Synthesis,Crystal Structure and Magnetic Properties of the New Quaternary Thiophosphate Rb2FeP2S6①

SU Yong-Fei JIANG Xiao-Ming ZENG Hui-Yi LIU Bin-Wen LIN Yang-Jie GUO Guo-Cong

(State Key Laboratory of Structural Chemistry,Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences,Fuzhou 350002,China)

A new quaternary thiophosphate Rb2FeP2S6was synthesized by the alkali metal halide flux method.Its structure features 1-D infinite1∞[FeP2S6]2-chains and counter cations Rb+.It shows a band gap of 2.03 eV.The nearest intra-chain Fe2+cations are antiferromagnetically coupled with each other, and there are weak ferromagnetic-like interactions between neighboring1∞[FeP2S6]2-chains, resulting in two successive magnetic phase transitions.

1INTRODUCTION

Metal thiophosphates[1]possess interesting properties,such as ion migration,nonlinear optical(NLO)behavior,phase transition,ferroelectricity,and magnetism[2].The diversity of their properties has structural origins,i.e.,various phosphorus-sulfur polyanions(PaSb)n–including(PS4)3–[3],(P2S6)4–[4],(P2S7)4–[5],(P3S10)5–[5c],and(P4S13)6–[6]can be formed in such type of compounds,as well as different assembling types and dimensions[5c,6,7].The A-MP-S(A=alkali metal,M=transition metal)type compounds are an important subgroup of metal thiophosphates and have received broad attention recently.For instance,LiZnPS4is a potential midinfrared nonlinear optical(NLO)material[8];Onedimensional AZrPS6(A=K,Rb,Cs)exhibits room temperature light emission[9];and 3-DLi2FeP2S6[10]and ATi2(PS4)3(A=Li,Na)[11]are promising electrode materials for high-energy density batteries.The high electropositive alkali metals in the A-M-P-S type compounds usually form strong ionic bonds with the surrounding anions,leading to low dimensional structures,which is particularly meaningful for the magnetic properties[7a,7c].Compounds containing 1-D magnetic chains in their structures,such as Li2(Li1-xFex)N[12],Co(H2L)(H2O)[13],Mn2Fe and Mn2Ni[14],(NEt4)[Mn2(5-MeOsalen)2Fe(CN)6][15],have attrac-ted much attention due to their high uniaxial magne-tic anisotropy and quantum tunneling relaxation,which may have potential applications in information storage and molecular spintronics[16].

During the exploration of low-dimensional magnetic compounds in the A-M-P-S system,we have gained a new quaternary thiophosphate,namely,Rb2FeP2S6(1),through the alkali metal halide flux reactions.It features 1-D infinite1∞[FeP2S6]2-chains,in which the Fe2+cations are antiferromagnetically coupled with each other,and there are weak ferromagnetic-like interactions between different1∞[FeP2S6]2-chains,resulting in two magnetic phase transitions.Here we report its synthesis,structure,electronic and magnetic properties,and theoretical calculations of its electronic structure and magnetic coupling constants were also performed.

2EXPERIMENTAL

2.1 Spectral measurements

The optical diffuse-reflectance spectrum of1(Fig.1)was measured using a Perkin Elmer Lambda 900 UV-vis spectrophotometer equipped with an integrating sphere attachment and BaSO4as the reference.The absorption spectrum was calculated from the reflection spectrum using the Kubelka-Munk formula:α/S=(1 – R)2/2R[17],in which α is the absorption coefficient,S is the scattering coefficient,and R is the reflectance.

Fig.1.UV-Vis diffuse reflectance spectrum of 1 and its absorption spectrum(inset)

2.2 Synthesis of Rb2FeP2S6

All reagents including iron powder(99.5%),sulfur powder(99.5%),phosphorus powder(99.5%)and RbCl powder(99.5%)were from Aladdin Chemistry Co.Ltd.and were used as received without further purification.For synthesizing the title compound,a stoichiometric mixture of the starting materials Fe,P,S and RbCl with the molar ratio of 3:5:14:15 was loaded into quartz tubes and then flame-sealed under vacuum of about 10-4Torr.The tubes were placed in a temperature-controlled muffle furnace,heated to 300oC in 5 hours and kept at that temperature for 10 hours,heated to 700oC in 5 hours and kept at that temperature for 10 hours,then increased to 1000oC in 10 hours and kept for 3 days,finally cooled down to 400oC in 200 hours before switching off the furnace.The products were washed with distilled water and dried with ethanol.Yellow crystals of1with a yield of 30%were obtained.The compound is stable in the air and water.

