Site selective behavior of alkali metals in binary doped C60

~ )

Solid State Communications, Vol. 82, No. 12, PP. 979-982, 1992. Printed in Great Britain.

0038-1098/9255.00+.00 Pergamon Press Ltd

SITE SELECTIVE BEHAVIOR O F A L K A L I METALS IN BINARY DOPED C60 I. Hirosawa, K. Tanigaki, J. Mizuki, T. W. Ebbesen, Y. Shimakawa, Y. Kubo and S. Kuroshima Fundamental Research Laboratories, NEC Corporation, 34, Miyukigaoka, Tsuknba, Ibaraki 305, Japan (

received 12 March 1992 by H. Kamimura ) (Accepted 14 April 1992) The site selective behavior of binary doped C6o superconductors with compositions of A2CsIC60 and A~Cs2C60 ( A=Na, K, Rb ) was observed by X-ray diffraction. All the A2Cs~C6o's indicate the ordering between octahedral and tetrahedral sites, while the situations for the AICs2C6o compositions are different depending on the kind of metal A. It is shown that the relationship between the ionic radius of metal A and the size of tetrahedral site plays an important role in site selectivity.

annealing of Rb and Cs doped samples was performed for 5 days at 350"C. For X-ray diffraction experiments, samples were transferred to thin glass capillary tubes (0.4mm¢ and thickness of 0.01mm) in a glove box. The Cu Kct powder diffraction patterns at room temperature were collected in the 0-20 mode with a diffractometer (Rigaku RINT attached RU-300) at 50kV and 250mA. Pyrolytic graphite (PG) crystal was set just before a scintillation counter as a monochrometer. A SQUID magnetometer (Quantum design MPMS) was used to measure the temperature dependence of d.c. magnetization of the samples.

Introduction The synthesis of alkali metal doped compounds of C6o as a new class of superconducting materials has been widely investigated ;-it. It is especially important to know the atomic structures for understanding the origin of superconductivity of these materials. By X-ray diffraction measurements, these superconducting compounds have fcc structures ( Fm3m ) and doped alkali metals occupy sites such as (1/4 1/4 1/4) ( tetrahedral site or T-site ) and (1/2 0 0) ( octahedral site or O-site )12. Up to now various supeconducting transition temperatures (Tc) have been reported depending on the dopant ~.3.to.u.The highest Tc of alkali metal doped C6o was recorded for the binary doped system RblCs2C60t°. In these binary doped superconductors, it has been speculated that the larger alkali metals occupy preferentially O-sites ~, but has not been clarified yet. In this report, we will show direct evidence of site selective behavior of alkali metals for A~CsC~o'S and A~Cs:C6o'S. Influences of ionic radii and the concentration between doped metals on the structure are also discussed in detail.

Results and discussion All the observed diffraction patterns can be indexed with the fcc structure. The arrangements of metals were determined by analyzing intensities of these X-ray diffraction data. Various arrangements were applied to fit the observed data and the most possible model was selected by comparing the value of ~2 defined as follows: Z2 = I/(N-P)Z(I~- I,,)2/c2. In this formula, 6 is the standard deviation that is proportional to counts, N is the number of data points and P is the number of optimized parameters. To fit the observed data, scale factors and temperature factors are treated as optimized parameters. In our calculations, the C~o molecules located at the corners and face centers in the unit cell are treated as charged hollow spheres of radius r=3.55A. This value was optimized to reproduce our diffraction data of pristine C6o. It is close to the previously reported value t4. Diffraction profile of Na2CstC6o has a characteristic feature: there is no (222)diffraction peak although all the diffraction patterns of previously reported systems ( K3C6o, Rb3C6o and Rb2CslC6o lu2) have clear (222) peaks. Observed and calculated intensities of Na2CslC6o are shown in Figure 1. Non-ordered structure ( space group Fm3m ) where all interstitial sites are occupied with Cs and Na ions randomly, gives a large structure factor for (222)(Z2=18.78). Another candidate is partially ordered structure ( space group Fm3m ) in which O-sites are occupied with Na ions and T-sites are occupied with Na and Cs ions in a random fashion.

Experiments Six samples were prepared with the following nominal compositions: Na2Cs~C6o, Na~Cs2C6o, K2CstC6o, KtCs2C6o, Rb2Cs:Co~ and Rb~Cs2C~o. These were synthesized as described below from stoichiometric amounts of alkali metals and C6o. C6o was purified from fullerene extract purchased from Texas Fullerene on a neutral aluminum column using hexane/toluene elutant followed by heat treatment under l x l 0 + torr pressure. Alloys of Na2Cs, NaCs2, K2Cs and KCs2 were made in a sealed quartz tube( 5mm in diameter) by heating to ca. 150°C under helium. Stable alloy phase between Na and Cs appears only in the composition of 2:1. Phase separation would occur in the NaCs2 system. Then the Cm was introduced into the tubes containing the prepared alkali metals in the glove box filled with helium gas. Rb2Cs~C6o and RbtCs2C~o were prepared as reported in elsewhere to. The quartz tubes containing Ca and dopant alkali metals were degassed to 10.2 torr and refilled with helium to 700 torr. These were heated in a furnace at 200°C for 24 hours, then 400°C for 72 hours. Further 979

980

ALKALI METALS IN BINARY DOPED C60 Na2CslC60 20

=

OBSERVEO

10

.,..

