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J. Phys. Chem. Solids Vol. 53, No. 1I, pp.1321-1332, 1992 0022-3697/92 $5.00 + O.CMl Printed in Great Britain. 0 1992 Pergamon Press Ltd

SYNTHESIS AND CHARACTERIZATION OF FULLERIDES: A$,,

D. W. MURPHY, M. J. ROSSEINSKY,,R. M. FLEMING, R. TYCKO, A. P. RAMIREZ, R. C. HADDON, T. SIEGRIST, G. DABBAGH, J. C. TULLY and R. E. WALSTEDT AT&T Bell Laboratories, Murray Hill, NJ 07974-2070, U.S.A.

(Received 4 February 1992)

Abstract-Alkali metal fullerides (AC,) are a subject of considerable current interest because of the occurrence of superconductivity for A$, at surpassed only by the high T, oxides. The preparation and characterization of A.$, (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR (“C, 23Na and *‘Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine C, or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A,C, compounds are metallic (and superconducting). The superconducting T, increases nearly linearly with unit cell size. EHT (Extended Hiickel Theory) calculations and “C NMR relaxation measurements indicate higher of states for the higher T, compositions.

Keywords: Superconductivity, Fullerenes, carbon, structures.

INTRODUCTION EXPERIMENTAL

Since the discovery of [ 1] of C, and higher fullerenes The Cso used in this work was purified by standard there has been considerable interest in the chemistry extraction and chromatography of soot prepared and physical properties of these molecular forms of in-house by the spark erosion technique [2] or pur- carbon. The isolation of macroscopic quantities of chased from Texas Fullerenes. Extract was also pur- Cm by the spark erosion technique [2] added the chased from MER. We have observed no differences possibility of investigating solid state properties. The in the materials prepared from any of these sources. molecular fullerene solids are held together by rela- A variety of techniques have been employed to tively weak van der Waal’s forces similar to those synthesize A,&, with the choice of technique depen- between layers in graphite. A rich variety of physical dent on both A and x. A&, (A = K,Rb,Cs), is best properties have already been identified for solid C, prepared by direct reaction with excess alkali metal and its compounds [3]. The discovery of supercon- vapor at 250-350°C for 10-15 days in sealed Pyrex ductivity in KC,, [4] and identification of K,C, as tubes [14], and for A = Na at 350-4OO”C for 6 days the superconducting phase [5,6] has led to a broad in stainless steel tubes [ll]. The A&,, and A&,, class of superconductors [4-l l] of composition A&, (A = K,Rb,Cs) phases can be prepared by several (A = alkali metals) with superconducting critical tem- methods, but the most reliable way to control stoichi- peratures, T,, surpassed only by the copper oxides. ometry is reaction of appropriate amounts of A&, The occurrence of superconductivity has focused and C, at 200-250°C for 10-15 days [16]. For considerable interest on A$, and a rich variety of A = Na, Na,C, is best prepared by reaction of NaH A&,, have already been reported with compositions or Na,Hg, with Cso at 350°C for nine days with two at x =2 [ll, 121, 3 [6, 8, 111, 4 [13] and 6 [11, 141 intermediate regrindings [ll]. Na,C,, is best syn- depending on A. Alkali metal intercalation in the thesized by reaction of Na,Hg, with C, at 330°C for fullerenes already appears to be more diverse than the 10-15 days followed by distillation of Hg away widely investigated graphite analogs [lS]. In this from the product [1 11. A,& with mixed alkalis paper we expand on our previous reports on bulk, are prepared either by reaction of appropriate quan- polycrystalline A,&, and integrate these with the tities of the different A&&s with Cso, or by reaction results of others to present an overall picture of the of A,&, C, and NaH for Na containing compo- A$, compounds. sitions. In each of these methods the reaction times

