The ε ε ε(E) were calculated within the independent-particle approximation as a sum over transitions between occupied and virtual electronic states, with the number of bands in the calculations increased by 2× and 3× for the Pnma and R3m phases, respectively, to ensure convergence. The static dielectric constants, elastic constants, and piezoelectric moduli were obtained using the finite-differences routines in VASP with PBEsol+D3. The POP frequency was obtained from the Γ-point phonon frequencies and the Z Z Z * determined using PBEsol+D3 and DFPT. We note that PIE scattering is only relevant for the R3m systems, since the Pnma structure is centrosymmetric and the piezoelectric moduli vanish by symmetry.