0 20

I

,,ll

,, . . . . . . . .

¢I

10 >I--

Z UJ I-Z

o

I

20

I

,I

II

11

I .1

...

PARTIALLY ORDERED

Zo°

10 . . . .

0 20

10

I..i..i

0

5

,

•

6O

20

Vol. 82, No. 12

Calculated intensities for partially ordered structure are also very different from observed ones (;(2=62.64). Furthermore this model gives too intense peaks of (111), (200) and (222). Only when the structure of Na2Cs~C6ois ordered with Cs ions in O-sites and Na ions in T-sites ( space group Fm3m) do the calculated and experimental patterns exactly match. Intensities of the (hkl)'s, including the absence of the (222) peak, can all be fitted well with the ordered model with lattice constant ao=14.134A, yielding the smallest ;(2 of 0.87. This samples show superconductivity at 12K with a very large shielding fraction volume (more than 50%) ~6. For the nominal compositions Rb2Cs~C6o and K2Cs~C6o, the fcc structures are also found to be ordered using the same type of analysis. These compositions are superconducting with Tc's of 31K for Rb2CslC6o and 24K for K2Cs~C6o~6. In the case of the nominal Na~Cs2C6o composition, it can hardly be expected that the structure is ordered since the number of the large ions is twice that of the Osites in the unit cell. Observed diffraction patterns of the nominal components of NalCs2C6o, KiCs2C~o and Rb~Cs2C6o are shown in Figure 2. The diffraction pattern for the nominal composition NatCs2C6o shows very similar features to those of Na2Cs~C6o. There is no appearance of (222) diffraction peak and the values of ;(2 are also smallest for the ordered structure of Na2Cs~C6o. Furthermore, the observed Tc is always 12K regardless of the nominal compositions. This behavior is very different from the situation, for example, of KxRbyCo's where Tc is continually changing depending on the component ratios 15. Furthermore since neither NaxC6onor CsxC6o are superconducting n, it can be concluded that

Figure 1. Observed and calculated X-ray diffraction intensities from Na2Cs~C6o. Background, including diffraction from the aluminum sample holder and unreacted pristine C6o, is removed.

DIFFRACTION

PATTERNS

~1Cs~,60 40O

Table 1. Values of ;(2 for various models. In the case of Na2Cs~C6o, the value of ;(2 decreases to 0.87 when the observed data are fit with a 9:1 mixture of ordered Na~CslC6o and unreacted pristine C6o.

20O

>I--

0

i

i

i

t

KICs2C6o

i

*

400

VALUES

OF

Z ILl I.Z

X,2

0

par.aly ordered

2O0

ordered

non
62.64

18.78

i

,

i

i

RblCs2C6O

1000 Na2CslC60

1.59 (0.87)

500 NalCs2C60

0.58

2.59

3.70

K2CslC60

2.48

29.29

8.33

~,s2C60

5.96

0.87

1.27

Rb2CslC60

1.87

17.68

5.77

RblCs2C60

187.34

3.66

6.43

0

5

2o

4o

6o

2@ Figure 2. Powder diffraction patterns of Na~Cs2C6o, K~Cs2C6o and Rb~Cs2C6o. Peaks indicated by an asterisk(*) come from the aluminum of sample holder.

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ALKALI METALS IN BINARY DOPED C60

Vol. 82, No. 12

Table 2. Observed and calculated intensities for Na2Cs,C+0, K2Cs,C~, Rb2CstC+oand Rb,Cs2C~. In the case of Na2Cs,C~o, the intensities are calculated with a 9:1 mixture of ordered Na2Cs,C+o and uureacted pristine C~. There are eight pairs of (hkl)'s with the same scattering angle, and are indicated by *1 - *8. Na2CsC60

K2CsC60

hk I

Io

Io

Ic

Io

(1 1 1)

52 55 83 993

i 28 14 830 14 68 27 55 262

26 18 131 861 121 140 48

(2 o o) (2 2 o)

(3 1 1)

(2 2 2) (4 0 O) (3 (4 (4 /~ (4 (5 (6 (4 (6 (5 (6

3 1) 2 O) 2 2) 1 1) "1 3 3) *a 4 O) 3 1) 0 O)"~' 4 2) "2 2 O) 3 3) Z 2) 1 5 4 4 5 3

1) °z 1) "3 O) 2) 3) "A i)'"

(8 0 O)

67 33 123 283

14 51 53

15 48 53

40 26

7 48 34

121 28

II 21 31

12 27 32

41 32 3O

4 29 22 0 4

123 44 27 58

29 0 18

-

5

39

8

1

4 55

41

18

1 21

31

(7 3 3) (6 4 4) "s

28 3 4

47

12

(6 6 O)"e

18

13

12

4 2 6

(7 5 1) *~" (5

(+ (8 4 O)

20 2 3 2

(8 2 0 ) * S

(8 2 2)'0!

