Physica C 221 (1994) 149-156 North-Holland PHYSICA

Structure of the Li 1 + xTi2_ x O 4 superconducting system A neutron diffraction study

M. Dalton l, I. Gameson 2, A.R. Armstrong 3 and P.P. Edwards 2 University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, UK and IRC in Superconductivity, Madingley Road, Cambridge CB3 0HE, UK

Received 16 September 1993 Revised manuscript received 14 December 1993

The location of the Li + ions in the superconducting Li~+ xTi2_ xO4 solid solution (0 < x < 0.33 ) has been determined by refine- ment of time-of-flight powder neutron diffraction data. The spinel structure is maintained throughout, with the substituting Li + directly replacing the Ti ions on the spinel B-sites (16d). AC susceptibility measurements of the superconducting transitions revealed a slight reduction in Tc and a rapid reduction in the apparent volume fraction of superconductor at the metal-insulator boundary (x~ 0.15 ), whilst for x< 0.1 little difference in the superconducting properties was observed.

1. Introduction

The L i - T i - O ternary system includes a fascinating solid solution with the spinel structure: Li~ +~Ti2_xO4 ( 0 < x < 0 . 3 3 ) [1] . The x = 0 end-member of this solid solution (LiTi204) was discovered by John- ston, in 1973, to display superconduct ivi ty below about 11 K [ 2 ]. Subsequent research has shown that the replacement of Ti by Li ( increasing x ) carries the system through a superconduc tor - insu la to r tran- sit ion at x ~ 0.1-0.15 [ 3-7 ], the nature of which is still not fully understood.

The replacement of Ti by Li has always been as- sumed to be direct, i.e. the substi tut ing Li is bel ieved to occupy the spinal B-sites (16d) - see fig. 1. The powder X-ray diffract ion pat tern of Li2Ti204, pro- duced by the intercalat ion o f a tomic l i th ium into LiTi204 [8 ], showed no addi t ional lines and only small intensi ty changes. This observat ion indicates that the space group is unchanged, but it also high-

Current addresses: Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W 1X 4BS, UK.

2 School of Chemistry, University of Birmingham, Edghaston, Birmingham BI5 8TT, UK.

3 ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK.

lights the insensitivity of X R D to l i thium content and position. Accurate Li posi t ions and occupancies for this mater ia l could only be de te rmined by neutron diffraction [ 9 ]. In this subsequent study, it was found that all of the Li + ions occupy the octahedral (16c) sites; none occupy the spinel A-sites (8a) , resulting in an ordered rocksalt- type structure. This observa- t ion casts a doubt over the assumed B-site occupa- t ion of Li in Lil+xTi2_xO4 ( x > 0 ) , and an explo- rat ion of all the al ternat ive interst i t ial sites (8b, 16c, 48f) by neutron diffract ion was therefore under- taken. Knowledge of the posi t ions of the substi tut ing Li ions in this system is crucial to the unders tanding of both superconduct ivi ty and the nature of the meta l - insu la to r t ransi t ion.

2. Experimental

2.1. Preparation

Lil +xYi2_xO4 samples were prepared by solid state react ion between Li/TiO3 (Alfa, 99.9%), TiO2 (Johnston Mat they 99.9%) and Ti203 (BDH, 99.9%). The appropr ia te quanti t ies (1 ) of the start- ing mater ia ls were mixed as a paste in cyclohexane

0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0921-4534(93)E08 14-H

150 M. Dalton et al. / Structure of the Li l + x Ti :_ .,04 superconducting system

~ Z

+ i r"

J J

j J

j J

-~-.-@--- _~__~ / "

f

( ~ Oxide ions (32e)

@ A-sites (8a)

O B-sites (16d)

"4"- Octahedral interstice (16c)

• Tetrahedral interstice (8b)

• Tetrahedral interstice (48f)

Fig. 1. Half of the unit cell of the spinel structure, showing the detail of the two types of quadrant present and identifying the positions of all interstitial sites.

under an atmosphere of dry argon, pelletised, and wrapped in copper foil.

1 + x LizTiOs + 1 +3x 1 2 3 x ~ - ~ TiO2 + - - T i 2 0 3

-~Lij +xTi2_xO4 . ( l )

Compositions with x< 0.33 were fired under 5% H2/ Ar for 24 h at 850°C. The x=0.33 sample was fired at the same temperature in air, ground, repelletised and fired a second time. The resulting samples were stored under vacuum in a desiccator.

