What is your synthesis method? Can you control the heat during the process? Or do you have catalyzer? Such parameters affect the grain size distribution. One of them can have smaller surface area where as another one should have larger. 

You should also compare d-spacings obtained from XRD results. Attached article have a similar sample set in which you mention.

Regards.

Dear Sir,

we have a different phase - grain size diagramm. This is a problem of cristal phase at the dependence of the T, t, x and it is not proportional. This is in the dependence of this values decrease and increase .

All the best

F. Gräbner