極性金屬間化合物LuSn2的合成、晶體結構及能帶特征

岳呈陽

(濟寧學院化學與化工系，曲阜 273155)

極性金屬間化合物LuSn2的合成、晶體結構及能帶特征

岳呈陽

(濟寧學院化學與化工系，曲阜 273155)

在惰性氣氛氬氣保護下，通過高溫固相反應合成得到了一個新的二元極性金屬間化合物LuSn2。經X-射線單晶衍射與元素分析等方法確定了其晶體結構。LuSn2屬正交晶系，空間群為Cmcm，晶體學參數a=0.435 11(10)nm，b=1.601 6(4)nm，c=0.42780(8)nm，V=0.29812(11)nm3，Z=4，R1=0.0170，wR2=0.0324。 LuSn2屬于 ZrSi2結構類型，其結構中包含有一維“之”字型 Sn鏈與二維四方格子狀Sn層，Lu原子排列在Sn鏈與Sn層的空隙中。能帶結構計算表明LuSn2呈金屬導電性。

極性金屬間化合物；高溫固相反應；晶體結構；Sn化物

Group 14 elements (Si,Ge,Sn,Pb)can form a large number of polar intermetallic compounds when alloyed with alkali or alkaline earth[1-2].Among these phases,tin can form a variety of phases based on tin rings,ladders,chains and oligomers as well as other tin cluster moieties[3].For example,BaSn5shows a 2D slab of face-sharing hexagonal prisms which are formed by two superimposed graphite-like layers of Sn atoms[4],SrSn4features a 2D net consisting of hexagons of tin atoms in boat conformation[5],and Ca31Sn20contains tin dimers and linear pentamers in addition to isolated tin atoms[6].Tin dimer,tetramer,pentamer and infinite chain can also be found in CaSn,Ca7Sn6and Ca36Sn23[7].The tin-rich binary or ternary phases are capable of forming typesⅠ,Ⅱ andⅢ clathrates featuring cages based on tin pentagons and possessing interesting transport properties[8-9].Furthermore,mixing two types of cations with different sizes,such as the combination of alkaline earth and alkali metals,two different alkali metals,or two different alkaline earth metals,caneffectively change the electronic concentration and form a large number of polar intermetallic compounds with unusual anionic clusters,chains and layers[10-11].The chemical bonding and electron counting for most of the above polar intermetallic compounds can be understood by using the Zintl-Klemm concept as well as Wade′s rules.

The binary rare earth-tin system has afforded various polar intermetallic phases with anionic building units and special properties.For example,the structure of La5Sn4contain isolated Sn4-ions and [Sn2]dimers,whereas the structure of Sc5Sn4only contain isolated Sn4-ions.Ce3Sn7features 1D zigzag Sn chain and 2D double Sn layer composed of two parallel Sn squares interconnected by Sn atoms via Sn-Sn bonds,whereas Ce2Sn5contain the same 1D zigzag Sn chain and similar 2D triple layer composed of three parallel Sn squares interconnected by Sn atoms;RESn3(RE=La,Ce,Nd,Eu,Gd,Tb-Er)feature 3D framework composed of crossed 2D Sn square sheet.The compound Gd5Sn3of hexagonal Mn5Si3type hasattracted considerable interest in recent years because of its outstanding magnetic property.

Intrigued by the rich structural types and physical properties of these binary rare earth stannides,we undertook systematic studies in the binary Lu-Sn system,about which no one compound and phase has been reported so far.Our exploratory studies afforded the first binary Lu-Sn phase,namely as,LuSn2.Its structure crystallize in the ZrSi2type,and features 1D zigzag Sn chain and 2D square Sn sheet.Herein,we reportitssynthesis,crystalstructureandbandstructure.

