Poole Ch.P., Jr. Handbook of Superconductivity

166

Chapter 6: Crystal Structures of Classical Superconductors

sites are occupied by single atoms. The resulting arrangement of metal atoms and

fullerene molecules, illustrated in Fig. 6.26 for K3C60, corresponds to the simple

structure of BiF3, of which MnCu2A1, described in the section on intermetallic

compounds, is an ordered substitution derivative. Sr and Yb atoms were found to

occupy off-centered positions in the octahedral voids. In substituted fullerides

such as Na2CsC60 , the larger metal atoms preferentially occupy the octahedral

sites, whereas in Na2KC60 a disordered arrangement of cations was found. In

"stuffed" Na2Cs(NH3)4C60 , part of the Na atoms are surrounded by four

ammonium molecules forming a tetrahedron. These complex cations occupy

the octahedral sites, whereas the remaining Na and Cs atoms are randomly

distributed over the tetrahedral sites. An ordered vacancy derivative is reported

for

Yb2.75C60 ,

where part of the tetrahedral sites remain empty so that the cell is

doubled in all directions and the overall symmetry lowered to orthorhombic.

Only the tetrahedral voids were found to be occupied in Na2C60.

For metal-to-fullerene ratios exceeding 3 : 1, two slightly different solutions

are adopted by different cations. In Na6C60 and CasC60 the octahedral voids are

occupied by up to four metal atoms each, forming a tetrahedron. In the Na-

richest compound Na9.7C60 this tetrahedron is extended to a cube, with Na-Na

distances of 3.2 A. For metal atoms such as K, Rb, Cs, Sr, or Ba, the arrangement

of the fullerene molecules changes from face-centered cubic (Cu type) to body-

centered cubic (W type). As described previously for the structures of the

elements, this change can be achieved by extending the cubic close-packed

Fig. 6.26.

Structure of K 3 C60 in a partial projection along [0 0 1]. Dark shading and fullerene molecules:

z = 0; light shading: z -- 88

H. Organic Compounds

167

structure in the plane perpendicular to one of its 4-fold axes. Such packing is less

dense and contains six tetrahedral voids per molecule. In the cubic body-centered

structure of Cs6C60 all these voids are occupied by metal atoms, which form a

pattern where all faces of the cubic cell contain a square of atoms. At the metal-

to-molecule ratio 4: 1, an ordered arrangement of metal atoms and vacancies is

found, with a square of metal atoms on two and a pair of atoms on the other four

faces of the cell. The symmetry of this structure is tetragonal and the

c/a

ratio

slightly lower than unity. For the ratio 3 : 1, one-half of the tetrahedral voids are

empty and all faces of the cell contain two metal atoms. The structure is cubic

primitive and the arrangement of metal atoms and fullerene molecules identical to

the one formed by the atoms in the Cr3Si (A 15) type.

Different kinds of rotational disorder are reported for the fullerene

molecules in different compounds, the exact situation being not always clear. The

rotational disorder in Li2CsC60 is considered to be spherical. K3C60 presents a

merohedral disorder, where the C60 molecules are randomly distributed over two

orientations. In both orientations, related by a 90 ~ rotation, eight of the 20

hexagonal faces are perpendicular to 3-fold axes. In LT-Na2CsC60 and

Na2RbC60 a preferred orientation is adopted, where the molecules are rotated

by 98 ~ around the body diagonals. The ordered atom arrangement is described in

Pa3,

the same space group as found for the LT-modification of C60. In the "A 15-

type" structure the molecules located at 0 0 0 and 89 1 i are rotated by 90 ~ with

respect to each other.

The alkaline metal atoms fully donate their electrons to the C60 unit.

Superconductivity is observed for a metal-to-fullerene ratio close to 3:1, with

critical temperatures near 30K measured for Na2Cs(NH3)4C60, Rb3C60,

Rb2CsC60 , and Cs2RbC60. A maximum value of 45.0 K is reported for nominal

Rb2.TT12.2C60.

