Structural Stability Diagram of ALnP2S6 Compounds (A = Na, K, Rb

Article pubs.acs.org/IC

Structural Stability Diagram of ALnP2S6 Compounds (A = Na, K, Rb, Cs; Ln = Lanthanide) Leslie M. Schoop,*,† Roland Eger,† Reinhard K. Kremer,† Alexander Kuhn,† Jürgen Nuss,† and Bettina V. Lotsch*,†,‡,¶ †

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany ¶ Nanosystems Initiative Munich (NIM) & Center for Nanoscience, Schellingstr. 4, 80799 München, Germany ‡

S Supporting Information *

ABSTRACT: Thiophosphate compounds have been studied extensively in the past for their rich structural variations and for a large variety of interesting properties. Here, we report 11 new phases with the composition ALnP2S6 (A = Na, K, Rb, Cs; Ln = lanthanide). These new thiophosphates crystallize in four different structure types, with the space groups Fdd2, P1̅, P21, and P21/c, respectively. All phases are insulating and the calculated band gaps range between 3 eV and 3.5 eV. Magnetic measurements on the compounds with open f-shells show paramagnetic behavior and magnetic moments that match the expected free ion values of the respective lanthanide cations. We present a structural stability phase diagram for the ALnP2S6 family of compounds, which reveals a clear relationship between ionic radii and the preferred crystal structure, as well as stability regions to form ALnP2S6-type phases.

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INTRODUCTION Rare-earth and transition-metal chalchophosphate compounds have been extensively studied in the past, not only because their structural variety is extremely rich,1 but also because they can exhibit many useful properties, such as ferroelectricity,2 valence fluctuations,3 one-dimensional antiferromagnetic chains,4,5 halfmetallicity,6 photoluminescence,7 nonlinear optics,8,9 or reversible redox chemistry.10 They also show potential for use as thermoelectrics,11 electrocatalysts,12 or cathode materials,13,14 as well as ionic conductors.15−19 Recently, a theoretical study suggested that many salts of thiophosphate anions are promising candidates for applications in nonlinear optics for mid-infrared coherent light generation.20 Discovering further thiophosphate compounds, and also understanding which structure a certain composition is most likely to adopt, is therefore of interest, to be able to tailor different properties in the future. Rare-earth metal thiophosphate compounds usually contain either the tetrahedral [PS4]3− or the ethane-like hexathiohypodiphopsphate anion [P2S6]4− as their central building block.1 Whereas a very large amount of [PS4]3−-containing phases is known, the number of phases containing the [P2S6]4− anion is limited so far. In the case of [PS4]3− containing A3Ln3(PS4)4 phases, structural trends that relate the dimensionality of the adopted crystal structure to the alkali metal radius have been established,1 revealing that smaller alkali ions prefer higher connectivity in the anionic sublattice of the crystal structure. This increase in layer connectivity is necessary to decrease the anionic volume and to balance the charge densities of the © 2017 American Chemical Society

cationic and anionic sublattice. Hence, with A = Na or K, A3Ln3(PS4)4 phases adopt a three-dimensional (3D) crystal structure, whereas for A = Rb, the adopted structure is twodimensional (2D). A similar trend has been proposed to appear for ALnP2S6 phases, but since there are not enough of these phases known to date, no general rule could be securely applied yet. So far, four different compounds with the composition ALnP2S6 are known, mostly with A = Na. KLaP2S621 and NaSmP2S622 crystallize in the monoclinic space group P21/c (No. 14), adopting an ordered variant of the 2D crystal structure of the ferroelectric material Sn2P2S6.2 NaYbP2S622 and NaErP2S623 are known to be triclinic (space group P1̅), and crystallize in a 3D distorted CaCl2-related structure. There are also transition-metal-containing compounds featuring the [P2S6]4− anion that are found in different structure types. NaAlP2S6,24 AgAlP2S6,25 and the 3D polymorph of NaCrP2S626 all crystallize in the orthorhombic space group Fdd2 and adopt a 3D crystal structure. KBiP2S6 and KSbP2S6, as well as their Na and Tl analogues, crystallize in the monoclinic space group P21 (No. 4) and have a layered 2D crystal structure.27−30 Here, we report on the preparation and the crystal structures, as well as some elementary properties, of 11 new ALnP2S6-type phases. In combination with the known ALnP2S6 compounds, we were able to construct a structure map that clearly shows that the ionic radii of the alkali metal and rare-earth metal ions Received: August 26, 2016 Published: January 9, 2017 1121

DOI: 10.1021/acs.inorgchem.6b02052 Inorg. Chem. 2017, 56, 1121−1131

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Inorganic Chemistry

KLaP2S621 and NaSmP2S6.22 The crystallographic details are shown in Tables 1 and 2. The structure (shown in Figure 1a)

determine the adopted crystal structure. Our investigation provides substantial understanding toward the stability and structural chemistry of ALnP2S6 phases.

