Record Superconductivity Near 157K in a High-VF Copper-Oxide

10 March 2025
Superconductors.ORG

        In the early 1990's ambient pressure superconductivity topped out at 138 Kelvin in a thallium-doped mercuric-cuprate. Though that superconductor's volume fraction (VF) was only around 15-30% of the bulk, it was deemed a world record. Then in 2016 electron-doping of a thallium-tellurium copper-oxide pushed the Tc record up to 147 K — again with a modest volume fraction (see graphic below right). Now, in 2025, combining planar weight disparity with a high dielectric constant has established a new milestone near 157 Kelvin with a much higher volume fraction. At page top the compound SnSbBa2(Ca0.5Tm0.5)Cu1.5Mg1.5O8+ displays a Meissner transition of almost 140 milliGauss, which equates to a volume fraction of 70-80% of the bulk.


       This new discovery by Superconductors.ORG applies a 50-50 mix of copper and magnesium to a material that recently confirmed a strong correlation between dielectric constant and transition temperature (Tc). Since Cu and Mg have previously been found to be doppelgangers in some high-temperature superconductors, this 50-50 modification was thought to be worth attempting. The resulting 157K Tc confirmed this thinking.


        


       The above left graphic shows the 3212 structure that likely resulted from this formulation. It contrasts sharply with the 2212 structure (above right) that hosted the previous record-holder. However, all of the copper-oxide structures that have seen Tc improvements since 2005 have had one thing in common: greater planar weight disparity (PWD). The atomic weight ratios of atoms on opposite sides of oxygen sites had all increased along the c-axis. In this latest discovery there is not only PWD between Tm and Ca, but now also between the Cu and Mg atoms.

       Stoichiometric amounts of the below chemicals were used in the synthesis of this compound:

BaCO3   99.95%   (Alfa Aesar)   (decomposes to BaO during calcination)
Tm2O3   99.99%   (Stanford Materials)
CaCO3   99.95%   (Alfa Aesar)   (decomposes to CaO during calcination)
SnO   99.9%   (Alfa Aesar)
Sb2O3   99.999%   (Alfa Aesar)
CuO   99.995%   (Alfa Aesar)
MgO   99.95%   (Alfa Aesar)

       The chemical precursors were mixed thoroughly and pelletized. Since antimony-oxide has a low melting point, the mix was pre-sintered for 11 hours at 640C. Then the pellet was calcined at 770C for 2 hours and sintered overnight at 880C. Lastly, it was annealed for 9+ hours at 500C in flowing O2. Testing temperatures were determined using an Omega type "T" thermocouple. The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/Gauss.


RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.

E. Joe Eck
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