J O U R N A L O F C H E M I S T R Y Materials Communication Rb2Cu3CeTe5: a quaternary semiconducting compound with a two-dimensional polytelluride framework Rhonda Patschke,a Paul Brazis,b Carl R. Kannewurfb and Mercouri Kanatzidis*a aDepartment of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA. Fax: Int. Code+(517) 3531793. E-mail: kanatzidis@argus.cem.msu.edu bDepartment of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA Received 28th August 1998, Accepted 30th September 1998 Rb2Cu3CeTe5 has been synthesized from the reaction of Cu and Ce in a molten alkali metal/polytelluride flux.The compound crystallizes in the monoclinic space group, C2/m (no. 12) with a=18.6884(1) A° , b=6.2384(2) A° , c= 12.5264(3) A° , b=112.795(1)°, V=1346.34(5) A° 3, and Z= 4.Rb2Cu3CeTe5 is two-dimensional with 1 2 [Cu3CeTe5]2- layers built from one-dimensional 1 2 [Cu2CeTe5]3- chains that are ‘stitched’ together by distorted tetrahedral Cu atoms; the compound is paramagnetic and a narrow-gap p-type semiconductor. Over the past decade, the polychalcogenide flux method has become an established technique for discovering new solid state chalcogenides.1 Although many of the compounds form completely new structure types, others are reminiscent of, or can be considered derivatives of known chalcogenides.This is particularly true when lanthanide and actinide metals are Fig. 1 ORTEP representation of the structure of Rb2Cu3CeTe5 as involved. The binary LnQ3 phases (NdTe32 and ZrSe33 type), seen down the b-axis (90% ellipsoids).The ellipses with octant shading for example, are quite stable. Several new ternary phases have represent Ce and Rb, the crossed ellipses represent Cu and the open recently been reported in which the structural motifs are ellipses represent Te. related to these LnQ3 binaries. While NaLnS3 (Ln=La,Ce)4 and ATh2Q6 (A=Cs,Rb,K; Q=Se,Te)5 represent two diVerent variations of the ZrSe3 structure type, ALn3Te8 (A=Cs,Rb,K; Conceptually, these one-dimensional chains derive from the ZrSe3 structure type.By replacing one (Q22-) unit in the ZrSe3 Ln=Ce,Nd)6 is closely related to the structure of NdTe3. In an eVort to access quaternary phases which are less structurally framework with a Q2- unit, the coordination environment of the metal changes from bicapped trigonal prismatic to pentag- related to the LnQ3 binaries, another element was introduced into the synthesis.Copper proved to be well behaved in this onal bipyramidal. This change in coordination is accompanied by a conversion from two-dimensional layers to one-dimen- respect and we were able to isolate several compounds, whereas other elements gave phase-separated ternary compounds.sional chains. Within the 1 2 [CeTe5]5- chains exist empty distorted tetrahedral pockets of Te atoms which are large Reactions in the A/Cu/Ln/Q (Q=S,Se) system have produced several quaternary compounds, including K2Cu2CeS4,7 enough to accommodate Cu atoms. Each Cu atom is bonded at two points to the axial positions of two neighboring KCuCe2S6,7,8 KCuLa2S6,8 CsCuCe2S6,8 CsCuCeS38 and KCuUSe3.8 Other investigators have identified such com- pentagonal bipyramids, and at the remaining sites to the closest edge between these axial positions.The chains, once pounds as BaErAgS3,9 CsCuUTe3,10 BaLnMQ3 (Ln= La,Ce,Nd;M=Cu,Ag; Q=S,Se)11 and KCuEu2S6.12 Although extended to include the Cu atom, can be written as 1 2 [Cu2CeTe5]3-.Finally, the layers are formed when the second many of these phases are structurally unique, some still retain the components of the LnQ3 motif. It is apparent that the type of Cu atom ‘stitches’ these chains together in the adirection by coordinating to neighboring chains in a distorted greater the amount of copper in the framework, the more profound the eVect of breaking up the LnQ3 structure.Along tetrahedral arrangement. A view perpendicular to the layers is shown in Fig. 2(B). It is interesting that if one removes the these lines, we examined the A/M/Ln/Te (M=Cu,Ag) system using polytelluride fluxes and discovered several novel Ce atoms from the structure, the remaining [Cu3Te3] substructure remains contiguous. In this sense, the Ce atoms are compounds including KCuCeTe4,13 K2Ag3CeTe414 and K2.5Ag4.5Ce2Te9.15 We report here on Rb2Cu3CeTe5,16 a low- situated on both sides