Initial preparations, carried out assuming that the starting materials were pure and stoichiometric, gave poor results, with impurity phases evident in the powder XRD patterns of the products. This prompted a study of the starting materials by powder XRD, which revealed that both TiO2 and Li2TiO3 were structurally pure, but that a substantial pro- portion ( > 5%) of the "Ti203" was actually T i s O 9.

The Li2TiO3 was analysed for Li content by atomic

absorption, and was found to have actual composi- tion LiLTsTi202.89. A study was then carried out into the effect of adjusting the assumed stoichiometry of "Ti203" (between TiO,.5 and T i O j . 6 ) o n the purity of the product spinel phase, this was optimum for the stoichiometry TiO1.56. Further preparations were therefore carried out using the adjusted stoichiom- etries, as below (2).

l + x ~ Lil.78TiO2.89 + (0.302+ t.846x)TiO2 1.1~5

+ 1.136( 1 - 3x)TiOi.56 -" L i l + . ~ T i 2 _ x O 4 . (2)

2.2. Analysis

Powder XRD was employed primarily as a means of checking the purity of the spinel phase in the re- action products and for unit cell parameter deter- mination. Patterns were collected on a Phillips PW 1050 vertical goniometer with a Cu Kct source at the University Chemical Laboratory, Cambridge. A 20 range of 15 °-96 ° was studied, with an increment size of 0.02 ° and collection time of 6 s.

The concentration of lithium in each sample was determined by atomic absorption using a Perkin El- mer 3100 analyser.

2.3. Neutron d(ffkaction

Time-of-flight powder neutron diffraction data were collected for samples with x=O, 0.1, 0.2 and 0.33 plus an additional sample of x=0.33 reduced in an atmosphere of 5% Hz/Ar at 920°C on the PO- LARIS diffractometer at the ISIS facility at the Rutherford Appleton Laboratory, Chilton, UK. Samples ( ~ 7 g ) were contained in cylindrical va- nadium cans. The powder profile refinement was performed by the Rietveld method [ i0] using the least-squares program TF14LS based on the Cam- bridge Crystallographic Subroutine Library [ 11,12 ]. Scattering lengths for Li, Ti and O were taken to be -0.203, 0.344 and 0.581 )<10 -12 cm, respectively [13].

Rietveld profile refinement was carried out on ob- served patterns usi, ng space group Fd3m, with atomic positions as listed in table 1. The closeness of the fit of the simulated spectrum to the data was described

M. Dalton et al. / Structure of the Li ~ + xTi 2_ ~O ~ superconducting system 151

Table 1 Space group and atomic positions for the Li~ +xTi2_xO4 spinel system

Space Group Fd3m Ti at 16d (0.625, 0.625, 0.625) Li( 1 ) at 8a (0, 0, 0) Li(2) at 16d (0.625, 0.625, 0.625) Oat32e (u,u, u) Other interstitial sites: 8b (0.5, 0.5, 0.5); 16c (0.125, 0.125, 0.125); 48f (0,0.5,0.25)

by the parameter )~2 _ the origin of which is: 2

Z/=(R~x~) , (3,

Rwp= lOO x / ( Z wi(Yi-Yci~ ~ wi~i2 .], (4)

1 0 0 . / ( N - P + C_'~ Rexp = v \ Z wiy 2 ,]' (5)

where R ~ is the weighted profile R-factor, Rexp is the expected R-factor, Yi is the observed intensity of the ith data point, wi is a weighting factor, w ~ 1/y~, N is the number of data points, P is the number of vari- ables, and C is the number of constants.

In the refinement, O and Ti positions were con- strained to be in sites 32e and 16d, respectively, with fractional occupancies based on nominal stoichi- ometry. All other parameters were varied, with the possibility of Li occupying 8a, 8b, 16c, 16d and 48f sites being explored. This was carried out by includ- ing two of the five possible sites in each refinement model, and studying all ten possible combinations of the two out of five, as listed in table 2.

3.4. Superconductivity

Measurements of the superconducting transitions in samples with x<0.15 were carried out using a Lakeshore 7000 AC susceptometer operating at a field of 0.1 G and a frequency of 333 Hz. Samples were cooled to 4 K and warmed through the transition at a rate of 0.3 K/min. The superconducting response signal was calibrated against a lead sphere of known mass.

