1 J. CHEM. SOC. DALTON TRANS. 1988Synthesis and Characterization of Ti,Te,-Type Molybdenum Cluster Compounds,A,Mo,As, (A = Cu, Al, or Ga)tKirakodu Seetharama Nanjundaswamy and Jagannatha Gopalakrishnan *Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-5600 12, IndiaA new series of molybdenum cluster compounds of the general formula AxMo5As, (A = Cu, Al, orGa) has been synthesized. They are isostructural with the host Mo5As, (Ti,Te,-type) consisting oftrans-vertex shared Mo, octahedral chains. Investigations by X-ray photoelectron and Augerelectron spectroscopies revealed a charge transfer from A to Mo,As, in AxMo5As,. The occurrenceof metallic (Cu,Mo,As,) and non-metallic (Al,Mo,As, and Ga,Mo,As,) properties in thisisostructural series of solids is consistent with the electronic structure of Ti,Te,-type solidsinvolving M-M bonding in the cluster chains.Transition-metal compounds containing metal-metal bondedclusters are of current interest in view of their unique structuresand properties.'g2 Well known among this class of solids are theChevrel phases3y4 A,Mo,X8 (A = Pb, Sn, Cu, Li, etc.; X = s,Se, or Te) where the binary chalcogenides Mo,X, act as thehosts.The host structure consists of a three-dimensionalarrangement of cubic Mo,X, units which contain distortedoctahedral Mo, metal clusters. This arrangement producesintersecting channels of vacant lattice positions, which areoccupied by the A atoms. Recently it has been shown5'6 thatMo& clusters can condense through opposite faces of theMo, octahedra to give a new family of condensed metal clustercompounds, Mo,(~+ 1)X3(n+ 1)+ (n 2 l), which also formChevrel-phase analogues.The end member of the series is theinfinite chain anion, [Mo,X3]- which is present for examplein KMo,S,., Condensation of M6X8 (M = transition metal)clusters can also occur through corners and edges of M,~ctahedra.~.~.' A typical example of a trans-vertex condensedmolybdenum cluster compound containing a Mo,X, unit isMo,As, which crystallizes in the Ti,Te, ~tructure.~ Weenvisaged that Mo,As, could act as host for the insertion ofelectropositive metal atoms giving rise to a new family ofsolids, A,Mo,As,, just as Mo& chalcogenides act as hostsfor the Chevrel-phase compounds. The existence of compoundssuch as Cu,Nb,Si, l o and Ni,Nb,P, adopting the Ti,Te,structure, wherein Cu/Ni atoms are inserted in the voidsbetween the cluster chains, strengthens this viewpoint.Theobjective of the present work has been to synthesize andcharacterize such insertion compounds of Mo,As,.We have been able to synthesize new A,Mo,As, withA = Cu (1 < .Y < 4) and Fe, Ga, or A1 (x = 2). Characteriz-ation of these solids by X-ray photoelectron spectroscopy (x.P.s.),Auger electron spectroscopy (a.e.s.), X-ray absorption spectro-scopy (x.a.s.), and electrical conductivity measurements showsthat there is a charge transfer from A to Mo,As, in these solidsand that their electronic properties are determined by thevalence-electron count (v.e.c.) on the cluster molybdenuniatoms.ExperimentalThe clusters A,Mo,As, (A = Cu, 1 < x < 4; A = Fe, Al, orGa, x = 2) were prepared by reaction of the correspondingelements in the required stoicheiometry in evacuated sealedsilica ampoules at 1 OOO-1 050 "C for about 15 d with onegrinding in between. Experimental procedures for recordingt Non-S.I.unit employed: eV z 1.60 x J.CU-K, 2e10Figure 1. X-Ray powder diffraction patterns of (a) Cu,Mo,As,, (b)AI,Mo,As,, and (c) Ga,Mo,As,X-ray powder diffraction patterns and measuring electricalconductivity were as reported in an earlier paper.12 Electrondiffraction patterns were recorded on a Philips EM 301 electronmicroscope.An