1321 1322 D. W. MURPHYet al. are dependent on amount and crystallite size of the STRUCTURES Cso and the size and ambient in the reaction tube. Reactions in this study used from 10 to 500 mg The room solid state structure of Cm C,. Appropriate physical properties (e.g. X-ray may be described as FCC packing of identical, spheri- diffraction, NMR, magnetization) must be monitored cal Cso . The observed X-ray powder data to insure phase purity. Even brief exposure to air are modeled well by using uniform shells of electron results in serious degradation of the superconducting (radius 3.55 A) for the Cso molecules at the 4u properties of A&. They should be maintained in an positions of a FCC cubic cell of symmetry Ftn%~ inert atmosphere at all times. [ 19,201. A van der Waal’s radius of 5.01 8, is calcu- Powder X-ray diffraction patterns were taken using lated from the crystallographic unit cell. Motional CuK, radiation from a high intensity rotating anode narrowing of the “C NMR spectra shows that the source with a singly bent pyrolytic graphite (PG) molecules rotate rapidly compared to the NMR focusing monochromator and a flat PG analyser. The timescale (x 100 ps) [21,22]. At 260 K there is a first resolution was enhanced over that provided by the order phase transition to a simple cubic cell [20] of 0.5” acceptance of PG by placing slits before the symmetry Pug, corresponding to rotational ordering monochromator and the analyser. This gives an of the molecules. The Pa3 structure has been refined instrument with a moderate angular resolution by X-ray [20] and neutron diffraction [23] on powders (-0.25” 2t?) but a high dynamic range (> 10’ for and X-ray diffraction on twinned single crystals [24]. powder diffraction). Samples for X-ray diffraction Continued motional narrowing of the “C NMR were sealed in Debye-Scherrer capillaries to protect spectra below 260 K leads to the conclusion that Cm them from air. The capillaries were then mounted in molecules continue to rapidly ‘ratchet’ between sym- a vacuum chamber with a beryllium window to metry equivalent orientations above about 140 K eliminate scattering from air at low angles. 1251. LAZY/PULVERIX [ 171 and the NRCVAX [ 181 The structures of all the alkali metal fullerides package were used in refinement and modeling of characterized to date may be regarded as intercala- intensity data after modification of the source code to tion compounds of the pristine FCC structure or of allow a form factor for C,, asinQR/QR, which hypothetical Cso structures with BCC (body centered models C, as a hollow spherical shell of radius cubic) or BCT (body centered tetragonal) ball pack- R containing 360 electrons. R was fixed at ing. The ideal sizes and coordination numbers of 3.55 A as determined from our previous single crystal interstitial sites for FCC and BCC packings of rigid analysis [19]. The value of R was manually varied spheres of radii 5.01 A are summarized in Table 1 for various A$, and found to be constant along with the ionic radii of the alkali metal ions. The for each. The NMR spectra were obtained at octahedral and tetrahedral interstices of FCC are 9.39T (100.5 MHz for 13C). Samples for NMR fixed on special positions. The octahedral site is larger were sealed in Pyrex under a He atmosphere, than any alkali metal ion, and the tetrahedral site is except for 23Na which were sealed in quartz to closest in size to Na + . (Ions can generally occupy minimize background signals. Superconducting sites smaller than their hard sphere sizes would shielding measurements were taken on a Quantum indicate.) These are the only interstitial sites generally Design SQUID at fields of 5-10 Oe. DSC data were considered for FCC packing, but the large size of the taken on a DuPont 1090 Thermal Analyser. Elec- Cso molecules makes consideration of other sites tronic structure calculations were carried out with the reasonable. The most likely of these is (x,x,x) which EHT (Extended Hiickel Theory) band structure for x = l/3 gives a trigonal site of appropriate size for programs. small cations (e.g. Li+). For BCC packing, the

Table 1. Interstitial sites in C, Interstitial sites Alkali metals Structure Unit cell type (A) (wJ) No. per C, CN Radius (A) A+ Radius (A) FCC 14.17 (& ;, ;) 1 6 2.06 cs 1.70 (s.g. Fmg) c, t, t, 2 4 1.12 Rb 1.49 (x, x,x)x = l/3 8 3 0.78 K 1.38 Na 1.02 BCC 11.57t (0, f, 2) z = $ 6 4 1.46 Li 0.69 (s.g. ImJ) (x, y, z) x = y = l/8, z = f 6 3 1.13 tcalculated lattice parameter for spheres of radius 5.01 A. Synthesis of alkali metal fullerides 1323

b)

Go (fed bet d

A4C60 A6C60 bet fee bee Fig. 1. Schematic structures of C, and A,C, with C,s as large spheres and A as the smaller spheres. (a) FCC C, drawn in an equivalent BCT representation. (b) The structure of Na,C, with Na ions in tetrahedral interstices. (c) A$, with A ions in both tetrahedral and octahedral interstices. (d) The A&,, structure exhibited by K, Rb and Cs. (e) The FCC A&, structure (A = Na, Ca) with the darker Nas 50% occupied. (f) The BCC A&, structure of K, Rb and Cs.

interstitial sites located at (0,1/2,z) are tabulated for Schematic representations of the known A,& z = 0.25, which is equidistant from four C&s. The structures are illustrated in Fig. 1. Tables 2 and 3 list sites for BCT packing are more complex, depending unit cell data for the various observed phases. The both on x and c/a, but are approximately the size of A&, structure was first refined by Stephens et al. [6] those for BCC. for K,C,. For the C,, to be completely ordered and

Table 2. Unit cell and T,s for FCC A$, Lattice “C shift parameter(s) (A) T,(K) (%)t (ppm vs TMS)

Na,C,S 14.189 (1) - 173 (2) Na& 14.183 (3) - 175 (2) NaZG 14.380 (8) 167 (2) Na,KC, 14.120 (4) Na,RbC, 14.091 (6) 180 (2) NarRb, Cs, ,C, 14.148 (3) 8.63) Na,CsC, No. 1Q 14.132(2) 10.5 (8) 182 (2) Na,CsC, No. 24 14.176(9) 14.0 (9) K3C, 14.253 (3) 19.3 (30) 183 (2) K,RbC, 14.299 (2) 21.8 (32) Rb,KC, No. 10 14.336 (1) 24.4 (34) Rb,KC, No. 23 14.364 (5) 26.4 (32) Rb& 14.436 (2) 29.4 (35) 171 (5) Rb,CsC, 14.493 (2) 31.3 (48) tT,s and shielding fractions (%) were measured by d.c. magnetization. @ample is simple cubic. &les labeled No. 1 and No. 2 have the same nominal composition. 1324 D. W. MURPHY et al.

