Pawley and Rietveld fitting implemented in Topas Academic software. O-enriched samples were obtained by heating as-synthesised powders to 900 °C under an atmosphere of 70% to 973 K were performed at the same field using a triple resonance 7 mm Bruker laser probe. Temperature calibration of the probe was carefully performed by measuring the 79 Br resonance of KBr over the entire temperature range, with KBr typically included in the same rotor as the sample as an internal temperature reference. 4 and PO 4 environments (O V and O P ) at ∼620 ppm and ∼130 ppm, respectively. The # symbol denotes the peak at 140 ppm assigned to a minor (∼1 wt. %) bismuth silicate impurity phase.