Physical properties of graphite-nitrate residue compound

Cohen.

1977, Vol. 1X pp. 181485.

Pergamon Press.

PHYSICAL

Printed in Great Britain

PROPERTIES OF GRAP~IT~NITRATE RESIDUE COMPOUND

M. INAGAKI,tJ. C. ROUILLON,G. FUG and P. DELHAE~ Centre de Recherches Paul Pascal, C.N.R.S., Domaine Universitaire, 33405Talence, France (Received 14 Februa~ 1977) A~t~ct-Therms expansion, specific heat and magnetic properties of the “residue” compounds of graphitenitrate were measured at low temperatures between 1.5 and 3OOK. Met&c behaviour of the compound is confirmed from high density of state at Fermi level which was evaluated from specific heat data and from magnetic susceptibility measurements. The effective modulation of structural parameters and of properties with doping of nitrate ions is shown. Phase transitions of the compound are found to occur at cu. 320 and 230K.

and left in the air. They decomposed to a higher stage compound (supposedly by a mixture of third and fourth stages judging from the positions of X-ray diffraction lines) and after that to the “residue” compound, of which X-ray powder pattern agrees with the previous studies[2,3]. The decomposition from the lamellar to a “residue” compound seems to depend on the condition of decomposition, such as temperature, humidity, size of the specimen, etc. From PGCCL, four “residue” compounds with different values of apparent interlayer spacing were prepared by washing the compounds slightly during their decomposition. For each compound thus prepared, the content of nitric acid was determined from the weight increase of the sample. The apparent interlayer spacing a.,? was measured from the diffraction line corresponding to 002 of graphite (hereafter denoted by “002” line) by referring to the inner standard of silicon. For the compounds obtained from thin flakes of PGCCL, thermal expansion along the c-axis was measured by a low temperature X-rays diffractometer [5] at the temperatures between 20 and 300K and the magnetic anisotropy by Krishnan’s method from 77 to 300K. For the compound in powder form prepared from Madagascar graphite, thermal expansions along the cand a-axes at 20- 3OOK, specific heat at 1.5- 270K, electron spin resonance (ESR) at 10 - 300K and average magnetic susceptivity at 3 - 300 K were measured.

The so-called residue compounds of graphite [ 11have been studied only by a few investigators. One of the authors has found a special compound of graphite-nitrate[2], which was prepared by the decomposition of the lamellar compound. Its X-ray powder pattern is similar to that of graphite, except for the exact position of diffraction lines and the existence of extra peaks. By calculation of the apparent interlayer spacing (T, from the peaks corresponding to 001 lines of graphite, a smaller value than that for graphite was obtained (down to 3.31A). Five extra peaks with small intensities were found at the diffraction angles lower than the diffraction corresponding to 002 line and their d-spacings to be interrelated with each other and with the apparent interlayer spacing smaller than graphitic. Their metallic behaviour in electronic properties has been reportedl31; iow resistivity along the layer planes (ea. 2 x 10-‘&Icm), a very small and negative Hall coefficient which is field inde~ndent and a negligibly small magnetoresistance at liquid nitrogen temperature. Possible explanation of the structure and electronic properties of the compound has been discussed [3]. In the present work, thermal expansion, specific heat, diamagnetic susceptibility, electron spin resonance and magnetic anisotropy were measured at temperatures between 1.5 and 300K on these special residue compounds of ~aphite-ni~ate. Prelimin~y investigations of phase transition of this compound were also carried out. 2 EXpF,RlMENTAL The “residue” compounds of graphite-nitrate were prepared from the powder of Madagascar natural graphite and from stress-annealed pyrolytic graphite PGCCLJ The original graphites were kept in 96% concentrated nitric acid at room temperature so as to obtain the second stage lame&r compounds of graphitenitrate. After 2 days, they were taken out from the acid tExchange visitor under the program between JSPS and CNRS. On leave of absence from Synthetic Crystal Research Laboratory, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan. *Made by “Le Carbone-Lorraine”, France.

s.REsuLTs 3.1 Thermal expansion The variations of interlayer spacings with ambient temperature are shown in Fig. 1 for the compounds prepared from PGCCL containing different contents of nitric acid. For the “residue” compounds the apparent spacing l&.. decreases with temperature in a manner more pronounced than for the original graphite. The variation of &W in the temperature range between 100 and 300°K is approximately linear. If we calculate the thermal expansion coefficient (Yealong c-axis from this linear portion, the “residue” compound from PGCCL with the highest content of nitric acid has the value of 7.4~

181

182

M.