For the alkaline-earth metals the charge transfer is not complete

and superconductivity is found for a higher metal-to-fullerene ratio, e.g., Ca5C60

(8.4 K), Sr6C60 (4 K) and Ba6C60 (7 K). T c increases monotonically with increas-

ing cell parameter for both the f.c.c, and the b.c.c, packing.

b. ET and Other Charge-Transfer Salts

A large number of crystal structures of charge-transfer salts containing the

molecule bis(ethylenedithio)tetrathiafulvalene, abbreviated BEDT-TTF or simply

ET, have been determined. The ET molecule, shown in Fig. 6.27, is rather fiat

because of the presence of an extended n-electron system, but deviations are

always observed, in particular in the ethylene end groups. ET salts contain slabs

of ET molecules separated by anion layers, as illustrated by the structure of

fl-(ET)2I 3 in Fig. 6.28.

The structures of the fl-(ET)2X 3 family of compounds are triclinic with one

donor-molecule layer per translation unit. All molecules are parallel and the

linear anions, which can be 13, IBr 2, or AuI 2, are located at inversion centers.

168

Fig. 6.27.

Chapter 6:

Crystal

Structures of Classical Superconductors

H,c.S..c~S S~ S H

H-I II \C--C / C / "C'H

II ~ .H

H "C C / \ C ,,C,H

H" "S" ~S S ~

"S

ET (BEDT- TTF)

H c /--C.H

H" \ S / \S

S" ~ ~ "H

MDT- TTF

Bis(ethylenedithio)tetrathiafulvalene (ET) and methylenedithiotetrathiafulvalene (MDT-TTF)

molecules.

fl-(ET)2I 3 undergoes a phase transition at 175 K to an incommensurate modulated

structure. Both the 13 units and the ET molecules retain their orientation but are

displaced from the original positions in the HT-modification. Applied pressure

suppresses the onset of the modulated structure and an ordered, so-called r*

phase with a superconducting transition temperature of 8 K is formed. Among

the fl phases, higher values of T c are observed for larger anions.

In tc-(ET)2Cu(NCS)2 (T c = 10.4 K) the donor molecules form face-to-face

dimers, where the two molecules are rotated by 90 ~ with respect to each

other. There are two dimers per cell, related by a 2-fold screw axis. The ET

molecule layer is essentially centrosymmetric, but the overall symmetry is

lower. The Cu atoms and the linear NCS- anions form infinite branched

zigzag chains where the Cu atoms are coordinated to two N and one terminal

S atom in an approximately trigonal planar configuration. The presence of

twisted dimers is a common feature to all tc phases. In the superconducting

compounds, the central C-C bond of one molecule is located above a five-

membered ring of the neighboring layer. Superconducting transition tempera-

tures up to 12.5 K are reached for tc-(ET)ECu[N(CN)2]X halides with anion layers

similar to those observed in tc-(ET)ECu(NCS)2. However, in the former structure

a network of S-S interactions extends in two dimensions within the donor layers,

whereas in the latter similar contacts lead to the formation of infinite ribbons.

H.

Organic

Compounds

169

Fig. 6.28.

Structure of

fl-(ET)2I 3.

Large spheres: I; medium spheres: S; small spheres:

C; H

atoms not

shown.

The structure of

O-(ET)2(I3)l_x(AuI2)x

(T c -- 3.6 K for x < 0.02) is ortho-

rhombic and centrosymmetric. The ET molecules within each layer are approxi-

mately parallel to each other, but tilted from the intersecting plane, the molecules

of alternating layers being tilted in opposite directions. The molecular packing

within the slabs is similar to that found in the e phases, which, however, have

triclinic symmetry. The anions are located in mirror planes between the

layers. The unit cell is enlarged and the symmetry lowered to monoclinic

when an ordered arrangement of 13 anions is taken into account.

The crystal structure of (ET)4Hg3_6X 8 (X--C1) is built up of two

substructures, one formed by the ET molecules and the halogen atoms and the

other by the Hg atoms, with incommensurate translation units in one direc-

tion. The halogen atoms form infinite channels, inside which the mercury atoms

are arranged with their own translation period. As in the K phases, the ET

molecules form face-to-face dimers. 2-(Se-ET)2GaC14, containing bis(ethylene-

dithio)tetraselenafulvalene molecules, becomes superconducting at 9.5K. The

slabs of seleno-substituted molecules are separated by tetrahedral GaC14 acceptors.

