In this section a review o f the structural and electronic properties o f P b2 Sr2(Y i_ x C ax )C u 3 0 g + ô and [Pbi_y(Cu,Sr)y]Sr2 ( Y l-xC ax)C u2 0 y_§ phases is presented, w ith the object o f assessing the inherent difficulties in obtaining optim um superconducting transition tem perature and minim al transitional width.
3.5.1 Pb2Sr2(Yi-xCax)Cu308+0 phase
Structural Properties
A property com m on to all the copper oxide superconductors (cuprate) is the high covalency o f the Cu-O bonds and tw o-dim ensional character o f the copper-oxygen fram ew ork closely related to that o f the perovskites. Thus a large fam ily o f layered cuprates w ith the general form ula [A C uO g-xlm -[A O jn can be considered. Such a class o f oxides corresponds to the intergrowth o f m ultiple oxygen-deficient perovskite layers [A CuOg-x], w ith rock-salt-type layers A O [60]. They can be represented by the sym bol [m, n], w here m and n denote the num ber o f copper layers and [AO] layers in the
perovskite and rock-salt slab respectively. D ivalent lead, P b ^^ is sim ilar to B i ( l l 1) and 2
T1(I), in the respect that it has a 6s lone pair and is a potential cation for the generation o f layered-structures [61]. Thus it is interesting in the synthesis o f cuprates for w hich tw o dim ensional property o f the structure appears as an im portant factor for superconductivity. C ava et al. [64] utilised this idea by the synthesis o f the P b2 Sr2Lni_ xC axC ugO g+g (Ln=Y, or rare earth elem ents) superconductor. The structure o f this phase consists o f double pyram idal layers [(C u02.5 )2 ]oo interfaced w ith calcium and yttrium ions, as already observed in the bism uth and thallium oxides and in Y B a2 C u 3 0 y , associated w ith single rock-salt-type layers [(Sr, P b )i O ] o o sim ilar to
those observed in L a2C u04 -typ e oxides. The rock-salt layers are them selves
show ing a great resemblance with Y B a 2C u 306. Fi gure 3 .5. a, sh ow s the Pb2Sr2Y C u3 0 8 structure which consists o f double oxygen-deficient perovskite layers and single perovskite layers intergrown with single rock-salt-type layers [(Pb, Sr)] 0]oo . The sequence o f layers is -(Y )o-(C u02)c-(SrO )o-(P bO )c-(C u')o-(P bO )c-(SrO )o- ( C u 0 2 ) c - ( Y ) o - , where the subscript o and c indicate whether the cation is at the origin or at the center o f the mesh, respectively.
Y, Ca O Sr P b } O O O © o o _ © ( 0 (1) # ! © #
o
i
O
!
0 (2) (D : (D O Cu(1) o Cu(2) 0)
o
(a) Layering sequence of (b)1212-PSYCCO Pb-2213 phase
F i g u r e 3.5. a) The structure o f P h 2 S r 2 Y C i i s O s showing the s e q u o i c e o f layers p e r p e n d i c u l a r to the c-axis (adapted f rom [62]}. h) The e le ment ar y cell o f ( Ph ] - yC ux )S r2( y I - z C a r j C i n O y where oxygen 0 ( 4 ) is the site o f the e xcess
oxygen (adapted f r o m [63]).
All oxygen atoms are absent in the (Y) and (Cu) layers. The (CuQa) (SrO) blocks create pyramidal C11O3 layers which represent one o f the com m on features for most of the cuprate superconductors. The Sr ions are surrounded by 9 oxygen atoms sim ilar to the La-polyhcdron in LagCuOq. The Y cations are surrounded by 8 o x ygen atoms forming a prismatic polyhedron com m on to all cuprates containing the (C u 0 2 )-(L n )- (CuO g) blocks in which Ln is either a rare-earth element or Ca. The Pb cations are surrounded by 5 oxygen atoms arranged as pyramid. The Pb-O distances vary over a large range with three short distances forming a triangle and two long ones, that is due
to the 'lone pair' o f electrons o f Pb^^. The copper cations o f the oxygen-deficient Cu layer inserted betw een PbO layers exhibit the stick (linear) co-ordination that may not be linear because o f the oxygen disordering exists in the PbO layers. It is w orth pointing out that such an oxide can be obtained as a superconductor w hen it is synthesised in a reducing am bient or either at higher tem perature (920°C) w here oxygen soaking is very sm all to keep the Cu as m onovalent in PbO -Cu-PbO layer.
