Physica C 161 (1989) 245-251 North-Holland, Amsterdam

Nb AND Ta SUBSTITUTIONS IN YBa2Cu307_ x A N D RELATED P H A S E S : S T R U C T U R A L C H A R A C T E R I S A T I O N O F Lal.lBal.9Cu2.1Mo.9Os ( M = N b , Ta)

C. GREAVES and P.R. SLATER School of Chemistry, Superconductivity Research Group, University of Birmingham, Birmingham B15 2TT, UK

Received 20 July 1989 Revised manuscript received 1 September 1989

Nb and Ta substitutions in YBa2Cu307 and related phases have been examined in an attempt to synthesise phases of formula LBa2Cu2MOs (L = rare earth, M = Nb, Ta), in which the square planar Cu I sites of the YBa2Cu307 structure are occupied by M, and complete oxygen occupancy in the basal plane of the unit cell provides octahedral coordination for M. Single phase materials of composition LaHBat.9Cu2.1M0.908 have been successfully synthesised and their structures examined by powder X-ray and neutron diffraction. The location of M atoms solely in the Cul sites with octahedral symmetry is confirmed. The square planar CuO4 units, which link the layers of square pyramidal CuO5 units in YBa2Cu307, are replaced by MO6 octahedra in the new phases. This new link provides interesting scope for the synthesis of new phases with potential as superconductors, but to date no superconducting properties have been confirmed, possibly due to insufficient oxidation within the layers of CuO5 square pyramids.

1. Introduction

The structure o f the high tempera ture supercon- ductor YBa2Cu30 7_x, and related phases in which Y has been subst i tuted by one of the lanthanides , con- tains two distinct crystallographic sites for Cu, as seen in fig. la: Cu l (4-coordinate and nominal ly square planar) and Cu2 (5-coordinate and nominal ly square pyramida l ) [1,2]. In order to gain insight into the possible role of these two sites in relat ion to super- conduct ing propert ies, considerable a t tent ion has been focused on the way in which cat ion substi tu- t ions on the Cu sites influence the crit ical temper- ature, To. A wide var ie ty o f such subst i tut ions has been reported, including Li, Zn, A1, Ga and many t ransi t ion metal ions, but in all cases a decrease in Tc has been observed, the magni tude being depen- dent on the par t icular subst i tuent [ 3-12 ]. Structural de te rmina t ions have revealed different site prefer- ences for the subst i tuent cations: Co, A1 and Ga oc- cupy the Cu I sites, whereas Ni and Zn prefer the Cu2 posi t ion and Li adopts an essentially r andom distri- but ion [ 4 ,8 ,10-13 ]. Although controversy has sur- rounded the site preference of Fe, it appears to ex- hibit a preference for Cu 1 sites [ 14], but the observed differential occupancy of the two sites may depend

on the condi t ions employed during sample prepa- rat ion [7,9 ]. The fact that Zn subst i tut ions on the Cu2 site produce the most p ronounced depression of Tc has been used to support the view that the C u 2 - O2 planes are of p r imary impor tance in determining superconduct ing propert ies, with the chains of cor- ner- l inked C u l - O 4 squares being of lesser signifi- cance [ 8 ]. However, such a picture is clearly an ov- ers implif icat ion, since Ni, which also substi tutes p redominan t ly on the Cu2 site, produces decreases in Tc that are considerably smaller, and comparable to those observed for Cu 1 site substi tutions. Fur ther studies are called for to improve our unders tanding of chemical substi tut ions, and complete rat ional is- a t ion may require a bet ter theoret ical basis for the origin o f superconduct iv i ty in mater ia ls of this type.

The study repor ted here has developed from a somewhat different but related approach, the objec- t ive o f which was to form a completely new type of link between the otherwise unchanged layers of CuO5 square pyramids . We have therefore examined the possibi l i ty of replacing all the Cu on the Cul sites with cat ions which exhibit a p ronounced tendency towards octahedral coordinat ion, in the hope that their preferred envi ronment can be achieved by complete ly filling all the O sites in the basal plane of

0921-4534/89 /$03 .50 © Elsevier Science Publishers B.V. ( Nor th -Hol land Physics Publ ishing Divis ion )

246 C. Greaves, P.R. Slater / Structure of La l jBa l.9Cu z lMo.9 08 (M= Nb, Ta)

the unit cell, as shown in fig. lb. In order to maintain charge balance, M 5+ cations were required (assum- ing Cu 2+ in the Cu2 sites), and Nb 5+ and Ta 5+ were consequently considered. For the parent material YBa2Cu307, such a phase would have the nominal composition YBa2Cu2NbO8. The possibility of syn- thesising a series of new phases LBa2Cu2MO8 (L = Y, La, Eu; M = N b , Ta) has been explored, and struc- tural characterisation by powder X-ray and neutron diffraction performed where single phase products were obtained.