of a two-dimensional [CuTe]- substrate.In fact, this copper telluride framework, albeit distorted, bears dimensional compound in which the basic LnQ3 structure is substantially disrupted. a close resemblance to the layers of NaCuTe18 [Fig. 2(C)]. The magnetic susceptibility of Rb2Cu3CeTe5 was measured Rb2Cu3CeTe5 consists of 1 2[Cu3CeTe5]2- layers separated by Rb+ cations (Fig. 1.) The Ce atom is seven coordinate, over the range 5–300K at 6000 G, and a plot of 1/xm vs. T shows that the material exhibits nearly Curie–Weiss behavior exhibiting a distorted pentagonal bipyramidal geometry in which one g2-(Te22-) unit17 and three Te2- anions comprise with only slight deviation from linearity beginning below 50 K.Such deviation has been reported for several Ce3+ compounds the pentagon and two Te2- anions occupy the axial positions [Fig. 2(A)]. The pentagonal bipyramids share monotelluride and has been attributed to crystal field splitting of the 3F5/2 ground state of the cation.19 At temperatures above 150 K, ions, forming 1 2 [CeTe5]5- chains parallel to the b-axis.J. Mater. Chem., 1998, 8, 2587–2589 2587Fig. 2 (A) Schematic comparison of the two-dimensional layers of ZrSe3, the one-dimensional 1 2 [CeTe5]5- chains and the 1 2 [Cu2CeTe5]3- chains in Rb2Cu3CeTe5. The dotted line highlights the pentagonal bipyramidal coordination around Ce. Selected distances (A° ) are as follows: Ce–Te1 3.161(1), Ce–Te2 3.2538(5), Ce–Te3 3.246(2), Ce–Te4 3.253(2) and Te1–Te1 2.771(2).(B) View perpendicular to the layers of Rb2Cu3CeTe5, illustrating how the second Cu atom stitches together the 1 2 [Cu2CeTe5]3- chains to form two-dimensional layers. The ditelluride groups above and below the anionic layers are omitted for clarity. Selected distances (A° ): Cu1–Te2 2.820(2), Cu1–Te3 2.591(2), Cu1–Te4 2.593(2), Cu1–Ce1 3.332(2), Cu1–Cu2 2.650(2), Cu2–Te3 2.721(2), Cu2–Te4 2.721(2) and 2.593(2).(C) The distorted [CuTe]-, PbO-like layer in Rb2Cu3CeTe5. a meff of 2.64 mB has been calculated, which is in accord with the usual range for Ce3+ compounds (2.3–2.5 mB). The presence of Ce3+ is confirmed by IR spectroscopy with shows one well defined, broad peak at ca. 3420 cm-1 (0.42 eV) This absorption is electronic in origin and is attributed to an f–f or f–d transition within the f1 configuration of Ce3+. From this we can conclude that Rb2Cu3CeTe5 is a valence precise compound, and thus we expect semiconducting properties. The formal oxidation states are (Rb1+)2(Cu1+)3(Ce3+)- (Te2-)3(Te22-). The electrical conductivity of Rb2Cu3CeTe5 as a function of temperature measured on single crystals suggests that the material is indeed a narrow gap semiconductor with a room temperature conductivity value of 0.05 S cm-1 [Fig. 3(A)]. The log s vs. 1/T plot is non-linear over the entire temperature range of 8–300 K, suggesting the conduction mechanism varies in diVerent temperature regions, possibly due to diVerent types of mid-gap states.Thermoelectric power data as a function of temperature show a large Seebeck coeYcient at room temperature of +275 mVK-1 [Fig. 3(B)]. The increasing Seebeck coeYcient with decreasing temperature and its positive sign are consistent with a p-type semiconductor. Note added in proof. By the time we received proofs of this manuscript we became aware of the syntheses of BaDyCuTe3, K1.5Dy2Cu2.5Te5 and K0.5Ba0.5DyCu1.5Te3 (F.Q. Huang, W. Choe, S. Lee and J. S. Chu, Chem. Mater., 1998, 10, 1320). These compounds are not structurally related to the one reported here; however, they do belong in the broad quaternary family of A/Cu/Ln/Q compounds. Acknowledgments Financial support from the National Science Foundation (DMR-9527347 MGK) and (DMR-9622025 CRK) is gratefully acknowledged.The authors are grateful to the X-ray Fig. 3 (A) Variable temperature, four-probe electrical conductivity Crystallographic Laboratory of the University of Minnesota data for a single crystal of Rb2Cu3CeTe5. (B) Variable temperature thermopower data for a single crystal of Rb2Cu3CeTe5. and to Dr. Victor G. Young, Jr., for collecting the single 2588 J.Mater. Chem., 1998, 8, 2587–25890.447 g Te (7.0 mmol) which was sealed under vacuum in a carbon crystal X-ray data set. M.G.K. is a Henry Dreyfus Teacher coated quartz tube and heated to 850 °C for 10 days. The tube was