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Table 1. Crystallographic Data and Details of Data Collection for NaPrP2S6, KPrP2S6, and RbLaP2S6

EXPERIMENTAL DETAILS

General Synthesis Conditions. All preparations were carried out under an argon atmosphere. For the solid-state synthesis, stoichiometric amounts of sodium (Alfa Aesar, 99.8%), potassium (Alfa Aesar, 98%), RbBr (Alfa Aesar, 99.8%) and CsBr (Alfa Aesar, 99.9%), red phosphorus (Alfa Aesar, 98.9%), sulfur (Alfa Aesar, 99.5%), and lanthanide (Sc, Y, La, Ce, Pr, Sm, Tb, Lu (Johnson Matthey, pieces, distilled dendritic, 99.99%)) were sealed, unmixed, in uncoated quartz tubes, under vacuum (length, 200 mm; inside diameter, 10 mm). [NOTE: It is important not mix the starting materials before f illing them in the reaction container, because this can cause the alkali metals to react spontaneously with phosphorus and sulf ur in an uncontrolled manner.] To prevent damage of the sealed quartz tube due the initial strongly exothermic reaction, the starting material was placed into a small quartz container inside the quartz tube (length: 80 mm, outside diameter: 8 mm). All products were characterized using single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD), and the composition was confirmed via scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX) spectroscopy. The 11 synthesized quaternary alkali-metal rare-earth hexathiohypodiphosphates are sensitive against moisture to a different extent and must be stored and handled under inert conditions. Detailed description of the synthesis conditions for each phase are given in the Supporting Information. X-ray Diffraction. For SXRD, crystallites were picked under dry petroleum and mounted and sealed in glass capillaries. SXRD of NaPrP2S6 and NaLuP2S6 was carried out using an X-ray diffractometer (Model SMART-APEX CCD, Bruker AXS) working with graphitemonochromated Mo Kα radiation. The SAINT software (Bruker AXS) was used to integrate the reflections. The data collection for the other ALnP2S6 phases was performed with a STOE IPDS II diffractometer working with graphite-monochromated Mo Kα radiation. The STOE X-Area 1.56 software was used for integrating the reflection intensities and calculating the reciprocal lattice planes. The structures were solved with direct methods and refined by least-squares fitting, using the SHELXTL program.31 The crystals of compounds that contain rubidium and cesium were extremely soft and fragile, which made it challenging to identify crystals of high quality for SXRD. For PRXD, the powders were sealed in glass capillaries and the diffraction pattern was measured in Debye−Scherrer geometry, using a STOE StadiP diffractometer working with Ge-monochromated Mo Kα radiation. Electronic Structure Calculations. The calculations were performed in the framework of density functional theory (DFT), using the WIEN2K32 code with a full-potential linearized augmented plane-wave and local orbitals [FP-LAPW + lo] basis33−35 with the modified Becke−Johnson (mBJ) parametrization of the exchangecorrelation potential, which has been shown to result in band gaps of semiconductors in agreement with experimental values.36 The plane wave cutoff parameter RMTKMAX was set to 7 and the Brillouin zone was sampled by 171−442 k-points. For compounds with heavier lanthanides present, spin orbit coupling (SOC) was included for the lanthanide as a second variational procedure. Measurements of Magnetic Properties. Temperaturedependent magnetic susceptibility measurements were performed on a magnetic property measurement system (MPMS-XL5) from Quantum Design. A field of 1000 Oe was applied. No difference in the temperature-dependent magnetic susceptibility for field cooling or zero field cooling was observed.

crystal color space group formula weight (g/mol) density (g/cm3) a (Å) b (Å) c (Å) β (deg) V (Å3) Z radiation difference e− density (e/Å3) Rint R1 (F0 > 4σ(F0)) wR2 absorption coefficient, μ (mm−1) goodness of fit, GoF deposition number

NaPrP2S6

KPrP2S6

transparent green P21/c (No. 14) 418.229

transparent green P21/c (No. 14) 434.341

transparent P21/c (No. 14) 478.712

RbLaP2S6

3.118 11.6056(4) 7.3991(2) 11.1533(4) 111.554(1) 890.85(5) 4 Mo Kα1 +1.143 to −1.050

3.017 11.979(2) 7.468(2) 11.332(2) 109.38(3) 956.3(3) 4 Mo Kα1 +1.251 to −1.008

3.147 12.327(3) 7.529(2) 11.492(2) 108.72(3) 1010.13(4) 4 Mo Kα1 +3.924 to −2.979

0.0198 0.0197 0.0492 7.197

0.0361 0.0183 0.0408 7.095

0.0976 0.0644 0.1624 10.498

1.117

0.955

1.015

CSD-431530

CSD-431527

CSD-431534

Table 2. Crystallographic Data and Details of Data Collection for CsCeP2S6, CsLaP2S6, and KCeP2S6 CsCeP2S6 crystal color space group formula weight (g/mol) density (g/cm3) a (Å) b (Å) c (Å) β (deg) V (Å3) Z radiation difference e− density (e/Å3) Rint R1 (F0 > 4σ(F0)) wR2 absorption coefficient, μ (mm−1) goodness of fit, GoF deposition number