Table-3.2. A com parison o f the calculated and observed x-ray spectra
o f P b2Sr2(Yo^5Cao^5)C u^O g+ ^ m aterial (After P.G oodm an et al[67J).
h k l
dobs.(A ) Ifcalc.'I Ito b s.lh k l
^nhs.(^ ) Itcalc.'l Ifobs.')0 0 1 15.82 31.9 28.1 2 0 2 2.56 9.0 2.7 0 0 2 7.91 6.5 14.1 2 0 3 2.40 10.19 1.4 0 0 3 5.27 5.6 9.4 0 2 4 2.25 4.7 3.6 0 0 4 3.95 2.9 12.5 2 0 4 2.24 4.6 0.7 1 1 0 3.82 11.0 9.8 1 1 6 2.17 10.9 11.6 1 1 1 3.72 19.7 21.4 0 2 5 2.09 5.4 1.1 0 0 5 3.15 17.5 17.0 2 0 5 2.06 5.4 16.1 1 1 3 3.10 11.8 11.6 0 0 8 1.91 8.7 10.7 1 1 4 2.74 100.0 100.0 2 2 0 1.97 31.8 32.1 2 0 0 2.71 35.2 32.1 1 3 4 1.59 16.8 15.2 3 1 4 1.57 16.6 17.0 0 2 1 2.69 6.6 28.6 2 0 1 2.67 6.5 2.7 0 0 6 2.63 1.8 5.4
N eutron diffraction techniques also show that the P b2Sr2 (Y ^.^CaJ CugOg+g (Pb-2213 phase) has an orthorhom bic unit cell [64, 65], and is m ore com plex than the earlier know n m aterials due to the interm ediary PbO -CuO g-PbO layer. The com pound deviates slightly from tetragonal symmetry, (space group: Cm m m ) w ith lattice param eters
a=0.53933(2), b=0.54311(2), and c=1.57334(6)nm [62]. The origin o f the
orthorhom bic sym m etry distortion o f these m aterials is evidently caused by the disordering in the a-b plane o f the oxygen atoms o f the PbO layers over the general position (0, 0, z) o f the space group w ith x = 0.0275(5) and y = 0.0402(5)nm . This disorder is considered to be due to the lone-pair of the electrons associated w ith the
59
2+
Pb cations w hich repel the oxygen atoms in the PbO layers away from their sym m etric
position, leading to the orthorhom bic distortion. Tw o opposite basal oxygen atom s o f the pyram id, surrounding the Pb^^ m ove closer to the cation w hile the other tw o m ove further away. This arrangem ent satisfies the irregular co-ordination requirem ent o f the Pb^^(lone-pair) ion w ithin its square pyram idal oxygen environm ent [62, 66] (See for
exam ple//gM re 3.5.a). As oxygen level in the structure increasing in Pb-2213 phase to
O9 4 7, disordering increases due to oxidation o f Pb^^to Pb"^^ and also the disordered
oxygen induces changes in the co-ordination o f all cations [67]. In these layers o f the Pb-2213, the Pb is in a divalent state, and the oxygen non-stoichiom etry has a value o f Ô =0 in the superconducting state. In this phase, the electronically active double CUO2 pyram idal layers are separated by m ore than 1.5nm, m aking the m aterial highly tw o dim ensional from a structural point o f view, and further can be increased by the substitution o f C a for Y as a bigger cation. A com parison o f the experim ental and
calculated pow der x-ray diffraction pattern (XRD) is given in table 3.1.
