Doping YBCO with calcium can improve its current-carrying capability across grain boundaries by as much as 35 percent – but at a sacrifice. Doping lowers its 91K Tc (critical temperature) to a level uncomfortably close to that of the 77K liquid nitrogen coolant.
To date there have been just two variants of YBCO discovered that will accommodate calcium without Tc loss: Y2CaBa4Cu7O16+ (Tc ~96K) and Y3CaBa4Cu8O18+ (Tc ~99K). Superconductors.ORG today reports a third calcium-doped YBCO formulation has been found that produces a Tc considerably higher than even those two compounds. YCaBa3Cu5O11+ displays a resistive Tc near 107K.
In addition to a resistive Tc near 107K, the R-T plot at page top also shows a rogue phase near 135K. This was likely produced by a 3423C structure that formed as a minority phase. This conclusion was reached by examining the results achieved with a 177K YBCO variant in March 2009. The 3423C has one more barium atom and one more copper atom in the upper part of the structure, further increasing planar weight disparity.