  The extremely large magnetoresistance (XMR) material LaBi was reported to
become superconducting under pressure accompanying with suppressed
magnetoresistance. However, the underlying mechanism is unclear. By using
first-principles electronic structure calculations in combination with a
semiclassical model, we have studied the electron-phonon coupling and
magnetoresistance of LaBi in the pressure range from 0 to 18 GPa. Our
calculations show that LaBi undergoes a structural phase transition from a
face-centered cubic lattice to a primitive tetragonal lattice at $\sim$7 GPa,
verifying previous experimental results. Meanwhile, LaBi remains topologically
nontrivial across the structural transition. Under all pressures that we have
studied, the phonon-mediated mechanism based on the weak electron-phonon
coupling cannot account for the observed superconductivity in LaBi, and the
calculated magnetoresistance for LaBi does not show a suppression. The distinct
difference between our calculations and experimental observations suggests
either the existence of extra Bi impurities in the real LaBi compound or the
possibility of other unknown mechanism.