A number of salts with the asymmetric MDT-TTF molecule (methylene-

dithiotetrathiafulvalene), shown in Fig. 6.27, have been synthesized. Among

170 Chapter 6: Crystal Structures of Classical Superconductors

these, tc-(MDT-TTF)2AuI 2 is found to become superconducting at 4.5 K. The

orthorhombic structure contains donor molecule layers with dimers that are

rotated with respect to each other, similar to those found in the tc phases of the ET

salts. There are, however, two layers in the translation unit, related by a mirror

plane.

Crystallographic Data Sets

a. Criteria for Selection

Complete crystallographic data sets, including atom coordinates, are given for

111 compounds, representative for structure types found among classical super-

conductors. As a general rule for selection, at least one compound with a

superconducting transition temperature exceeding 4.2K should have been

reported for the structure type. Whenever one or more isotypic superconductors

are known, the set of atom coordinates is followed by a table listing cell

parameters and superconducting transition temperatures for selected isotypic

compounds. Compounds becoming superconducting at temperatures lower

than 4.2 K, including particular nonsuperconducting materials, have also been

considered in these tables. However, a lower limit around 1 or 2 K has sometimes

been fixed, without special indication.

Preference has been given, when possible, to recent literature references

and to references where the cell parameters and the critical temperature are

determined for the same sample. The historical aspect has thus not been

emphasized, which means that the original publications stating for the first

time the existence of a particular phase or its superconductivity may not be

mentioned. As a further consequence, the superconducting transition tempera-

tures listed here do not always correspond to the highest values reported in the

literature. Differences that were too large were, however, avoided. For systems

with a strong dependence on the chemical composition, an effort was made to

select a close-to-optimal composition, but also to respect the correspondence

between composition and cell parameters. Transition temperatures determined

for high pressure or on thin films were considered only occasionally.

The cell parameters listed in each table were taken from the first reference

indicated on the same line and the critical temperature, if not from the first, then

from the second reference. One or two literature references reporting structure

refinements, marked with an asterisk, are sometimes added. The term structure

refinement is here understood in a broad sense, depending on the class of

compound and the structure type. In some cases publications with additional data

are mentioned in remarks, without any claim on completeness. To avoid

differences due to small variations in experimental conditions used by different

!. Crystallographic Data Sets 171

authors, cell parameters for series of isotypic compounds were sometimes

preferably taken from the same reference.

b. Presentation and Notation

The general presentation of a data set is explained in Fig. 6.29 and the few

abbreviations used here are listed in Table 6.3. The Pearson code and the

Wyckoff sequence are defined in Section A, where the use of the

International

Tables for Crystallography

is also briefly explained. In a few particular cases a

slightly different presentation has been chosen. This is the case for the structures

Fig.

6.29.

Structure [Colloquial Strukturbericht

I p~rsonl spa~ Wyckoff]

type [ name notation I code ] group]

s~qu~nr I

MgAI204 type, spinel, H1 l,

cF56,

(227)

Fd3 m - ecb

Refined

Superconducting [Diffraction Temperature for Reliability

composition transition temperature [ method data collection factor

Li0.7sTi20 4,

Tc =

13.2 K, SX,

T=

223 K, R =0.0109

Cell Formula

parameters units in cell

a=8.4030A, Z=8

Site

label

Atom

Multiplicity and

Wyckoff letter

r \

WP x

Fractional

coordinates

Y

Occupation

factor

Occ.

Li 8(b) 3/s % 3/8

Ti

16(c) 0 0 0

O 32(e) 0.23685 0.23685 0.23685

0.75

Extract from the tables with explanation of items. For some tables reference is made to figures

in which the structure type is presented.

172 Chapter 6:

Crystal Structures of Classical

Superconductors

TABLE 6.3

Notations used in the data sets.

Bravais lattice

Diffraction data

Reliability factor

Other

aP

mP

mS

oP

oS

oF

oI

tP

tI

hP

hR

cP

cF

cI

SX

PX

PN

R

Rw

R8

Rwp

n.o.