Scholar 1993–1998. This work made use of the SEM facilities then cooled to 400 °C at -3 °C h-1, and then quenched to room of the center for Electron Optics at Michigan State University.temperature. The excess RbxTey flux was removed, under nitrogen At Northwestern University, this work made use of the Central atmosphere, with dimethylformamide to reveal black needle- Facilities supported by NSF through the Materials Research shaped crystals in 45% yield (based on Cu). The crystals are air and water stable. Phase homogeneity was confirmed by comparing Center (DMR-9632472).the power X-ray diVraction pattern of the product against that calculated using the crystallographically determined atomic coordinates. Microprobe analysis carried out on randomly selected Notes and references crystals gave an average composition of Rb2.46Cu3.29Ce1.0Te5.55. A Siemens SMART Platform CCD diVractometer was used to 1 M. G.Kanatzidis and A. C. Sutorik, Prog. Inorg. Chem., 1995, 43, collect data from a crystal of 0.160×0.035×0.010 mm dimensions 151 and references therein; M. G. Kanatzidis, Curr. Opin. Solid using Mo-Ka (l=0.71073 A° ) radiation. SMART16b software was State Mater. Sci., 1997, 2, 139; M. A. Pell and J. A. Ibers, Chem. used for data acquisition and SAINT16c for data extraction and Ber./Recueil, 1997, 130, 1.reduction. An absorption correction was performed using 2 B. K. Norling and H. Steinfink, Inorg. Chem., 1966, 5, 1488. SADABS.16d 3 V.W.Kro� nert and K. Plieth, Z. Anorg. Allg. Chem., 1965, 336, Crystal data at 173 K: a=18.6884(1), b=6.2384(2), c= 207. 12.5264(3) A° , b=112.795(1)°, V=1346.34(5) A° 3, Z=4, Dc= 4 A. C. Sutorik and M. G. Kanatzidis, Chem. Mater., 1997, 9, 387. 5.623 g cm-3, monoclinic, space group C2/m (no. 12), m= 5 J. A. Cody and J. A. Ibers, Inorg. Chem., 1996, 16, 3273; E. J.Wu, 25.741 mm-1, index ranges -22h20, 0k7, 0l14, M. A. Pell and J. A. Ibers, J. Alloys Compd., 1997, 255, 106; 2hmax=50°, total data 3427, unique data 1307 (Rint=0.044), data K.-S. Choi, R. Patschke, S. J. L. Billinge, M. J.Waner, M. Dantus with Fo2>2s(Fo2) 1087, no.of variables 60, final R/wR2= and M. G. Kanatzidis, J. Am. Chem. Soc., in press. 0.0461/0.1182, GOF 1.041. The structure was solved and refined 6 R. Patschke, J. Heising, J. Schindler, C. R. Kannewurf and using the SHELXTL-5 package of crystallographic programs;16e M. G. Kanatzidis, J. Solid State Chem., 1998, 135, 111. SHELXTL refines on F2. (b) SMART: 1994, Siemens Analytical 7 A.C. Sutorik, J. Albritton-Thomas, C. R. Kannewurf and Xray Systems, Inc., Madison, WI 53719 USA; (c) SAINT: Version M. G. Kanatzidis, J. Am. Chem. Soc., 1994, 116, 7706. 4, 1994–1996, Siemens Analytical Xray Systems, Inc., Madison, 8 A. C. Sutorik, J. Albritton-Thomas, T. Hogan, C. R. Kannewurf WI 53719 USA; (d) SADABS: G.M. Sheldrick, University of and M.G. Kanatzidis, Chem.Mater., 1996, 8, 751. Go� ttingen, Germany, to be published. (e) SHELXTL: Version 5, 9 P. Wu and J. A. Ibers, J. Solid State Chem., 1994, 110, 156. 1994, G.M. Sheldrick, Siemens Analytical X-ray Instruments, Inc. 10 J. A. Cody and J. A. Ibers, Inorg. Chem. 1995, 34, 3165. Madison, WI 53719. Full crystallographic details, excluding 11 A. E. Christuk, P. Wu and J.A. Ibers J. Solid State Chem., 1994, structure factors, have been deposited at the Cambridge 110, 330; P. Wu and J. A. Ibers, J. Solid State Chem., 1994, Crystallographic Data Centre (CCDC). Any request to the CCDC 110, 337. for this material should quote the full literature citation and the 12 W. Bensch and P. Du� richen, Chem. Ber., 1996, 129, 1489. reference number 1145/124. 13 R. Patschke, J. Heising, P. Brazis, C. R. Kannewurf and 17 The Te–Te stretch exhibits a Raman shift at ca. 160 cm-1. M. G. Kanatzidis, Chem.Mater., 1998, 10, 695. 18 G. Savelsberg and H. Scha�fer, Z. Naturforsch., Teil B. 1978, 33, 14 R. Patschke, P. Brazis, C. R. Kannewurf and M. G. Kanatzidis, 370. Inorg. Chem., in press. 19 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 15 R. Patschke, P. Brazis, C. R. Kannewurf and M. G. Kanatzidis, Pergamon Press, New York, 1984, p. 1443. submitted for publication. 16 (a) Rb2Cu3CeTe5 was synthesized from a mixture of 0.448 g Rb2Te (3.0 mmol), 0.095 g Cu (3.0 mmol), 0.070 g Ce (1.0 mmol) and Communication 8/06729E J. Mater. Chem., 1998, 8, 2587–2