2. Experimental

2. I. S a m p l e synthesis

High purity Y203, Eu203, La203, BaCO3, CuO, NbzO5 and Ta2Os were used to prepare samples with the following nominal stoichiometries: YBaECu2NbO8 (sample 1 ), YBazCu2TaO8 (sample 2), EuBazCu2NbO8 (sample 3), EuBazCu2TaO8 (sample 4), LaBa2CuzNbO8 (sample 5), La- Ba2CuETaO8 (sample 6).

Stoichiometric mixtures of the intimately ground powders were heated for 12 h at 940°C in air, and the products were ground and examined by powder X-ray diffraction (XRD) . The samples were re- heated at 1030°C in flowing dry oxygen for 12 h, slowly cooled (20 h) to ambient temperature and re- examined by powder XRD. For samples 1-4, XRD

9

o . f_ / o "o-

o

[o1 0 Bo • Cu

"~- "" 0

o ~ °

o

,,, ,'( 0

• NblTo

o 0 (bl

Fig. 1. The structures of (a) YBa2Cu307, L=Y and (b) LBa2Cu2MOs, M = Nb/Ta.

after each heat treatment implied the absence of any phase with the YBa2Cu307 structure. The main products appeared to be CuO and a cubic perovskite of the form Ba2LMO6 (L=Y, Eu; M = N b , Ta). For samples 5 and 6, on the other hand, a multi-phase XRD pattern was observed after heating in air, but after the oxygen treatment a pattern characteristic of the YBa2Cu307 structure was obtained, with only minor amounts of Ba3CuNb209 or Ba3CuTa209 being present as an impurity. Since these impurities ap- parently contained no La, it was assumed that the principal phase must be correspondingly rich in this component, with the excess La presumably substi- tuting onto the Ba site. Accordingly, two additional samples were synthesised using similar procedures but with compositions LaL1Bal.gCu2.1Moo.908 ( M = N b , sample 7; M = T a , sample 8). Due to the increased La content, the M:Cu ratio was reduced since an oxygen content in excess of 8 per unit cell would otherwise have been required to balance the cationic charges. XRD data confirmed samples 7 and 8 to be single phase.

The oxygen contents for samples 7 and 8 were es- timated by thermogravimetric analysis (TGA) using 10% H2/90% N2 in a Stanton Redcroft STA 780 and heating to 930°C at 10°C m i n - k

2.2 Structure determinat ion

Time-of-flight powder neutron diffraction data were collected for samples 7 and 8 on the diffrac- tometer POLARIS at ISIS, Rutherford Appleton Laboratory, using cylindrical vanadium containers. The Rietveld method was employed for structure re- finement and the program was based on the Cam- bridge Crystallography Subroutine Library [ 15 ]. Neutron scattering lengths of 0.827, 0.525, 0.7718, 0.7054, 0.691 and 0.5805 (all × l 0 -12 cm) were as- signed to La, Ba, Cu, Nb, Ta and O, respectively.

Step-scan X-ray diffraction data (14-78 ° two theta, step 0.025 ° ) were collected for sample 8 using a Picker diffractometer and quartz monochromated Cu-Ko~ 1 radiation. A well documented program [ 16 ] based on the Rietveld method was used for structure refinement, and a pseudo-Voigt peak shape function was adopted.

C. Greaves, P.R. Slater / Structure of La l.lBa l.9Cu2.1Mo.90s (M= Nb, Ta) 247

3. Results

The oxygen contents of samples 7 and 8 were de- termined by TGA to be 7.94(5) and 7.92(5) re- spectively. When heated in air, negligible oxygen loss was found to occur up to 930°C, and even in nitro- gen only a slow oxygen loss was observed at 930°C. This behavior contrasts markedly with that of un- doped samples of YBaECU307 and LaBa2Cu3OT, and reflects the fact that Nb 5+ and Ta 5+ are more stable to reduction than is Cu 3+.

