magnetic ternary rare-earth rhodium stannides

~

Solid State Co~anunications, Voi.42, No.2, pp.97-I02, 1982. Printed in Great Britain.

0038-1098/82/140097-06503.00/0 Pergamon Press Ltd.

Structural Distortion in the Primitive Cubic Phase of the Superconducting/Magnetic Ternary Rare-Earth Rhodium Stannidea

J. L. Hodeau, M. Marezio Laboratoire de Cristallographie, CNRS, Grenoble, France

J. P. Remeika and C. H. Chen Bell Laboratories, Murray Hill, NJ 07974 USA (Received Received

by E. F. Bertaut)

7 December 1981 by E . F . BEgTAUT

A structural distortion in the primitive cubic phase of the SnM3Rh4Snl2 compounds with M = La, Ce, Pr, Nd, Sm, and Gd has been detected by electron diffraction and studied by X-ray diffraction. On the contrary, no distortion has been detected for the compounds with M = Eu, Yb, ca, Sr, and Th. The La, Yb, ca, Sr, and Th compounds become superconductors with T c ranging between 8.7 K and 1.9 K, whereas those of Eu and Gd have a magnetic transition at about 11 K. Long exposure (200 hrs.) precession photographs revealed the existence of superstructure spots which can be indexed either on a body-centered cubic cell with al = 2a% or on a tetragonal cell with a T = V~acp and CT = C%. In this latter case the sample must be considered as twinned and the extra spots would come from three different individuals each having the tetragonal axis along one of the three cubic fourfold axes. However, electron microscope photographs have failed so far to reveal the existence of domains. From the systematic absences it has been determined that the cubic distortion belongs to space group I2t3 while the tetragonal one to space group P4222. The main effect of the distortion is the lowering of the site symmetries which favors the disorder between the cationic tin and the M atoms. The size of these latter atoms does not seem to be an important factor for the distortion. The only feature which separates the distorted compounds from the undistorted ones is the valence of the M atoms. However, no explanation can be offered why the valence plays an important role for the distortion.

Introduction

different chemical behavior as the first type has a metallic or a cation-like character while the second type has a metalloidal or an anion-like character. From the crystal chemical point of view the structure of this series of compounds is quite unique since it contains the same element with opposite chemical behavior.

Compounds such as SnM~Rh~Sni2 (M = La, Ce, Pr, Nd, Sm, Gd, Eu, Yb, Ca, Sr, and Th) have been found to crystallize with a cubic structure, space group Pm3n, 2 molecules per unit cell, and a lattice parameter of about 9.7 ~.1-5 Remeika et al I reported that among the eleven compounds, those of La, Yb, Ca, Sr, and Th became superconductors with Tc ranging from 8.7 K for the Ca and Yb compounds, to 1.9 K for the Th one. On the contrary, for the Eu and Gd compounds a magnetic transition was found to occur at about 11 K. For the other four compounds the same authors did not find any anomaly in the electric and magnetic behavior down to T = 1.1 K. The crystal structure of these compounds can be described as containing two interpenetrated structures, namely M3Sn and Rh4Snl2. The former has the arrangement of the A15 compounds while in the latter the tin atoms form a tridimensional array of corner-sharing trigonal prisms whose centers are occupied by the Rh atoms. Figure 1 represents a projection of four unit cells of the SnM3Rh,Sn12 structure on the (100) plane. The tridimensional array of corner-sharing prisms is outlined. The M and the Sn atoms of the A15 structure occupy the cuboctahedral and icosahedral holes of this array, respectively. The two types of tin atoms, one forming the A15 substructure and the other forming the prisms array, are crystallographically independent and have

An analogy between the SnM3Rh4Sn12 compounds and the ternary oxides A'A'3B40~2 with perovskite-like arrangement can be drawn. The result is that the ternary stannides can be regarded as the covalent counterparts of the ionic ternary oxides. The former contains a tridimensional array of corner sharing RhSn6 trigonal prisms while the latter contains the same type of array, but of BO6 octahedra. Both networks produce icosahedral, tetracapped rhombic prismatic, and cuboctahedral sites. The first and the second type of sites are occupied in the ternary oxides by the A' and A" atoms while the first and the third types are occupied in the ternary stannides. The shift of the M atoms with respect to the sites occupied by the A" atoms in the structure of the ternary oxides is important because it gives rise to the A15 arrangement between the M and the Sn atoms. A departure from the cubic symmetry of some of the SnM3Rh4Snt2 compounds has been detected by electron diffraction techniques and confirmed by singie-crystal X-ray diffraction. We report herein the characterization of this distortion.

