Li₉Yb₂[PS₄]₅ and Li₆Yb₃[PS₄]₅: two lithium-containing ytterbium(III) thiophosphates(V) revisited

1 Introduction

Since the first representatives of rare earth metal(III) thiophosphates(V) were discovered in 1984 for the examples Pr[PS₄] and Tb[PS₄] [1], a whole new field of thiophosphate chemistry was opened up. Today, all of the ortho-thiophosphates(V) with the general formula RE[PS₄] (RE = Sc, Y, La, Ce–Lu) have been accurately characterized [1], [2], [3], [4]. Members of this family may be promising candidates for applications such as optical devices [2], solid electrolytes [3] or semi-conductors [5]. The plain RE[PS₄] phases (RE = La–Yb and Y) crystallize tetragonally in space group I4₁/acd with the Pr[PS₄]-type structure and the lattice parameters a = 1087.7(5) and c = 1932.0(9) pm for the prototypic praseodymium compound Pr[PS₄] with Z = 4 and coordination number (C.N.) of RE³⁺ = 8 [6]. With the smallest Ln³⁺ cation of the lanthanoid series (Ln = Ce–Lu), a monoclinic crystal structure in space group P2₁/n for Lu[PS₄] (a = 813.73(11), b = 642.50(9), c = 1062.89(15) pm, β = 97.62(2)°; Z = 4) occurs as singularity, providing it with C.N.(RE³⁺) = 7 [7]. Another exception is the scandium-containing representative, which crystallizes triclinically in space group P1‾ with a = 644.5(5), b = 665.9(5), c = 621.1(4) pm, α = 91.36(5), β = 98.76(5), γ = 86.22(5)° for Z = 2, but again with C.N.(RE³⁺) = 7 in Sc[PS₄] [8].

If such a structure with [PS₄]^3– anions is padded with extra cations like those of the alkali metals (A = Li–Cs), the three-dimensional structure of the tetragonal RE[PS₄] representatives becomes less cross-linked and structures with different dimensionalities emerge. The same holds for the monoclinic and triclinic RE[PS₄] examples, where the dimensionality is already reduced to layer-like. But in all cases phases with the general formula (A₃[PS₄])_x(RE[PS₄])_y can be derived [2].

Further studies on the implementation of alkali metals into rare earth metal(III) thiophosphates(V) yielded several new compounds including those of potassium, rubidium and cesium, for example K₉Ce[PS₄]₄ [9], K₃Ce[PS₄]₂ [10], K₆Yb₃[PS₄]₅ [11], K₃Nd₃[PS₄]₄ [12], Rb₃Sm[PS₄]₂ [13], Rb₃Pr₃[PS₄]₄ [14], Cs₃Sm[PS₄]₂ [13], Cs₃Nd[PS₄]₂ [12] and Cs₃Pr₅[PS₄]₆ [15]. Interestingly, for sodium the complex anion hexathiodiphosphate(IV) [P₂S₆]^4– has been observed instead of the ortho-thiophosphate(V) anion [PS₄]^3–, for example in NaLa[P₂S₆] [16], NaSm[P₂S₆] [17] or NaTb [P₂S₆] [18].

In 2006 and 2007, respectively, the first lithium-containing compounds Li₉Nd₂[PS₄]₅ (a = 1502.64(6), b = 989.31(4), c = 2083.37(9) pm, β = 95.913(4)° for Z = 4) [19] and Li₆Gd₃[PS₄]₅ (a = 2832.7(2), b = 1007.4(10), c = 3382.2(2) pm, β = 114.30(7)° for Z = 12) [20], both crystallizing monoclinically in space group C2/c, were published. They both can again be attributed to the general formula (A₃[PS₄])_x(RE[PS₄])_y since they exhibit discrete [PS₄]^3– anions. The title compounds crystallize with the prototypic structures of Li₉Nd₂[PS₄]₅ and Li₆Gd₃[PS₄]₅, but show slight differences, making them worthwhile for a revision [19], [20].

2 Experimental section

All preparations were carried out under inert conditions in an argon-filled glove box (GS Mega E-Line, Glovebox Systemtechnik). For the synthesis, stoichiometric amounts, according to the reaction Eqs. (1) and (2), of lithium hemisulfide (Li₂S: Sigma Aldrich, 99.98% on trace metal basis), elemental sulfur (S: Johnson Matthey, puratronic), red phosphorus (P: ChemPur, 99.995%) and ytterbium metal (Yb: ChemPur, 99.9%) were mixed and sealed under dynamic vacuum in fused glassy silica ampoules. The reactants were then heated to 650 °C for seven days to yield small single crystals of dark red color. These crystals were highly sensitive towards moisture and oxygen, so inert atmosphere was crucial.

Selected crystals were examined by single-crystal X-ray diffraction on a STOE & Cie StadiVari diffractometer using monochromatized MoKα radiation (λ = 71.07 pm). The structures were determined using the program package Shelx-97 for solution and refinement. The program package Shelxs-97 was used for direct method calculations to solve the structures, whereas the package Shelxl-97 was used to refine the structure calculation on F² values using full-matrix least-squares algorithms.

In Table 1, the crystallographic data for the structures of Li₉Yb₂[PS₄]₅ and Li₆Yb₃[PS₄]₅ are displayed.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49 7247 808 666; E-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-2064321 (Li₉Yb₂[PS₄]₅) and CSD-2064322 (Li₆Yb₃[PS₄]₅).