Both samples 7 and 8 were tetragonal and the space group P4 /mmm was assumed with initial parame- ters for structure refinement based on previous structure determinations of Fe-doped YBa2Cu307, which is also tetragonal. Since the neutron scattering lengths of Cu, Nb and Ta are similar, refinement of site occupancies to determine their distribution was not attempted, but instead a separate XRD deter- mination was performed on the Ta containing sam- ple, since this was considered to be of greater reli- ability. For the neutron diffraction refinements therefore, the Nb/Ta unit cell contents were con- strained at their expected value (0.9), and were lo- cated entirely on the Cu 1 site. This latter assumption was subsequently demonstrated to be valid by the XRD refinement. Since the chemical composition implied the presence of excess La on Ba sites, the Ba site occupancy was allowed to vary. Anisotropic thermal motion for O atoms was allowed since sig- nificant decreases in the residuals R1 and Rwv re- sulted [ 17 ]. For both samples, the thermal ellipsoid for O 1 (0, 1/2, 0), displayed anomalously large val- ues for B~ (around 12 A2). As a result, the O1 at- oms were allowed to move off the special site along [ 100]. Due to space group restrictions, refinement was based on an average unit cell in which half the O 1 atoms were shifted to (x, l /2, 0 ) and half to ( - x, 1/2, 0); the result implied a static displacement of around 0.2 ~ from the 0, 1/2, 0 site. The refined structural parameters, tables I and II, confirm the structure shown in fig. lb. The oxygen labels O1, 02 and 04 have been used in this work to simplify com- parison with related structures, e.g. YBa2Cu307. The observed, calculated and difference neutron profiles are shown in figs. 2 and 3.

Sample 8 was selected for detailed XRD exami- nation due to the large difference between Ta and Cu

scattering factors. Owing to the insensitivity of X-rays to O atoms in such a sample, the oxygen parameters were fixed in accordance with the values in table II (the use of compatible isotropic temperature factors for O atoms was considered appropriate for the XRD refinement). Within the limitations of the chemical constraint that the unit cell contained 2.1 Cu and 0.9 Ta, the distribution of Ta between the two possible sites was refined. A subsequent refinement in which the chemical constraint on the Cu: Ta ratio was lifted confirmed the validity of the initial constraint, since deviations from the expected ratio were insignifi- cant. Since Ba 2+ and La 3÷ are isoelectronic, the pos- sible location of some La on Ba sites could not be examined. The refined parameters are given in table III, and the corresponding diffraction profiles in fig. 4. The refinement shows conclusively that Ta oc- cupies only the Cul site, and we assume a similar preference obtains for Nb in sample 7. Relevant bond distances for samples 7 and 8 are given in table IV.

4. Discussion

The products from the attempted substitution of Nb and Ta into LBaECU307 (L=Y, Eu, La) suggest that only a small energy difference separates the two possible products: 1. LBa2Cu2MOs, 2. Ba2LMO6+CuO (M--Nb, Ta). Since the phases Ba2LMO6 are based on the perovskite structure with the rare earth cation in the small octahedral site, it is probable that their stability will decrease for large L 3+ ions. This is consistent with the observation that substituted phases based on the YBa2Cu307 struc- ture were obtained only for the largest rare earth, La, whereas for Eu and Y BaELMO6 and CuO were formed.

The refined O1 site occupancies for the substi- tuted phases LaHBal.9CUE.1MOs, tables I and II, sug- gest completely filled sites and unit cell oxygen con- tents of 8, as expected, with approximately octahedral coordination around the Cu I (strictly Nb I and Ta 1 ) positions. The oxygen stoichiometries are therefore in good agreement with the TGA results. The Ba site occupancy from neutron diffraction proved insuffi- ciently precise to determine whether partial occu- pancy with La was occurring. The presence of 0.1 La atoms in this position would be reflected in an ef-

248 C. Greaves, P.R. Slater / Structure o f La l.~Ba ~.gCu z lMo.90s (M= Nb, Ta)

Table I Neutron diffraction structural parameters for Lat. ,Ba LgCu2.1Nbo.908.

