Furthermore, comparing the µ as a function of n indicates the mobilities are also an order of magnitude lower in the π phases (Figs S9/S10 † ). The µ of both the Pnma and π phases is limited by polar-optic phonon (POP) scattering (Figs S9-S12 † ), for which the scattering rates are proportional to the POP frequency ω po and inversely proportional to the high-frequency and static dielectric constants ε ∞ /ε S . The π phases have larger ω po and smaller ε, which both favour stronger scattering. The lower σ of the π phases can therefore be explained by larger carrier masses and stronger (POP) carrier scattering.