Table 3.3. X R D data fo r a Pb2Sr2(YQ^CaQ 4 ) C u ^ 0 ç .^ phase obtained on p o w d er
ground fro m a single crystal (A fter J.S.Xue et al. [68]).
h k l
d(À)
h k l
d(Â)
h k l
d(Â)
h k l
d(Â)
0 0 2
7.906
0 0 6
2.628
3 10
1.703
2 3 5
1.355
0 0 3
5.254
0 0 7
2.254
3 1 1
1.694
4 0 0
1.345
0 0 4
3.947
0 2 4
2.233
1 1 9
1.593
4 1 3
1.268
1 1 0
3.822
1 16
2.165
0 0 10
1.576
1 1 12
1.242
1 1 1
3.716
0 2 5
2.005
1 3 4
1.572
1 3 9
1.225
1 1 2
3.440
2 0 5
2.047
3 14
1.563
2 4 0
1.211
1 1 3
3.093
0 0 8
1.971
2 2 6
1.545
4 2 0
1.205
1 1 4
2.745
2 2 0
1.910
2 2 7
1.456
4 17
1.130
0 2 0
2.710
0 0 9
1.752
2 3 3
1.444
2 0 0
2.692
0 2 7
1.732
0 0 11
1.433
0 2 1
2.672
2 2 4
1.719
2 2 8
1.371
2 0 1
2.654
1 3 0
1.713
0 2 10
1.363
D ivergence betw een these results arise from (i) a possible preferred orientation effects in the sintered pellet used and
(ii) from the assum ption m ade in the calculations o f a random distribution o f species w ithin the C a-Y plane. The X RD data for the Pb2Sr2(Y o.6Cao.4 )C u309_5 m aterial obtained from pow der (ground single crystals ) has also been included w ith the lattice
electron m icroscopy o f polycrystalline sam ples and single crystals o f the Pb-2213
phase generally show a plate-like m orphology [64]. H igh resolution electron
m icroscopy (HREM ) analysis of this com pound by means o f m ultislice im age m atch confirm ed the structure m odel of Cava et al., and also provided the c-axis value of
1.58nm [67]. The investigation was carried out using a JE O L 40000EX electron
m icroscope operated 400kV and fitted with a top-entry stage, w hich had a capability of lattice resolution o f 0.12nm and point-to-point resolution o f 0.14nm . The preparation o f sam ples was perform ed by crushing under a m ixture o f ethanol and liquid nitrogen to m ake a suspension which was dried onto holey carbon films. It has also been shown
that these com pounds are not c-centred. W eak, but sharp reflections violating c-
centring w ere observed, indicating long-range ordering o f oxygen in the PbO layers. It is also pointed out that this m aterial is the m ost stable i.e. never degrades w ith tim e, in the 400kV electron beam than other cuprate superconductors [67].
3.5.2 Atomic ordering and charge localisation
The P b 2 S r2 Y i-x C ax C u 3 0 8 + ô structure exhibits one way in w hich local charge distribution controls the electronic properties o f layered cuprate superconductors. To
show the local charge distribution, one can rew rite the form ula as
Sr2 (Y ,C a)C u2 0 6 [Pb2C u0 2 + 0]; w here the structural com ponent in square brackets acts as a charge reservoir w hich controls the charge on the superconducting Cu-O planes. W hen x=0 and 0=0 the m aterial is an antiferrom agnetic sem iconductor in w hich Pb is in a divalent state, the copper between the PbO layers is m onovalent and in-plane
copper atom s are divalent. The PbO -C uO g-PbO sandw ich can accom m odate a
significant charge density (5 electronic charges p er form ula unit) by the oxidation o f
the C u ^ ^ to C u ^ ^ and P b ^ ^ to P b ^ ^ [68]. C hem ical analysis o f the Pb-2213 phase by the selective dissolution m ethod and the inductively coupled plasm a technique as well as by N M R have confirm ed that the average valence o f all the copper atoms increases up to 4-2.0 w ith increasing Ô for 0.5 < 8 < 1 .8 [69, 70].
On the basis o f the chem ical analysis and H all effect m easurem ents [68], it has also been show n that holes introduced by extra oxygen first enter the CuO § layer and then the PbO layer only after the average valence o f the copper changes to 4-2. This charge rem ains locked and never gets transferred to the Cu0 2 layers w hilst the divalent
61