3 Results

3.1 Crystal structure of Li₉Yb₂[PS₄]₅

Li₉Yb₂[PS₄]₅ crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 1487.98(9), b = 978.63(6), c = 2046.75(12) pm and β = 96.142(3)° for Z = 4 at room temperature. The crystallographic data is given in Table 1. Its structure is basically isotypic to the literature-known Li₉Nd₂[PS₄]₅ [19] featuring a crystallographically singular Yb³⁺ position, along with three unique P⁵⁺, five unique Li⁺ and 10 unique S²⁻ positions (Table 2). The crystal structure is shown in Figure 1 with highlighted [PS₄]^3– tetrahedra.

Table 2:

Fractional atomic coordinates, Wyckoff positions and (equivalent) isotropic displacement parameters for Li₉Yb₂[PS₄]₅.

Atom	Wyckoff site	x/a	y/b	z/c	Ueq/iso/pm2
Li1	4c	1/4	1/4	0	986(71)
Li2	8f	0.4560(6)	0.4868(8)	0.3875(4)	339(17)
Li3	8f	0.2769(9)	0.4170(15)	0.4664(7)	804(36)
Li4	8f	0.4628(9)	0.1999(16)	0.1592(8)	912(43)
Li5	8f	0.0344(6)	0.3299(9)	0.0730(4)	398(19)
Yb	8f	0.232425(11)	0.02861(2)	0.298387(7)	151(1)
P1	4e	0	0.01712(14)	1/4	168(3)
P2	8f	0.37509(6)	0.13516(9)	0.43820(5)	166(2)
P3	8f	0.24891(6)	0.18938(9)	0.14814(5)	154(2)
S1	8f	0.05846(7)	0.12555(11)	0.32762(5)	191(2)
S2	8f	0.40208(7)	0.39301(11)	0.28128(5)	239(2)
S3	8f	0.11969(7)	0.49341(11)	0.48377(5)	259(2)
S4	8f	0.45717(7)	0.29587(11)	0.46564(5)	292(2)
S5	8f	0.24602(7)	0.19509(11)	0.40957(5)	237(2)
S6	8f	0.41311(7)	0.03232(11)	0.35806(5)	218(2)
S7	8f	0.32752(7)	0.03243(11)	0.18391(5)	218(2)
S8	8f	0.18894(7)	0.26458(11)	0.22495(5)	183(2)
S9	8f	0.32384(7)	0.33979(11)	0.11124(5)	200(2)
S10	8f	0.15549(7)	0.13412(11)	0.07170(5)	210(2)

Figure 1:

Crystal structure of Li₉Yb₂[PS₄]₅ with highlighted [PS₄]^3– tetrahedra as viewed along [010].

Each Yb³⁺ cation is eightfold coordinated by sulfur atoms building up a bicapped trigonal prism as the coordination polyhedron (Figure 2; d(Yb–S) = 277.9–289.7 pm, d‾ = 283.0 pm). The slightly shorter distances as compared to the isotypic Li₉Nd₂[PS₄]₅ (d(Nd–S) = 289.8–295.0 pm, d‾ = 292.5 pm) [19] are due to the smaller ionic radius of Yb³⁺ in contrast to Nd³⁺ [21]. The bicapped trigonal prism [YbS₈]¹³⁻ is realized by four edge-grafting [PS₄]^3– tetrahedra (Figure 2), a very common feature in the crystal chemistry of rare earth metal(III) ortho-thiophosphates(V) [6], [22].

Figure 2:

Coordination environment for the crystallographically unique Yb³⁺ cation in Li₉Yb₂[PS₄]₅.

Regarding the environment of the phosphorus atoms, the distances d(P–S) = 202.0–205.5 pm (Table 3) are in the same range as those of other ortho-thiophosphates. The sulfur tetrahedra around the three unique phosphorus atoms are slightly distorted, as indicated by the range of deviations (∢(S–P–S) = 104.1–116.3°, Table 4) from the ideal tetrahedral angle. P1 is located on the special Wyckoff site 4e with four covalently bonded sulfur atoms and two ytterbium and two lithium cations in distant positions, complemented by another set of even more remote Li⁺ cations (Figure 3, top). The environment of P2 is quite similar to the one of P1, though one ytterbium is substituted by another lithium cation, but there are eight atoms again in the extended environment of the [(P2)S₄]^3– anion (one Yb³⁺ and six plus one Li⁺ cations (Figure 3, mid). The edges of the [(P3)S₄]^3– tetrahedron are bridged by three lithium and two ytterbium cations and additionally three Li⁺ cations end-on connected at S10 can be attributed to the extended environment of P3 (Figure 3, bottom). So the situation of the phosphorus atoms in Li₉Yb₂[PS₄]₅ is very much the same as in the prototypic Li₉Nd₂[PS₄]₅ [19].

Table 3:

Selected interatomic distances (d/pm) in Li₉Yb₂[PS₄]₅.