Table 3. Unit cell and alkali sites for A&, and A&, 13C shift Phase a(A) c(A) cla rJt (ax) r&S (avg) (ppm vs TMS) K,C, BCT 11.886 (7) 10.774 (6) 0.906 ;:;;} (1.40) ;.;} (1.53)

BCT 11.962 (2) 11.022 (2) 0.921 RW, ;:g} (1.48) ;:;} (1.58) 181 (2)

Cs,C, BCT 12.057 (18) 11.443 (18) 0.949 ;:;;} (1.60) ;‘;;} (1.67)

K&x, BCC 11.3905 1.00 ;I;;} (1.36)

Rb,C, BCC 11.5489 1.00 ::;;} (1.45) 154 (5)

BCC 11.7905 1.oo Cs,C@l ;.;;} (1.59)

tDistance from center of A ion site to center of Cm minus the van der Waal’s radius of C, (5.01 A). tTbe 0 denotes a vacancy. §Data from Ref. 14. remain FCC, one expects the space group Fm3. In Na on the tetrahedral sites and 50% occupancy of a this packing all molecules are identical when viewed cubic cluster of sites at 32fpositions (0.43, 0.43, 0.43) down (100) directions. Stephens et al. [6j found that centered on the octahedral site. An arrangement of the observed space groups is Fm3m indicating that Na consistent with half occupancy of these the molecules are disordered over two orientations sites is the presence of Na, tetrahedra formed by Na differing by 7[/2 rotations about (100). A11 A$& atoms at opposite corners of the cube. These tetrahe- that have been structurally characterized exhibit FCC dra would then be disordered between parent octa- structures at room temperature. hedral sites. This structure has also been proposed for Powder X-ray diffraction patterns for Na,C, are Ca,C, [26]. The diffraction pattern of Na,C, at room shown in Fig. 2. Refinement of the patterns [l l] temperature fits the normal FCC A,&, structure. indicate that Na,C, has Na in tetrahedral sites of a Although Na,C, refines well for Na on both octa- primitive cubic lattice. The Na saturated phase has a hedral and tetrahedral sites, the possibility of Na composition near Na&,,, which refines well for two displacement to a more general position (e.g. 32f) is likely. Below 260 K there is a structural transition in I I I I I I I I Na,C,, resulting in an X-ray pattern (Fig. 2) that appears to be a two-phase mixture arising from disproportionation into compositions with lattice parameters close to those of Na,C, and Na,C,. Remarkably this transition is reversible with tem- perature. Since it is hard to imagine facile Na motion over large distances to produce a macroscopic two phase material, we envisage a shorter range diffusion of Na from the tetrahedral site to form clusters about the octahedral site. Preferred compositions (still to be determined) such as Na,C, with all Na clustered about the octahedral site could account for formation of two phases. T= 180K The BCC A&, (A = K, Rb, Cs) structure, first I reported by Zhou et al. [ 141,is based on BCC packing of Cm molecules with A ions occupying four coordi- nate interstices between rotationally ordered C, molecules. For A = Cs, refinement located A at (0,1/2,0.28), which may be described as a distorted 10 20 30 40 50 tetrahedron with A closer to two C&s. Assuming the 28 same fractional coordinate for other A, the radii of Fig. 2. Powder diffraction patterns of Na,C,. Intensities are normalized between samples, but the 300K and 180K the sites are given in Table 3. The average of the four patterns of Na,C, are on the same scale. distances to C,s are close to the A+ ionic radii. The Synthesis of alkali metal fullerides 1325

