Composition and effect on electronic properties 111 5.3 Oxygen and effect on electronic properties

5.4. Average structural disorder 117

5.41. Rocksalt layer

5.42. Phase composition and oxygen content 5.43. Resonant X-ray diffraction studies

5.5. Local structure/structural disorder 134

5.51. In situ EXAFS/XRD study of 1212 phase 5.52. Solid state NMR studies on 1212 phase

5.6. Summary 157

5.1. Structural overview 5.11. Idealised structure of 1212

First discovered by Subramanian et al*^^ and Lee et al*^^, the so-called 1212 phase has shown some particularly exciting chemistry, especially from addition of Tl. The structure itself is quite interesting, but the simplest way to describe it is to start from the well known 123 compound YBa2Cu3 0 ?^, figures 5.1 and 5.2.

Cu-O "chains" Ba-O

Cu-O "planes" Y

%

_r

#

4

f

4

ilk

Figure 5.2. Structure of YBuzCusO?

Each Cu atom on the CuO: plane is co-ordinated to five oxygen atoms forming a [CuOs] pyramid. The CuO: plane is sandwiched between Y and BaO planes. In its fully oxygenated form, the oxygen in the charge reservoir layer preferentially occupies the (0,14,0) position and the structure is determined to be orthorhombic (space group Pmmm) with a » 3.82Â, b » 3.88Â and c % 11.68Â. It is possible to remove the oxygen fully from the charge reservoir layer to leave YBa:Cu3 0 6, which has a

f l

4

4

%

%

Figure 5.3. Structure of YBa2Cu306.

Replacing Ba^^ (1.47Â at CN=10) with (1.32Â at CN=10) does not lead to the formation of YSr:Cu3 0 7 This phase is not stable and may only be stabilised under

high pressures*^^ or by substituting elements such as for Cu^^l Replacing Cu with some Pb on the charge reservoir layer does stabilise the structure; however, this layer changes structurally from perovskite type to a "rocksalt" configuration. The difference between the perovskite and rocksalt monolayers lies in the position of the oxygen. In YBa:Cu3 0 7 this is (0,V4,0) and in (Pb,Cu)Sr2YCu: 0 7 this is (16,14,0). This latter

%»

% f

% ib

%»

Figure 5.4. Idealised 1212 structure, atoms labelled.

The Stacking sequence 1212 is essentially the same as that in YBazCugO?, but the structure belongs to a tetragonal system with space group P4/mmm. YBazCuiO? could also be described as a 1212 structure and if so is written not only as 1212 but also additionally as 1212P to show that it has a perovskite type charge reservoir layer.

The ideal structure is that shown in figure 5.4, with some important parameters summarised in table 5-1.

Atom Site X _y z Pb/Cu(l) la 0 0 0 Sr 2h 0.5 0.5 0.21 Y/Ca Id 0.5 0.5 0.5 Cu(2) 2g 0 0 0.36 0(1) 4i 0 0.5 0.375 0(2) 2g 0 0 0.165 0(3) Ic 0.5 0.5 0

Table 5-1. Ideal atomic positions for a (Pb,Cu)Sr2(Y,Ca)Qi2 0 7.

5.12. Rocksalt layer and cation disorder within structure

Studying the rocksalt layer more closely, figure 5.5, and assuming the standard cell edge of 3.82Â (AB and BC on diagram), then the length AC is 5.4Â. This leaves abnormally long bond lengths (2.7Â) between the Pb, O and/or Cu, thus the rocksalt oxygen is displaced from its (14,16,0) to a fourfold (x,14,0) site. Also, figure 5.6, there is evidence that the Pb and Cu atoms are displaced from their ideal (0,0,0) site to a (x,0,0) site, figure 5.6. In (Pb,Cu)Sr:(Y,Ca)Cu20y it is possible to introduce a small excess of oxygen into the rocksalt layer. Maeda et al^^^ showed this to be the 2f (0,16,0) site and estimated its amount to be approximately 0.1 oxygen's per formula unit. This excess proves to be detrimental to superconductivity.

3.82 Â

B

3.82 Â

B

Figures 5.5. AB fffojection of rocksalt layer structure.

Pb/Cu(1) (x,0,0) 0(3 (x,%,0)

0(4)

Of course, it is possible to have a rocksalt configuration in which the Cu has been replaced by a number of different atoms, this was discussed in section 2.12. These elements are presented below in table 5-2.

Element Charge VI fold (Â) vn ifoid (Â ) X fold (Â)

Fe 3+ 0.645 HS Ni 2+ 0.70 Mg 2+ 0.720 Cu 2+ 0.73 - Bi 5+ 0.74 Zn 2+ 0.745 Fe 2+ 0.77 HS Pb 4+ 0.775 0.94 Tl 3+ 0.88 Cd 2+ 0.95 Ca 2+ 1.00 1.12 1.28 Bi 3+ 1.02 Hg 2+ 1.02 Sr 2+ 1.16 1.25 1.32 Pb 2+ 1.18 - Y 3+ N/A 1.015

Table 5-2. Effective ionic radii for a series of cations in different co-ordination environments. Data from

Slîamion‘^^1

With the exception of Fe and Ni that tend to occupy the Cu-O planes, it is possible to induce superconductivity with the elements above in the rocksalt layer in addition to Pb. The best onset temperatures have been found using elements which have an ionic radius larger than that of Pb^^ and it has never been clear whether or not Pb^^ is favoured in these cases. This research is not dealing with the size of rocksalt layer cation and effects on superconductivity and shall not be discussed further.

The (Y,Ca) site is an eight fold site, and column four in table 5-2 shows the ionic radius of some elements which can adopt this co-ordination. In a PbCu:1212 sample, there would appear to be the possibility for Sr and Pb to substitute onto this site. On the Sr ten-fold co-ordination site then the only possibility for substitution is Ca. For a sample of nominal composition (Pb,Cu)Sr:(Y,Ca)Cu20y, then the possible cation disorder may be as follows table 5-3.

Layer/Site Disorder

Rocksalt layer Ca, Sr and possibly Y

(Y,Ca) site Pb and possibly Sr

SrO layer Ca

CuOz planes None of these other atoms prone to taking the pyramidal co-ordination that Cu does

Table 5-3. Possible cation disorder in (Pb,Cu)Sr2(Y,Ca)Cu2 0y.

What the above tries to show is the numerous possibilities that might exist for cation disorder in such a complex system. Whether or not this disorder enhances or degrades superconducting properties cannot be exactly determined; however samples containing several different elements would realistically be expected to have varying degrees of cation disorder. Disorder in (Pbfi+x]/2Cu[i-x]/2)Sr2(Yi.xCax)Cu2 0 7 is considered in

sections 5.4 and 5.5.