Atoms	d/pm	Atoms	d/pm
Yb–S2	277.9(1)	P1–S1 (2×)	202.8(1)
Yb–S5	278.9(1)	P1–S2 (2×)	205.1(1)
Yb–S8	279.3(1)
Yb–S9	280.5(1)	P2–S3	202.8(1)
Yb–S6	283.2(1)	P2–S4	203.3(1)
Yb–S7	286.4(1)	P2–S5	203.3(1)
Yb–S1	287.9(1)	P2–S6	205.5(1)
Yb–S8′	289.7(1)
		P3–S7	202.0(2)
		P3–S8	202.7(2)
		P3–S9	204.0(2)
		P3–S10	205.1(2)
Li1–S10 (2×)	242.1(1)	Li2–S10	241.3(8)
Li1–S9 (2×)	257.5(1)	Li2–S2	241.7(9)
Li1···S3 (2×)	316.8(1)	Li2–S4	245.9(8)
		Li2–S1	246.3(8)
Li3–S5	248.3(14)
Li3–S10	251.1(14)	Li4–S6	252.2(15)
Li3–S3	251.7(13)	Li4–S9	258.4(15)
Li3–S5	281.9(14)	Li4–S7	268.6(15)
Li3–S4	293.4(13)	Li4–S2	292.8(16)
		Li4···S4	307.9(15)
Li5–S6	250.8(9)	Li4···S2	333.4(15)
Li5–S10	263.3(9)
Li5–S4	266.8(9)
Li5–S3	290.4(9)
Li5–S3′	293.1(9)
Li5···S1	326.3(9)

Table 4:

Selected interatomic angles (∢/deg) in Li₉Yb₂[PS₄]₅.

Atoms	∢/deg
S1–P1–S1′	116.89(9)
S1–P1–S2 (2×)	107.15(4)
S1–P1–S2′ (2×)	108.96(4)
S2–P1–S2′	107.38(9)
S3–P2–S4	106.93(6)
S3–P2–S5	111.18(6)
S3–P2–S6	109.52(6)
S4–P2–S5	112.17(6)
S4–P2–S6	113.00(6)
S5–P2–S6	104.09(6)
S7–P3–S8	106.31(6)
S7–P3–S9	111.39(6)
S7–P3–S10	113.26(6)
S8–P3–S9	109.18(6)
S8–P3–S10	111.63(6)
S9–P3–S10	105.10(6)

Figure 3:

Extended environments for all three [PS₄]^3– tetrahedra in Li₉Yb₂[PS₄]₅.

Figure 4:

Infinite layers with the composition  ∞2{[Yb[PS4]3]6−} running parallel to the (001) plane in the crystal structure of Li₉Yb₂[PS₄]₅.

The [PS₄]^3– tetrahedra build up infinite layers together with the [YbS₈]¹³⁻ polyhedra, which spread out parallel to the ab plane exhibiting the composition  ∞2{[Yb[PS4]3]6−} as depicted in Figure 4. The five crystallographically different lithium cations are located in and between these layers. The (Li1)⁺ cation is the only one located on the special Wyckoff position 4e, whereas the other four reside on the general 8f sites (Table 2). The coordination environment of (Li1)⁺ is consisting of four coplanar sulfur atoms (d(Li1–S) = 242.1–257.5 pm) and two sulfur atoms further away (d(Li1–S′) = 316.7 pm), so the coordination number is thus C.N. = 4 + 2 defining an elongated octahedron. (Li2)⁺ is surrounded by four sulfur atoms in a slightly distorted tetrahedron (d(Li2–S) = 241.3–246.3 pm). The coordination environment of the (Li3)⁺ position appears as a trigonal bipyramid (d(Li3–S) = 248.3–293.4 pm) and the (Li4)⁺ cation is surrounded by 4 + 2 sulfur atoms (d(Li4–S) = 252.2–292.8 pm plus 307.9 and 333.4 pm), creating a trigonal prism. For the (Li5)⁺ position, six coordinating sulfur atoms can be found (d(Li5–S) = 250.8–293.2 pm plus 326.6 pm), so that the environment can be described with a highly distorted octahedron showing a very inhomogeneous contact distribution for C.N. = 5 + 1 (Table 3).

Figure 5:

Extended unit cell of Li₆Yb₃[PS₄]₅ with highlighted [PS₄]^3– tetrahedra as viewed along [010].

Figure 6:

Coordination polyhedron of the (Yb5)³⁺ position, which in Li₆Yb₃[PS₄]₅ is half occupied by Li⁺ (Li11).

The Li⁺ cations were not refined anisotropically, but only isotropically (Table 2) due to their high displacement parameters, hinting at possible cation mobility and thus ion conductivity. Further studies of the conductivity issue are thus necessary.

Different from Li₉Nd₂[PS₄]₅, (Li1)⁺ was not refined with a split position, but the two highest peaks in the map of the residual electron density stem from Li5 ghosts (site Q1: 0.0384, 0.3560, 0.0488, d(Li5···Q1) = 56 pm; site Q2: 0.0384, 0.3098, 0.0524, d(Li5···Q2) = 45 pm).

3.2 Crystal structure of Li₆Yb₃[PS₄]₅

Li₆Yb₃[PS₄]₅ also crystallizes in the monoclinic space group C2/c (no. 12). The lattice parameters are a = 2814.83(16), b = 997.34(6), c = 3338.52(19) pm and β = 113.685(3)° for Z = 12. Crystallographic data are given in Table 1. The crystal structure of Li₆Yb₃[PS₄]₅ (Figure 5) is almost isotypic with the Li₆Gd₃[PS₄]₅-type structure [20], but the description presented here differs from the prototypic one in such a way that one of the ytterbium positions (Yb5) is occupied with half ytterbium and half lithium in contrast to the partial occupation of 65% found for Gd5 in the Li₆Gd₃[PS₄]₅ analogue published in 2007 [20]. Nevertheless, the metric of the unit cell does not differ much from the one (a = 2817.8(3), b = 997.7(1), c = 3339.2(4) pm and β = 113.65(1)°) presented before by Mewis et al. for Li₆Yb₃[PS₄]₅ [20].