A,& structure, described by Fleming el al. [13], has between ions with the largest size differences, i.e. a BCT structure and was refined in the I/4mmm space Na,CsC, and Na,RbC, [ 111. It should be noted that group with C, as a shell of electron density. The true the large size of the octahedral site compared to that symmetry must be lower if the Cd are completely of the A + ions is reflected in large thermal factors in ordered due to the absence of a four-fold axis in the the structure refinements of A&,. The larger unit C, . The sizes of the A+ sites in A.&,, are cells of Na,C, and Na,KC, compared to Na,RbC, given in Table 3. The sizes of the A + ion sites in both are attributed to Na+ occupation of sites displaced A,Cso and A&, closely match the ionic radii of the from the centers of the octahedral sites. The larger A+ ions. The A&,, structure may be viewed as a of A&,, compared to the A&, of the same defect A,&, structure. The six identical sites in the A reflects the disproportionation of site sizes in the BCC structure disproportionate into four slightly former with the vacant sites being larger as shown in smaller sites and two larger ones for BCT structures Table 3. The difference is most pronounced for the with c/a < 1.0. The A ions occupy the smaller sites. smallest ion, K+ . The observed structures can all be rationalized on Electrostatic calculations shed light on the exist- the basis of alkali ion size and electrostatic consider- ence of the observed phases and their detailed struc- ations. A plot of the unit cell of the A$, tures [13,27]. The calculations assume point charges normalized per Cm molecule versus the total cation for A+ and C&, experimental lattice parameters for volume calculated from the ionic radii, shown in known phases, and an electrostatic energy of putting Fig. 3, makes a useful comparison both within and x electrons on C, of E, = - EA + CJR, where the between structure types based on alkali ion sizes. For electron affinity, E,, is taken as 2.6 eV. For C, with the FCC series, compositions exist with both larger radius R = 3.5 A, CJR is the minimum Coulomb and smaller cell volumes than for the pristine host. repulsion of x point charges on the sphere. For This is also true for the BCC series, based on the Rb,C, the known phases (x = 3, 4 and 6) are the estimated volume of a hypothetical ‘pristine BCC ones found to be most stable with these simple host’ with spheres of the same radius as C,. For assumptions and have electrostatic energies within A& the cell is contracted for A = K (rx+ = 1.38 A) 0.1 eV of each other. Two other structures, Rb,C, which is smaller than the hard sphere site size (Rb in tetrahedral sites) and Rb,C, with the Al5 (r = 1.49 A), whereas Rb (rRb+ = 1.49 A) is nearly a structure (Fig. 4) are less stable by only about 0.2 eV. perfect match and Cs (rcs+ = 1.70 A) causes a lattice Other calculations suggest that the Al5 structure is expansion. The A& data are more complicated energetically comparable [28]. The A&,, phase is because there are two different types of sites. The experimentally observed for A = Na, but the A15 smaller tetrahedral site is ‘lattice expanding’ for ions structure has yet to be observed. The most likely larger than Na, and the octahedral site is ‘lattice place to find an Al5 structure would be for A = Cs, contracting’ for all A ions, giving rise to a minimum in the lattice parameter as a function of cation size. Refinements of X-ray powder intensity data for mixed alkali A&, show distinct site ordering only