CsLaP2S6

KCeP2S6




3.357 12.686(3) 7.495(2) 11.485(2) 107.18(3) 1043.3(4) 4 Mo Kα1 +5.749 to −3.827

3.305 12.732(3) 7.535(2) 11.554(2) 107.45(3) 1057.4(4) 4 Mo Kα1 +2.471 to −2.021

2.978 11.993(2) 7.517(2) 11.383(2) 109.58(3) 996.9(3) 4 Mo Kα1 +0.967 to −1.417

0.0945 0.0519 0.1176 9.237

0.0788 0.0545 0.1678 8.849

0.0706 0.0446 0.0550 6.686

0.985

1.556

0.810

CSD-431524

CSD-431525

CSD-431526

−

consists of ∞2[Ln(P2S6)] layers that are separated by the alkali cations. The alkali ions are 8-fold coordinated by sulfur (Figure 1c). The ionic interlayer distances between Na and S range from 2.96 Å to 3.08 Å, whereas they are larger in the case of K, Rb, and Cs (3.20−3.35 Å, 3.33−3.44 Å, and 3.48−3.62 Å, respectively). Within the layers, the lanthanide ions are linked through [P2S6]4− anions in such a way that each lanthanide ion is coordinated by nine S atoms. The LnS9 polyhedra (Figure 1b) take the shape of capped quadratic antiprisms and share edges and corners with the [P2S6]4− anions. Since the

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RESULTS NaPrP2S6, KPrP2S6, KCeP2S6, RbLaP2S6, CsCeP2S6, and CsLaP2S6. NaPrP2S6, KPrP2S6, KCeP2S6, RbLaP2S6, CsCeP2S6, and CsLaP2S6 all crystallize in the monoclinic space group P21/c (No. 14) and adopt the same 2D crystal structure as 1122


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Figure 1. (a) Crystal structure of NaPrP2S6, KPrP2S6, KCeP2S6, RbLaP2S6, CsCeP2S6, and CsLaP2S6; the two-dimensional (2D) layers stack along the a-axis. (b−d) The different coordination polyhedra for the lanthanide (panel (b)), the alkali (panel (c)), and the [P2S6]4− anion (panel (d)). (e) Photograph of transparent green crystals of NaPrP2S6.

Figure 2. (a) Total density of states (DOS) and partial density of states (PDOS) of RbLaP2S6, La states are shown in red, S states are shown in blue, P states are shown in green, and Rb states are shown in orange. (b) Band structure plot of RbLaP2S6, showing that the conduction band consists of highly localized La f-states. (c) Total DOS and PDOS of CsLaP2S6, La states are shown in red, S states are shown in blue, P states are shown in green, and Cs states are shown in orange. (d) Band structure plot of CsLaP2S6; the electronic structures of both compounds are closely related.

between 2.92 Å and 3.22 Å, whereas Ce−S and La−S bond lengths are between 2.98 Å and 3.30 Å. The P−P and P−S bond lengths of the [P2S6]4− anion are very similar in all six

coordination number of 9 is preferred for larger lanthanide cations, it can be expected that this structure type is more favored for the early lanthanides. The Pr−S bond lengths range 1123


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Figure 3. (a) Temperature-dependent magnetic susceptibility, as well as Curie−Weiss fit of NaPrP2S6 and (b) temperature-dependent magnetic susceptibility of KPrP2S6. No χ0 was used for the Curie−Weiss analysis of NaPrP2S6. For KPrP2S6, a Curie−Weiss analysis was not possible, because of the curvature.

Figure 4. (a) Crystal structure of KSmP2S6, the two-dimensional (2D) layers stack along the c-axis. (b−d) Different coordination polyhedra for (b) the lanthanide, (c) the alkali, and (d) the [P2S6]4− anion. (e) Photograph of a yellow transparent crystal of KSmP2S6.

slightly smaller band gap, because of the larger and softer Se atoms. The partial density of states (PDOS) plot (Figures 2a and 2c) shows that the valence band is composed of states of all elements, with sulfur having the largest contribution, in agreement with a −2 oxidation state of sulfur. The conduction band is almost exclusively composed of La f-states that are separated by a gap from other states in the conduction band. These states are very flat, nondispersive, and located 3.0− 3.2 eV above the Fermi level (Figure 2b). Since in La3+ the f-shell is empty but would be populated next upon reduction of La3+, this type of band structure is not surprising. The band gap seems to be of direct nature at the Γ-point; however, since the conduction band is extremely flat, other indirect transitions are also possible.

compounds (∼2.2 and 2.0 Å, respectively). For detailed bond lengths and atomic coordinates please refer to Tables S12 and S5−S10 in the Supporting Information. PXRD patterns of all compounds can be found in Figure S1 in the Supporting Information. While the La- and Ce-containing phases are colorless, the Pr-containing phases appear as transparent green crystals (Figure 1e), because of the open f-shell of Pr. Electronic structure calculations predict band gaps of 2.99 eV for RbLaP2S6 and 2.96 eV for CsLaP2S6, which is not only in agreement with the lack of color but also within the range of the band gaps in related selenidophosphates, where a gap of 2.7−2.8 eV has been experimentally determined for the isotructural KLaP2Se6.37 As expected, the Se compound has a 1124