2.3 Crystal data and structure determination

Single-crystal XRD data of1were collected by a Rigaku Pilatus CCD diffractometer using a graphite-monochrmated MoKa radiation(λ=0.71073 ?)at 293(2)K.The intensity data sets were measured using an ω-scan technique in the range of 3.3°<θ<25.49°for1at 293(2)K and reduced using the CrystalClear package[18].The structure determination was carried out by direct methods,and the refinements were performed using the full-matrix leastsquares method on F2with anisotropic thermal parameters for all atoms.For compound1,a total of 4091 reflections were recorded and 1023 were unique(Rint=0.0326),among which 858(–7≤h≤7, –14≤k≤11,–10≤l≤7)were observed.Compound1cry-stallizes in monoclinic,space group P21/c with a=6.042(3),b=12.338(6),c=9.045(4)?,β =124.25(2)o,V=557.3(5)?3,Z=2,F(000)=452,Dc=2.867 g·cm-3and μ =11.376 mm-1,(Δρ)max=0.542,(Δρ)min= –0.468 e/?-3.Final Ra/wRb(I>2σ(I))=0.0254/0.0550,R/wR(all data)=0.0349/0.0574(aR=||Fo|–|Fc||/|Fo|,bwR2=[w(Fo2–Fc2)2]/[w(Fo2)2]1/2).

2.4 Computation procedure

Theoretical calculations including band structure and magnetic properties based on the method of density-functional theory(DFT)[19]were performed by using the Vienna ab initio simulation package[20].Spin-polarized DFT calculations employing the projector augmented wave method adopt a plane-wave energy cutoff of 500 eV and a 4×2×3 Monkhorst-Pack grid of Brillouin-zone k-point sampling.The LDA plus on-site repulsion U method LDA+U[21]was employed to properly describe the electron correlation associated with the Fe 3d states with U of 3.0 eV.It’s worth noting here that the value of U doesn’t significantly affect the calculation results,as can be seen from the fact that the calculated band gaps are almost the same for1when using different U.The calculated band gaps are 1.95,2.00 and 2.10 eV for U=3.0,4.0 and 5.0 eV,respectively.

3 RESULTSAND DISCUSSION

3.1 Crystal structure

The title compound crystallizes in space group P21/c.Its asymmetric unit contains crystallographically independent two Rb,one Fe,one P and three S positions,among which the Fe atom was coordinated by six S atoms to form a distorted octahedral FeS6unit(Fig.2).Two neighboring phosphorus atoms each possess tetrahedral coordination of one P and three S atoms,joining into pairs by P–P bond.Each pair is surrounded by six S atoms,forming a distorted P2S6octahedron.All octahedral FeS6and P2S6units with a molar ratio of 1:1 share two opposite faces with each other to form infinite one-dimensional1∞[FeP2S62-]chains extending along the a direction,between which the counter cations Rb+are embedded.The Fe–S and P–S bond lengths in1(Table 1)range from 2.5530 to 2.5980 ? and from 2.0173 to 2.0214 ?,respectively,which lie in the normal ranges for Fe–S and P–S bond lengths in known iron thiophosphates[22].

Fig.2. (a)Structural framework of 1 showing isolated[FeP2S6]nn-chains along the a direction.(b)A1∞[FeP2S62-]octahedral chain viewed perpendicularly to its longitudinal direction.(c)Octahedral P2S6and FeS6units

Table 1.Selected Bond Lengths(?)for 1

3.2 Magnetic property

The variable-temperature magnetic susceptibilities of compound1were measured in the range of 2～300 K under the external magnetic field of 1000 Oe(Fig.3).The cm·T value per Fe2+(d6)unit at room temperature is 3.33 emu·K·mol?1,which is close to the spin only value of 3.0 emu·K·mol?1for uncoupled high spin Fe2+(S=2)in the octahedral coordination geometry.With decreasing temperature,thecmvalue of Fe2+increases gradually and receives the highest peak of 0.044 emu·mol?1at~27 K.After that,thecmvalue starts to decrease at a high speed and reaches its lowest point with the value about 0.032 emu·mol-1at 9 K,indicating the presence of antiferromagnetic coupling between the nearest intrachain neighboring Fe2+centers with the transition temperature of~27 K.Then it increases again to a value of 0.038 emu·mol-1at 2.0 K,implying the ferromagnetic-like coupling between neighboring1∞[FeP2S62-]chains.The magnetic susceptibility of Fe2+conforms well to the Curie-Weiss law in the high temperature range of~50～300 K and gives the negative Weiss constants(?)–48 K for Fe2+,and Curie constants 3.71 emu·K·mol-1for Fe2+,further demonstrating the presence of antiferromagnetic alignments between the nearest intrachain neighboring spin centers in compound1.