There are five crystallographically different Yb³⁺ positions, where one of them, (Yb5) as mentioned, is partially occupied with lithium (Li11). Moreover there are eight unique phosphorus, 10 different lithium and 30 distinct sulfur positions (Table 5). Some of the Li⁺ sites are underoccupied to secure the charge balance, as it is the case in the ternary Li₃[PS₄] [23], but with all specific restraints given in Table 5, the highest peak in the residual electron density map stems from an ytterbium ghost (site Q1: 0.0920, 0.3567, 0.3274; d(Yb1···Q1) = 99 pm).

Table 5:

Fractional atomic coordinates, Wyckoff positions and (equivalent) isotropic displacement parameters for Li₆Yb₃[PS₄]₅.

Atom	Wyckoff site	x/a	y/b	z/c	Ueq/iso/pm2
Li1	8f	0.4591(11)	0.111(3)	0.3051(9)	470(68)
Li2	8f	0.3295(11)	0.119(3)	0.3257(9)	543(76)
Li3	8f	0.0419(12)	0.290(3)	0.1760(11)	639(86)
Li4	8f	0.1821(12)	0.271(3)	0.2115(11)	668(89)
Li5	8f	0.0870(14)	0.267(4)	0.4442(12)	869(112)
Li6a	8f	0.2374(17)	0.181(5)	0.4574(15)	921(139)
Li7a	8f	0.2402(17)	0.353(5)	0.0550(15)	926(140)
Li8a	8f	0.011(2)	0.024(6)	0.3052(19)	984(193)
Li9a	8f	0.210(2)	0.053(6)	0.0978(18)	903(165)
Li10a	8f	0.211(3)	0.009(9)	0.179(3)	904(259)
Li11a	8f	0.08438(4)	0.36233(11)	0.05365(3)	178(5)
Yb1	8f	0.128390(19)	0.34440(5)	0.331055(16)	148(1)
Yb2	8f	0.336525(19)	0.35085(5)	0.205456(16)	165(1)
Yb3	8f	0.384015(19)	0.25625(5)	0.067250(16)	148(1)
Yb4	8f	0.461951(19)	0.38179(5)	0.427795(17)	204(1)
Yb5a	8f	0.08438(4)	0.36233(11)	0.05365(3)	178(5)
P1	4e	0	0.3259(4)	1/4	151(9)
P2	8f	0.18517(12)	0.4606(3)	0.44057(10)	167(7)
P3	8f	0.48971(12)	0.0554(3)	0.06885(10)	178(7)
P4	8f	0.13465(11)	0.0262(3)	0.37534(10)	154(7)
P5	8f	0.20743(12)	0.3494(3)	0.13185(10)	181(7)
P6	8f	0.40012(12)	0.4405(3)	0.31528(10)	162(7)
P7	8f	0.09384(11)	0.0427(3)	0.01005(10)	164(7)
P8	8f	0.33950(12)	0.0188(3)	0.24334(10)	172(7)
S1	8f	0.02144(11)	0.4346(3)	0.30609(10)	181(7)
S2	8f	0.06218(12)	0.2109(3)	0.25589(10)	240(7)
S3	8f	0.18036(13)	0.3270(3)	0.48418(11)	288(8)
S4	8f	0.28729(11)	0.1386(3)	0.02875(10)	174(7)
S5	8f	0.22436(12)	0.3773(3)	0.40681(10)	225(7)
S6	8f	0.38517(11)	0.0108(3)	0.10752(10)	191(7)
S7	8f	0.00749(12)	0.3646(3)	0.08952(11)	228(7)
S8	8f	0.45771(14)	0.1200(3)	0.45406(12)	355(9)
S9	8f	0.41684(12)	0.0728(4)	0.02201(11)	316(9)
S10	8f	0.49204(11)	0.1949(3)	0.11538(10)	187(7)
S11	8f	0.17632(11)	0.0889(3)	0.34158(10)	182(7)
S12	8f	0.31967(11)	0.4776(3)	0.06182(10)	180(7)
S13	8f	0.08579(12)	0.1795(3)	0.37227(10)	205(7)
S14	8f	0.40432(12)	0.3559(3)	0.15357(10)	197(7)
S15	8f	0.15143(14)	0.2182(4)	0.12878(11)	388(10)
S16	8f	0.17709(14)	0.4869(4)	0.08341(12)	444(11)
S17	8f	0.23703(11)	0.4418(3)	0.19101(10)	196(7)
S18	8f	0.26794(12)	0.2476(3)	0.12738(10)	276(8)
S19	8f	0.16007(12)	0.0383(3)	0.23064(10)	232(7)
S20	8f	0.04311(12)	0.0558(3)	0.14065(10)	233(7)
S21	8f	0.43003(12)	0.3377(3)	0.27775(10)	217(7)
S22	8f	0.37507(12)	0.3312(3)	0.35503(10)	256(8)
S23	8f	0.15449(13)	0.1655(3)	0.02226(10)	294(8)
S24	8f	0.38565(13)	0.3643(3)	0.46019(12)	295(8)
S25	8f	0.44071(11)	0.4782(3)	0.05644(10)	193(7)
S26	8f	0.03865(12)	0.1253(3)	0.02877(10)	211(7)
S27	8f	0.29370(12)	0.1737(3)	0.24465(10)	212(7)
S28	8f	0.20003(12)	0.3567(3)	0.29015(10)	215(7)
S29	8f	0.11365(12)	0.4687(3)	0.19376(10)	235(7)
S30	8f	0.38229(12)	0.0863(3)	0.21022(10)	200(7)

