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

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 [5]. 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. [8].

Fig. 1:

Pale-yellow crystals of TmPS₄ (left) and dark, reddish brown crystals of YbPS₄ (right).

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 [11] (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).

Fig. 2:

(a) Crystal structure of TmPS₄ and YbPS₄ (a axis is the vertical axis, Tm/Yb atoms in grey, P atoms in red, S atoms in yellow, and isolated [PS₄]³⁻ tetrahedra in green). (b) Coordination environment of the two crystallographically different Ln³⁺ cations.

The lattice parameters of TmPS₄ are a=10.598(2), c=18.877(4) Å with V=2120.2(6) ų and agree well with the ones reported by Huang et al. (a=10.5962(2), c=18.881(1) Å) [21] 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) ų. They are consistent with the powder lattice parameters reported by Le Rolland et al. (a=10.571(1), c=18.817(2) Å) [8], but significantly smaller than the powder lattice parameters reported by Palkina et al. (a=10.664(2), c=18.989(4) Å) [6], [22]. Based on their 2004 study [9], Schleid et al. rationalized in Ref. [6] 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) [6].

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 [23] resulting in 2.11 eV, which is in good agreement with the red color of YbPS₄.

Fig. 3:

Calculated electronic band structure of YbPS₄ with an indirect band gap of 2.11 eV.

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 T_d 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 T_d 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) [5], [8], [11], [20].

Fig. 4:

Raman spectra of TmPS₄ and YbPS₄ with DFT-calculated Raman-active modes and assigned vibrational modes of the [PS₄]³⁻ anion (cf. text).

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 Г_Raman=7 A_1g+9 B_1g+9 B_2g+19 E_g 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 [24], BPS₄ melts congruently at 780 K [3], and Ag₃PS₄ melts incongruently at 800 K [25]. High melting points are also found in ht-AlPS₄ (1070 K) [2] and Cu₃PS₄ (1240 K) [26]. Note that melting points of other lanthanide family members LnPS₄ are unknown.

4.1 Syntheses

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.

4.2 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 [27].

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: deposit@ccdc.cam.ac.uk, 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. [2]. After thermal analysis, the products were again characterized by XRD and SEM-EDX analysis.

4.6 Calculations

The electronic band structure calculations were carried out with the Vienna Ab initio Simulation Package (VASP) [28], [29], [30] based on density functional theory using projector-augmented-wave (PAW) potentials [31], [32] 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) [33]. Pseudopotentials for Tm and Yb were chosen that treat the 4f electrons as part of the atomic core. The Monkhorst-Pack [34] 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 [23]. The density of phonon states were derived using the direct method [35], as implemented in Phonopy [36], [37].