Physica C 185-189 (1991) 425-426 North-Holland
CALORIMETRICOBSERVATIONOF A STRUCTURALPHASETRANSITIONAT ELEVATEDTEMPERATURES IN SINGLECRYSTAL(360 N.A. Fortune(a), K. Murata(a), F. Igala), Y.Nishihara(a), K. Kikuchi(b), S. Suzuki(b), I. Ikemoto(b),andY.Achiba(b) (a) Electrotechnical Laboratory, Tsukuba, Ibaraki 305 Japan (b) Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan We present the first observation of multiple structural phase transitions in single crystal undopod C60. Crystals annealed in He gas at 500 K (225 °C) exhibit transitions at 250 K and 425 K (150 oC). The 250 K transition has been attributed to an orientational ordering of individual C60 molecules in powdered crystal C60. A third (metastable) transition at 185 K in as-grown crystals is eliminated by annealing at temperatures above 425 K. These results have direct relevance to vapor phase doping of alkali metals at elevated tempera',ures to induce bulk superconductivity in C60.
1. INTRODUCTION The recent developmentI of a method for synthesizing large quantities of C60 has led to a number of exciting discoveries, including the observation of superconductivity 2 in alkali-metal-doped C60. Since vapor phase doping of C60 powders and crystals requires elevated temperatures (>160-200 °C) to induce bulk superconductivity, 3,4 a knowledge of any structural changes in crystalline C60 above room temperature is particula,ly important. We present here the first evidence for a structural phase transition at 150 °C (425 K), indicating that the structural phase at which vapor phase doping of powders and crystals occurs is not the known room temperature structure. 2. EXPERIMENTAL METHOD We have used the at-calorimetric method5 to measure the relative temperature dependence of the heat capacity of C60 single crystals between 80 K and 500 K. C60 powder was prepared using standard techniques, 1o4 then separated using high precision liquid chromatography (HPLC) using CS2 as an effluent. The resulting C60 powder was recrystallized in CS2 by pumping to evaporate the solvent. 4 The specific heat was measured with an ac-calorimeter 5 by bonding 10 crystals with flat crystal habits totalling 0.60 mg with Ag paste to a 10 I.tm thick 0.50 mg 99.99% purity Au platform.
3. RESULTS AND DISCUSSION In Fig. 1 we show the total specific heat divided by temperature (in relative units) of the (360 single crystals (plus addenda) from 300 K to 500 K, after annealing in He between 425 and 500 K for 1 hour. A clear, reversible transition is apparent at 425 K.
FIGURE 1 Total specific heat divided by temperature for single crystals of C6o (plus addenda) annealed in He at 425-500 K for 1 hour. A structural phase transition occurs at 425 K (150 °C). As-grown crystals atso exhibit a transition at the same temperature.
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N.4. Fortune et a L / Calorimetric observation of a structural phase transition
The addenda, measured separately, shows only a smoothly varying background.The specific heat between 310 K and 375 K is slightly hysteretic. Preliminary x-ray measurements 4 also indicate an irreversible change in the crystal structure of asgrown crystals at this temperature, but the precise crystal structure has not yet been determined. We suggest that the need for high temperature treatment (>150 °C) to induce bulk superconductivity with large Meissner fractions in powder and crystal alkali-metal-doped C60 is related to the structural phase transition at 150 °C observed here. In Fig. 2 we show the total specific heat (in relative units) of the annealed C60 single crystal plus addenda from 80 K to 300 K. The phase transition beginning below 250 K and peaking at 230 K here is in good agreement with the 249 K structural phase transition previously observed by x-ray diffraction and differential scanning calorimet,'y in powdered fcc C60 .6 The second peak at 245 K may be indicative of a slightly higher transition temperature in one or more of the single crystals measured. Similar low temperature specific heat measurements (80 K - 300 K) on as-grown pre-annealed C60 single crystals (not shown) exhibit a second transition at 185 K This transition disappears upon annealing of the crystals, suggesting that the 185 K feature is related to a meta-stable phase in the asgrown samples. Preliminary x-ray diffraction measurements at 300 K indicate that the disordered, predominantly crthorhombic structure of the asgrown crystals differs significantly from the annealed crystal structure, but the exact post- annealed structure has not yet been determined. The convergence of our results on annealed single crystals and powered fcc C60 suggests that annealing above 150 °C may remove any metastable phases in the as-grown samples and promote formation of the expected disordered fcc structure. We note, however, that recent molecular energy calculations 7 predict that the most energetically stable configuration for C6o crystals is an orthorhombically distorted tcc structure.
FIGURE 2 Total specific heat for the same samples as in Fig. 1 plotted versus temperature for 80 K -300 K. The transition beginning just below 250. K is in good agreement with a previous observation s of a structural transition in powdered fcc Cs0 at 249 K.
REFERENCES 1. W. Kratschmer et aL, Nature 347 (1990) 354. 2. A.F. Hebard etaL, Nature 350 (1991) 600. 3. K. Holczer etaL, Science 252 (1991) 1154. 4. K. Kukuchi et aL, these proceedings. 5. N.A. Fortune et al., Solid State Communications 79 (1991) 265. 6. P. heiney etal., Phys. Rev. Lett 66 (1991)2911. 7. Y. Guo et al., Nature 351 (1991) 464.