97

98

SUPERCONDUCTING/MAGNETIC

TERNARY RARE-EARTH RHODIUM STANNIDES

O

©

O

Vol. 42, No. 2

@

L,

@

@

I

" ©

@

Fig. 1

©

@

Projection of the Sn(1)M3Rh4Sn(2)]2 structure on the (100) plane. The threedimensional array of corner-sharing trigonal prisms is outlined. The smallest, the medium, and largest open circles represent the Sn(2), Rh, and M atoms, respectively. The shaded circles represent the Sn(1) atoms.

Experimental The preparation of all ternary stannides and their chemical analysis has been reported in References 1 and 2. The stoiehiometric formula SnM3Rh,Snl2 has been quantitatively determined by single-crystal X-ray diffraction performed on the ytterbium compound. A small amount of disorder was found to exist between the metallic tin and the rare-earth which improved the agreement between the chemical analysis formula and that based on X-ray data. The crystal structures of the other members of the series have not been refined yet. However, the amount of disorder should vary across the series, which would reconcile the differences in the chemical formula for the members of the series. For the electron microscope/diffraction studies three different instruments were used: a Siemen's Elmiskop-102 of the CSIC at Madrid, a JEM-100Cx of the DRF at Grenoble and a JEM-200B at Bell Laboratories. Each microscope was equipped with a double tilting goniometer stage in order to reach different planes in reciprocal space with the same sample. The first two microscopes were operated at 100 KV while the third was operated at 200 KV. Samples for study were crushed in an agate mortar and suspended in n-butanol then transferred to holey carbon-coated copper grids. Single-crystal X-ray diffraction patterns were obtained by the precession method and Zr-filtered Mo Ka radiation. Exposure times greater than 200 hours were required to register the superstructure reflections.

Figure 2 shows an electron diffraction pattern of the [0011% zone for the gadolinium compound. Extra reflections are ~-0

and related positions.

camera. It can be seen that when a long exposure time ( ~ 200 hrs.) is employed the X-ray precession photograph shows the same type of extra reflections observed in the electron diffraction pattern, but the intensity distribution, as expected, is significantly different. The same extra spots were observed in the precession photograph of the [0101% zone. In order to determine the supercell and its symmetry, precession photographs corresponding to the [ h k l ] ,

taken.

Figure 4 shows the

/ sootioo k

A.

(hkl),

oxt*a

1

reflections could be indexed on a cubic body-centered cell with aci = 2a%, namely they were either of or

type.

~- l

k ~- %

However, all extra spots could also be

¢p

explained by a tetragonal cell with a'T =a'%--b%, ~r=a'%+bep, and ~CT=--C% with the assumption that the sample was a twin crystal comprised of three individuals, each corresponding to an orientation of the tetragonal axis along one of the three fourfold axes of the cubic cell. Each individual gave rise to one type of extra spots,

1,

k

, and

~--~-,

respectively. This tetragonal indexing implies that the metric symmetry of the lattice remains cubic. A powder film taken with a Guinier camera and Fe Ka radiation did not reveal any

Observations

observed at the

®

Figure 3

ep

shows the same zone taken by a precession X-ray diffraction

v5

line splitting due to the difference between T

aT and CT. On

the other hand it was observed that the extra spots of the electron diffraction photographs appeared and disappeared by changing the region of the crystal being explored and/or by tilting the sample in order to see different sections of the

Vol.

42, No.

2

SUPERCONDUCTING/MAGNETIC

TERNARY

,,

•

•

•

•

•

RARE-EARTH

Cp

•

RHODIUM

STANNIDES

99

•

10 G

0

I •

0

V

m

L

• :

x

G Cp

100

•

•

•

•

•

•

•

•

•

Fig. 2

0

S

Electron microscope photograph of the [001]cp zone.

0 e

,w

aL

Fig. 3

Precession photograph of the zero layer perpendicular to the c*cp obtained with Zrfiltered Mo radiation. The exposure time is about 200 hrs. The primitive cubic lattice is outlined.

reciprocal lattice. This could be explained by the presence of domains in the sample, which would favor the tetragonal indexing. The domain size would be larger than the electron beam and by moving the sample, one would observe

sequentially domains with the tetragonal axis perpendicular to each other. Consequently the extra spots in the (001) plane would only be observed for those domains whose tetragonal axis is parallel to the electron beam. We are planning to make