1 Experimental

1.1 Materials and instrumentation

All manipulations were performed inside an argonfilled glove box with moisture level below 1 ppm.All chemicals were used as received:lutetium bricks(Acros,99.99%)and tin granules(99.998%,Shanghai fourth chemical reagent company).Elemental analyses for Lu and Sn were performed on a JSM-6700F scanning electron microscope equipped with an energy dispersive X-ray spectroscope (EDS,Oxford INCA).Data were acquired with an accelerating voltage of 20 kV and SEM of 40°.Visibly clean surfaces of crystals were selected for analysis and high magnifications were used during data collection in order to obtain accurate results.X-ray powder diffraction patterns were collected at room temperature on an XPert-Pro diffractometer using Cu Kα radiation(λ=0.154 06 nm)in the 2θ range of 5°～85°with a step size of 0.04°and 10 s·step-1counting time.The data analysis was performed using the JADE 5.0 software package.

1.2 Synthesis of LuSn2

Single crystals of LuSn2were initially obtained from a mixture of Lu and Sn metals in a molar ratio of 1∶5,and the large excess of tin metal was used as the flux.The lutetium bricks were scraped clean with a scalpel before the cutting and weighing.The mixture of Lu(174.97 mg,1 mmol)and Sn (593.5 mg,5 mmol)was loaded into a niobium tube already welded at one end,and the other end was subsequently arc-welded under an argon atmosphere.For tube fumace reactions,the Nb containers were enclosed in fused quartz tubes,which were flame-baked under a good vacuum (～0.013 3 Pa)beforesealing.Thetubewasputintoahightemperature furnace and heated to 980℃during 5 hours.Then it was allowed to react at 980 ℃ for 6 days,and annealed at 650℃for 7 days,and allowed to cool at a rate of 0.1 ℃·min-1to room temperature.The reaction products were opened in argon-filled glove box and observed under microscope.Prism-shaped gray single crystals of LuSn2could easily be selected from reaction products and used for structure determination.Microprobe elementalanalyseson severalsingle crystals indicated the presence of Lu and Sn elements in an average molar ratio of 1.1(2)∶2.2(9),which was in agreement with the results from single crystal X-ray diffraction studies.The compound is stable in moist air for several weeks.After the structure and composition analyses,a lot of efforts were subsequently made to prepare a single phase of LuSn2.The reactions were carried out with stoichiometric mixture of the elements.The samples were heated at 980℃for 24 h,quenched in cold water and annealed at different temperatures(550,650,700 and 750 ℃,respectively)for 15 days.The highest yield of～60%for LuSn2was obtained by annealing at 650℃based on X-ray powder diffraction studies.

1.3 Crystal structure determination of LuSn2

A gray prism-shaped single crystal of LuSn2(0.20 mm×0.05 mm×0.03 mm)was selected from the reaction products and sealed within a thin-walled glass capillary under an argon atmosphere.The crystal was mounted on a Rigaku Mercury CCD and the data were collected by using a graphite-mono-chromatic Mo Kα radiation(λ=0.071073 nm)at 293(2)K.In the range of 2.54°＜θ＜27.41°,a total of 1 122 reflections were collected and 214 were independent(Rint=0.0429),of which 210 with I＞2σ(I)were observed.The data set was corrected for Lp factors,air absorption and absorption due to the variations in path length through the detector faceplate.Absorption correction based on Multi-scan method was also applied.

Its structure was solved using direct methods(SHELXL-97),and refined by least-squares methods with atomic coordinatesand anisotropic thermal parameters[12].Based on systematic absences and E-value statistics,the space groups of Cmcm was selected and gave satisfactory refinement.All atomic sites in LuSn2were fully occupied according to the site occupancy refinements and no abnormal behaviors were found for their thermal parameters.In the asymmetric unit of LuSn2,all the Lu(1),Sn(1)and Sn(2)atoms fill the 4c positions with occupancies of 0.25,respectively.Final difference Fourier maps showed featureless residual peaks of 1 020 and-1 510 e·nm-3(0.092 nm and 0.159 nm from Lu(1)and Sn(2)atom,respectively).Some of the data collection and refinement parameters are summarized in Table 1.The important bond lengths and angles are listed in Tables 2.

ICSD:422965.