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Inorganic Chemistry The plot of the inverse magnetic susceptibility versus temperature displayed in Figure 3 reveals a slight curvature typical for the magnetic susceptibilities of Pr3+ cations, because of crystal field splitting effects of the 3H4 groundstate multiplet.38 The effective magnetic moments at 300 K amount to 3.68 μB and 3.89 μB for NaPrP2S6 and KPrP2S6, respectively, in agreement with the value of 3.6 μB typical for Pr cations in a +3 oxidation state. The slight excess, especially found for KPrP2S6, may be due to a population of the excited 3H5 multiplet typically located only 2000 cm−1 above the ground state.39 The magnetic susceptibility data down to 1.8 K give no indication of long-range magnetic order, despite the slightly negative Curie−Weiss temperature (−20.7 K) obtained when extrapolating the low-temperature inverse susceptibility data. In the NaPrP2S6 structure, the Pr ions form a network of distorted tetrahedra that are arranged in a triangular lattice. The distances between the individual Pr atoms are within the range of 5.02−5.49 Å. This range is very large, and observing the magnetic order or the effects due to magnetic frustration in these materials is therefore unlikely, despite the negative Weiss temperature. For comparison, in the frustrated pyrochlore derivatives Ln3Sb3Zn2O14 and Ln3Sb3Mg2O14,40,41 the shortest lanthanide distance amounts to ∼3.7 Å. These compounds order below 1 K and have comparable Weiss temperatures (−38.4 K for Pr3Sb3Zn2O14).42,43 KSmP2S6. KSmP2S6 crystallizes in the monoclinic space group P21 (No. 4) and adopts a layered 2D crystal structure that is closely related to the monoclinic structure adopted by NaPrP2S6, KPrP2S6, KCeP2S6, RbLaP2S6, CsCeP2S6, and CsLaP2S6. In fact, the compound NaSbP2S6 is known to exist in both structure types.29 The structure (shown in Figure 4a) − also consists of ∞2[Ln(P2S6)] layers that are separated by the alkali cations; the difference between the two monoclinic structure types can be understood as a stacking difference of − the ∞2[Ln(P2S6)] layers. The alkali ions are 9-fold coordinated by sulfur (Figure 4c). The ionic interlayer distances between potassium and sulfur range from 3.16 Å to 3.7 Å. Again, the Ln ions are linked through [P2S6]4− anions within the layers; however, in the KSmP2S6 structure, each Ln ion is coordinated by only eight S atoms. In this structure type, the anions are translated into each other by a 2-fold screw axis, rather than a center of inversion, which results in the noncentrosymmetric space group. The LnS8 polyhedra (Figure 4b) take the shape of a distorted square antiprism and share corners and edges with the [P2S6]4− anions. The Sm−S bond lengths range between 2.90 Å and 3.01 Å, and a ninth S atom is 3.83 Å away from the Sm atom. Therefore, samarium, which is the smallest of the lanthanides that adopts a monoclinic ALnP2S6-type phase, can undergo a change in its coordination number, based on the A cation present (NaSmP2S6 crystallizes in the P21/c space group).22 KSmP2S6 appears as transparent yellow crystals. Detailed crystallographic information can be found in Table 3; atomic coordinates and a detailed list of bond distances can be found in Tables S11 and S12, respectively, in the Supporting Information. A PXRD pattern of KSmP2S6 can be found in Figure S1. Magnetic measurements (not shown) reveal a very small magnetic moment, which is expected for Sm3+ compounds. No sign of magnetic order was observed down to 1.8 K. Because of the typical large temperature-independent Van-Vleck paramagnetism contribution from the first excited J = 7/2 multiplet of Sm3+,44 Curie−Weiss fits were not advisible for this material.

Table 3. Crystallographic Data and Details of Data Collection for KSmP2S6 KSmP2S6 crystal color space group formula weight (g/mol) density (g/cm3) a (Å) b (Å) c (Å) β (deg) V (Å3) Z radiation difference e− density (e/Å3) Rint R1 (F0 > 4σ(F0)) wR2 absorption coefficient, μ (mm−1) goodness of fit, GoF Flack parameter, x deposition number

transparent yellow P21 (No. 4) 338.39 3.096 6.495(1) 7.381(2) 9.938(2) 92.52(3) 476.0(2) 2 Mo Kα1 +1.472 to −1.655 0.0461 0.0258 0.0500 8.178 0.936 −0.017(9) CSD-431528

NaYP2S6, NaTbP2S6, and NaLuP2S6. NaYP2S6, NaTbP2S6, and NaLuP2S6 all crystallize in the triclinic space group P1̅ (No. 2) and adopt the same 3D crystal structure as NaErP2S623 and NaYbP2S622 (see Table 4 for crystallographic details). Table 4. Crystallographic Data and Details of Data Collection for NaYP2S6, NaTbP2S6, and NaLuP2S6 NaYP2S6 crystal color space group formula weight (g/mol) density (g/cm3) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z radiation difference e− density (e/Å3) Rint R1 (F0 > 4σ(F0)) wR2 absorption coefficient μ (mm−1) goodness of fit, GoF deposition number

NaTbP2S6

NaLuP2S6

transparent P1̅ (No. 2) 366.227 2.732 6.870(1) 7.114(1) 9.128(2) 87.63(3) 87.59(3) 88.11(3) 445.1(1) 2 Mo Kα1 +1.311 to −0.602 0.0469 0.0283 0.0454 8.280

transparent P1̅ (No. 2) 436.246 3.263 6.849(1) 7.124() 9.107(2) 87.71(3) 87.54(3) 88.19(3) 444.01(15) 2 Mo Kα1 +4.477 to −1.749 0.0243 0.0271 0.0685 9.697

transparent P1̅ (No. 2) 452.292 3.440 6.8185(3) 7.204(3) 9.1370(4) 87.232(1) 87.633(3) 88.287(1) 436.55(3) 2 Mo Kα1 +1.789 to −2.765 0.0184 0.0202 0.0517 13.070

