Sinclair- Superconducting Transition Metal Oxides (BiSCCO and YBCO) Flashcards
The form of the formula for BiSCCO
[AO]m[ABO3]n
For BiSCCO m=3 and n=1, 2 or 3
La2CuO4 was an example of m=1 and n=1
The three types of BiSCCO
Bi-2201: Bi2Sr2CuO6+δ. CN of Cu is 6 in CuO6 octahedra, n=1, Tc=20K.
Bi-2212: Bi2Sr2CaCu2O8+δ. CN of Cu is 5 with square pyramids (half octahedra), n=2, Tc=90K.
Bi-2223: Bi2Sr2Ca2Cu3O10+δ. CN of Cu is 4 with square planes, n=3, Tc=110K
What is true between the 3 types of BiSCCO?
Tc increases with n (number of CuO2 planes).
For each compound Tc varies with δ (excess oxygen).
Tc max for Cu2.15+.
Electrical properties are highly anisotropic, SC resides in ab-planes of CuO2 sheets.
Large, complex and anisotropic unit cells, a not b roughly 5Å, c>20Å
Resistivity vs T for BiSCCO in ab planes and c direction
For ab-planes, diagonal line from top right down towards bottom left then almost straight line down to 0 at Tc.
For c direction, line almost straight up from origin, then exponential like decay stabilising at some finite value.
Resistivity always mischief higher in c direction
When is Tc optimised for families of compounds?
When n=3 (peaks then). At higher or lower values of n Tc is lower
Types of doping in homologous series of [AO]m[ABO3]n families
n-type doping replaces an ion with a higher charge with one of a lower charge (donor doping) resulting in Cu2+/Cu+.
p-type doping replaces an ion with a lower charge with one of a higher charge (acceptor doping) resulting in Cu2+/Cu3+
Formula of YBCO and Tc
YBa2Cu3O7-δ (where δ between 0 and 1)
92K
Structure of YBCO
Orthorhombic unit cell of 3 perovskite blocks. a=3.82, b=3.88, c=11.67Å (less anisotropy than BiSCCO). Central block has Y in centre and block above and below have larger Ba ion in centre. Ba is larger so has higher CN to oxygens than Y (10 to 8). There are 2 missing O in Ba perovskite (two top/bottom edges) and 4 missing in Y perovskite (all vertical edges). The corners of blocks are Cu, edges normally have O. The O in middle block closer to Y and Ba in adjacent blocks
Planes and chains in YBCO
Consider one vertical edge of the full unit cell. The bottom left (nearest) Cu in the top perovskite block has CN of 5 to O in a square pyramid (accounting for next unit cells). The Cu just above has CN of 4 to O forming a plane of CuO4 which lies in the plane of the left hand face. This mirrors on the bottom half of the unit cell so you have the bases of two square pyramids (SC CuO2 planes) next to each other with their top/bottom corners connected to CuO4 chains
Resistivity vs T for YBCO in a, b and c directions
All follow same shape with same Tc. Diagonal lines down from top right until Tc ruched then almost straight down. b is below a but both in μΩcm range. c above but only in mΩcm range
Copper ions in YBa2Cu3O7
2Cu2+ + Cu3+ = Cu2.33+
The Cu2+ have CN 5 for square pyramid of O.
The Cu3+ have CN 4 for plane of CuO4
What happens if the O content of YBCO drops below 7?
Can go as low as 6 depending on how it’s treated. Tc drops steeply as it decreases. Can reduce it by heating it at higher temperatures in N2.
Now have 2Cu2+ + Cu+ = Cu1.67+
Cu2+ still have square pyramids but Cu+ has CN=2 (above and below) forming Cu+ dumbbells between corners of pyramids. Makes it an insulator
Crystal symmetry changes in YBCO with oxygen content
As δ increases (less oxygen) lattice parameter b decreases and a increases until they join at about δ=0.6 and the unit cell becomes tetragonal. c also steadily increases. Grain susceptibility trends also reach not as negative values at the lower temperatures as δ increases.
Synthesis of YBCO
Reaction is
1/2Y2O3 + 2BaCO3 + 3CuO -> YBa2Cu3O7-δ + 2CO2
Reaction performed at 925C to ensure homogenous reaction of cations. Depending on cooling rate from 925C, δ between 0 and 1 (maybe 0.5 using equation). Therefore need to post-anneal in O2 at 400-500C for several hours to get δ about 0 and Tc about 92K