3. Results and discussion

XRD showed that all Li~ +xZi2_xO4 samples were almost single phase, possessing the spinel structure; tiny quantities of an impurity phase with the dis- ordered rocksalt structure were observed in all sam- ples, and a trace amount of TiO2 (rutile) was pres- ent in Lil.aaTil.670 4. A least-squares refinement was carried out on the XRD data; the results are sum- marised in table 3. Lattice parameters (a) were found to vary linearly with composition (x): no deviation from Vegard's law was observed (fig. 2). In addi- tion, lattice parameters for x<0.33 are consistently higher than those reported previously [ 1,3,5,7 ]. The oxygen position parameter, u, was observed to have no compositional dependence, in agreement with previous results. Attempts were made to refine both oxygen and lithium occupancies. For the oxygen, no significant improvement in the fit was achieved, and errors in derived occupancies were of the same order as their departure from the ideal (04) occupancy. Lithium site refinement proved impossible due to the insensitivity of calculated diffraction peak inten- sities to large occupancy changes.

Li analysis revealed a slight deficiency ( < 7%) in all samples. I f this is assumed to be due to volatil- isation at high temperature, two different reactions are possible:

Li loss: Li l+xTi2_xO4 ~l

Lil +x_yTi2_xO4 + yLi , ( 6 )

Li20 loss: Lil +xTi2_xO4

~Li l +z_2zTi2_xO4_z + zLi2 O • (7)

These lead to the derived formulae listed in table 4. Powder neutron diffraction patterns confirmed the

high degree of purity evident from XRD. On mag- nification of the baseline region, however, small amounts of impurity phases were observed in all pat- terns. A phase with the ordered rocksalt structure is present ( a~4 .14 A); this is most noticeable in the reduced x=0.33 sample. In addition, a small num- ber of unidentified peaks were observed at d-spac-

~1 Li~÷xTi2_xO4 refers here to the stoichiometric mixture of at- oms present in the sample at elevated temperature; it does not necessarily imply that volatilisation takes place from the spi- nel phase.

152

Table 2 Models

M. Dalton et al. / Structure o f the Li l + ~7"i e_ ~04 superconducting system

for Li positions used in the refinement of neutron diflYaction data

Sites Minimisation? Comments

8a/16d yes 8a/16c no 8a/8b yes 8a/48f no 16d/16c yes 16d/8b yes 16d/48f no 16c/8b no 16c/48f no 8b/48f no

Good fit to the data, sensible occupancies obtained (see text for details )

High Z 2, negative 8b occupancy.

High Z 2, negative 16c occupancy. Very. high Z 2, negative 8b occupancy, impossibly high 16d occupancy.

Table 3 Results of refinement of powder XRD data for samples

Lil + ~Ti2_ ,04

x a ( A ) u Rou~/R¢~p

0.00 8.413(1 0.01 8.411(1 0.02 8.410(1 0.05 8.404(1 0.10 8.399(1 0.20 8.377(1 0.33 8.358(1

0.386(1) 1.71 0.388(1) 1.58 0.387(1) 1.64 0.389(1) 1.40 0.388(1) 1.33 0.389(1) 1.77 0.386(1) 1.76

E r~

8.42

8,41

8.4

8.39

8.38

N. 3 7

8.36

8.35

-0.05

• • 0 - Johnston . ~ - - ~ - I)escham res

0 0.05 O.I 0.15 0.2 0.25 0.3 0.~,5

x m 1.i I ~ l i 2 ~()4

Fig. 2. Unit cell parameter (a) against composition (x) in Li 1 + ~Ti2 _ xO4.

ings o f 2.35, 2.23 and 2.14 A. Re f ined va lues o f a for

the spinel phase ( table 5) show excel lent ag reemen t

wi th those ob t a ined f r o m X R D data and are p lo t ted

in fig. 2. It can also be seen that r educ t ion o f the

.v= 0.33 sample causes a substant ia l expans ion o f the

lat t ice f rom a = 8 . 3 5 6 8 to 8 .3670/~ .