0.763 CSD-431533

1.026 CSD-431532

1.027 CSD-431529

The structure (Figure 5a) consists of a 3D network of − 3 ∞[Ln(P2 S6 )] that has tunnels along all three axes hosting the Na ions. Although the structure is 3D, there are only a few connections between layers that stack along the [011] direction (see Figure 5). Therefore, the structure can also be seen as an intermediate between a 2D crystal structure and 3D crystal structure. The Ln ions are coordinated by eight S atoms in a 1125


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Figure 5. (a) Crystal structure of NaYP2S6, NaTbP2S6, and NaLuP2S6. Ln3+ and [P2S6]4− ions form a 3D network. (b−d) Different coordination polyhedra for (b) the lanthanide, (c) the alkali, and (d) the [P2S6]4− anion. (e) Photograph of a transparent crystal of NaYP2S6.

being located at the L point. However, the difference between the indirect band gap and the direct gap at M is minimal. In the case of NaYP2S6, a gap of 3.35 eV was found, and the band structure plot (Figure 6d) is very similar to that of NaLuP2S6, showing the same type of indirect band gap. The PDOS plot (Figure 6c) for NaYP2S6 shows that the upper valence band is also composed of sulfur states that are only slightly mixed with states of the other elements. The lower conduction band consists mostly of Y states, which matches the expected oxidation state of +3 for Y. The band gap of isostrucutral NaYbP2S6 has previously been measured to be 1.85 eV;22 the much lower value probably arises due to the open f-shell of Yb3+. Magnetic susceptibility measurements of the Tb compound indicate paramagentic behavior with no indication of long-range order above 1.8 K, similar to the situation observed for the monoclinic compounds. Figure 7 shows the temperaturedependent magnetic susceptibility, as well as the temperaturedependent inverse susceptibility of NaTbP2S6. The small feature at 230 K likely arises from a small impurity of Tb metal that orders antiferromagnetically around this temperature.45 The Curie−Weiss fit indicated in Figure 7 results in an effective magnetic moment of 9.54 μB, which matches the expected free ion value of Tb3+ very well (9.7 μB). The Weiss temperature is found to be 17 K, which suggests predominant ferromagnetic interactions of the spins. In NaTbP2S6, the Tb3+ ions form zigzag chains along the c-axis, with Tb−Tb distances of ∼4.85 Å, which, however, is again too large to expect cooperative order between the Tb spins. NaScP2S6. For Ln = Sc, only one ALnP2S6-type compound could be obtained, with A = Na. It crystallizes in the orthorhombic space group Fdd2 (No. 43) and is isotypic to NaAlP2S6,24 AgAlP2S6,25 and 3D-NaCrP2S6.26 Crystallographic details are given in Table 5. The structure (see Figure 8) can be described as a stuffed variant of the TiP2S6 structure46,47 and

heavily distorted square prismatic fashion (Figure 5b). If only the six shortest bond lengths are considered, the Ln ions are coordinated in a trigonal antiprismatic fashion; however, since two more S atoms are only 0.05−0.2 Å farther away, the polyhedra is better described by a coordination number of 6 + 2. The Ln−S bond lengths found in all three structures are in good agreement with those observed in related structures.22,23 Please refer to Table S12 in the Supporting Information for a detailed list of bond lengths and Tables S2−S4 in the Supporting Information for information on the atomic coordinates for all phases. The LnS8 polyhedra share edges with each other to form a zigzag structure along the c-axis. These zigzag chains are connected by [P2S6]4− ions, sharing edges and corners with the LnS8 polyhedra. The Na atoms are coordinated by eight S atoms that form a distorted square antiprismatic polyhedra (Figure 5c). As in the monoclinic phases, the P−P and P−S bond lengths of the [P2S6]4− anion are ∼2.2 and 2.0 Å, respectively. PXRD patterns of all compounds can be found in Figure S1. The smaller coordination number for the Ln ion found in the triclinic structure, compared to the monoclinic structures, agrees well with our observation that the triclinic structure seems to be preferably formed with smaller lanthanides. Electronic structure calculations predict a band gap of 3.48 eV for NaLuP2S6, which agrees well with the optical transparency of the material. The PDOS analysis (Figure 6a) shows that the upper valence band consists mostly of sulfur states, which agrees well with an expected −2 oxidation state for sulfur. The Lu f-states are well-localized below the Fermi level, as expected for a completely filled f-shell of Lu. The lower conduction band is composed of states of all elements, with Lu and P states having the largest contribution. The band gap is indirect (Figure 6b), with the valence band maximum being located at the M point and the conduction band minimum 1126


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Figure 6. (a) Total DOS and PDOS of NaLuP2S6; Lu states are shown in red, S states are shown in blue, P states are shown in green, and Na states are shown in orange. (b) Band structure plot of NaLuP2S6, showing that the band gap is indirect. (c) Total DOS and PDOS of NaYP2S6, Y states are shown in red, S states are shown in blue, P states are shown in green, and Na states are shown in orange. (d) Band structure plot of NaYP2S6; the electronic structures of both phases are very similar.