ESCA-3 Mark I1 spectrometer (VG Scientific Ltd.) wasused to record X-ray photoelectron spectra and X-ray initiatedAuger electron spectra.Freshly prepared samples were used andexposure to the atmosphere was kept to a minimum. Powderedsamples pressed into thin pellets and coated with silver paintwere mounted on the P8 probe in an argon atmosphere. Sincewe were interested in determining the charge state of the metalatoms, etching with argon ion was avoided. The compounds arefairly good conductbrs, therefore there was no shift in bindingenergies due to charging. The peak energies reported here ar2 J. CHEM. SOC. DALTON TRANS. 1988Table 1. Lattice parameters, v.e.c., and room-temperature resistivities ofA,Mo,As,Lattice parameters/A& P (300 K)/Mo ,AS, 9.600( 1) 3.278(2) 3.6 2.0 x lo-,Cu,Mo,As, 9.642(2) 3.282(2) 4.0 1.8 xCu,Mo,As, 9.644(3) 3.284(2) 4.4 1.0 xAl,Mo,As, 9.643(2) 3.283(3) 4.8 2.0Ga,Mo,As, 9.641(2) 3.282(3) 4.8 1 .oCompound a C v.e.c.ohm cmTable 2. Core-level binding energies (eV) of A,Mo,As, compoundsCompound Core levelMo,As, MO 3d2MO 3d:As 3dCu,M o ,As, Cu 2p;MO 3d+MO 3d3As 3dGa,Mo,As, Ga 3dMO 3d5Mo 3 4As 3dBinding energy228.623 1.841.6933.0228.5231.741.619.6228.4231.541.6Table 3. L,VV (L3M45M45) Auger-electron kinetic energies (eV) of Cuand Ga in Cu,Mo,As,, Ga,Mo,As,, and related solids*Compound L,VV energy Compound L,VV energyc u 9 18.8 Ga 1 068.1Cu,Mo,As, 918.0 Ga,Mo,As, 1 066.9Cu AgSe 917.6 GaAs 1 066.4Cu,Se 917.5 GaP 1 066.2c u , s 917.4* L,VV energies of elemental Cu, Ga, and related solids are taken fromref.15.Figure 2. Electron-diffraction patterns of (a) Mo,As, and (h)Cu,Mo,As, along the [OOl] zone axeswith reference to Au(4f;) which occurs at 83.7 eV and areaccurate within k0.2 eV. The Cu-K and Mo-K edges wererecorded with a bent-crystal spectrograph and energyanalysis of the spectra was carried out with the help of a Carl-Zeiss G-I1 type photometer.Results and DiscussionWe have investigated the formation of A,Mo,As, phases forA = Pb, Cu, Fe, Co, Ni, Al, or Ga with different values of x byallowing the elements to react in evacuated sealed silica tubesat elevated temperatures. Powder X-ray diffraction of theproducts (Figure 1) revealed that A,Mo,As, isostructural withMo,As, are formed only with Cu(0 < x < 4), Fe(x = 2),Al(x = 2), and Ga(x = 2).Refined lattice parameters for thenew phases are listed in Table 1. We have examinedCu,Mo,As, and Mo,As, by electron diffraction to providefurther evidence for the structural similarity. Several crystals ofboth compounds were examined in the microscope. The mostcommon orientation was [OOl] of the tetragonal cell; the dif-fraction patterns (Figure 2 ) are similar showing the tetragonalsymmetry of Cu,Mo,As,. Lattice parameters of A,Mo,As,(Table 1) reveal that, while the c remains nearly the same, thereis an increase in a as compared to Mo,As,, on insertion of Aatoms.By analogy with Chevrel phases, one would expect a chargetransfer from A to the molybdenum cluster in these solids.Wehave investigated the charge transfer and oxidation states of themetal atoms in A,Mo,As, by x.P.s., a.e.s., and x.a.s. The core-level binding energies of Mo, As, Cu, or Ga in Mo,As,,Cu,Mo,As,, and Ga,Mo,As, as determined by X.P.S. are givenin Table 2. The core-level shifts relative to elemental solidsprovide information about the charge transfer and oxidationstates in favourable cases.14,15 The Cu 2p3 binding energy inCu,Mo,As, (933 eV) is higher than that of elemental copper(932.6 eV). We see no shake-up satellite in the Cu 2p spectrum ofCu,Mo,As, (Figure 3). A similar shift in Ga 3d binding energyis noticed in Ga,Mo,As,. That the charge transfer from Aatoms