Recent predictions and experimental observations of high Tc superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using pulsed laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization, including four-probe electrical transport measurements, of lanthanum superhydride samples that display a significant drop in resistivity on cooling beginning around 260 K and pressures of 190 GPa. Additional measurements on two additional samples synthesized the same way show resistance drops beginning as high as 280 K at these pressures. The drop in resistance at these high temperatures is not observed in control experiments on pure La as well as in partially transformed samples at these pressures, and x-ray diffraction as a function of temperature on the superhydride reveal no structural changes on cooling. We suggest that the resistance drop is a signature of the predicted superconductivity in LaH10 near room temperature, in good agreement with density functional structure search and BCS theory calculations. *

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. X-ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H-H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.

Recent computational studies have predicted that rare-earth superhydrides are promising high-temperature superconductors. Of these phases having very high hydrogen content (XH n , n >6, where X is the rare-earth atom) a cubic phase of lanthanum hydride, been recently synthesized at 170 GPa and identified as LaH 10±x in good agreement with theoretical predictions. The experiments found that the stability of the phase extended to lower pressure and a distorted form was found on decompression. Here we examine the nuclear quantum and anharmonic dynamics on LaH 10 , including the behavior of the hydrogen sublattice in comparison with the predicted atomic metallic hydrogen at higher pressure. We also examine the vibrational dynamics and electronic properties of the lower pressure phase of LaH 10 and find that superconducting T c decreases relative to cubic but remains relatively high (i.e., 229-245 K).