Atom Position x y z B Unit cell (~2) occupancy

La ld 1/2 1/2 1/2 0.29(2) 1.0 Ba 2h 1/2 1/2 0.1918(2) 0.43(4) 2.01(2) Cul la 0 0 0 0.30(3) 0.1 Nbl la 0 0 0 0.30(3) 0.9 Cu2 2g 0 0 0.3521(1) 0.24(1) 2.0

Bl i B22 B33

O1 4n 0.0628(7) 1/2 0 1.1(1) 0.08(7) 1.4(1) 2.02(3) 02 4i 1/2 0 0.3691(1) 0.12(3) 0.43(3) 1.06(5) 4.0 04 2g 0 0 0.1625(2) 0.98(5) 0.98(5) 0.66(8) 2.0

Tetragonal, P4/mmm, a=b= 3.970( 1 ) A, c= 11.989(3) A, R~= 3.50%, Rp= 2.77%, Rwp= 3.01%, Rcx~=2.21%.

Table II Neutron diffraction structural parameters for Lal.iBal.9Cu2ATao.90 s.

Atom Position x y z B Unitcell (A 2 ) occupancy

La ld 1/2 1/2 1/2 0.31(2) 1.0 Ba 2h 1/2 1/2 0.1931(2) 0.47(5) 2.03(3) Cul la 0 0 0 0.16(3) 0.1 Tal la 0 0 0 0.16(3) 0.9 Cu2 2g 0 0 0.3520(1) 0.22(2) 2.0

Bl I B22 B33

O1 4n 0.0563(8) 1/2 0 0.9(2) 0.00(6) 1.6(1) 2.04(4) 02 4i 1/2 0 0.3694(1) 0.09(3) 0.40(4) 1.12(5) 4.0 04 2g 0 0 0.1619(2) 0.94(6) 0.94(6) 0.54(8) 2.0

Tetragonal, P4/mmm, a=b= 3.965( 1 ) A, c= 12.029(3) A, RI= 4.55%, Rv= 3.10%, R,~,= 3.40%, Rcxa= 2.33%.

fective unit cell occupancy for Ba of 2.06, which dis- agrees with the ob ta ined values by only 1-2 s tandard deviat ions.

The high B~1 values observed for both samples, which were subsequently t ranslated into static dis- placements , are interest ing since s imilar effects have been repor ted in related systems. In YBa2Cu3OT, for example, the mot ion of O 1 also appears anisot ropic but the effect is less p ronounced than observed in the present study. However, in a s tudy o f La3Ba3Cu6O14+x [ 18], in which one quar ter o f the Ba sites o f the YBa2Cu307 type structure are occu- pied by La, the repor ted values for B, , are very sim- ilar to those de te rmined for samples 7 and 8. A co- operat ive rota t ion of the Cu 1-O6 octahedra a round

the [001] axis could account for the O1 displace- ments, and to examine a possible dr iving force for such distort ions, bond strength calculations were per formed for the Cu and N b / T a a toms in samples 7 and 8 [ 19]. Assuming Cu 2+ bond strength param- eters, the Cu2 site valence was calculated as 1.90 for both samples, which supports the absence o f any Cu 3+ in this location. For Nb and Ta, the ra ther high values o f 5.08 and 5.24 were de te rmined for O 1 con- s t rained to the special site at 0, 1/2, 0 whereas al- lowing displacements along [ 100 ] reduced these val- ues to 4.95 and 5.12 respectively. Such a lengthening o f the N b / T a - O 1 bonds, which may relate to small rotat ions o f the octahedra, could therefore provide a stabil isat ion. However, this precise mechanism

C. Greaves, P.R. Slater / Structure of Lal iBa~,9Cue.lMo.908 (M=Nb Ta) 249

2 CL

L 5

0 5

1 . 4

eN £ 2 .

n 13 8 s

/ n 6

O 4

o ' 0 2

e

8 n - 0 2

A

d/A

N O 8

r

°n 0 6 s

/

o 0

c

d

076 o'.8 ~o

0 , 8

0 . 7

N 0.6~

t 0 5

n 6 4-

t n 3

~ O . Z c r

e

8 -@

, , , i , i , i , i , , , i , J , i

o5. o:6 o'.8 ~ '.o dA&

Fig. 2. Observed (dots), calculated and difference neutron dif- fraction profiles for Lal.|Bal.gCu2jNbo.9Os.

cannot explain the high O l thermal parameters in La3Ba3Cu6Ol4+x [18], since Cu is now located in the Cul sites.

The indication from bond strength data that the Cu2 site is occupied by Cu 2+ allows the oxidation state of the residual Cu on the Cul sites to be con- sidered. Given that the presence o f Cu 2+ or Cu 3+ would result in overall unit cell oxygen contents of 7.90 and 7.95, the neutron diffraction refinements and thermal analysis results suggest that Cu 3+ ions accompany NbS+/Ta 5+ in the Cul sites.