Table 5. Crystallographic data and details of data collection for NaScP2S6 NaScP2S6 crystal color space group formula weight (g/mol) density (g/cm3) a (Å) b (Å) c (Å) V (Å3) Z radiation difference e− density (e/Å3) Rint R1 (F0 > 4σ(F0)) wR2 absorption coefficient, μ (mm−1) goodness of fit, GoF Flack parameter, x deposition number

Figure 7. Temperature-dependent magnetic susceptibility, as well as Curie−Weiss fit of NaTbP2S6. No χ0 was used for the Curie−Weiss analysis. The small anomaly at 230 K is likely due to a small impurity of Tb metal that orders antiferromagnetically around this temperature.45

consists of ScS6 octahedra that share three edges with [P2S6]4− anions, forming an anionic 3D open network [ScP2S6]−. The Na+ cations occupy holes of the anionic network and are coordinated by six S atoms in a distorted octahedral fashion (Figure 8c). The Na−S bond distances are between 2.87 Å and 2.98 Å (see Tables S1 and S12 in the Supporting Information for a detailed list of bond distances, as well as atomic coordinates). The fact that the very small Sc3+ ions are coordinated by only six S atoms fulfills the general trend that smaller rareearth metal ions prefer 6-fold coordination rather than 9- or 8-fold, as observed in the monoclinic or triclinic structures. In addition, of the four observed structure types in the ALnP2S6 phases, this is the most 3D-like structure. The [P2S6]4− anion is very similar shaped in all structure types, i.e., the bond lengths and angles do not vary by more than ±2° and ±0.02 Å (see the

transparent Fdd2 (No. 43) 322.278 2.192 8.024(4) 11.200(2) 21.735(4) 1953.3(7) 8 Mo Kα1 0.281 to −0.261 0.0327 0.020 0.034 2.33 0.888 0.04(4) CSD-431531

Supporting Information for a detailed list of bond lengths and bond angles). The anion itself is centrosymmetric in all structures. A PXRD pattern of NaScP2S6 can be found in Figure S1. NaScP2S6 crystals are colorless and transparent. The band gap is calculated to be 3.31 eV, again agreeing with the lack of color. Note that the band gap is in the desirable range for mid-infrared nonlinear optics applications and the compound is noncentrosymmetric. Therefore, future investigations of the second harmonic generation (SHG) coefficients of these materials are of interest. The upper valence band mostly consists of 1127


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Inorganic Chemistry

Figure 8. (a) Crystal structure of NaScP2S6. The structure is three-dimensional. (b−d) Different coordination polyhedra for (b) the lanthanide, (c) the alkali, and (d) the [P2S6]4− anion. (e) Photograph of a transparent crystal of NaScP2S6.

Figure 9. (a) Total DOS and PDOS of NaScP2S6; Sc states are shown in red, S states are shown in blue, P states are shown in green, and Na states are shown in orange. (b) Band structure plot of NaScP2S6, showing bands with very low dispersion.

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DISCUSSION ALnP2S6-type phases crystallize in four different structure types, ranging from 2D structures, to an intermediate 2D-like structure, to finally a 3D structure. Table S13 in the Supporting Information gives an overview of all phases reported here. Often, ionic radii sizes of either alkali metal or lanthanide ions, or their ratios, are a crucial factor in determining which crystal structure is preferred. Therefore, we derived a structure map by plotting the Shannon ionic radii48 of the Ln ions versus the Shannon ionic radii of the alkali-metal cations (see Figure 10). Similar diagrams have been constructed previously for LnAuX (where X = As, Sb, Bi)49 and XYZ (where X = Mg, Ca, Sr, Ba; Y = Cu, Ag, Au; and Z = P, As, Sb, Bi) phases50 and allowed to identify regions of stability of different crystal structures. Similar to the previous diagrams, we observe clear areas in which the different phase are preferred. While both monoclinic structures (P21/c and P21) seem to be adopted for larger lanthanides, the triclinic phase (P1̅) appears for the smaller ones. Only for Sc (r = 74.5 pm), which is much smaller than the smallest

sulfur states, similarly to other ALnP2S6 phases, again agreeing with a −2 oxidation state for sulfur (Figure 9a). The conduction band consists mainly of Sc states, as expected for Sc3+. Phosphorus states seem to have a higher contribution to the conduction band than to the valence band, which suggests a positive charge on the P atoms, as expected for [P2S6]4− anions. The band structure (Figure 9b) shows that the bands around the Fermi level are extremely flat, suggesting weak bonding. The band gap seems to be slightly indirect with the valence band maximum located at the Y point and the conduction band minimum found at the Γ point. However, the difference in direct transition between Y−Y or Γ−Γ is very small; it will most likely also be observed in an optical experiment. The electronic structure of NaScP2S6 is generally similar to that of triclinic NaYP2S6, suggesting that the impact of the crystal structure on the electronic structure is minor in ALnP2S6-type phases. The electronic structure seems to be rather dominated by the number of f-electrons present in the respective lanthanide. 1128


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conditions (such as, for example, high pressure), phases are formed in the empty region of the phase diagram. It is interesting that the noncentrosymmetric structures seem to only appear close to the phase boundary to the area that does not allow for stable ALnP2S6-type phases. These trends also match the observations in related ALnP2Se6 selenidophophate compounds. KLaP2Se6, KCeP2Se6, KPrP2Se6, and NaCeP2Se6 are all known to crystallize in the 2D P21/c monoclinic structure.37,51 Those four compounds contain large Ln ions, and they crystallize in the same structure as their thiophosphate analogues. In contrast to the thiophosphates, however, the two compounds KYP2Se6 and KGdP2Se6 are also known to exist. These seleniodophosphates are outside the stability range of the phase diagram for the thiophosphates and crystallize in a fifth, different structure type in the orthorhombic space group P212121.37 This structure is 2D and has an 8-fold coordination for the Ln ion, which generally matched the trends noted above. Although several attempts have been made to synthesize analogous thiophosphates, we could not find any compounds with this structure type. Hence, for selenidophosphates, the structural stability map seems to be slightly different, which is most likely related to the larger size and polarizability of the Se atoms, which reduces the Ln−Se bond distances and changes the ionic character of the compounds.