HT

LT

RT

WP

anorthic (triclinic) primitive (lattice point at 0 0 0)

monoclinic primitive (0 0 0)

monoclinic side-centered (0 0 0, 1, 1 0 for C-centered)

orthorhombic primitive (0 0 0)

orthorhombic side-centered

(0 0 0, 1 89 0 for C-centered)

orthorhombic face-centered (0 0 0, 0 89 89 89 0 1, 1 89 0)

orthorhombic body-centered (0 0 0, 1 89 89

tetragonal primitive (0 0 0)

tetragonal body-centered (0 0 0, 89 89

hexagonal primitive (0 0 0)

hexagonal rhombohedral

cubic primitive (0 0 O)

cubic face-centered (0 0 O, 0 89 89 89 0 89 89 89 O)

cubic body-centered (0 0 O, 89 89 89

single-crystal X-ray

powder X-ray

powder neutron

single-crystal conventional

single-crystal weighted

powder Bragg

powder weighted profile

superconductivity not observed

high temperature

low temperature

room temperature

Wyckoff position

reference containing structure refinement

of the fullerides, which have been subdivided into structures with fullerene

molecules in f.c.c, and b.c.c, packing, respectively. The Chevrel phases with

interstitial atoms are represented by four complete data sets. Compounds with

rhombohedral and triclinic structures are listed separately, with T c indicated only

in the former, when relevant. A table with information about the space group and

cell parameters of charge-transfer salts is presented at the end of this section, but

no complete data set with atom coordinates was selected.

The data sets are subdivided into element, binary, ternary, and quaternary

structure types. Within each category they are listed according to the coefficients

in the name of the representative compound, indicated as the first item in each

data set. Structure types with the same coefficients are ordered alphabetically.

The structure type names are written with bold-faced characters in the text and in

J. Tabulated Data 173

Tables 6.1 and 6.2. References for type-defining compounds can be found in

TYPIX

(Parth6

et al.,

1993/1994; Cenzual

et al.,

1995), a compilation of

standardized data for inorganic compounds defining structure types, accompanied

by a brief crystal chemical characterization.

Tabulated Data

a. Element Structure Types

Cu type, c.c.p., f.c.c., A1,

cF4,

(225)

Fm~3m-a

La, T c = 6.06 K, PX

a - 5.290A, Z -- 4, Figs. 6.2, 6.3, 6.4b, 6.16 (Heiniger

et al.,

1973)

Atom WP x y z

La 4(a) 0 0 0

Compound a(A) Tc(K ) Ref.

La 5.290 6.06 a 1

Th 5.0861 1.38 2, 3

A1 4.0495 b 1.175 4, 3

Pb 4.9508 7.22 5, 6

aT c

--

11.93 K at 14 GPa (Maple

et aL,

1969).

bValue taken from

figure.

References: 1, Heiniger

et aL

(1973); 2,

Harris and

Raynor (1964); 3, Roberts (1976); 4,

Bandyopadhyay and Gupta (1978); 5, Klug (1946); 6, Claeson (1966).

cx-Ga type, A 11,

oS8, Cmca-f

Ga, T c = 1.083 K, SX, R = 0.143

a = 4.523, b - 7.661, c = 4.524 A, Z = 8 (Sharma and Donohue, 1962; Roberts, 1976)

Atom WP x y

Ga 8(f) 0 0.1549

0.0810

~-Ga type,

mS4,

(15)

C2/c-e

Ga, T c = 6.2 K, PX, T = 248 K, R = 0.08

a = 2.766, b = 8.053, c = 3.332 A, fl = 92.03 ~ Z = 4 (Bosio

et al.,

1969; Roberts, 1976)

Atom WP x y

Ga 4(e) 0 0.131

174

Chapter 6: Crystal Structures of Classical Superconductors

~/-Ga type, oS40, (63)

Cmcm-g3fc 2

Ga, T c=7K,SX, T=220K, R=0.057

a = 10.593, b = 13.523, c = 5.203 A, Z = 40 (Bosio

et al.,

1972; Roberts, 1976)