INAGAKI et al.

origlnal 7

3.32_ OQ =; 3.310" I< 330 _ I

0

3.29_

5

IO

I5

20

25

T2, K2

Fig. 2. Thermal variation of specific heat C, in the helium temperature range for the “residue” compound with 31 wt% nitric acid.

3.29 3.27,

3.251 0

I 200

I 100

I 300

7;K

Fig. 1. Changes of spacing a,.. with temperature for the “residue” compounds with different contents of nitric acid made from the stress-annealed pyrolytic graphite PGCCL. A: 27 wt%, B: 23 wt%, C: 17.7wt%, D: 27.5wt%. 10m5K-‘, much larger than the value of 2.7 x 10e5K-’ for the original graphite[5]. For the residue compound from Madagascar graphite with 31 wt% nitric acid, a large value of oc was observed. With decreasing the content of nitric acid, the spacing CT.,.. at room temperature in-

creases, the thermal expansion coefficient cu, decreases and tends toward the value of the original graphite. For the extra peaks at low angle side, the same changes in d-spacing with temperature were observed. The thermal expansion coefficients determined from their linear portion in the temperature range from 100to 300°K are approximately the same as the coefficients ac measured on the same compounds. The lattice constant of graphite along the u-axis (&,J decreases with increase in temperature within the examined range of temperature[6,7]. For the “residue” compound of graphite-nitrate, however, it was difficult to check the same on dloo. The experimental points seem to suggest a slight increase in &, with temperature, although the points are strongly scattered.

C(HNO-i)o.W according to the content of nitric acid 31 wt%. Even if the content of nitric acid would be ignored, no significant differences in values would be found. The values of the parameters are summarized in Table 1, as compared with the published data for pure graphite[8]. Table 1. Characteristics evaluated from specific heat

Graphite y(lJ/mole.K*) N(E,) (atom*eV)-’ a(&mole*K4) 00(K) Characteristic temperature for acoustic vibrations 8,(K) 0,(K) 82(K)

“Residue” compound of graphite-nitrate

13.8181 0.0058 27.7 413

384 0.16 99.8 270

238[5] 25.7 120

170 58.4 290

In Fig. 3, the specific heat due to lattice vibration which is obtained by subtraction of the electronic term from the observed value is shown as a function of temperature. It is much larger than that of pure graphite[g]. The observed specific heat for acoustic vibration was analyzed according to Tarasov’s model191 and the characteristic temperatures obtained are also summarized in Table 1, together with those for graphite 151.

3.2 Specific heat Below Y’K, the following linear relation between C,/T

and T* is observed (Fig. 2), CJT = y t aT*.

(1)

The electronic contribution y in specific heat was obtained by the extrapolation of the observed linear relation to WK. From the value of 7, the density of states at Fermi level N(&) is estimated. The slope of the linear relation gives the coefficient (Yof lattice vibration term which gives the three-dimensional Debye temperature &. In these estimates we took the molecular weight at

3.3 Magnetic properties The average magnetic susceptibility of the compound from Madagascar graphite was -0.30 x 10e6CGS emu/g and independent of temperature between 3 and 300°K. The paramagnetic susceptibility obtained from ESR of the same compound was estimated as of the order of 3.5 x lo-’ CGS emu/g. Its value showed no change with ambient temperature between 10 and 3OO”K,suggesting a Pauli paramagnetism. The signal of the electron spin resonance was of dysonian shape like for metals with a very small anisotropy. The shift in g-value was 4 x 10e4. The magnetic anisotropy Ax = x11-x1 of the com-