In o rder to carry out r e f inemen t o f the u param-

eter, site occupanc ies and Li d is t r ibut ion , it is pref-

erable that the fit ( Z 2) should be sens i t ive to small

ad jus tmen t s in these variables . The neu t ron diffrac-

t ion pa t te rns o f all samples are d o m i n a t e d by the in-

tense (222) peak, which wou ld be expec ted to bias

the prof i le r e f inemen t s ignif icantly, leading to po-

tent ia l ly unre l iable and inaccura te values o f the

a f o r e m e n t i o n e d pa ramete r s and occupancies . A sec-

ond p r o b l e m is that 6Li (which accounts for ~ 7% ot

the Li) is a s t rong absorber o f neut rons ; the absorb-

ing power being greater for lower energy (s lower

neu t rons ) . Fo r these two reasons, the neu t rons with

t i m e o f flight greater than 10 000 gs ( co r r e spond ing

to d-spacings greater than about 1.7 /~) were ex-

c luded f rom the re f inement .

The results o f Rie tve ld profi le re f inement f rom the

: , = 0 . 3 3 sample for all the mode ls o f Li site distri-

bu t ion are s u m m a r i s e d in table 2. Only one mode l

p roduced m i n i m i s a t i o n with sensible Li occupan-

cies, co r re spond ing to those expec ted f rom nomina l

and expe r imen ta l ly d e t e r m i n e d fo rmulae ; that where

the Li + ions are d i s t r ibu ted be tween the A ( 8 a ) and

B ( 1 6 d ) sites o f the spinel lattice. Other models e i ther

converged for imposs ib le site occupancies , or did not

converge at all. The d i f fe rence plot ob ta ined for

x = 0 . 3 3 is i l lustrated in fig. 3. Di f fe rence Four i e r

m a p s were analysed, and revea led no s ignif icant

anomal i e s be tween obse rved and s imula t ed pat terns:

there was no ev idence o f scat ter ing due to Li in any of the a l t e rna t ive inters t i t ia l sites (8b, 16c, 48 f ) .

Con t r i bu t i ons to Z 2 are f rom small a m o u n t s o f im-

M. Dalton et al. I Structure of the Li ~ +~Ti 2_~O~ superconducting system

Table 4 Nominal and experimentally determined formulae of Li1+~Ti2_~O4 samples

153

Nominal From Li analysis From Li analysis (Li loss ) (Li20 loss )

From refinement

Li~.ooTi2.000 4 Li0.92 ( 1 )Ti2.0004 Li0.92( ~ )Ti2.02 (2)O4 Lil.LoTil.9004 Li1.030 )Til.9004 Li1,04( 1 )Til.92( 1 )04 Li1.20Til.8004 Liz.13( 1 )Til.8004 LiE 14( 1 )Til.s:~( 1 )04 Lit.ssTit.6704 - - Lil.2a(l)Ti1.68(1)04

Lio.92(s)Ti2.ooO4 Li1.os(s)Ti1.9004 Li1.t4(s)Til.soO4 Li1.26(s)Til.6704

Table 5 Results of profile refinement of neutron diffraction data obtained from Lil +xTi2 _xO4 samples

Ti

Li(1)

Li(2)

0

x 0 0.1 0.2 0.33 a (A) 8.41134(1) 8.40099(2) 8.38040(2) 8.35685(2) occupancy 1.00 0.95 0.90 0.83 B (A2) 0.28(3) 0.29(3) 0.35(3) 0.53(3) occupancy 0.92(2) 0.91(2) 0.94(2) 0.96(2) B (~2) 0.60(6) 0.53(7) 0.53(7) 0.54(8) occupancy - 0.06(1) 0.10(1) 0.15(1) B (~2) _ 0.29(3) 0.35(3) 0.53(3) u 0.38760(4) 0.38765(3) 0.38762(3) 0.38763(3) occupancy 1.00 1.00 1.00 1.00 B (A2) 0.30(2) 0.33(1) 0.35(1) 0.45(2)

Z 2 10.10 4.65 5.14 7.00

I00-

80

60-

40

20

0.8 1.0 1.2 2 ,4 1.6

D-spacing (Angstrom)

Fig. 3. Observed, calculated and difference neutron diffraction patterns for Li1.aaTil..6704.

154 M. Dal ton et al. / S t r u c t u r e o f the L i l + ~ Ti2_ ~04 superconduct ing sys tem

purity (at d ~ 1.25 ~ ) , and from energy-dependent ab so rp t i on - reflected in the evolution of (Y~.,-Yi) from positive to negative as the effective wavelength of the neutron increases. Attempts to correct for ab- sorption, using the alternative refinement program TF12LS, gave similar crystallographic parameters and no significant improvement in the fit.

