An overall comparison between the principal-axis orientations of the -tensors and the second-order zero-field-splitting (ZFS) tensors for four and four centres reported on in our previous studies has been made. One direction of the principal axes of the -tensor is nearly the same as one of those of the second-order ZFS tensor for centres, but a similar direction does not exist for centres. It is also found that one direction of the principal axes of the second-order ZFS tensors for centres is similar to one of those for the centres. It is suggested that ions substituted for in tetrahedra play the role of positive monovalent charge compensators for both and , replaced at the sites Ti(1) and Ti(2). Possible charge-compensation models related to ions for providing an explanation of the origins of the two different centres arising from the and at crystallographically equivalent Ti sites are also suggested.