Table 1 Crystal and structure refinement data for LuSn2

Table 2 Selected bond lengths(nm)and bond angles(°)for LuSn2

1.4 Computational details

To better understand the electronic structure of LuSn2,ab initio electronic structure calculation was performed with both the full-potentiallinearized augumented plane wave plus the local basis(FPLAPW+lo)method(WIEN2K)and extended Hückel tight-binging(EHTB)method.

WIEN2K:The WIEN2K program is based on the density functional theory(DFT)and the highly accurate FPLAPW+lo method,using the general gradient approximation (GGA-PBE)to treat the exchange and correlation potential[13].In order to obtain as precise result as possible,we expanded the basis function up to RMT×KMAX=7,where RMTis the smallest muffin-tin spherical radius present in the system,and KMAXis the maximum modulus for the reciprocal lattice vector.We adopted the values of 0.25 and 0.224 nm for Lu and Sn atoms,respectively,as the RMTradii.The valence states set included the 4f,5d,6s orbitals for Lu and 5s,5p orbitals forSn atoms,respectively,which were expanded using the basis functions.Convergence of the self-consistent iterations was performed inside the reduced Brillouin Zone to be within 0.000 1 c.c with a cutoff energy of-8.0 Ry between the valence and core states.We calculated the total energy with the 100,200,500 k points for the title compound,and the results suggested that the calculated results did not change much from 100 to 500 k-points.So in this study,we used the 200 k-point in the complete Brillouin zone,and the Brillouin zone integration was carried out with a modified tetrahedron method.The Fermi level was selected as the energy reference(EF=0 eV).

EHTB Calculations:The Crystal Orbital Overlap Population (COOP)curves were performed using the Crystal and Electronic Structure Analyzer(CAESAR)software package[14].The following orbital energies and exponents were employed in the calculations (Hij=orbital energy (eV),ξ=Slater exponent):Lu 6s:Hij=-6.050, ξ=1.666;6p:Hij=-6.050, ξ=1.666;5d:Hij=-5.120, ξ=2.813;4f:Hij=-22.400, ξ=9.136;Sn:5s,Hij=-16.160,ξ=2.120;5p,Hij=-8.320,ξ=1.820.

2 Results and discussion

2.1 Crystal structure

LuSn2crystallizes in the orthorhombic system,space group Cmcm.The structure of LuSn2belongs to the ZrSi2type.It is isostructural with RESn2(RE=Tb～Tm),featuring 1D zigzag Sn chains and 2D square Sn sheets,spaced by the lutetium cations (Fig.1).Furthermore,it is found that all the cell parameters of RESn2(RE=Tb～Tm)decrease from the early to the late RE members,which is also accordance with the decreasing radii of rare earth ions(Fig.2).

There are one Lu and two Sn sites in an asymmetric unit of LuSn2.As shown in Fig.3a,the Sn(1)atoms form 1D zigzag chain via Sn(1)-Sn(1)bond,which is similar to those in AeSn (Ae=Ca,Sr,Ba),Yb3CoSn6and Sm2NiSn4[15-16].Within the zigzag chain,all the Sn-Sn bonds are of 0.29775(10)nm,which are slightly longer than the Sn-Sn single bond distance of 0.281 0 nm observed in element α-Sn.These Sn-Sn bonds are accordance with those in other stannides,such as Eu3Sn5,Yb5Ni4Sn10and K3Mg18Sn11[17-19].The Sn-Sn-Sn angle of 91.85(0)°is comparable to those in RESn2(RE=Tb～Tm),which is much smaller than that for an ideal tetrahedral geometry due to the presence of the lone pair electrons.

The 2D square Sn sheet is composed of Sn(2)atoms as shown in Fig.3b,which is similar to the square Bi sheet in EuBi2and LaLiBi2[20-21].The Sn(2)-Sn(2)distance of 0.305 12(5)nm is comparable to those in RE3MnSn5-x(RE=Tm,Lu)and Ca2Cu6Sn7[22].The Sn(2)-Sn(2)-Sn(2)angle is 89.02(0)°and 90.96(0)°,which are close to that in standard square.All the Lu atoms are located in the space of 1D Sn chains and 2D square Sn sheets.Lu atom is coordinated by six Sn(1)and four Sn(2)atoms with Lu-Sn distances of 0.309 81(5)～0.339 74(8)nm,which are comparable with the sum of covalent radii of Lu and Sn atoms(0.3011 nm)(Fig.4).