840 -

26 50 75 100 126 A&o A Cation Volume (is) Al5 Fig. 3. The crystallographic volume of A$, per C, (from Fig. 4. A hypothetical A&,, with the Al5 structure. The Tables 1 and 2) versus the sum of the A+ ion volume (per structure can be seen to be an ordered defect structure of C,). The lines are a guide to the eye. ‘W,. 1326 D. W. MURPHYet al. where there would be a good size match and the FCC Thus, there must be a K,C, phase with x being structure would be destabilized by the size mismatch sufficiently small that the i3C NMR chemical shift is between Cs and the tetrahedral sites. The report by unaffected. Our experience with these materials has Kelty et al. [lo], of superconductivity in Cs,$, could been that there is no single technique capable of be due to an Al 5 phase. We have also observed small demonstrating phase purity for all AC,,, but rather amounts of superconductivity in Cs,C,, but have only a combination of techniques is essential. Thus, we observed Cs,C, and Cs,C, by X-ray diffraction. expect the exact determination of phase limits in these The octahedral and tetrahedral sites in FCC lat- materials will be the subject of considerable future tices are electrostatic saddle points rather than min- work. Our current view of the phase diagrams for the ima, and the total energy can be lowered by AC,, is illustrated in Fig. 5. We use gradually shaded displacement of the cations to positions of lower regions around Cm and A&, to reflect our uncer- symmetry. Uncorrelated cation displacements would tainty about the exact location of the phase bound- preserve FCC symmetry, but correlated displace- aries and our prejudices about which ions might ments would be symmetry lowering. Both Na,KC, exhibit larger solid solution regions. and Na,C, exhibit symmetry lowering transitions below room temperature [l 11. Displacement of Na CHEMICAL PROPERTIES OF A&, from the octahedral site becomes massive for Na,C,, resulting in a new phase. The minimum electrostatic Knowledge of the chemical properties of C& and its energy for the A&, structures, employing exper- compounds with alkali metals is crucial to the syn- imental lattice parameters, occurs for cation position thesis of single phase materials, their handling in the about (0.21,0,1/2), in good agreement with the refined measurement of physical properties, and the eventual value of (0.22,0,1/2) for Rb,C, [ 131. The electrostatic synthesis of new materials or crystals. In addition to calculations similarly predict the optimal position the reactions outlined above that we consider the best (0,1/2,z) to be z = 0.29 site for the BCC A&,, current methods of synthesis for A&,,, we have structure in agreement with the refined Cs position explored several other reactions that may prove (z = 0.28) of Cs,C, [14]. The success of the simple useful in the preparation of new materials or the electrostatic model for most AC,,, supports the de- growth of crystals. scription of these compounds as fulleride salts. The All of the alkali metals react directly with C,,, but Na,C,, phase and its Ca analog, are notable in being the reactions are most facile for the heavier alkali, the only A$,, structures observed so far that do not presumably due to higher alkali metal vapor pressure, correspond to electrostatically favorable structures, higher diffusivity of A in the solid A,&, and lower leading to the conclusion that they are stabilized by reactivity with glass. Direct reaction is the most covalent interactions either within the A, cluster or convenient route to the highest stoichiometry fulle- between A and C,. Tetrahedral Na, clusters are also rides, A,&, (A = Na, K, Rb, Cs). For K, Rb and Cs found in (CH,Na), [29] and an Na:+ cluster has been implicated in zeolite Y [30]. One might expect extensive solid solutions in the A$, based on partial filling of interstitial sites. However, the bulk of the data reported to date Na, 1 Na, Na 1 phase 1 phase indicate that most of the observed phases for A = K, ? ? Rb and Cs are line phases or nearly so. This was first shown in connection with superconductivity by K Holzcer et al. [S], who realized that different nominal compositions of K,& with the same T,s and volume fractions peaking at x = 3 indicated a line phase Rb superconductor at that composition. From X-ray powder diffraction Zhu et al. [31] have reported that for A = Rb, the x = 0 and x = 6 phases show vari- cs ations in lattice parameters indicative of a broader region of homogeneity. They suggest that Rb,C, 0 1 2 3 4 5 6 forms a solid solution (BCC) for 5 < x < 6 and that x in A,Cs, a dilute FCC phase is present for 0 < x < 1. “C NMR supports a two phase description of K,,,C,, as Fig. 5. A proposed phase diagram for A$,. The notation A, is equivalent to A$,. Gradually shaded regions are C,, + K,C,, [27], but DSC of the same sample shows meant to indicate uncertainty about the actual location of no evidence of the Fm3m to Pa3 transition at 260 K. the phase boundary. Synthesis of alkali metal fullerides 1327 the A,&, and A&, phases are more conveniently routes are also soluble in THF). The solubility in THF prepared by reaction of Cso with A&& as first is further evidence that these compounds behave as described by McCauley et al. [16]. This is especially fulleride salts. However, heating to -300°C was true for small quantities because the difficulty of necessary to remove solvated solvent from the reac- accurately weighing small quantities of A is circum- tion products. Nominal compositions K3C6, and vented. McCauley et al. [16] noted that the actual Rb3C, prepared in this way showed only small stoichiometry by weight uptake was As&,, rather amounts of superconductivity and were non-crys- than the crystallographically limiting composition talline, but showed 13CNMR spectra similar to those A,&. We have also found this to be the case and of material prepared by solid state routes. Reaction attribute the difference to alkali adsorbed on the with excess alkali metal in either NH, or THF gave surface of crystallites or associated with defects. insoluble products, presumably a solvated A.$,. because of the heterogeneous of the reactions, Wudl et al. 1371 have reported that C,, reacts with the conditions necessary for equilibration vary con- amines in a nucleophilic addition and we cannot siderably and monitoring of several physical proper- exclude the possibility that NH3 is added to the C, ties is necessary to assure complete reaction. For direct and is then eliminated on heating. reaction with 20-50mg of C, powder in tubes with Since the A+ ions coordinate strongly to THF and a volume of N ICC, measurements of X-ray powder NH,, we attempted a modification of the McCauley patterns, NMR and superconducting flux exclusion prep. using C, in a toluene solution (eqn (3)) indicate that 7-14 days at 250-350°C may be needed for completion. In one experiment, the superconduct- ing shielding fraction increased with increasing reac- tion time and temperature before leveling off at 35% The characteristic color of C, in toluene disappears shielding after 21 days [32]. The thermal stability in after approximately 30 min and all the C,, precipitates. the absence of or moisture is not known with The as-precipitated material does not show supercon- any precision. Superconducting shielding fractions of ductivity. ‘H NMR and weight gain measurements K&, begin to decrease on annealing above 450°C indicate a composition Rb&,, . (CsH,CH3)3,, for the but this is likely dependent on annealing time and may reaction with A = Rb. It is presumably the C& anions be different for other alkalis. K,C, decomposes by that are solvated rather than the cations in this case. 550°C to give K,C, [ 131with the liberated K reacting Powder X-ray diffraction shows that this material is with the glass. For molecular compounds, all of the crystalline. The powder pattern has not been indexed, A,C,s are remarkably stable thermally. but shows a prominent 10.0 8, peak previously associ- The molecular nature of C, and its solubility in ated with solvated Cm phases [38]. DSC measurements organic solvents makes solution chemistry routes to show an endotherm at 350°C presumably associated A$& tempting. In addition, solution with solvent loss. The strong tendency for solvation shows that Cm is reduced in a series of one electron of both anions and cations in this system makes reductions [33] (so far five le reductions have been finding a true solution route challenging. reported) differing by -0.5 V with the first reduction We have prepared A,& (A = Na, x = 2,3; A = K, near -0.25 V vs SCE. Bausch et al. [34] obtained x = 3) by reaction of solid C, with solid AH or ABH, solutions of C;$- anions by sonication of C, and Li as shown in eqns (4) and (5) in THF that had a “C NMR of 157 ppm. Wang et al. obtained superconducting K,Cso [35] and Rb,C, xAH+C,+A,Cm+;H1 (4) [36] by reaction of toluene solutions of C,,, with the alkali metal followed by filtration of the resulting precipitate. We have obtained A3C, by reaction of C,, x ABH, + C, + A&, + ; H, + ; (B2H6). (5) with alkali metals in liquid ammonia or THF, fol- lowed by heating to N 300°C (eqns (l)-(2)) These reactions are convenient because it is much easier to handle small quantities of AH or ABH, than THF 300°C Chins) + 3K,io,,-3K& + C&j=,,- [K3GoI (1) the alkali metal. The reactions were run in sealed Pyrex ampoules after grinding the together in a mortar and pestle. K,C, prepared by eqn (5) (200°C for 12 h, 275°C for 4 h, 350°C for 72 h) gave as sharp a superconducting transition as we have seen The first point of note concerning these reactions is from any reaction. These reactions are closely related that A,C, (at least for K and Rb) is soluble in NH, since the reaction temperature is near the temperature and in THF (the pure phases prepared by the standard that borohydrides decompose to give hydrides. It is 1328 D. W. MURPHY et al. noteworthy that graphite reacts with KH to form the ternaries KH,JZ8 and KHO.& [39]. No tendency to form ternary hydrides was observed for C, under our conditions, but Wudl et al. [371 have shown that alkyl borohydrides add H to Cso. These routes are particu- larly attractive for Na since its low vapor pressure 1:: and reactivity with Pyrex present problems for the %%Rb,%o direct reaction of Na and Cm. Kelty et al. [lo] used Hg amalgams to prepare A,C, with K, Rb, Cs and mixed alkalis. We have also used the amalgam Na,Hg, to prepare Na,C,s. Analogous reactions -A._d!!L280 200 120 PPM with graphite again lead to ternary phases [39] which we have not observed for C,.