1. ^aSite occupation probability (s.o.p.) by free refinement: s.o.p.(Li6), 85%; s.o.p.(Li7), 85%; s.o.p.(Li8), 65%; s.o.p.(Li9), 70%; s.o.p.(Li10), 45%; s.o.p.(Yb5/Li11), 50:50% with standard deviations of about 3% for the pure Li⁺ and less than 1% for the mixed Li⁺/Yb³⁺ site.

Table 6:

Selected interatomic distances (d/pm) in Li₆Yb₃[PS₄]₅.

Atoms	d/pm	Atoms	d/pm
Li1–S21	245(3)	Li2–S22	246(3)
Li1–S1	248(3)	Li2–S29	246(3)
Li1–S29	250(3)	Li2–S17	247(3)
Li1–S10	259(3)	Li2–S27	254(3)
Li1···S30	304(3)
Li3–S1	255(3)	Li4–S19	255(3)
Li3–S29	258(3)	Li4–S17	257(3)
Li3–S2	261(3)	Li4–S15	260(3)
Li3–S20	262(3)	Li4–S28	261(3)
Li3–S7	275(3)	Li4–S29	265(3)
		Li4···S27	304(3)
Li5–S3	249(4)	Li6–S3	235(4)
Li5–S13	254(4)	Li6–S12	250(5)
Li5–S7	262(4)	Li6–S5	252(5)
Li5–S25	298(4)	Li6–S3	258(5)
Li7–S18	245(5)	Li8–S14	278(6)
Li7–S12	249(5)	Li8–S21	280(6)
Li7–S4	259(5)	Li8–S20	281(7)
Li7–S16	268(5)	Li8–S13	284(6)
Li7–S4	283(5)	Li8–S2	292(6)
Li7–S23	289(4)
Li9–S18	247(5)	Li10–S19	265(9)
Li9–S5	259(6)	Li10–S28	275(8)
Li9–S23	263(5)	Li10–S15	278(9)
Li9–S15	281(6)	Li10–S22	284(8)
		Li10–S27	297(8)
Yb1–S13	271.3(3)	Yb2–S18	274.6(3)
Yb1–S30	273.4(3)	Yb2–S27	274.8(3)
Yb1–S6	278.1(3)	Yb2–S21	276.9(3)
Yb1–S2	279.0(3)	Yb2–S11	278.9(3)
Yb1–S11	283.8(3)	Yb2–S17	279.5(3)
Yb1–S28	285.6(3)	Yb2–S19	280.9(3)
Yb1–S5	288.3(3)	Yb2–S30	291.3(3)
Yb1–S1	292.4(3)	Yb2–S14	305.0(3)
Yb3–S9	275.7(3)	Yb4–S22	271.4(3)
Yb3–S4	276.2(3)	Yb4–S25	274.8(3)
Yb3–S6	278.7(3)	Yb4–S24	277.1(3)
Yb3–S12	281.5(3)	Yb4–S8	277.2(3)
Yb3–S25	283.5(3)	Yb4–S20	282.7(3)
Yb3–S23	284.8(3)	Yb4–S26	283.3(3)
Yb3–S10	287.7(3)	Yb4–S10	295.4(3)
Yb3–S14	288.0(3)	Yb4–S26	319.1(3)
Yb5/Li11–S9	259.5(4)
Yb5/Li11–S26	266.0(3)
Yb5/Li11–S16	269.4(4)
Yb5/Li11–S8	279.8(4)
Yb5/Li11–S15	284.9(4)
Yb5/Li11–S7	286.4(4)
Yb5/Li11–S23	324.2(4)
Yb5/Li11···S8′	377.8(4)
P1–S1 (2×)	203.4(4)	P2–S3	201.8(5)
P1–S2 (2×)	203.5(4)	P2–S4	204.1(4)
		P2–S5	204.3(5)
		P2–S6	204.8(4)
P3–S7	201.8(4)	P4–S11	202.5(4)
P3–S8	202.5(5)	P4–S12	203.0(4)
P3–S9	202.6(4)	P4–S13	203.2(4)
P3–S10	206.7(4)	P4–S14	204.1(4)
P5–S15	202.0(5)	P6–S19	202.1(4)
P5–S16	202.8(5)	P6–S20	203.8(4)
P5–S17	203.1(4)	P6–S21	204.4(4)
P5–S18	203.9(5)	P6–S22	204.9(5)
P7–S23	200.5(4)	P8–S27	202.4(4)
P7–S24	200.6(4)	P8–S28	202.6(4)
P7–S25	204.5(4)	P8–S29	204.3(4)
P7–S26	206.5(4)	P8–S30	204.8(4)

Table 7:

Selected interatomic angles (∢/deg) in Li₆Yb₃[PS₄]₅.

