  The search for room temperature superconductivity has accelerated
dramatically in the last few years driven largely by theoretical predictions
that first indicated alloying dense hydrogen with other elements could produce
conventional phonon-mediated superconductivity at very high temperatures and at
accessible pressures, and more recently, with the success of structure search
methods that have identified specific candidates and pressure-temperature (P-T)
conditions for synthesis. These theoretical advances have prompted improvements
in experimental techniques to test these predictions. As a result, experimental
studies of simple binary hydrides under pressure have yielded high critical
superconducting transition temperatures (Tc), of 260 K in LaH10, close to the
commonly accepted threshold for room temperature, 293 K, at pressures near 180
GPa. We successfully synthesized a metallic La-based superhydride from La metal
and ammonia borane, NH3BH3, and find a multi-step transition with a Tc of 294 K
for the highest onset. When subjected to subsequent thermal excursions to
higher temperatures that promoted a chemical reaction to what we believe is a
ternary or higher order system, the transition temperature was driven to higher
temperatures. Although the reaction does not appear to be complete, the onset
temperature was pushed from 294 K to 556 K before the experiments had to be
terminated. The results provide evidence for hot superconductivity well above
room temperature, in line with recent predictions for a higher order hydride
under pressure.