Refined occupancies, oxygen positions (u) and isotropic temperature factors (B) are listed for all compositions in table 5. Values of u agreed very well with those determined from X R D refinement and again showed no dependence on composition across the solid solution (fig. 4). This result seems rather surprising, since, as the amount of Li substitution (,v) increases, the ratio o f the radii of B-site to A-site cat- ions (r(B)/r(A)) decreases (the lattice parameter decreases with x, but the A-sites remain unchanged). It is well known that r(B)/r(A) has a strong influ- ence on the oxygen packing and, hence, the u pa- rameter [ 14 ]. The constant u parameter across the solid solution implies that both A - O and B-O bond lengths shorten with x, so that the size of the LiO4 A-site tetrahedron (and therefore the effective ra- dius of Li + ) is composition dependent.

Values of Li site occupancies were strongly cor- related with isotropic temperature factors, so that uncertainty values are quite large for each of these parameters. This might be expected, since both re- duced occupancies and increased atomic vibration would lead to lower (less negative) scattering from the Li sites. The occupancy values reveal that all samples appear to be Li deficient by between 5% and

O.38800

0.38775

0.38750

0.38725

O.387O0

-0.05 0 0.(}5 0.1 0.15 0.2 0.25 0.3 11.35

x m l.il+xTi2 xO4

Fig. 4. Oxygen pa ramete r (u) agains t x in Lij +xTi2_ ~O4.

8%, in excellent agreement with the experimentally determined values (see table 4). In all cases, the de- ficiency occurs predominantly in the A-sites (8a), Isotropic temperature factors show a general in- crease with Li substitution (.r in Lil+,Ti2_,O4) in all sites except 8a, for which no significant depen- dence on x is observed. Attempts to include O site occupancies in the refinement produced a margin- ally better fit, but little deviation from stoichiometry was observed - there was no evidence for oxygen de- ficiency and uncertainty values were rather higher.

It may be expected from a consideration of the minimisation of free energy under equilibrium con- ditions that the Li and Ti cations on the spinel B-sites (for x > 0) should be ordered. Such ordering would result in a lowering of the symmetry of the spacc group and /o r the presence of supercells; both would lead to additional reflections in diffraction patterns. No such additional reflections - consistent with the unit cell parameter, or a multiple t h e r e o f - were ob- served in either X-ray or neutron diffraction pat- terns. The possibility of B-site ordering was further explored by searching for superlattice reflections in a sample with composition x=0 .33 , using selected area electron diffraction (SAED). However, all pat- terns observed showed no evidence of satellite dif- fraction spots, implying the absence of any super- lattice. Hence, under the chosen conditions of preparation, the Li and Ti in the spinel products ap- pear to be randomly distributed over the B-sites.

AC susceptibility measurements revealed that su- perconductivity occurred in samples with .v_< 0.15. The transition onset temperatures ( Tcon ) are plotted in fig. 5, and it can be seen that Tco,~ remains con- stant at ~ 12.3(3) K for .v_<0.1, with a slight drop for .v =0.15 as the metal-insulator transition is approached.

The change in transition width (defined here as the temperature at 10% of the full transition minus the temperature at 90% of full transition) with com- position is illustrated in fig. 6 and shows a steep- ening increase with x, similar to the results of Ueda et al. [7].

The superconducting response signals (fig. 7 ) shou a variation with x which is rather different from the approximately linear decrease reported previously [5,7]. The values for x < 0 . 0 5 are effectively con- stant at ~2.5 m V c m -3, which corresponds to the

M. Dalton et al. /Structure o f the Li ~ + xTi e_ xO~ superconducting system 155

13

12

~ 11

10

% %

%

-0.05 0 0.05 0.1 (I.15 0.2

x in Lil+xTi 2 xO4

Fig. 5. Superconducting onset temperature, as measured by AC susceptibility, against composition in Li~ +~Ti2_xO4.

3

E 2.5

&

~. 1.5

o.5 I i 1

-0.05 0 0.2

, ! I I i I

0.05 0.1 0.15 x in Lil+xTi 2 xO4

Fig. 6. Superconducting transition width, as measured by AC susceptibility, against composition in Lil +xTi2_xO4.