Atoms	∢/deg
S1–P1–S1	115.6(3)
S1–P1–S2	106.61(12)
S1–P1–S2	106.61(12)
S1–P1–S2	108.36(13)
S1–P1–S2	108.36(13)
S2–P1–S2	111.4(3)
S3–P2–S4	109.52(19)
S3–P2–S5	109.7(2)
S4–P2–S5	116.7(2)
S3–P2–S6	113.8(2)
S4–P2–S6	103.49(17)
S5–P2–S6	103.48(18)
S7–P3–S8	108.1(2)
S7–P3–S9	112.3(2)
S8–P3–S9	110.4(2)
S7–P3–S10	116.69(19)
S8–P3–S10	105.21(19)
S9–P3–S10	103.83(19)
S11–P4–S12	112.39(18)
S11–P4–S13	105.74(18)
S12–P4–S13	111.50(19)
S11–P4–S14	108.38(19)
S12–P4–S14	106.68(18)
S13–P4–S14	112.21(18)
S15–P5–S16	109.1(2)
S15–P5–S17	110.5(2)
S16–P5–S17	110.2(2)
S15–P5–S18	109.4(2)
S16–P5–S18	112.0(2)
S17–P5–S18	105.60(18)
S19–P6–S20	116.76(19)
S19–P6–S21	101.73(18)
S20–P6–S21	109.2(2)
S19–P6–S22	110.3(2)
S20–P6–S22	101.93(19)
S21–P6–S22	117.6(2)
S23–P7–S24	112.89(19)
S23–P7–S25	106.11(19)
S24–P7–S25	111.35(19)
S23–P7–S26	112.2(2)
S24–P7–S26	107.82(19)
S25–P7–S26	106.35(18)
S27–P8–S28	114.07(19)
S27–P8–S29	108.57(19)
S28–P8–S29	110.88(19)
S27–P8–S30	106.19(18)
S28–P8–S30	106.01(19)
S29–P8–S30	111.02(19)

Four of the five crystallographically unique Yb³⁺ cations reside in eightfold sulfur coordination, where Yb1, Yb2 and Yb3 are surrounded by sulfur atoms forming bicapped trigonal prisms (d(Yb–S) = 271.3–288.3 pm; (Table 6). Yb4 is also eightfold coordinated by sulfur atoms, but now in the shape of a square antiprism (d(Yb–S) = 271.4–319.1 pm). The fifth ytterbium position is partially occupied with lithium (s.o.p.(Yb5)/(Li11) = 50%:50%) and the coordination polyhedron is rather unusual. It resembles that of a trigonal prism (d(Yb–S) = 259.5–286.4 pm) with two additional sulfur atoms further away (d(Yb–S23) = 324.2 pm and d(Yb···S8′) = 377.8 pm), so that in total six plus one sulfur atoms are involved in the direct coordination sphere of Yb5 and Li11 (Figure 6) The distance to the closer extra one is a little shorter than in the Li₆Gd₃[PS₄]₅ case (d(Gd–S) = 328.4 pm) [20], but the distance to the extra one further apart is significantly longer when compared to the gadolinium representative (d(Gd–S′) = 361 pm) [20]. Based on calculations of the Effective Coordination Number (ECoN) [24], we have decided to take only S23 with ECoN = 0.156 into account, but leave S8′ with ECoN = 0.002 behind in the construction of the capped trigonal prismatic coordination sphere of the (Yb5/Li11)²⁺ cation.

Figure 7:

Depiction of the two different kinds of strings of Yb³⁺-centered sulfur polyhedra with Yb1 and Yb2 (left) as well as with Yb4 and Yb5/Li11 (right), which are connected through [(Yb3)S₈]¹³⁻ polyhedra in Li₆Yb₃[PS₄]₅ to form a three-dimensional network.

The ytterbium-sulfur polyhedra are connected into two different kinds of strings, which build up the framework of the Li₆Yb₃[PS₄]₅ structure. The polyhedra surrounding Yb1 are connected via corners to the coordination polyhedra of Yb2 and build up a chain of sulfur-connected [YbS₈]¹³⁻ polyhedra along [010] (Figure 7). The [(Yb4)S₈]¹³⁻ antiprisms occur as dimers by edge-sharing and are connected via [(Yb5/Li11)S₆₊₁]¹²⁻ prisms to broad bands propagating along [010] (Figure 7). The strings of Yb1- and Yb2-containing polyhedra are connected to the ribbons of Yb4- and Yb5/Li11-centered polyhedra through the polyhedra around Yb3 and the [PS₄]^3– tetrahedra which produces a three-dimensional framework  ∞3{[Yb3[PS4]5]6−} (Figure 8).

Figure 8:

Connection of the sulfur polyhedra centered by Yb2, Yb3, Yb5/Li11 through P5 to form a three-dimensional framework in the Li₆Yb₃[PS₄]₅ structure as viewed along [010].

