Structural Stability Diagram of ALnP2S6 Compounds (A = Na, K, Rb, Cs; Ln = Lanthanide)

KREP2S6 (RE = Sm, Gd, Tb, Dy): A Series of Polar Rare-Earth Thiophosphates with High Laser-Induced Damage Thresholds and Moderate Nonlinear-Optical Responses Synergistically Contributed by Isolated P2S6 Units and {[RE2S15]24-}∞ Layers

As one of the potential candidates of nonlinear-optical (NLO) materials, rare-earth chalcophosphates have demonstrated promising properties. Here, KREP2S6 (RE = Sm, Gd, Tb, Dy) were synthesized using the facile RE2O3-B-S solid-state method. They crystallize with a monoclinic chiral P21 structure, and their layer structures are built by isolated ethane-like P2S6 dimers and RES8 bicapped trigonal prisms built {[RE2S15]24-}∞ layers. By comparing the structures with related ones, the change of the alkali metal or RE3+ ions can cause structural transformation. Their band gaps are tunable between 2.58 and 3.79 eV, and their powder samples exhibit good NLO properties. Theoretical calculations suggest that the NLO properties are mainly contributed by P2S6 units and {[RE2S15]24-}∞ layers synergistically, in which {[RE2S15]24-}∞ layers and P2S6 units dominate the contribution to the band gap and second-harmonic-generation response, respectively. This work enriches the application of rare-earth chalcophosphates as NLO materials.

NaREP2Se6 (RE = Y, Sm, Gd-Dy): Quaternary Rare-Earth Selenophosphates with Unique 3D {[REP2Se6]-}∞ Framework Built by RESe8 and P2Se6 Motifs and Multiple Properties

Rare-earth (RE) chalcophosphates have been widely studied because of their abundant structures. Here, five new RE selenophosphates, NaREP2Se6 (RE = Y, Sm, Gd-Dy), were synthesized by a facile RE oxide-boron-selenium solid-state route. They crystallize in the triclinic P1̅ space group, featuring three-dimensional (3D) structures constructed by RESe8 and P2Se6 motifs, different from common 2D RE chalcophosphates A-RE-P2-Q6 (A = alkali metal; Q = S, Se) system. Their structural chemistry and relationship with related phases are analyzed. Both the size of A and the coordination geometry of RE have important influences on the system's structures. Their optical band gaps are tunable from 1.79 to 2.50 eV, and they exhibit diverse magnetic behaviors, including Van-Vleck-type paramagnetism, antiferromagnetism, and ferromagnetism. Their photocurrent responses and thermal stabilities are analyzed as well. Calculation results suggest that the RESe8 and P2Se6 units make a great contribution to the optical properties. This work enriches the chemistry and multifunctional properties of RE chalcophosphates.

KREP2 Se6 (RE = Sm, Gd, Tb): The First Rare-Earth Selenophosphates with Remarkable Nonlinear Optical Activities Realized by Synergistic Effect of RE- and P-Based Motifs

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials because of their nice integrated performances; however, very few RE chalcophosphates are involved for this topic. Here, three quaternary RE selenophosphates, KSmP₂ Se₆ (1), KGdP₂ Se₆ (2), and KTbP₂ Se₆ (3), are profoundly studied for their NLO potentials. Their noncentrosymmetric P21 structures feature RESe8-bicapped trigonal prisms and ethane-like [P₂ Se₆ ]₄ - dimers built {[REP₂ Se₆ ]-}∞ layers. As the first studied NLO-active RE selenophosphates, 1-3 exhibit second harmonic generation (SHG)responses ≈0.34-1.08 × AgGaS₂ at 2.10 µm and laser-induced damage thresholds (LIDTs) ≈1.43-4.33 × AgGaS₂ , and they all show phase-matchable behaviors, indicating their wonderful balanced NLO properties. Theoretical calculations demonstrate that the synergistic effect between RESe₈ and P₂ Se₆ units makes the major contribution to the SHG responses.

Manganese(ii) thiophosphate (MnPS3) intercalates with lanthanide (PrIII and NdIII) complexes: optical and magnetic properties

We report the intercalation of PrIII and NdIII macrocyclic complexes into the layered hexahypothiophosphate MnPS3 and the effect of this process on the optical and magnetic properties of the layered host.