Fig.3.(a)Variable-temperature χm(left)and χmT(right)curves.(b)Variable-temperature 1/χmcurve

3.3 Theoretical calculation

In order to get further insights into the magnetic properties of1,the band structures and density of states(DOS)of complex1were calculated based on density-functional theory(DFT),and the magnetic exchange coupling constants in1were calculated through the energy-mapping method[23].The electronic structure of ferromagnetic state for1is calculated and presented in Fig.4a,which exhibits that ferromagnetic state is insulated with indirect band gap.The calculated bandgap of1is 1.95 eV,which is close to the experimental values of 2.03 eV.The calculated spin magnetic moment per Fe2+is 3.58,which can be gotten from the difference between the integration of DOS of up and down spin,and they are consistent with the pictures of high-spin Fe2+(d6,S=2)in1.

The bands can be assigned according to the total and partial densities of states(DOS,Fig.4b).The band just above the Fermi level(or the bottom of conduction band)is predominately derived from unoccupied Fe-dstates.The band just below the Fermi level is composed of S-p states,mixed with a small amount of Fe-d states.Therefore,the optical absorption of1is mainly ascribed to the charge transitions from S-p and Fe-d states to Fe-d states.

Fig.4a).Band structures of up-spin(red line)and down-spin(blue line)for the ferromagnetic state of 1 along the high symmetry k-points:Z(0,0,0.5),G(0,0,0),Y(0,0.5,0),A(–0.5,0.5,0),B(–0.5,0,0),D(–0.5,0,0.5),E(–0.5,0.5,0.5),C(0,0.5,0.5).b)Total and partial DOS for the Rb,Fe,P and S atoms in 1.The Fermi level is set at 0 eV for the band structures and DOS.The calculated band gap of 1 is 1.95 eV

Fig.5.Six ordered spin states employed to extract the spin exchange parametersJa,JbandJc.For simplicity,only the magnetic cations Fe2+are shown.The spin-up and spin-down magnetic sites are represented by blue and red spheres,respectively.The yellow and green lines represent magnetic coupling between the same and opposite spins,respectively

The magnetic exchange parameters including one between the nearest intrachain neighboring Fe2+ions labeled byJaand two other ones between the nearest interchain neighboring Fe2+ions labeled by JbandJccan be evaluated by examining six ordered spin states,namely,the FM and AFM1-5 states,among which two redundant spin states are added to check the consistency of calculation.The six ordered spin states are defined in Fig.5 in terms of 2×1×1 supercell.The total spin exchange interaction energies of the six ordered spin states are expressed in terms of the Hamiltonian:

where H0is related to non-spin variables and is the same for all the sixspin states.Jij=Ja,b,cis the spin exchange parameter for the spin exchange interaction between the spin sites i and j,andandare the spin angular momentum at the spin sites i and j,respectively.The energy expressions of the six ordered spin states obtained per 2×1×1 supercell(S=2 for Fe2+)can be expressed as:

E(FM)=E0+S2(2Ja+2Jb+Jc)

E(AF1)=E0+S2(Jc)

E(AF2)=E0+S2(–Jc)

E(AF3)=E0+S2(–2Ja+2Jb–Jc)

E(AF4)=E0+S2(2Ja– 2Jb–Jc)

E(AF5)=E0+S2(–2Ja– 2Jb+Jc)

Table 2.Relative Energies in meV of the Six Ordered Spin States of 1 Determined from LDA+U Calculations

Table 2 summarizes the relative energies of the six ordered magnetic states determined from LDA+U calculations.When the relative energies of these spin states are mapped onto the corresponding energies determined from the spin Hamiltonian,the values ofJa,Jb,andJccan be obtained.The calculatedJais 1.4 meV while bothJbandJcare close to 0,indicating the intrachain spin exchangesJaare antiferromagnetic,and the interchain interactions are very weak,which are consistent with the experimental results.

4 CONCLUSION

In summary,a new quaternary thiophosphate Rb2FeP2S6was obtained via high-temperature solidstate reactions.Its structure is constructed by 1-D infinite1∞[FeP2S6]2-chains and countercations Rb+.The optical diffuse reflectance spectrum reveals its band gap of 2.03 eV.Variable-temperature magnetic susceptibility measurements indicate the antiferromagnetic coupling between the neighboring Fe2+centers and the ferromagnetic-like interchain coupling.Electronic structure and DOS calculations show that complex1exhibits an indirect band gap of 1.95 eV for the ferromagnetic state and its optical absorptions are mainly ascribed to the charge transitions from S-p and Fe-d states to the Fe-d states.Magnetic coupling constant of the intrachain Fe2+?Fe2+pair was derived to be 1.4 meV by LDA+U calculations and energy-mapping analysis.

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