In addition to the O 1 shifts, a significant displace- ment also seems to have occurred for the Ba position relative to YBazCu30 7 (z (Ba) =0.1841 ), [ 1,2 ], and LaBa2Cu307 (z (Ba) = 0.1801 ), [ 20 ]. This observed shift away from the z = 0 plane has been noted in a previous study of Ca substituted YBa2Cu3OT. Ca substitution on Y sites results in a reduction of the oxygen content in the z = 0 plane, and subsequent shifts of Ba and O ions to compensate the effective transfer of positive charge from the Y sites to the z = 0 plane [21 ]. By analogy, we might therefore have ex-

Fig. 3. Neutron diffraction profiles for LamBal.gCu2.tTao.9Os.

peered a decrease in z (Ba) for samples 7 and 8, as- sociated with the increase in oxygen content to around 8. However, for these materials the negative charge o f the additional oxygen is balanced in the same plane by M 5+ cations, which should cancel the electrostatic effect. The displacement would there- fore appear to have a different origin, and possibly relate to high repulsions between Ba 2+ and the M 5+ cations.

The presence of Nb or Ta in the Cu 1 sites also re- suits in an increase in z (O4) relative to that ob- served in YBa2Cu307 (0.158) since M s + - O bonds are longer than C u - O distances. Nb and Ta substi- tutions cause an expansion of the trait cell dimen- sions, and this gives rise to longer Cu2-O2 bond dis- tances relative to those in YBa2Cu3OT.

AC susceptibility measurements indicated that neither sample 7 nor sample 8 was superconducting above 8 K. A likely reason for the absence of super- conducting properties may relate to the presence of pure Cu 2+ in the Cu2 sites. Various substitutions have therefore been examined in an effort to raise the Cu oxidation state, including Ca 2+ and Sr 2+ for

250 C. Greaves, P.R. Slater / Structure of La~.~Bal.gCuz~Mo 90s (M=Nb Ta)

Table III X-ray diffraction structural parameters for Lal.lBaLgCu2.1Tao.908

Atom Position x y z B Unit cell (A 2 ) occupancy

La ld 1/2 1/2 1/2 0.08(10) 1.0 Ba 2h 1/2 1/2 0.1938(2) 0.39(8) 2.0 Cul la 0 0 0 0.13(11) 0.101(8) Tal la 0 0 0 0.13(11) 0.899(8) Cu2 2g 0 0 0.3529(5) 1.0(2) 1.999(8) Ta2 2g 0 0 0.3529(5) 1.0(2) 0.001(8) O1 4n 0.0564 1/2 0 1.0 2.0 02 4i 1/2 0 0.3694 1.0 4.0 04 2g 0 0 0.1619 1.0 2.0

Tetragonal, P4/mmm, a=b= 3.9658 ( 1 ) A, c= 12.0304(3) A, R~= 5.97%, Rp= 5.54%, R~p= 7.58%, Rexp= 1.98%.

90000.

80000

70000

SO000

50000

40000

30000

20000

10000

REFLS 0

DIF 0

~o

. , i l •, l • ,

~o ~o

--~ . . . . . 3,- . . . . .

~o ~o ~o

DEG 2THETA

Fig. 4. Observed (dots), calculated and difference X-ray diffraction profiles for LaL i Bal.9Cu2.~Tao.9Os.

Table IV Selected bond distances (A).

Lal.lBal.9Cu2nNbo.90 s LaHBal.gCu2.1Tao.9Oa

Cul (Nb) -Ol 2.001(1) [×4] Cul(Ta)-O1 1.995(1) [X4] - 0 4 1.948(2) [×2] - 0 4 1.947(2) [×2]

Cu2-O2 1.995(1) IX4] Cu2-O2 1.994(1) [×4] - 0 4 2.273(2) - 0 4 2.287(2)

La 3+, and Ti 4+ for NbS+/Ta 5+. However, in all cases,

substitution was unsuccessful since impuri ty phases were observed. An alternative route would involve increasing the C u : N b / T a ratio, whilst maintaining

an oxygen stoichiometry o f around 8. Unfor tunate ly

this approach also proved unsuccessful since the ox- ygen content was found to decrease as the Cu con-

tent increased. This presumably reflects the insta- bility of Cu 3÷ in octahedral environments , and its preference for square planar stereochemistry.