Figure 10. Structural map for ALnP2S6-type phases. The Shannon ionic radius of the Ln ions is plotted on the y-axis and the radius of the alkali-metal ions on the x-axis. The differently colored regions represent the different structure types adopted, indicated by the respective space group. The dimensionality and the coordination number (CN) of the lanthanide is also indicated for each region. Data for NaYbP2S6, NaErP2S6, KLaP2S6, and NaSmP2S6 were taken from refs 21−23.

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CONCLUSION We investigated ALnP2S6-type phases with A = Na, K, Rb, and Cs and Ln = Sc, Y, La, Ce, Pr, Sm, Tb, and Lu, and we found 11 new phases with this composition. All phases are insulating with band gaps of >3 eV if the f-shell is either full or empty, which makes the noncentrosymmetric phases possible candidates for mid-infrared nonlinear optics. The compounds with partially filled f-shells show paramagnetic behavior at >1.8 K. ALnP2S6-type phases crystallize in four different structure types; we showed that the preference for the structure types is dependent on the sizes of the lanthanide metal and alkali-metal ion radius. Our structure map clearly shows that the determining factor, with regard to whether a structure is two dimensional (2D) or three dimensional (3D), is the radius of the Ln ion. 3D structures are only found for the smaller lanthanides (Ln = Gd−Lu, Sc), which can be linked to the coordination number of the lanthanide. 2D structures are found for all alkali metals studied here (Ln = Na−Cs), but only for the larger lanthanides (Ln = La−Sm). Therefore, our structure map shows ranges of stability for the ALnP2S6-type phases, providing rules for which structure type will be adopted and also showing the limits for the formation of the ALnP2S6-type phases. Understanding ranges of phase stability is of crucial importance for potential future guided material design to tailor materials with desired properties.

lanthanide Lu (r = 86.1 pm), the orthorhombic phase is adopted. This partially agrees with the trend observed for A3Ln3(PS4)4 phases, stating that, for larger alkali ions, structures with a lower dimensionality are preferred.1 Both 3D structures (P1̅ and Fdd2) seem to form only with A = Na, whereas larger alkali cations seem to exclusively adopt the 2D monoclinic structures. However, in contrast, a 2D monoclinic structure is also found with Na on the A-site, suggesting that this rule does not capture all details. Figure 10 indicates that the Ln radius is also of crucial importance for the adopted crystal structure, and this is observed to an even to larger extent than the alkali-metal radius. A 2D monoclinic structure is only adopted with large Ln cations (Ln = La−Sm), whereas the smaller lanthanides (Ln = Gd−Lu, Sc) seem to prefer a 3D structure. This observation agrees well with the coordination number of the lanthanide in the different structure types. Whereas in the monoclinic 2D structures, the lanthanide is coordinated by nine or eight S atoms, the 3D triclinic structure features a coordination number of 6 + 2. Therefore, it seems obvious that this structure type is preferred by smaller lanthanides. The orthorhombic structure, which features the smallest coordination number of 6, is only adopted by very small Ln cations. The structure map shown in Figure 10 features an empty region where rLn is small and rA is large. In this region, despite many attempts, we could not prepare any ALnP2S6-type phases under the synthesis conditions applied here (all failed attempts are marked with an “X” in the structural map). This suggests that, in this region, the Ln ions are too small to adopt the 9-fold coordination required by the 2D structure; however, at the same time, the alkali ions are too large to accommodate a 3D crystal structure. Since there is no further 2D ALnP2S6-type phase known that has a lower coordination number for the lanthanide, only phases with different stoichiometry seem to be formed in this region. The KSmP2S6 structure allows for 8-fold coordination, so this structure type could be a possible solution for even smaller lanthanides; however, we could not find any evidence for this phase being formed with lanthanides smaller than Sm. It is however possible that, under different synthesis

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02052. Detailed tables of atomic coordinates; detailed lists of bond lengths and bond angles; detailed description of synthesis conditions; powder diffraction patterns (PDF) Crystallographic data for CsCeP2S6 (CIF) Crystallographic data for CsLaP2S6 (CIF) Crystallographic data for KCeP2S6 (CIF) Crystallographic data for KPrP2S6 (CIF) 1129


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Inorganic Chemistry

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Crystallographic Crystallographic Crystallographic Crystallographic Crystallographic Crystallographic Crystallographic