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Physical properties of graphite-nitrate residue compound

IO

1 I

L

L

IO

Id

1

I03

T.K Fig. 3. Thermal variation of specific heat C, with temperature for the “residue” compound with 31 wt% nitric acid. ----, pure graphite. pounds from PGCCL was measured from 77 to 300K. The smallest value of -0.35 x 10e6CGS emu/g was obtained for the compound with largest content of nitric acid. This value of Ax was comparable to that obtained for a second-stage lamellar compound of graphite-nitrate. With decrease of the content of nitric acid, the value of AX increases and approaches that of the original PGCCL - 21 x 10e6 CGS emu/g.

3.4 Phase transitions When the compound with 31% nitric acid made from PGCCL was slowly heated on the hot stage of the X-ray goniometer, a change in powder pattern at diffraction angle 20 between 25 and 30” was observed, as illustrated in Fig. 4. At 320”K, the starting compound gradually transformed into a structure with peaks at ca. 25.7” and 28.6” in 28. The new structure looks as a mixture of regular lamellar compounds of higher stages with the well-established spacing between-graphite basal planes separated by nitrate ions (7.84A). In continuing the heating at 320”K, all parts of the compound seemed to change to the mixture of such lamellar compounds. At the same time, however, the lamellar compounds start to decompose because of their instability in the air. So, the diffraction line at about 25.7” starts to shift to higher angle side and to broaden, as shown in Figs. 4(c) and (d). After 72 hr heating, the whole sample converted into the real residue compound of graphite-nitrate, which gives a little larger value of apparent interlayer spacing than natural graphite. The transformation from the “residue” compound to a

Fig.4. Changeof X-raypowderpatternwithholdingtimeat 320K. -, calculatedcurve. mixture of lamellar compounds is extremely slow, as can be seen from Fig. 4. If heated quickly, only a trace of lamellar compounds is observed and the process looks as a direct transformation from the “residue” to the real residue compound. The transition from the “residue” to lamellar compounds is influenced by many factors, such as temperature, humidity of the atmosphere and evaporation of nitric acid during heating. Partial return of the lamellar compound to the “residue” compound is observed by cooling down to room temperature (Figs. 4a and b). A specific heat anomaly was observed at 232K, as shown in Fig. 5. This anomaly can be compared with the transitions found by Ubbelohde and his collaborators [ 101 and by Kawamura et al[ll]. But in our case this transition exhibits a hysteresis effect and is, therefore, a first order transition. Furthermore, it seems that differences in behaviour are found depending on the starting compound.

3.5 Relations between physical properties

the structural parameters

and

In Fig. 6, the thermal expansion coefficient values which were obtained from the “002” line and from the extra peak at 17.9”in 20, the diffraction intensity of the extra peak relative to the “002” line, the content of nitric acid and magnetic anisotropy Ax are shown as a function of the spacing 2.~ at room temperature. The spacing ci..,.. has a close relation with other structural parameters and properties. From these relations and the