The [PS₄]^3– tetrahedra are comparable to similar tetrahedra in other ortho-thiophosphates(V) (Tables 6 and 7). With P–S bond lengths varying between 201.8 and 206.7 pm (Table 6), they fall just short of the usual range of distorted [PS₄]^3– tetrahedra, when compared with Li₃[PS₄] (d(P–S) = 204.2–206.9 pm, ∢(S–P–S) = 106.2–113.9°) [23] or Yb[PS₄] (d(P–S) = 202.7–203.7 pm, ∢(S–P–S) = 106.4–116.4°) [25]. The sulfur tetrahedra around the P5 atoms function as a link between the different strands present in the structure. As shown in Figure 8, they connect the Yb1/Yb2- and Yb4/Yb5-centered chains.

The 10 crystallographically different Li⁺ cations reside in four- to sixfold sulfur coordination in the described framework structure. The polyhedra can be addressed as strongly distorted tetrahedra or trigonal pyramids (Li1, Li2, Li5, Li6 and Li9), trigonal bipyramids (Li3, Li4, Li8 and Li10) or octahedra (Li7). The bond lengths vary in the range of d(Li–S) = 245–304 pm, which is a larger interval than usually found in lithium thiophosphates, when compared e. g. with Li₃[PS₄] (d(Li–S) = 242–261 pm) [23] or Li₄[P₂S₆] (d(Li–S) = 263 pm, 6×) [26], but a similar situation is found in the other lithium ytterbium thiophosphate Li₉Yb₂[PS₄]₅ (d(Li–S) = 242–294 pm, Table 3) with the structure type of Li₉Nd₂[PS₄]₅ (d(Li–S) = 243–294 pm) [19].

Since the Yb5 site is half occupied with lithium, the lithium sites Li6–Li10 are only partially occupied, which leads to a full compensation of charges (s.o.p.(Li⁺) = 45–85%, Table 5). This clarifies the assumption of Mewis et al. in 2007 [20] that the RE5 site is only partially occupied by the rare earth metal (site occupation probability (s.o.p.) of RE = 44–65%) [20], and consequently some of the lithium sites must not be fully occupied. It was now possible to refine the structure to an extent, such that the mixed occupancy has been confirmed and the partial occupation of several lithium sites has been revealed (Table 5).

4 Discussion

The two compounds presented in this work both crystallize monoclinically and even in the same space group type, namely C2/c, but with two different structure types as described in the previous sections. They were prepared following reaction Eqs. (1) and (2), with Yb[PS₄] and Li₄[P₂S₆] found as by-products in both reactions. In Yb[PS₄] the two Yb³⁺ sites are both eightfold coordinated by sulfur atoms (d(Yb–S) = 280.4–300.9 pm) with distances very much comparable to those found in Li₉Yb₂[PS₄]₅ (d(Yb–S) = 277.9–289.7 pm), where the unique Yb³⁺ cation is eightfold coordinated as well, and in Li₆Yb₃[PS₄]₅ (d(Yb–S) = 271.3–319.1 pm), with C.N. = 8 for Yb1–Yb4. The mean Yb–S distance in Li₉Yb₂[PS₄]₅ amounts to d‾ = 283.0 pm, which perfectly agrees with the two different values for Yb[PS₄] (d‾ = 286.7 pm for Yb1 and d‾ = 272.8 pm for Yb2) [4], [27], but is longer than the three ones for the trigonal-dodecahedrally surrounded Yb³⁺ cations in Li₆Yb₃[PS₄]₅ (d‾ = 281.5 pm for Yb1, d‾ = 282.7 pm for Yb2 and d‾ = 282.0 pm for Yb3) and almost the same as for the square-antiprismatically coordinated one (d‾ = 285.1 pm for Yb4). Similar, yet shorter distances are found in the sesquisulfides, for instance the T-type Yb₂S₃ (d(Yb–S) = 269.7–278.8 pm) [28, 29, 30] or the E-type Yb₂S₃ (d(Yb–S) = 265.6–273.6 pm) [31, 32, 33], but only six sulfur atoms reside here in closer proximity to the Yb³⁺ cations. The Yb5 position, which shows a mixed occupancy with lithium (Li11) in Li₆Yb₃[PS₄]₅, is also only sixfold coordinated. The comparison with Yb5/Li11 is handicapped by the mixed occupation on the one hand and the lower coordination number (C.N. = 6 + 1) on the other. The three short distances to sulfur ranging from 259 to 269 pm are acceptable for Li⁺, but too short for Yb³⁺, while the four long ones (279–314 pm) appear too long for Li⁺, but suitable for Yb³⁺ on this compromising site. Mewis et al. discussed even an eighth sulfur ligand at 361 pm for Li₆Gd₃[PS₄]₅ [20], which turns into 377.8 pm for d((Yb5/Li11)–S8’) in Li₆Yb₃[PS₄]₅ and therefore into something negligible for the smaller Yb³⁺ cation as compared to Gd³⁺ [21]. The mean values for d‾ ((Yb5/Li11)–S) of 280.0 pm for C.N. = 7 and 274.3 pm for C.N. = 6 reflect this trend and agree quite well with the one found for NaCl-type LiYbS₂ (d(Li/Yb–S) ≈ 270 pm) [34] with the same 1:1 mixed occupation for Li⁺ and Yb³⁺ at the same crystallographic site, while α-NaFeO₂-type LiYbS₂ shows a cation ordering with shorter, d(Li–S) = 265.1 pm, and longer, d(Yb–S) = 275.7 pm, cation-anion distances [35], [36]. However, one should keep in mind that this LiYbS₂ situation applies for C.N. = 6 in both polymorphic structures.