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for for for for for for for

chemical Hydrogen Evolution over Wide pH Range. ACS Energy Lett. 2016, 1, 367−372. (13) Brec, R.; Schleich, D.; Ouvrard, G.; Louisy, A.; Rouxel, J. Physical properties of lithium intercalation compounds of the layered transition-metal chalcogenophosphites. Inorg. Chem. 1979, 18, 1814− 1818. (14) Barj, M.; Sourisseau, C.; Ouvrard, G.; Brec, R. Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds. Solid State Ionics 1983, 11, 179−183. (15) Liu, Z.; Fu, W.; Payzant, E. A.; Yu, X.; Wu, Z.; Dudney, N. J.; Kiggans, J.; Hong, K.; Rondinone, A. J.; Liang, C. Anomalous high ionic conductivity of nanoporous β-Li3PS4. J. Am. Chem. Soc. 2013, 135, 975−978. (16) Lin, Z.; Liu, Z.; Fu, W.; Dudney, N. J.; Liang, C. Lithium Polysulfidophosphates: A Family of Lithium-Conducting Sulfur-Rich Compounds for Lithium−Sulfur Batteries. Angew. Chem. 2013, 125, 7608−7611. (17) Richards, W. D.; Wang, Y.; Miara, L. J.; Kim, J. C.; Ceder, G. Design of Li1+2xZn1−xPS4, a new lithium ion conductor. Energy Environ. Sci. 2016, 9, 3272−3278. (18) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A lithium superionic conductor. Nat. Mater. 2011, 10, 682− 686. (19) Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.; Yonemura, M.; Iba, H.; Kanno, R. High-power all-solid-state batteries using sulfide superionic conductors. Nat. Energy 2016, 1, 16030. (20) Kang, L.; Zhou, M.; Yao, J.; Lin, Z.; Wu, Y.; Chen, C. Metal Thiophosphates with Good Mid-infrared Nonlinear Optical Performances: A First-Principles Prediction and Analysis. J. Am. Chem. Soc. 2015, 137, 13049−13059. (21) Evenson, C. R., IV; Dorhout, P. K. Thiophosphate Phase Diagrams Developed in Conjunction with the Synthesis of the New Compounds KLaP 2 S 6 , K 2 La(P 2 S 6 ) 1 / 2 (PS 4 ), K 3 La(PS 4 ) 2 , K4La0.67(PS4)2, K9−xLa1+x/3(PS4)4 (x = 0.5), K4Eu(PS4)2, and KEuPS4. Inorg. Chem. 2001, 40, 2884−2891. (22) Goh, E.-Y.; Kim, E.-J.; Kim, S.-J. Structure Modification on Quaternary Rare Earth Thiophosphates: NaYbP2S6, NaSmP2S6, and KSmP2S7. J. Solid State Chem. 2001, 160, 195−204. (23) Komm, T.; Schleid, T. Drei Alkalimetall-Erbium-Thiophosphate: Von der Schichtstruktur bei KEr[P2S7] zur dreidimensionalen Vernetzung in NaEr[P2S6] und Cs3Er5[PS4]6. Z. Anorg. Allg. Chem. 2006, 632, 42−48. (24) Kuhn, A.; Eger, R.; Nuss, J.; Lotsch, B. V. Synthesis and Crystal Structures of the Alkali Aluminium Thiohypodiphosphates MAlP2S6 (M = Li, Na). Z. Anorg. Allg. Chem. 2013, 639, 1087−1089. (25) Pfitzner, A.; Andratschke, M.; Rau, F.; Brunklaus, G.; Eckert, H. Präparation, Kristallstruktur und NMR-Spektroskopie an AgAlP2Q6 (Q = S, Se). Z. Anorg. Allg. Chem. 2004, 630, 1752. (26) Coste, S.; Kopnin, E.; Evain, M.; Jobic, S.; Brec, R.; Chondroudis, K.; Kanatzidis, M. G. Polychalcogenophosphate flux synthesis of 1D-KInP2Se6 and 1D and 3D-NaCrP2S6. Solid State Sci. 2002, 4, 709−716. (27) Manriquez, V.; Galdamez, A.; León, D.; Garland, M.; Jimenez, M. Crystal structure of potassium bismuth hexathiodiphosphate, KBiP2S6. Z. Kristallogr.New Cryst. Struct. 2003, 218, 151−152. (28) Manríquez, V.; Galdámez, A.; León, D. R.; Garland, M. Crystal structure of potassium antimony hexathiodiphosphate, KSbP2S6. Z. Kristallogr.New Cryst. Struct. 2003, 218, 435−436. (29) Manríquez, V.; Galdámez, A.; Ruiz-León, D. Preparation, crystal structure and characterization of α-NaSbP2S6 and β-NaSbP2S6 phases. Mater. Res. Bull. 2006, 41, 1337−1344. (30) Gave, M. A.; Malliakas, C. D.; Weliky, D. P.; Kanatzidis, M. G. Wide Compositional and Structural Diversity in the System Tl/Bi/P/ Q (Q = S, Se) and Observation of Vicinal P-Tl J Coupling in the Solid State. Inorg. Chem. 2007, 46, 3632−3644. (31) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

KSmP2S6 (CIF) NaLuP2S6 (CIF) NaPrP2S6 (CIF) NaScP2S6 (CIF) NaTbP2S6 (CIF) NaYP2S6 (CIF) RbLaP2S6 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L. M. Schoop). *E-mail: [email protected] (B. V. Lotsch). ORCID

Bettina V. Lotsch: 0000-0002-3094-303X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Eva Brücher for magnetic measurements, Willi Hölle for SXRD measurements, Viola Duppel for taking images of the crystals, and Claudia Kamella for SEM/EDX analysis. We gratefully acknowledge the financial support by the Max Planck Society, the Nanosystems Initiative Munich (NIM) and the Center for Nanoscience (CENS). L.M.S. gratefully acknowledges financial support by the Minerva Fast Track Fellowship.

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REFERENCES

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Structural Stability Diagram of ALnP2S6 Compounds (A = Na, K, Rb

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