the direct method , as implemented in Phonopy , .
5 Supporting information
Atomic coordinates and equivalent isotropic displacement factors are given as supplementary information available online (Tables S1, S2). Further, the electronic band structure as well as DFT-calculated density of phonon states of TmPS4 are illustrated in Figs. S1 and S2 (DOI: 10.1515/znb-2019-0217).
Dedicated to: Professor Arndt Simon on the occasion of his 80th birthday.
The authors acknowledge financial support by the BMBF (3XP0177B (FestBatt), the Max Planck Society, and the Center for Nanoscience (CeNS). We thank V. Duppel for taking images of the crystals and for SEM-EDX analyses, A. Schulz for performing the single-crystal Raman spectroscopy, W. Hölle for single-crystal X-ray diffraction measurements and the Computer Service group at MPI-FKF for providing computational facilities.
 B. Le Rolland, P. McMillan, P. Molinie, P. Colombet, Eur. J. Solid State Inorg. Chem.1990, 27, 715–724.Search in Google Scholar
 K. K. Palkina, S. I. Maksimova, N. T. Chibiskova, T. B. Kuvshinova, A. N. Volodina, Inorg. Mater.1984, 20, 1338–1341.Search in Google Scholar
 M. Gjikaj, Habilitation thesis, University Clausthal, Clausthal, 2008.Search in Google Scholar
 Z. Huang, V. Cajipe, P. Molinié, J. Rare Earths1998, 16, 167–171.Search in Google Scholar
 Z. Huang, V. Cajipe, P. Molinié, J. Rare Earths1999, 17, 6–11.Search in Google Scholar
 K. K. Palkina, T. B. T.B. Kuvshinova, S. I. Maksimova, N. T. Chibiskova, Izv. Akad. Nauk SSSR Neorg. Mater.1998, 25, 1555–1556.Search in Google Scholar
 S. Löken, W. Tremel, Eur. J. Inorg. Chem.1998, 1998, 283–289.10.1002/(SICI)1099-0682(199802)1998:2<283::AID-EJIC283>3.0.CO;2-LSearch in Google Scholar
The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2019-0217).
©2020 Walter de Gruyter GmbH, Berlin/Boston