Discovery of an olivine-type lithium manganese thiophosphate, LiMnPS4, via a building block approach

An olivine-type orthothiophospate LiMnPS₄ has been synthesized for the first time through a building block approach by reacting preformed ternary lithium thiophospate with MnCl₂. Diffuse reflectance measurements show an optical band gap of 2.36 eV, which is further confirmed by DFT calculations. Irreversible weak ferromagnetic ordering and metamagnetism are verified through preliminary magnetic measurements.

Revisiting thiophosphate Pb3P2S8: a multifunctional material combining a nonlinear optical response and photocurrent response

Pb₃P₂S₈ was structurally characterized three decades ago with a second harmonic generation response. In this work, Pb₃P₂S₈ was revisited to investigate its electronic structure via DFT calculations and optical properties by UV-vis measurements, second harmonic generation tests, laser damage threshold tests, and photocurrent measurements. Pb₃P₂S₈ is constructed by [PbS₇] polyhedra and [PS₄] tetrahedra, which was supported by crystal orbital Hamilton population (COHP) calculations. The electron localization function (ELF) simulations revealed the dominantly covalent and ionic bonding nature of P-S interactions and Pb-S interactions, respectively, both of which are strongly polarized. Pb₃P₂S₈ is an indirect n-type semiconductor of 1.8 eV and 2.4(1) eV, which are obtained from DFT calculations and UV-vis measurements, respectively. Pb₃P₂S₈ is a non-type-I phase matching material with a good balance of second harmonic generation (SHG) and laser damage threshold (LDT) of 3.5 × AGS and 2.6 × AGS, respectively (SHG based on 38-50 μm particle size sample). Pb₃P₂S₈ exhibits an intriguing photocurrent response of 45 μA cm^-2 under light irradiation. Pb₃P₂S₈ is a new multifunctional material combining a nonlinear optical response and photocurrent response.

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

The lithium ytterbium ortho-thiophosphates Li9Yb2[PS4]5 and Li6Yb3[PS4]5 were prepared through the reaction of stoichiometric amounts of ytterbium metal, elemental sulfur, red phosphorus and lithium hemisulfide at elevated temperatures in sealed silica tubes. The compounds occur as dark red single crystals which crystallize monoclinically in space group C2/c with the lattice parameters a = 1487.98(9), b = 978.63(6), c = 2046.75(12) pm and β = 96.142(3)° for Li9Yb2[PS4]5 (Z = 4) and a = 2814.83(16), b = 997.34(6), c = 3338.52(19) pm and β = 113.685(3)° for Li6Yb3[PS4]5 (Z = 12). Li9Yb2[PS4]5 can be assigned to the structure type of Li9Nd2[PS4]5, whereas the structure of Li6Yb3[PS4]5 the structure is similar to that of the prototypic Li6Gd3[PS4]5. Both structures feature discrete [PS4]3– tetrahedra (d(P–S) = 202–207 pm) and strands of [YbS8]13− polyhedra (d(Yb–S) = 271–319 pm) propagating along [010]. When attributed to the general formula (Li3[PS4]) x (Yb[PS4]) y , ideas of the dimensionality of both structures can be derived. Whilst the lithium-richer Li9Yb2[PS4]5 (x/y = 1.5) develops planes with the composition   ∞ 2 { [ Y b [ P S 4 ] 3 ] 6 − } ${}_{\infty }^{2}\left\{{\left[\mathrm{Y}\mathrm{b}{\left[\mathrm{P}{\mathrm{S}}_{4}\right]}_{3}\right]}^{6-}\right\}$ , Li6Yb3[PS4]5 (x/y = 0.667) exhibits a rather complex three-dimensional network of ytterbium-centered polyhedra connected via [PS4]3– tetrahedra with lithium cations in the framework structure   ∞ 3 { [ Y b 3 [ P S 4 ] 5 ] 6 − } ${}_{\infty }^{3}\left\{{\left[\mathrm{Y}{\mathrm{b}}_{\mathrm{3}}{\left[\mathrm{P}{\mathrm{S}}_{4}\right]}_{5}\right]}^{6-}\right\}$ . These Li+ cations are hard to locate in both compounds, but reside in four- to sixfold sulfur coordination (d(Li–S) = 235–304 pm). Some Li+ positions are underoccupied and some Li+ cations share sites with Yb3+ cations in Li6Yb3[PS4]5, and even in Li9Yb2[PS4]5 their high displacement values suggest Li+ cation mobility. According to the empirical formulae, three Li+ cations have to be replaced with one Yb3+ cation to reach the lithium-poorer compound and structure (Li6Yb3[PS4]5) starting from the lithium-richer one (Li9Yb2[PS4]5).