NMR

13C NMR has proven a useful spectroscopy for fullerenes, affording information on the structure, RW%a dynamics and electron structure [21,22,25,27,40]. n For pristine C,,, the room temperature 13C spectrum consists of only one motionally narrowed line at 143 ppm corresponding to freely rotating molecules 180 0 [21,22,25]. Transition to the ordered Pa3 structure 280 200 120 320 PPM PPM at 260 K is accompanied by a sudden increase in the Fig. 6. Representative “C NMR spectra spin-lattice relaxation rate, (1 /T,) [25], but continued for A&, (100.5 MHz). Shifts are relative to TMS. motional narrowing indicates that molecular rotation remains fast on the NMR timescale down to w 140 K. the A&,, make NMR a useful tool for evaluating Analysis of the temperature dependence of the relax- phase purity. ation rate indicates energy barriers to molecular In addition to “C, we have used 23Na and 87Rb reorientation of 42 meV above 260 K and 250 meV spectra to gain information about the alkali sites. The below 260 K [25]. resonances for both 23Na and 87Rb are very close to Room temperature 13C NMR spectra of a variety the values for the ions in aqueous solution in agree- of A,C& compounds are shown in Fig. 6. The pos- ment with the description of these compounds as itions of the centers of gravity (isotropic shifts) of the A,+ C’&. For Rb,C, there are two resonances at NMR lines are listed in Tables 2 and 3. The i3C room temperature with an approximately 2: 1 ratio at resonances of the A&, are shifted downfield from - 5 and - 130 + ppm, which we assign to the tetrahe- that of pure Cm. The overall shifts are the sum of dral and octahedral sites, respectively. Powder X-ray chemical shifts and (for the conducting phases) refinements indicate that K and Rb are disordered in Knight shifts. The magnitudes and signs of the K,.5Rb,,SC60, but 87Rb NMR finds approximately Knight shifts are currently under study. It is apparent equal amounts of Rb on both sides, indicating that from the 13C shifts that there is no simple monotonic there is substantially more than a statistical occu- correlation of the shifts with the formal charge on pation of the octahedral site by Rb as expected based C,. At room temperature, the 13C spectra of Na&,, on the larger size of Rb+ than KC. Na,C,, K3Ca, Na,CsC,, Rb&, and Na,C, show significant motional narrowing, indicating that C& ELECTRONIC PROPERTIES anions reorient on a timescale less than h 100 ps. At low temperatures, where molecular rotation is very The overriding interest in A$, is the occurrence slow, crystallographically inequivalent carbons gen- of superconductivity for compounds with x = 3. erally have different r3C chemical and Knight shift Superconducting transitions in A,C, are readily tensors. The NMR spectra are therefore superposi- measured by d.c. magnetization. The susceptibility tions of powder patterns from the inequivalent sites. measured on warming, after cooling in zero field, Differences in the shift tensors for different sites lead gives a diamagnetic shielding. Representative shield- to a residual line broadening in A,&, (e.g. compared ing curves for superconducting A,C, are shown in with pure C,) even when molecular reorientation is Fig. 7 and the T,s are tabulated in Table 2. The rapid. The various chemical shifts and lineshapes of fraction of observed diamagnetism to that expected Synthesis of alkali metal fullerides 1329

RbzCg RbS.