The Li–S distances found in the by-product Li₄[P₂S₆] (d(Li–S) = 263.0 pm, 6×) [26] are well within the range of the Li–S distances found in the two structures (Li₉Yb₂[PS₄]₅: d(Li–S) = 241–293 pm; Li₆Yb₃[PS₄]₅: d(Li–S) = 235–304 pm) described here and just slightly longer than in lithium ortho-thiophosphate(V) Li₃[PS₄] (d(Li–S) = 242–261 pm) [23]. Some Li⁺ positions are underoccupied and others share sites with Yb³⁺ cations as in Li₆Yb₃[PS₄]₅. Even in Li₉Yb₂[PS₄]₅ the high displacement values suggest a certain Li⁺ cation mobility, which should be investigated by conductivity measurements as soon as pure material of these highly moisture sensitive compounds is available.

Both structures presented here can be regarded as a combination of Yb[PS₄] and Li₃[PS₄] in an integer ratio. Thus they can be attributed to the general formula (Li₃[PS₄])_x(Yb[PS₄])_y with x/y = 1.5 for Li₉Yb₂[PS₄]₅ and x/y = 0.667 for Li₆Yb₃[PS₄]₅, which can be explained as Li₃[PS₄]-filled framework structures of Yb[PS₄]. Following this, it becomes apparent that Li₉Yb₂[PS₄]₅ is generally lithium-richer than Li₆Yb₃[PS₄]₅. The values for x and y and the ratio of those can hint at the dimensionality of the framework structure [2]. The higher the fraction of Li₃[PS₄] within the structure, the lower the dimensionality. This is true for both compounds presented here, as there are layers with the composition  ∞2{[Yb[PS4]3]6−} within the Li₉Yb₂[PS₄]₅ structure and a three-dimensional framework  ∞3{[Yb3[PS4]5]6−} in the Li₆Yb₃[PS₄]₅ structure. Due to the slightly different occupation of certain cation sites with lithium or ytterbium, the [PS₄]^3– tetrahedra are shifted, so that the channels and layers within both structure types differ from each other. In both structures, chains of sulfur-connected [YbS₈]¹³⁻ polyhedra are found. In Li₉Yb₂[PS₄]₅ there is only one unique Yb³⁺ position, so the chain consists only of this one, whereas in Li₆Yb₃[PS₄]₅ the similar chains are made from Yb1 and Yb2. A very suitable comparison between the crystal structures for the formula types Li₉RE₂[PS₄]₅ and Li₆RE₃[PS₄]₅ was already presented by Mewis et al. [20] 14 years ago, so it must not be repeated here. The only thing to emphasize is the replacement of three Li⁺ cations of the lithium-rich case with one RE³⁺ cation in the lithium-poor case, which not only is reflected by the empirical formulae, but also by a view along [010] (b ≈ 979 pm for Li₉Yb₂[PS₄]₅ versus b ≈ 997 pm for Li₆Yb₃[PS₄]₅) at the crystallographic ac planes of the two monoclinic structures in space group C2/c as is shown in Figure 4 in reference [20].

Comparisons with the binary components have their limitations, since fourfold coordinated Li⁺ cations in Li₂S (anti-fluorite structure, d(Li–S) = 247.8 pm, 4×) [37] and sixfold coordinated Yb³⁺ cations in Yb₂S₃ (bixbyite structure: d(Yb–S) = 269.7–278.8 pm [30], corundum structure: d(Yb–S) = 265.6–273.6 pm [33]) have to be juxtaposed. Only for the P₂S₅ structure [38] with its P₄S₁₀ molecules [39] there is a good match, for tetrahedrally coordinated P⁵⁺ centers are present here with d(P–S) = 189.0–212.0 pm) as also found in Li₉Yb₂[PS₄]₅ (d(P–S) = 202.0–205.5 pm) and Li₆Yb₃[PS₄]₅ (d(P–S) = 200.5–206.7 pm) although three times vertex-sharing [PS₄]^3– tetrahedra in the supertetrahedral P₄S₁₀ cage with one short terminal and three long bridging P–S bonds [38] are compared with isolated ones in both lithium ytterbium(III) ortho-thiophosphates(V).

The crystals of both compounds presented in this paper appear in a deep red color, just like Yb[PS₄], which can be attributed to a ligand-to-metal charge transfer process (S²⁻(3p) → Yb³⁺(4f)) in the sense of a prereduction of the Yb³⁺ (4f¹³) to the Yb²⁺ cation (4f¹⁴). This process becomes less efficient as the coordination number of Yb³⁺ decreases or the coordination sphere narrows, since T-Yb₂S₃ is orange [30] and E-Yb₂S₃ [33] as well as LiYbS₂ [35] are lemon yellow.

Meanwhile, besides Li₉Yb₂[PS₄]₅ and Li₆Yb₃[PS₄]₅, several other Li₉RE₂[PS₄]₅ (RE = Pr, Sm, Dy, Ho, Er and Lu) [40], [41] and Li₆RE₃[PS₄]₅ (RE = Gd, Dy, Ho, Lu and Y) [20], [40] phases are known, on the basis of which we could have written this paper, but the ytterbium examples were chosen, since an esteemed colleague and Hessian compatriot is turning 70 in May 2021, and ytterbium happens to have this atomic number.