Momentum-dependent spin polarization exists in ferroelectric materials with strong spin-orbit coupling (SOC), which can be controlled by ferroelectric switching, promising for nonvolatile applications in spintronics. In the present work, we investigate the SOC-induced spin texture in orthorhombic $\mathrm{B}{\mathrm{i}}_6{\mathrm{O}}_9$, a recently discovered ferroelectric material with robust polarization down to the atomic limit. Utilizing first-principles calculations, we reveal the SOC-induced giant band splitting around the Γ point and notable spin texture, which can be well described by a k·p Hamiltonian with sizable Rashba parameters of ${\lambda }_R=0.65\mathrm{eV}\AA$ and Dresselhaus parameters ${\lambda }_D=-0.23\mathrm{eV}\AA$ at the conduction-band minimum. We also find that the SOC parameters can be effectively modulated by strain, resulting from band splitting and band anticrossing collectively, supported by substrate and strain-dependent calculations. The highly tunable spin texture indicates the opportunities for using $\mathrm{B}{\mathrm{i}}_6{\mathrm{O}}_9$ for the nonvolatile charge-spin conversion useful in spintronics.

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• Dresselhaus coupling
• Rashba coupling
• Spintronics
• Ferroelectrics