RbzK{:

KZRb. T,(K) ” t KS-

-1 10 . Na,Rb.,Cs r. T(K) t Fig. 7. Shielding measurements for A&,, normalized to the t shielding at 5K. 01 tjas Ns,l$ Na,Rq I J 0 10 20 30 40 50

Catlon VoIume(is) for the same volume of perfect diamagnetism is the Fig. 9. Superconducting T,s plotted against calculated cat- shielding fraction, or volume fraction. The shielding ion volume for A&,. fraction is a quantitative determination of the phase purity of a superconductor only if the sample mor- phology is known. For example, anomalously large been noted above. A plot of the T,s against the shielding fractions can be obtained if a closed shell of intercalated cation volume (Fig. 9) extrapolates to superconductor surrounds a non-superconductor. T,= 0 near 20-22 8, [3] corresponding to the compo- For C, there is another effect, related to the finite size sitions Na,KC,-Na,RbC,. T, extrapolates to zero of the crystallites. Low values for the shielding frac- for about the same cation size for which the A,&, tion are obtained unless the grain size of the super- unit cell reaches a minimum (Fig. 3). Furthermore, conductor is > 10 times the London penetration compositions on the low cation volume branch of depth. For these materials the particle size is of the Fig. 3 are also apparently non-metallic (based on ‘% order of 1 p and the penetration depth of K,C, is TlT measurements). Disordered displacements of ~44800 A [41]. A two fluid model suggests the maxi- Na+ ions on octahedral sites could contribute to mum shielding for a loose powder to be near 35% for electron localization for the low cation volume com- these parameters [42]. The superconducting T,s of positions. In addition, the Na,C, structure suggests A,C,s are plotted against lattice parameter in Fig. 8 covalent interactions and the possibility of only par- along with T,s[4345] and lattice constants [46] for tial charge transfer. If partial charge transfer does K&, and Rb$$, determined under pressure. For play a role it appears to be more significant for Na a > 14.2 A the 1 atm and pressure data are in excel- on the octahedral site, since the T,of Na,CsC, is as lent agreement, with substantial differences only for expected based on extrapolated values. It is interest- the smallest cations where structural anomalies have ing to note that the cell volumes of the A&,, has a very similar dependence on intercalant volume to the A&s, leading to the possibility of higher Tcsfor this

l K,C6, PRESSURE series if the electron count could be manipulated to a Rb3C6,, PRESSURE half-fill the ti, band (three electrons per C,). 0 A3Ce0 ONEATM. 0 30 m In order to understand the origin of the supercon- ductivity in A&, and the magnitude of the Tcs,a $ more complete picture of the electronic properties is needed both above and below T,.In addition, prop- erties of the other A,& are of intrinsic interest and may add insight into superconductivity. The un- availability of single crystals, or even dense pellets, combined with the air sensitivity of the compounds have limited the number and variety of experiments 0 0 ’ ’ ’ q ’ ’ ’ ’ ’ ’ relevant to the elucidation of electronic properties. 14.5 13.7 13.9 14.1 14.3 14.7 Conductivity measurements have been reported only LATTICE PARAMETER, a ( A ) for thin films [47-511. These measurements show Fig. 8. The relationship of the superconducting T, to the small increases in the resistivity with decreasing tem- unit cell size of A&,,. 0 is data from Table 2. l and A are data from K,C, and Rb,C,, respectively, under press- perature for K,C, and Rb,C, with depressed Tcs ure as given in Ref. 46. attributed to granularity [SO]. It has further been 1330 D. W. MURPHY et al. suggested that the conductivity is near the Mott minimum metallic limit [49]. These studies also show that the A&s are insulating. A number of other techniques such as “C NMR [27,40], bulk magnetic susceptibility [5 11, EPR spin susceptibility [51, 521 and photoemission [12,53-561 have been used to address the question of metallic conductivity and density of states. There is a general consensus from all of these techniques that the A,&+ (with the exception of those with Na on octahedral sites) are metallic with the other structures being non-metallic. Calculation of densities of states for A3Cso from each of these techniques, however, involve approximations and vary considerably from each other. The EHT band structures [57] calculated for FCC Cso, Pa? Cm and A$, (X = 3, 4, 6) are shown in Fig. 10. As discussed previously [47], the EHT calcu- lations provide a qualitatively correct description of the conduction bands of Cso The band structure calculations indicate that the DOS distributions of DENSITY OF STATES / eV &=30 the t,,_ and t,,_ derived bands are relatively constant Fig. 10. EHT band structures of A$, from Ref. 57. across the different phases. The band widths and energy spectra remain qualitatively unchanged, and thus the molecular features of C, dominate the origin of insulating behavior for x = 2 or 4 is not electronic structure of the doped phases. It is clear known, but could arise from a localized Jahn-Teller from Fig. 10 that the A&, phases will be insulators distortion on C*,-, a collective charge density wave, a (filled conduction band), and that phases with x = 3, Mott-Hubbard insulator or carrier 4 and (presumably) 2 are all predicted to have below the Mott limit. partially filled bands. Of significance to the density of A useful basis for discussion of the magnitude and states, the band width for A,&, decreases with range of T, in the A,&, and of possible pair mediat- increasing unit cell parameter resulting in an increase ing excitations is the McMillan equation [59] in the density of states of z 15% for Rb,C, over -1.04(1 +n) K3Cm t81. T,=ho 1.2k, exp 1 -p*(l +0.621) Information on metallic conductivity and density 1’ of states may be obtained from NMR using the where 1 = N(E,)P, V is the strength of the coupling Korringa relationship. The Korringa relaxation of the conduction electrons to the mediating exci- mechanism predicts that T,T should be constant for tation of frequency w and p* is the renormalized a metal [58]. Both K3Cm and Rb3C6, exhibit 13C T,T Coulomb repulsion between conduction electrons. values that are only weakly dependent on tempera- The relationship is valid for weak or intermediate ture [27,40]. Just above T,, T,T = 100K-s for Rb,C, coupling (i 5 1.5). Superconductivity in the A,C,s and 165K-s for K3C,. Assuming relaxation by a has been attributed to phonon mediated pairing contact hyperfine interaction between nuclei and non- either via purely intramolecular C& modes [6@62] interacting electrons, with a magnitude of 0.6 Gauss, (Z 1000-2000 K) or low energy A-C,, optic modes the T,T values imply densities of states at the Fermi [63] (~200 K). Purely electronic coupling has also energy of roughly 22eV-’ and 17 eV-’ for Rb3C, been proposed [64-661. The modes calculated to have and K3Cso (per Cso per spin state). For Na,C, T,T strong coupling to the conduction electrons are Ra- increases by a factor of six with decreasing tempera- man active. In thin film samples, these modes [67] ture from 373 K to 93 K and that of Na,RbC, (predominantly H, symmetry) become unobservable increases by more than a factor of two over this on formation of A,& then reappear on formation temperature range, suggesting loss of metallic con- of A&,,. Inelastic neutron scattering on bulk samples ductivity for the A,C,s with the smallest cations. For is consistent with this observation [68]. The fact that the A$& A& and A&, phases examined to date, both K3Cso and Rb3Ceo have virtually the same T,s Tl T is strongly dependent on temperature, indicating when they have the same lattice constant (under that they are not metallic. For x = 6 the insulating pressure) is strong evidence against involvement of behavior is expected from filling of the t,, band. The modes containing the A ions. r3C effect Synthesis of alkali metal fullerides 1331