100

SUPERCONDUCTING/MAGNETIC TERNARY RARE-EARTH RHODIUM STANNIDES

Fig. 4

Precession photograph of I = 1/2 layer obtained with Zr-filtered Mo radiation. exposure time is about 200 hrs. The primitive cubic lattice is outlined.

electron microscope studies in order to prove the existence of domain boundaries. However, preliminary bright-field and dark-field images at low and high magnifications have failed to reveal these boundaries, which indicate that the samples are monodomain crystals. This would favor the cubic indexing. Since with the present data we cannot decide which is the correct symmetry we give herein the description of the distortion in the two symmetries. The same type of superstructure was found to exist for five other members of the SnM~Rh4Sn12 series, namely those for which M = La, Ce, Pr, Nd, and Sin, whereas the compounds of Eu, Yb, Ca, Sr, and Th did not show any extra spots in precession photographs exposed for about 200 hrs. Discussion

The body-centered cubic structure can be described in several space groups, namely Ia3d, I43d, Ia3, I4132, and I2t3, but since the only systematic absences among the (hk/) reflections, as deduced from precession photographs, are those due to the body-centered lattice ( h + k + / = 2n+l), it belongs to space group I213. The approximate positional parameters and the site symmetries in this space group are given in Table 1. It must be pointed out that there is only one, rather weak, reflection of h00 type (1400) which destroys the systematic absence corresponding to the 41 screw axis, i.e. h ~ 4n for (h00). If one disregards this reflection the body-centered cubic structure belongs to I4132 space group. The tetragonal distortion can be described in space groups P4z/nnm, P42nm, P4222, and P42. Since for this symmetry the crystals are considered to be twinned, the systematic absences can only be inferred. The possible ones are among the (001) for l = 2n+l, which lead to space groups P4222 and P42. The distortion in the former space group is illustrated in Table 1. Note that a shift of the origin equal to

-~-

must be

applied on going from the primitive cubic structure to the

Vol. 42, No. 2

The

tetragonal one. The physical properties of the SnM3Rh,Sn12 series along with the lattice parameters of the primitive cubic lattice, the covalent and ionic radii of the atoms, and the Sn/M ratio as obtained from chemical analysis, are reported in Table 2. It can be seen that there is no correlation between the distortion and the physical properties. Since M is the only atom which varies across the series, the parameter of the primitive cubic lattice gives indirectly a measure of the M atom size. No correlation can be established between the lattice parameters and the distortion. This indicates, therefore, that the distortion is not entirely a size effect. In the last column of Table 2 the Sn/M ratio, as determined from chemical analysis, is given. It varies across the series, being smaller than the stoichiometric value (4.33) for the distorted compounds and larger for the undistorted ones. This indicates that some M atoms substitute for some Sn(1) cations in the distorted compounds whereas some Sn(1) cations substitute for some M atoms in the undistorted ones. These two types of disorder were proved to exist in the undistorted ytterbium compound. However, the second type of disorder was found to occur about five times more than the first. One of the main effects of the distortion is to lower the site symmetries. This favors the disorder between the Sn(l) and the M atoms, because the symmetries of the sites occupied by these two atoms become similar in the cubic distortion and equivalent in the tetragonal one. It is not clear at this point why the substitution of M atoms for Sn(1) cations causes the distortion whereas the opposite substitution does not. A feature which seems to separate perfectly the distorted compounds from the undistorted ones, is the valence of the M cations. All trivalent cations correspond to the first class of compounds while the divalent ones and tetravalent thorium correspond to the second class. Again, this cannot be explained with the present data. The importance of the M cation valence is confirmed by the results obtained for the GeM3Ru4Gel2 and GeM3Os4Gel2 series (M = rare earth, except La for the osmium series and La and Eu for the ruthenium one). 7 These compounds are all isostructural with the stannides. The lattice parameter variation

SUPERCONDUCTING/MAGNETIC

Vol. 42, No. 2

TERNARY RARE-EARTH RHODIUM STANNIDES

I0

Table 1 Body-centered cubic and tetragonal distortions of primitive cubic SnM3Rh4SnI2 Compounds 1213

Pm3n

Sn(ll)

8a

3

xxx

x=.O

(12)

8a

3

xxx

x=.25

M(1)

24c

1

xyz

x~.125

y=.25

z~.O

(2)

24c

1

xyz

x~.125

y=.25

z=.50

Rh(1)

8a

3

xxx

Sn(1)

Rh

(2)

8a

3

xxx

x=-.125

24c

1

xyz

x=.125

(4) Sn(21)

24c

I

xyz

24c

1

xyz

x=,125

y~.625

m3

P4222

000

i I

x=,125

(3)