It should be noted that all the Sn-Sn bonds in LuSn2are smaller than the corresponding bonds in RESn2(RE=Tb～Tm),which is accord with the radii of rare earth ions.Furthermore,the nearest distance between 1D Sn chain and 2D Sn sheet are also decrease with the decreasing of radii of rare earth ions.Hence,the radii of rare earth ions are capable of tuning the crystal structures of RESn2.

2.2 Electronic structure calculations

To further investigate the nature of the bond character and transport property of LuSn2,we carried out highly accurate band structure calculation by using FPLAPW method.The spin splitting has not occurred in LuSn2.It is seen that the Fermi level falls in a nonzero area with large density of states,indicating that LuSn2is metallic.From the calculated total and partial DOS with no spin-polarization(Fig.5),one narrow peak observed at-5 eV below the Fermi level belongs to the Lu-4f states,which is fully occupied.The Lu 6s states are well above the Fermi level as found in typical electron donors.Furthermore,the calculation converge to theoretical saturation moment of almost 0μBfor the Lu cation in LuSn2.Hence the Lu cation is most likely in an oxidation state of+3.The states in the energy range of-10 to-5 eV are essentially dominated by Sn5selectrons,and the states between the-5 to 0 eV are mainly contributed by the Sn5pelectrons,and the states above the Fermi level mainly result from Lu6sorbitals.The DOS of two crystallographically different Sn atoms completely overlap between each other in the range of-10～0 eV.Mulliken overlap population(MOP)analyses based on EHTB calculations are also performed,and such calculation strategy has been effectively used in many cases[23-24].The Sn(1)-Sn(1)and Sn(2)-Sn(2)bonds are significantly bonding with MOP values of 0.837 5 and 0.767 4,respectively,which are comparable with their bond distances(0.0297 75(10)and 0.0305 12(5)nm,respectively).The Lu-Sn bonds(0.030 981(5)～0.033 974(8)nm)are very weak accor ding to the bond structure calculations(with an average MOP value of 0.247 3 at the Fermi level),which show the existence of bonding states just above the Fermi level.Results of our calculations indicate that the strong Sn-Sn bonds,as well as weak Lu-Sn bonding interactions,are mainly responsible for the metallic behavior of LuSn2.Such phenomena has been reported in some similar phases[25-27].

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Synthesis,Crystal Structure and Band Structure of LuSn2

YUE Cheng-Yang

(Dep artmentof Chem istry and Chemical Engineering,Jining University,Qufu,Shandong 273155,China)

A new binary intermetallic compound,LuSn2,has been synthesized by solid-state reaction of the corresponding pure elements in a welded niobium tube at high temperature.Its crystal structure was established by single-crystal X-ray diffraction.LuSn2crystallizes in the orthorhombic space group Cmcm with a=0.43511(10)nm,b=1.6016(4)nm,c=0.42780(8)nm，V=0.29812(11)nm3,Z=4,R1=0.0170 and wR2=0.0324.The structure of LuSn2belongs to the ZrSi2type.It is isostructural with RESn2(RE=Y,Gd-Tm),featuring one-dimensional(1D)zigzag Sn chains and two-dimensional(2D)square Sn sheets,spaced by the lutetium cations.Band structure calculations indicate that LuSn2is metallic.ICSD:422965.

intermetallic;high-temperature solid-state reaction;crystal structure;stannide

O614.43+2；O614.347

A

1001-4861(2011)11-2245-06

2011-04-21。收修改稿日期：2011-07-30。

國家自然科學基金青年基金(No.21101075)，山東省優秀中青年科學家科研獎勵基金(No.BS2011CL009)，濟寧學院“國家級基金計劃預研項目(No.2011YYJJ06和2011YYJJ07)”和山東省高等學校科技計劃(No.J11LB52)資助項目。

E-mail：yuecy@fjirsm.ac.cn