C. Greaves, P.R. Slater / Structure of La l lBa l.9Cuz lMo 90s (M= Nb, Ta) 251

Acknowledgements

We t h a n k t he Sc ience a n d E n g i n e e r i n g R e s e a r c h

C o u n c i l for f i n a n c i a l s u p p o r t , t h e p r o v i s i o n o f n e u -

t r o n d i f f r a c t i o n fac i l i t ies a n d a s t u d e n t s h i p to P .R.S.

We are a lso g ra te fu l to I .C.I . fo r a d d i t i o n a l f u n d i n g .

We t h a n k J. M a y e r s a n d S. Hu l l for e x p e r i m e n t a l as-

s i s t ance w i t h t he co l l ec t i on o f n e u t r o n d i f f r a c t i o n

da ta .

References

[1 ] J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyen, B. Raveau, J.L. Soubeyroux, J.L. Tholence and R. Tournier, Europhys. Lett. 3 (1987) 1301.

[ 2 ] W.I.F. David, W.T.A. Harrison, J.M.F. Gunn, O. Moze, A.K. Soper, P. Day, J.D. Jorgensen, D.G. Hinks, M.A. Beno, L. Soderholm, D.W. Capone II, I.K. Schuller, C.U. Segre, K. Zhang and J.D. Grace, Nature 327 ( 1987 ) 301.

[ 3 ] I. Sankawa, M. Sato and T. Konaka, Jpn. J. Appl. Phys. 27 (1988) L28.

[4] T. Kajitani,K. Kusaba, M. Kikuchi, Y. Syono and M. Hirabayashi, Jpn. J. Appl. Phys. 27 (1988) L356.

[5] Y. Maeno, T. Tomita, M. Kyogoku, S. Awaji, Y. Aoki, K. Hoshino, A. Minami and T. Fujita, Nature 328 ( 1987 ) 512.

[6] P. Strobel, C. Paulsen and J.L. Tholence, Solid State Commun. 65 (1988) 585.

[7] P. Bordet, J.L. Hodeau, P. Strobel, M. Marezio and A. Santoro, Solid State Commun. 66 (1988) 435.

[8] C.L. Chien, G. Xiao, M.Z. Cieplak, D. Musser, J.J. Rhyne and J. Gotaas, Proc. 2nd. Ann. Conf. on Superconductivity and Applications, eds. D.T. Shaw and H.S. Kwok (Elsevier, 1988).

[ 9 ] A.M. Balagurov, G.M. Mironova, A. Pajaczkowska and H. Szymczak, Physica C 158 (1989) 265.

[10]H. Maeda, A. Koizumi, N. Bamba, E. Takayama- Muromachi, F. Izumi, H. Asano, K. Shimizu, H. Moriwaki, H. Maruyama, Y. Kuroda and H. Yamazaki, Physica C 157 (1989) 483.

[ 11 ] C. Greaves and P.R. Slater, Physica B 156&157 (1989) 888. [ 12] T.J. Kistenmacher, Phys. Rev. B 38 (1988) 8862. [13] R.S. Howland, T.H. Geballe, S.S. Laderman, A. Fischer-

Colbrie, M. Scott, J.M. Tarascon and P. Barboux, Phys. Rev. B39 (1989) 9017.

[ 14] B.D. Dunlap, J.D. Jorgensen, C. Segre, A.E. Dwight, J.L. Matykiewicz, H. Lee, W. Peng and C.W. Kimball, Physica C 158 (1989) 397.

[ 15 ] J.C. Matthewman, P. Thompson and P.J. Brown, J. Appl. Crystallogr. 15 (1982) 167.

[ 16 ] D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 ( 1981 ) 149.

[ 17] W.C. Hamilton, Acta CrystaUogr. 18 (1965) 502. [ 18 ] W.I.F. David, W.T.A. Harrison, R.M. Ibberson, M.T. Weller,

J.R. Grasmeder and P. Lanchester, Nature 328 ( 1987 ) 328. [ 19 ] I.D. Brown and D. Altermatt, Acta Crystallogr. B 41 ( 1985 )

244. [20] M. Izumi, K. Uchinokura, A. Maeda and S. Tanaka, Jpn. J.

Appl. Phys. 26 (1987) L1555. [21 ] C. Greaves and P.R. Slater, Supercond. Sci. Technol. 2

(1989) 5.