2a

8e

1 1 1 ~ ~ ~

32

z=.625

y~-.125

z=.125

4n

2

x x ~

M(1)

2c

222

1 0 y 0

(2)

2d

222

i 1 0 ~

(3)

40

2

x x ~

24k

m

Oyz

y~.31

3

3

x=.25

x=.88

(4)

40

2

x x ~

x=,38

Rh(1)

4J

2

x00

x=.75 x=.75

(2)

4~

2

x 0 ~I

(3)

4m

2

x 1 0

x=.75

4k

2

x 1 21

x=.75

80

1

xyz

(4) Sn(2)

1

Sn(1)

x=.O

y=.155

z=.075

x=.41

y=.41

z=.lO

(22)

x=.25

y=.175

z=.O95

(22)

x=.09

y=.09

z=.10

(23)

x=.O

y=-.155

z=-.075

(23)

x=.83

y=.33

z=.06

(24)

x=.25

y=.325

z=.405

(24)

x=.83

y=.33

z=.44

z=.15

Sn(21)

(25)

x=.O

y=.155

z=-.075

(25)

x=.17

y=.52

z=.75

(26)

x=.25

y=.325

z=.095

(26)

x=.33

y=.98

z=.75

(27)

x=.O

y=-. 155

z= 075

(28)

x=.25

y=.175

z=.405

Table Properties Lattice Parameter

Crystal Structure

La

9.745

I1

Ce

9.710

I1

--

Pr

9.693

I1

Nd

9.676

I1

Sm

9.657

I1

Gd

9.638

I1

M

Eu

9.749

I

Yb

9.676

Ca

9.702

Sr

Th

M Covalent Radius

M Ionic Radius**

1.69 A

1.16

3+

3.9

1.65

1.14

3+

4,0

--

1.65

1.13

3+

3,9

--

1.64

1.11

3+

4,1

--

1.62

1.08

3+

4,3

1.61

1.05

3+

M ~ii

1.85

1.25

2+

I

S

8.6

1.80

1.14

2+

4,6

I

S

8.7

1.74

1.12

2+

4,5

9.800

I

S

4.3

1.91

1.26

2+

9.692

I

S

1.9

1.65

1.05

4+

*S and M i n d i c a t e **Yhese

values

Physical Property*

2

of S n M 3 R h 4 S n I 2

S

superconducting

correspond

3.2

11.2

and m a g n e t i c

to 8 - c o o r d i n a t e d

transition

cations

(6).

temperatures,

Valence of M A t o m s

respectively.

Sn/M Ratio from chem. a n a l y s i s

4,3

102

SUPERCONDUCTING/MAGNETIC TERNARY RARE-EARTH RHODIUM STANNIDES

and the values of the effective magnetic moment per rare earth atom across the two series show that the germanides contain trivalent rare earth atoms except those of cerium and europium. The structural refinement performed on GeY3Ru4Ge12 indicates that its symmetry is lower than cubic. We believe that several structures must be refined before making any conclusion.

Vol. 42, No. 2

Acknowledgments The Grenoble authors would like to thank M. Alario-Franco and J. Gonzalez-Caibet for their collaboration in obtaining the electron diffraction patterns at the Chemistry Department of the University of Madrid and for helpful discussions. We also thank, from BTL, G. P. Espinosa for help in crystal growth, A. S. Cooper for monitoring with X-ray powder diffraction, and H. Barz for superconducting Tc measurements.

REFERENCES 1. J . P . Remeika, G. P. Espinosa, A. S. Cooper, H. Barz, J. M. Roweil, D. B. McWhan, J. M. Vandenberg, D. E. Moncton, Z. Fisk, L. D. Woolf, H. C. Hamaker, M. B. Maple, G. Shirane, and W. Thomlinson, Solid State Commun. 34, 923 (1980).

4.

2.

G . P . Espinosa, Mat. Res. Bull. 15, 791 (1980).

7.

3.

A.S. Cooper, Mat. Res. Bull. 15, 799 (1980).

J . M . Vandenberg, Mat. Rcs. Bull. 15, 835 (1980).

5. J. L. Hodeau, J. Chenavas, M. Marezio, and J. P. Remeika, Solid State Commun. 36, 839 (1980). 6.

R . D . Shannon, Acta CrystaUogr. A32, 751 (1976). C. U. Segre, H. F. Braun, and K. Yvon, in Ternary Shenoy, Dunlap, and Fmdin [Elsevier North Holland, 1981], p. 243.

Supercotuluctors, eds.