experiments, on the other hand, show the involve- 6. Stephens P. W., Mihaly L., Lee P. L., Whetten R. L., Huang S.-M., Kaner R., Diederich F. and Holczer K., ment of C based modes. Three experiments have been Nature 351, 632 (1991). reported with as (T,ocM-“) of 1.4f0.5 [69] and 7. Rosseinsky M. J., Ramirez A. P., Glarum S. H., 0.37 f 0.05 [70] for Rb& and 0.30 f 0.06 [71] for Murphy D. W., Haddon R. C., Hebard A. F., Palstra T. T. M., Kortan A. R., Zahurak S. M. and Makhija K,C,. In our experiment [70] a decrease of T, by A. V., Phys. Rev. L&t. 66, 2830 (1991). 0.65 K for Rb&, (a = 0.37 f 0.05) was found for a 8. Fleming R. M., Ramirez A. P., Rosseinsky M. J., sample with 75% enrichment with i3C. Measurements Murphy D. W., Haddon R. C., Zahurak S. M. and Makhija A. V., Nature 352, 787 (1991). of the superconducting gap are similarly divergent: 9. Tanigaki K., Ebbesen T. W., Saito S., Mizuki J., Tsai values of 5.3 have been suggested from STM point J. S., Kubo Y. and Kuroshima S., Nature 352, 222 contact tunneling [72,73] although optical data [74] (1991). 10. Keltv S. P., Chen C.-C. and Lieber C. M., Nafure 352, suggest a gap of 3-5kT, and NMR data suggest 223 (1991). 34kT, [40]. Definitive and reproducible measure- 11. Rosseinsky M. J., Murphy D. W., Fleming R. M., ments of physical properties critical to a complete Tvcko R.. Ramirez A. P., Siegrist T., Dabbagh G. and Barrett S: E., Nature, 356, 4i6 (1992). - understanding of superconductivity in A,C,, are still 12. Gu C., Stepniak F., Poirier D. M., Jost M. B., Benning needed. P. J., Chen Y., Ohno T. R., Martins J. L., Weaver J. H., Fure J. and Smalley R. E., Phys. Rev. B45,6348 (1992). SUMMARY 13. Fleming R. M., Rosseinsky M. J., Ramirez A. P., Murphy D. W., Tully J. C., Haddon R. C., Siegrist T., Tycko R., Glarum S. H., Marsh P., Dabbagh G., In this paper we have described the synthesis, Zahurak S. M., Makhija A. V. and Hampton C., Nature structures and physical properties of A&,. Most of 352, 701 (1991). the compounds can be synthesized by direct reaction 14. Zhou O., Fischer J. E., Coustel N., Kycia S., Zhu Q., McGhie A. R., Romanow W. J., McCauley Jr., J. P., of Cm with alkali metals at 20&3OO”C, although a Smith A. B. III and Cox D. E., Nature 351,462 (1991). wider variety of chemistry is available with the excep- 15. Dresselhaus M. S. and Dresselhaus G., Ado. Phys. 30, tion of the FCC A&& structure. The compounds are 139 (1981). 16. McCauley J. P., Jr., Zhu Q., Coustel N., Zhou O., best described as salts of c”& and have structures Vaughan G., Idziak S. H. J., Fischer J. E., Tozer S. W., consistent with those predicted on the basis of ion size Groski D. M., Bykovetz N., Lin C. L., McGhie A. R., and electrostatic considerations. The A,Cas form a Allen B. H., Romanow W. J., Denenstein A. M. and Smith A. B. III, J. Am. Chem. 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