Ic

51 35 168 267

-

52 74 46 991

RbCs2C60

72 13 48 828 65 49 25 50 275

(4 4 4)

(7 (5 (6 (6 (5 (7

Ic

Rb2CsC60

25

12

(7 5 3) ''3 (1 1 9) "B

the only superconducting phase prepared from Na and Cs is the ordered Na2Cs,C+o. As for the Rb,Cs2C~, ordered structure cannot be expected for the same reasons as given for Na, Cs2Ceo. The large value of Z2 ( Z2=187 ) indicates that there is no possibility of the ordered structure. Non-ordered structure also gives a large Z2 of 6.43. Observed intensities are reproduced well by the partially ordered structure of Rb, Cs2C+o with lattice constant ao=14.555A ( • Z2=3.66 ). In this partially ordered structure of Rb, Cs2C60 all O-sites are occupied by Cs ions and T-sites are occupied by Rb and Cs ions randomly. It is also possible to consider that the ions in T-sites are also ordered. In that case, the space group has lower symmetry. There is one possible location of ions with zincblende structure ( space group F,~3m ) as follows: the sites of (1/4 1/4 1/4),(3/4 3/4 1/4),(1/4 3/4 3/4) and (3/4 1/4 3/4) are occupied with Cs ions and the other sites are occupied with Rb ions. This structure can also reproduce the observed data. It is difficult to distinguish between the

ii 3 1 5

-

574 -

4 7 153 960 91 152 39 3

561 9

Io 25 20 171 797 188 129 27 -

571 -

Ic 9 45 276 921 151 181 39 3

620 7

128 44

122 20

144 33

1

20 133 31 37 66

8 116 25 13 20

76

48

79

16

40

14

30

-

8

8

20

6

4

-

0

16

3

60

43

55

39

-

102 33 7 18

4

6

44

0 23

39

0 19

2

two possibilities since the values of Z2 are very close to each other ( Z2=3.64 for ordering in T-site ). Recent mCs NMR measurements show that only one signal originated from O-sites is observed for Rb2CslC6o and both signals originated from O- and T-site are observed for Rb,Cs2C6o ,3. These results strongly support our conclusion for the structures for RbxCsyC+o. We have also studied the nominal composition of K,Cs2C60. However the observed diffraction peaks were too weak and broad to determine atomic arrangements. This can possibly be explained in terms of ionic radii of alkali metals, 0.98A for Na, 1.33A for K, 1.49A for Rb, and 1.65A for Cs. The O-site, whose radius is 2.06A +, could accommodate all the alkali metals mentioned above while the T-sites (1.12A 6) would be selective depending on the ionic radius. Considering the lattice constants of alkali metal doped C+o's, Rb ions in T-sites would enlarge the space of T-sites, so that Cs ions can be introduced into T-sites together with Rb ions. On the other hand, Na ions are smaller than T-site and would

982

ALKALI METALS IN BINARY DOPED C60

shrink the size of T-site. This would make it very difficult for Cs ions to locate in the T-sites in the case of Na~Cs~C6o. The case of K ions is very complex. K ions will probably enlarge the T-sites but not sufficiently to easily allow Cs ions. This might explain why it is difficult to prepare high quality KtCs2C60 phase. In conclusion, we have showed clear evidence of site selective behaviors for superconducting compounds A2Cs~C+o and A1Cs2C+o ( A=Na, K and Rb ). All A2Cs;C+0 formed ordered structures, where O-sites are occupied with Cs. On the other hand, A,Cs2C6obehaviors are more complex. There is no stable Na~Cs2C+o phase while

Vol. 82, No. 12

RblCs2C+o is partially ordered. This is explained by the relationship between the ionic radius and the size of Tsite. It is also noted here that there is a possibility of ordering of Rb and Cs ions in the T-sites themselves for RblCs2C6o. Further experiments are in progress to clarify this possibility. Finally the site selectivity of alkali metals discussed in the present paper gives important information for understanding alkali metal doped C+o superconductors. Acknowledgement - We are grateful to Drs. S. Saito, A. Oshiyama and Y. Miyamoto for their helpful discussions.

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