Bulk-Rashba Effect with Suppressed Spin Relaxation in a Polar Phase of Bi_{1- x}In_{1+ x}O₃

The Rashba effect enables control over the spin degree of freedom, particularly in polar materials where the polar symmetry couples to Rashba-type spin splitting. The exploration of this effect, however, has been hindered by the scarcity of polar materials exhibiting the bulk-Rashba effect and rapid spin-relaxation effects dictated by the D'yakonov-Perel mechanism. Here, a polar LiNbO₃-type R3c phase of Bi_{1- x}In_{1+ x}O₃ with x ≈0.15-0.24 is stabilized via epitaxial growth, which exhibits a bulk-Rashba effect with suppressed spin relaxation as a result of its unidirectional spin texture. As compared to the previously observed non-polar Pnma phase, this polar phase exhibits higher conductivity, reduced bandgap, and enhanced dielectric and piezoelectric responses. Combining first-principles calculations and multimodal magnetotransport measurements, which reveal weak (anti)localization, anisotropic magnetoresistance, planar-Hall effect, and nonreciprocal charge transport, a bulk-Rashba effect without rapid spin relaxation is demonstrated. These findings offer insights into spin-orbit coupling physics within polar oxides and suggest potential spintronic applications.