Atom WP x

Ga(1) 8(g) 0.2206 0.0504

Ga(2) 8(g) 0.2282 0.3612

Ga(3) 8(g) 0.3744 0.2062

Ga(4) 8(f) 0 0.1053

Ga(5) 4(c) 0 0.2853

Ga(6) 4(c) 0 0.5009

1

4

1

4

1

4

0.0000

1

4

1

4

8-Ga type,

hR66,

(166)

R3m-h 3 ea

Ga, T c = 7.85K, SX, T = 191K, R = 0.09

a = 9.087, c = 17.02A, Z = 66 (Bosio

et al.,

1973; Roberts, 1976)

Atom WP x

Ga(1 ) 18(h) 0.434 0.566 0.2437

Ga(2) 18(h) 0.507 0.493 0.3971

Ga(3) 18(h) 0.566 0.434 0.1562

0 0

Ga(4) 9(e)

Ga(5) 3(a) 0 0 0

a-I-Ig type, A 10,

hR3,

(166)

R3m-a

Hg, T c = 4.154 K, PX, T = 5 K

a -- 3.4574, c = 6.6634 A, Z = 3 (Barrett, 1957; Roberts, 1976)

Atom WP x y

Hg 3(a) 0 0 0

In type, A6, t/2, (139)

I4/mmm-a

branch ~-Pa,

A a

In, T c = 3.408 K, PX

a = 3.2530, c = 4.9455 A, Z = 2 (Smith and Schneider, 1964; Roberts, 1976)

Atom WP x y

In 2(a) 0 0 0

Compound a(A) c(A) T c (K)

Ref.

Hg 3.995 2.825 a 3.949

In 3.2530 4.9455 3.408

1,2

3,2

aAt 77 K.

References: 1, Donohue (1974); 2, Roberts (1976); 3, Smith and Schneider (1964).

J. Tabulated Data 175

Mg

type, h.c.p., A3, hP2, (194)

P63/mmc-c

Tc, T c=7.9K,PX, T=4.2K

a--2.7364, c- 4.3908A, Z = 2, Figs. 6.2, 6.16 (Marples and Koch, 1972; Giorgi and Szklarz,

1970a)

Atom WP x y z

1 2 1

Tc

'3[ "~

Compound a a(A) c(A) T c (K) Ref.

Tc 2.740 4.399 7.92 1

Tc0.94Nb0.06 2.757 4.432 12.8 2

TC0.gzMoo.18 2.752 4.443 13.7 3

TCo.82W0.18 2.755 4.449 11.3 1

Re 2.7550 b 4.4437 b 1.697 4, 5

Reo.88Wo.12 2.7575 b 4.4590 b 7.5 4, 6

T1 3.463 5.539 2.38 7, 5

aData for

TCl_xTx

(T = Ru, Os, Rh, Pd, Sn; T c < Tc(Tc)) reported in Alekseyevskiy

et al.

(1975).

bValue taken from figure.

References: 1, Giorgi (1985); 2, Giorgi and Szklarz (1970a); 3, Stewart and Giorgi (1978); 4, Savitskiy

et al.

(1969); 5, Roberts (1976); 6, Chu

et al.

(1971); 7, Staun Olsen

et al.

(1994).

o~-Nd type, d.h.c.p., A3',

hP4,

(194)

P63/mmc-ca

La, T c = 4.88 K, PX

a -- 3.770, c = 12.159 A, Z -- 4, Fig. 6.2 (Spedding

et al.,

1956; Maple

et al.,

1969)

Atom WP x y

1 2 1

La(1) 2(c) 5 5 a

La(2) 2(a) 0 0 0

I~-Sn type, white tin, A5,

tI4, (141) 141/amd-b

Si, T c = 7.1 K, PX, p = 13 GPa

a = 4.686, c = 2.585 A, Z= 4 (Jamieson, 1963; Roberts, 1976)

Atom WP x

1 3

Si

4(b) 0 ~ g

aOrigin at center (2/m).

(continued)

Poole Ch.P., Jr. Handbook of Superconductivity

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