Closer examination of the isotropic peaks (i.e., the central transition, or CT, centreband) corresponding to the O V environments in the O spectra of the V-substituted samples shows that, for Bi 0.913 V 0.087 O 1.587 , the CT centreband can be fit to a single Voigt function, corresponding to a single type of O V environment. On the other hand, the fit to the CT centreband for Bi 0.852 V 0.148 O 1.648 requires two such functions with relative intensity varying with temperature (Figure ), indicating the presence of two distinct types of O V environments in the more highly V-substituted material.