A series of Rb4Ln2(P2S6)(PS4)2 (Ln = La, Ce, Pr, Nd, Sm, Gd) rare earth thiophosphates with two distinct thiophosphate units [PVS4]3- and [PIV2S6]4

A series of rubidium rare earth thiophosphates with the formula Rb4Ln2(P2S6)(PS4)2 (Ln = La, Ce, Pr, Nd, Sm, and Gd) were synthesized using the high temperature molten flux crystal growth method utilizing a RbBr flux. Single crystals of all title compounds, as well as phase pure powders of the La-, Ce-, and Sm-containing compositions, were obtained. Single crystals of the title compounds were characterized by single crystal and powder X-ray diffraction for structure and phase identification. Rb4Ln2(P2S6)(PS4)2 crystallizes in the monoclinic crystal system adopting the P21/n space group for the large rare earths (Ln = La, Ce, Pr) and the C2/c space group for the smaller rare earths (Ln = Nd, Sm, Gd). This Rb4Ln2(P2S6)(PS4)2 series is a rare example of thiophosphates containing both tetrahedral [PVS4]3- and dimeric [PIV2S6]4- thiophosphate units that, in this structural family, link corrugated rare earth sulfide chains into sheets. The band gaps of the materials were determined from UV-Vis data and the fluorescence spectrum of Rb4Ce2(P2S6)(PS4)2 was collected. Optical band gaps were estimated to be 2.9 and 2.4 for the Nd and Sm analogues, respectively.

Trends in rare earth thiophosphate syntheses: Rb3Ln(PS4)2 (Ln = La, Ce, Pr), Rb3−xNaxLn(PS4)2 (Ln = Ce, Pr; x = 0.50, 0.55), and RbEuPS4 obtained by molten flux crystal growth

Crystal structures of new rubidium rare earth thiophosphates with the formulas Rb₃Ln(PS₄)₂ (Ln = La, Ce, Pr), Rb_3−xNa_xLn(PS₄)₂ (Ln = Ce, Pr; x = 0.50, 0.55), and RbEuPS₄ crystallized out of a molten RbCl flux.

Dimensional reduction upon calcium incorporation in Cs0.3(Ca0.3Ln0.7)PS4 and Cs0.5(Ca0.5Ln0.5)PS4

A series of Ca-containing lanthanide thiophosphates has been obtained and their structural evolution from 3D for LnPS₄ and Cs_0.3(Ln_0.7Ca_0.3)PS₄ to 2D in Cs_0.5(Ln_0.5Ca_0.5)PS₄ was shown as a function of Ca content.

Three of a kind? The non-isotypic triple CsCe[P2Se6], CsSm[P2Se6] and CsEr[P2Se6]

Three new compounds of the CsLn[P2Se6] family with Ln = Ce, Sm and Er have been prepared and structurally characterized. Plate-shaped, amber-colored single crystals of these cesium lanthanoid(III) hexaselenodiphosphates(IV) were obtained by heating stoichiometric amounts of Ln, P and Se with CsCl as a reactive flux in fused silica ampoules at 800 °C for four days. CsCe[P2Se6] crystallizes monoclinically in space group P21/c with a = 1297.86(9), b = 776.24(5), c = 1198.43(8) pm, β = 106.589(3)° and Z = 4. The structure is isotypic with that of KLa[P2Se6], the Cs+ cations being ten-fold coordinated by selenium atoms to form double layers of condensed [CsSe10]19− polyhedra. Ce3+ resides in a nine-fold coordination and the [CeSe9]15− polyhedra also form double layers parallel to (100). CsSm[P2Se6] crystallizes in the orthorhombic space group P212121 with a = 688.67(5), b = 754.48(5), c = 2215.21(15) pm and Z = 4. Its structure is isotypic with that of KY[P2Se6] and the Cs+ cations reside in an eleven-fold coordination of selenium atoms constituting monolayers of condensed [CsSe11]21− polyhedra within the (001) plane. Sm3+ exhibits an eight-fold coordination sphere of selenium atoms and the [SmSe8]13− polyhedra are also linked to build up parallel monolayers. CsEr[P2Se6] crystallizes in the monoclinic space group P21/c again, but forms its own structure type with the lattice parameters a = 753.81(5), b = 1281.92(9), c = 1276.47(9) pm and β = 106.898(3)° and Z = 4. The Cs+ cations are twelve-fold coordinated by selenium atoms and erects a three-dimensional framework of condensed [CsSe12]23− polyhedra. The Er3+ cations show seven selenium atoms as neighbors and the [ErSe7]11− polyhedra are edge-connected to form discrete dimers [Er2Se12]18−. All three structures have similar ethane-like [P2Se6]4– anions in staggered conformation with bond lengths of 219–226 pm for d(P1–P2) and 213–222 pm for d(P–Se), which connect the Cs+ and Ln 3+ coordination polyhedra into three-dimensional crystal structures.

Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho)

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs₂NaLn(PS₄)₂ (Ln = La-Nd, Sm, and Gd-Ho) were grown as large single crystals (∼0.1-1 mm³) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs₂NaLn(PS₄)₂ series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm³⁺ and Gd³⁺ causes a subtle change in coordination number from 9 (for Ln = La-Sm) to 8 (for Ln = Gd-Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs₂NaLn(PS₄)₂ (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.

Size-Driven Stability of Lanthanide Thiophosphates Grown from an Iodide Flux

To determine the influence of the lanthanide size on the structures and properties of thiophosphates, a thiophosphate series containing different lanthanides was synthesized via high temperature flux crystal growth and their structures and physical properties analyzed and compared. Layered thiohypophosphates NaLnP₂S₆ (Ln = La, Ce, Pr) and thiopyrophosphates CsLnP₂S₇ (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, Y) were grown out of an iodide flux using consistent reaction conditions across both series. Under the mildly reducing iodide flux reaction conditions, a rather rare example of phosphorus reduction from the +5 to the +4 oxidation state was observed. Both resultant structure types are based on lanthanide thiophosphate sheets with the alkali cations located between them. Magnetic susceptibility measurements were conducted and revealed Curie-Weiss behavior of the samples, with a Van Vleck contribution in the CsSmP₂S₇ sample. UV-vis data was found to be in good agreement with the literature, indicating little influence of the sulfide environment on the localized 4f orbitals.

Locally Controlled Cu-Ion Transport in Layered Ferroelectric CuInP2S6

Metal thiophosphates are attracting growing attention in the context of quasi-two-dimensional van der Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes, and transition-metal thiophosphates are explored as a new class of ferroelectric materials. For the latter, a representative copper indium thiophosphate is ferrielectric at room temperature and, despite low polarization, exhibits giant negative electrostrictive coefficients. Here, we reveal that ionic conductivity in this material enables localized extraction of Cu ions from the lattice with a biased scanning probe microscopy tip, which is surprisingly reversible. The ionic conduction is tracked through local volume changes with a scanning probe microscopy tip providing a current-free probing technique, which can be explored for other thiophosphates of interest. Nearly 90 nm-tall crystallites can be formed and erased reversibly on the surface of this material as a result of ionic motion, the size of which can be sensitively controlled by both magnitude and frequency of the electric field, as well as the ambient temperature. These experimental results and density functional theory calculations point to a remarkable resilience of CuInP₂S₆ to large-scale ionic displacement and Cu vacancies, in part enabled by the metastability of Cu-deficient phases. Furthermore, we have found that the piezoelectric response of CuInP₂S₆ is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInP₂S₆, and more generally ionically conducting thiophosphates, posits new prospects for their applications in van der Waals heterostructures, possibly in the context of catalytic or electronic functionalities.

Metal Thio- and Selenophosphates as Multifunctional van der Waals Layered Materials

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow-on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase-transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate- to wide-bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought-after properties of complex oxides with electronic functions of 2D and quasi-2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.

Tuning the electronic and vibrational properties of Sn2P2Se6and Pb2P2S6crystals and their metallization under high pressure

Ferroelectric Sn₂P₂Se₆and paraelectric Pb₂P₂S₆demonstrate the remarkable responses of their electronic band structures to applied high pressures.