is essentially to the Mo is seen from the shift of the Mo 3dbinding energy as compared to that in Mo,As, (Figure 4).Thearsenic levels essentially remain unaffected; for instance, the As3d binding energy is 41.6 eV in Mo,As, as well as inCu4Mo5As4 and Ga,Mo,As,.The changes in chemical state which give rise to a shift in thecore-level X.P.S. also produce a shift in the Auger spectra.',These shifts which are much larger than X.P.S. shifts and aremainly determined by changes in the polarizability of theenvironment of the ionized atom have been used to characterizethe chemical state of atoms in solid^.'^-'^ L,VV Auger elec-tron energies of Cu and Ga in Cu,Mo,As, and Ga,Mo,AsJ. CHEM. SOC. DALTON TRANS. 1988 3Figure 3. X.P.S.Ga(3d 1J.930 93 5 940 94 5 950 955 960Binding energyIeVof (a) Cu 2p and (b) Ga 3d core levels in Cu,Mo,As, and Ga,Mo,As, respectivelyI l l l l l l l l l l l l l225 230 235 240Binding energyIeVFigure 4.X.P.S. of Mo 3d in (a) Mo5As4, (6) Cu,Mo,As,, and (c)Ga, Mo, As,recorded by using Al-K, radiation in the N(E) us. E mode aregiven in Table 3 [N(E) = Auger electron counts, E = kineticenergy]. For comparison, corresponding data for similar Cu-and Ga-containing solids are also listed. It is seen that the L,VVTable 4. Chemical shifts" in the K-absorption edges of Mo and Cu inA,Mo,As, and related solidsChemical shift (eV) inI 1Mo-K edge Cu-K edgeMo,As, 4.8Mo6S8 6.5Cu1.8M06S8 6.1M06Se8 5.2Cu,Mo,As, 4.5Cu1.8M06Se8 4.9-1.922.101.77--" Measured relative to the metals. The data for Mo,X, and Cu,Mo,X8(X = S or Se) are taken from S.Yashonath, M. S. Hegde, P. R. Sarode,C. N. R. Rao, A. M. Umarji, and G. V. Subba Rao, Solid State Commun.,1981, 37, 325.energy of Cu in Cu,Mo,As, is lower than that of Cu metal butis comparable to the Cu(L,VV) values of CuAgSe and Cu,Se.Similarly, the Ga(L,VV) energy of Ga,Mo,As, is significantlylower than that of elemental gallium but is comparable to thecorresponding value for GaAs. These results may be taken toindicatethat thechemicalnature ofCuinCu,Mo,As,is similar tothat in CuAgSe, and that of Ga in Ga,Mo,As, is similar to thenature of Gain GaAs. Further evidence was provided by chemicalshifts in the K-absorption edge (Table4). The K-edge shift ofCu inCu,Mo,As, is of the same order of magnitude as in the Chevrelphases, CU,.6Mo6Sg and Cu,,,Mo,Se,.Thus it is reasonable toassume that in A,Mo,As, the A atoms act as electron donors'transferring' their valence electrons to Mo atoms of the Mo,As,host; the electrons result in an increase in the v.e.c.* of themolybdenum atom in the cluster. Although it is difficult todetermine quantitatively the extent of electron transfer and the* The valence electron count (number of valence electrons) per metalatom in the cluster which participate in M-M bonding42.0 T0 - 0001-5 - 0- 0 0 ( C ) - o o o o o o o o o o 0I I I I I I I I 3 40 380 420 460 500 - - ‘ e 3000o oo o - 0 -000 -0 -- 00 - 00 0 - O m0 - - 0 ‘ e0 ‘ m00 - -0 - - 00 ( b ) - - (a 1- - o00 - 00I l l l r l r l ! I l l l l l .40 80 120 160 200 240 280 0J. CHEM.SOC. DALTON TRANS. 198811105 9EI\ a“ 8 9765Figure 5. Resistivity us. temperature plots of (a) Cu,Mo,As,, (6) AI,Mo,As,, and ( c ) Ga,Mo,As,1.901-85: 1.80.c0\a1.751.701.65oxidation states of the insertion atoms in AxMo,As,, it is mostlikely that theformaloxidationstatesofCuandGainCu,Mo,As,and Ga,Mo,As, are I and I11 respectively.Electrical resistivity measurements indicate that Mo,As, andCu,Mo,As, are metallic but Al,Mo,As, and Ga,Mo,As, aresemiconducting. While the room-temperature resistivities ofMo,As, and CuxMo,As,, measured on sintered polycrystallinepellets, are around lop2 ohm cm, those of Al,Mo,As, andGa,Mo,As, are around 1-2 ohm cm (Table 1).The tempe-rature dependence of the resistivities (Figure 5) clearly showsthat Cu,Mo,As, is metallic and Ga,Mo,As, and Al,Mo,As,are semiconducting. The occurrence of metallic and non-metallic behaviour in this isostructural series of solids probablysignals the influence of cluster v.e.c. on the electronic properties.This behaviour may be understood in terms of the electronicband structure of Ti,Te,-type solids.’ In this model (Figure 6),which emphasizes the M-M bonding in the cluster chain, eachcluster forms four normal M-M bonds in the equatorial plane;in addition, there are four three-centre M-M bonds involvingthe bridging M atoms at the vertex. The ordering of metal d-likestates shows gaps at 8, 16, and 24 electrons per cluster. Thispicture is consistent with the generally observed v.e.c.of 2.4-3.6 for the Ti,Te, structure., It also explains the formation andproperties of A,Mo,As, reported in this paper. The clusterMo,As, has a v.e.c. of 3.6 at molybdenum [(5 x 6 - 12)/5]assuming that each molybdenum atom ‘transfers’ three of itsvalence electrons to arsenic in forming the compound. Thus,with 18 electrons per cluster available for M-M bonding, thehighest occupied M-M band is partially filled and thereforeMo,As, is metallic. Insertion of four copper atoms in Mo,As,in forming Cu,Mo,As, would increase the v.e.c. to 4.4, makingavailable 22 electrons per cluster for M-M bonding. (Thisassumes that copper is formally 1+ in the solid.) With 22electrons per cluster, the highest occupied M-M band is stillpartially filled in Cu,Mo,As,.In Al,Mo,As, and Ga,Mo,As,the v.e.c. would be 4.8 (assuming that each Al/Ga provides threeelectrons to the cluster). With 24 electrons per Mo, cluster,the highest occupied band would be full and therefore thealuminium and gallium derivatives are semiconducting. It issignificant that a similar behaviour is seen in the M6X8 family ofisolated cluster compounds: the 24-electron compounds,’ *-*’MO,Re& and Mo,Ru,Se,, are semiconducting, while all theother Mo6X8 and A,Mo6X8 Chevrel phases with electroncounts less than 24 per cluster are metallic.,The present investigation has shown that using Mo,As, asthe host it is possible to prepare metal insertion compoundsof formula A,Mo,As,, which are analogous to the Chevrelphases.Just as in the Chevrel phases, the inserted metal atoms‘transfer’ their valence electrons to the host, increasing thenumber of electrons available on molybdenum for M-Mbonding. Strikingly, when this number is 24, as in Al,Mo,As,and Ga,Mo,As,, the material becomes semiconducting, thebehaviour being reminiscent of M6X8 cluster compounds with24 electronsJ. CHEM. SOC. DALTON TRANS. 1988 5Energyt24168.LFigure 6. Electronic structure of Ti,Te,-type condensed metal cluster compounds. (a) Atomic structure of M,X4 chain resulting from thecondensation of M,X, units. (b) Schematic energy levels of the d states involved in M-M bonding. (c) Band structure of a typical M,X4 solid for thewave vector along the chain, k,.The number of electrons per unit cell is indicated (from ref. 7)AcknowledgementsWe thank Professor C. N. R. Rao for valuable advice andencouragement. Our thanks are also due to Dr. M. S. Hegde forrecording X.P.S. and a.e.s. and Dr. P. R. Sarode for x.a.s.measurements. The University Grants Commission, New Delhiand the Department of Science and Technology, Governmentof India, are thanked for support of this research.References1 J. Lewis and M. L. H. Green, Philos. Trans. R. SOC. London, Ser. A,2 A. Simon, Angew. Chem., Znt. Ed. Engl., 1981, 20, 1.3 0. Fischer and M. B. 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