184

M. INAGAKI ef at.

model for such compounds. It must be emphasized that such “residue” compounds cannot be explained by structures of usual graphite-nitrate lamellar compounds studied by many invest~tors[l2]. However, it will be necessary to assume the intercalation of nitrate ions between graphite layer planes because of effective m~ulation of structural parameters and properties with the content of nitrate ions, as shown in Fig. 6. According to Fuzeliiert, the relative intensities of the extra peaks and of the “002” line of such “residue” compound are explainable by assuming a second stage lamellar compound with an intercalated spacing of about 6.6 A, much smaller than the value of 7.84A of the usual lamefiar compounds [ 121. Fig. 5. Thermal variation of specitic heat C, for the “‘residue” Our “residue” compounds have relatively large specompound with 31 wt% nitric acid, showing a phase ~sitioR at cific heat due to lattice vibrations. The observed Debye 230°K. temperature of 270°K is much lower than that of the I I graphite (Table I), this being consistent with the metallic behaviour of physical properties of this compound. However, the vibrational anisotropy as defined by the ratio 6J& is roughly constant. S~~~Iy the substitutional doping of 0.23 at% boron in graphite reduces the Rebye temperature to 4OS*K[8]. The specific heat of “residue” compounds has a very large ~on~bution from conduction electrons. The evaluated density of states at Fermi level N(&) of O.l6/atm. eV, (Table 1) is very much larger than the value for the graphite O.OOSS/atm*eV. From this N(B) the susceptibility for P&i p~ama~etism is estimated as 3.3~ IO-’ CGS emu/g, one order of magnitude higher than the susceptibility actually observed by ESR. This discrepancy might indicate that the sample is non-homogeneous and that onIy a part of the susceptib~ity is detected by ESR, or that y observed in specific heat is not all due to dww at room temperature, 8 conduction electrons. Fii. 6. Changes of structural parametqs and other properties The measurements of specific heat and magnetic with the apparent interlayer spacing d.,.+ for the “residue” properties at low temperatures have demonstrated the compounds. I,, is the intensity of the extra peak at 17.9’ in “residue” 28 relative to that of the “‘002”line, e= and a,, are the thermal metallic ~haviour of our ~phite-crate expansion coefficients determined from the “002” line and the compound. It may be called a synthetic metal, as proextra peak,respectively,and Ax is the magneticanisotropy.The posed by Ub~iohde[l3]. We have also shown the exisopen marks stand for the “residue” compoundsmade from the pyrolytic graphite PGCCL and the filled marks for that made tence of two phase ~nsitions for our compound: at ca. 320°K a transition from the “residue” to mixed lame&r from the natural graphite. compound and at cu. 230K a first order transition. At present, the lack of knowledge of the detailed experimental results described above, it may be recognized that the amount of doping of graphite with nitric structure of these “residue” compounds precludes a acid governs defi~tely the structure and the physical complete unders~nding of their properties and of these observed phase transitions. properties of the final compound.

We have considered our compounds of graphite-nitrate as being residue compounds. However, they are far from the residue compounds as defined by Hen&[11 because of their large contents of nitric acid (maximum 31 wt%; ClsHN03), at maximum almost comparable with the nitric acid contents of the second stage Iamellar compound (33 wt%; C&NO,), and also because of the remarkable shrinkage in the apparent spacing &BY with decreas~g ambient temperat~e. We have not yet established a comprehensive crystallographic structural tPrivate communication.

RFSERENCES 1. G. Henuig, J. Chem. Phys. 19,922 (1951);20, 1438and 1443 (195’2). 2. M. Inagaki, Car&on 5,317 (1967). 3. Y. His~y~a, A. Ono, hf. Inagaki and T. Tsuzuku, Jup. I. A&. Plrys. 8, 1189(1969). 4. M. Inagaki, Tunso 1%7(49)21. 5. G. Fllg, Thesis, 1975, Bordeaux I; G. Filg, H. Gasparoux and P. Delhaes, 1. Phys. B (to be public). 6. J. B. Nelson and D. P. Riley, Proc. Phys. Sot. (London) 57, 477 (1945). 7. B. T. Kelly, 4th London International Carbon and Graphite Conference paper 71 (1974).

Physical properties of graphite-nitrate residue compound 8. B. J. C. van der Hoeven, Jr., P. H. Keesom, J. W. McClure and G. Wagoner, Phys. Rev. 152, 7% (1%6). 9. V. V. Tarasov. Zh. Fir. Khim. 27.1430 (1953): V. V. Tarasov and G. A. Yunitskii, Zh. Fiz. I&n. 39; 2077.(1%5). 10. K. Bottomley, G. S. Parry and A. R. Ubbelohde, Proc. Roy. Sot. (London) A279, 291 (1964); D. E. Nixon, G. S. Parry

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and A. R. Ubbelohde, ibid. A291,324 (1%6). 11. K. Kawamura, T. Saito and T. Tsuzuku, Carbon 13. 452 (1975). 12. N. Pl&er, Les Carbones II p. 555. Masson & Cie (1965). 13. A. R. Ubbelohde, Chem. lnd. 588 (1972);Carbon 14, 1 (1976).