4

3

m , "

o

I[ i~. - ° °

2 °° o"

°o

i i l l "

0 ~ ~ ' ~ ~ ~ [

-0.05 0 0.05 0.1 0.15 0.2

x in Lil +xTi2_xO4

Fig. 7. Superconducting response signal per unit volume, as mea- sured by AC susceptibility, against composition in Lil +xTi2_xO4.

response expected for a 100% superconductor with approximately spherical particles. For x> 0.05, a de- crease in the response signal is observed, which has previously been interpreted as a decrease in the ap- parent "volume fraction" due to a smaller quantity of superconducting phase [ 5 ]. An alternative expla- nation, which allows for a chemically homogeneous solid solution, would be an increase in the London penetration depth (2L) - t o the order of the grain diameter at x=0.15. This would be expected from the sharp increase in the normal state resistivity (,~LOt:p i/21 from a p ( T = 1 5 K ) of ~4 m ~ c m for

x = 0 - corresponding to a 2L of 2730A [5] -- to p ( 15 K) ~ 4 D cm towards the metal-insulator tran- sition at x = 0.125 [ 7 ] - corresponding to a 2L of ~ 6 g m .

4. Conclus ions

The results described in this work can be sum- marised as follows: (1) It has been shown that the substituting Li ions in the Lil+xTiz_xO4 system directly replace the Ti ions on the 16d (B-sites) of the Fd3m unit cell, maintaining the spinel structure for all compositions within the solid solution range (0 < x < 0.33 ). (2) The Li and Ti ions are randomly distributed over the B-sites. (3) Lithium deficiency occurs predominantly in the A-sites of the spinel; these 8a vacancies may facili- tate the observed Li + mobility in LiTi204 [15-17 ]. (4) The oxygen positions within the unit cell (u) are independent of composition (x), even though the apparent ratio of B to A-site ionic radii decreases with X.

(5) For the x=0.33 end-member, a substantial ex- pansion of the lattice is observed upon reduction in 5 % H 2.

(6) AC susceptibility measurements support pre- vious findings of relatively constant Tco,~t for x < 0.1, but "volume fraction" measurements indicate a non- linear decrease with x; samples with x < 0.05 are al- most "100% superconducting".

The disrupting effect of the Li + ion on the con- ducting/superconducting Ti sublattice is considera- ble, for it is this effect which is primarily responsible

156 M. Dalton et al. / Structure of the Lit + ,Ti 2_ x04 superconducting system

for the onse t o f a m e t a l - i n s u l a t o r t r ans i t ion for

x ~ 0.15 [ 5,18 ]. Chemica l l y and s t ructura l ly (wi th in

the cons t ra in ts o f neu t ron d i f f r ac t ion) , the system

appears homogeneous . Elect ronical ly , however , the

subs t i tu t ion o f l i t h ium on the t i t a n i u m subla t t ice is

p rofound; subst i tut ion leads to the p roduc t ion o f non-

conduc t ing doma ins and clusters. Lamber t et al. [ 18 ]

( a n d ear l ie r Ha r r i son et al. [5] ) have m o d e l l e d this

b e h a v i o u r in t e rms o f subla t t ice c luster evo lu t ion ,

i n v o l v i n g a m o d e l in which an ef fec t ive " r a n g e " is

g iven to the d i s rup t ion o f the Ti subla t t ice v ia Li +

subs t i tu t ion . The repuls ive po ten t ia l o f the " i m p u r -

i ty" ion was t aken such that six su r round ing Ti sites

are ef fec t ive ly b locked o f f f r o m conduc t ion . The su-

pe rconduc t ing p roper t i es o f the sys tem are then mo-

del led in t e rms o f pe rco la t ion pa ths th rough con-

d u c t i n g / s u p e r c o n d u c t i n g domains .

Fu r the r s tudies o f the s u p e r c o n d u c t i n g and nor-

mal state p roper t i e s are u n d e r way in o rde r to pro-

v ide a be t te r u n d e r s t a n d i n g o f the unusua l e lec t ron ic proper t ies o f this system.

Acknowledgement

The f inanc ia l suppor t o f the Science and Engi-

neer ing Resea rch Counc i l ( S E R C ) is grateful ly ac-

knowledged . We are also grateful to the S E R C for the

p rov i s ion o f neu t ron b e a m facil i t ies.

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