The two types of O V environments display different temperature-dependent behaviours and thus provide insights into the underlying oxide-ion motional mechanisms that distinguish the Vsubstituted samples. Both V-substituted compounds share a common O V1 environment centred at ∼620 ppm at room temperature, which narrows as a function of increasing temperature, reaching a minimum CT linewidth between 473 and 523 K (Figure ). This environment therefore undergoes thermally-activated ionic motion with a motional rate eventually exceeding the range to which the CT linewidth is sensitive, with no further narrowing above 523 K until coalescence occurs (see SI). By contrast, the linewidth of the second, higher-frequency O V2 peak (at ∼630 ppm at room temperature), which is only seen for Bi 0.852 V 0.148 O and Figure ). Both features undergo linewidth narrowing with increasing temperature (see Figure and the SI) and move to higher chemical shifts. As described later (see Discussion), the observed resonances correspond to subtly different local O P environments that persist at high temperatures, a phenomenon which may also occur for the O V environments in Bi 0.852 V 0.148 O 1.648 but cannot be directly observed because of coalescence with the O Bi feature.