Anomalous stability of a double perovskite

Cation ordering at A or B sites of the ABO3 perovskite structure gives AA’B2O6 or A2BB’O6 double perovskites (DPvs) and 1:1 cation ordering at both sites can give AA’BB’O6 double double perovskites (DDPvs). During our high–P,T (HPHT) synthesis studies of Mn-based DPvs and DDPvs, we discovered an anomalous result – that CaMnMnTeO6 gave only a DPv (Ca0.5Mn0.5)2MnTeO6 structure whereas CaMnMnReO6 (where Re is slightly smaller than Te) and CaMnMnWO6 (where W is slightly larger than Te) both gave DDPvs. We hypothesise that significant formation of Te4+ occurs under HPHT synthesis conditions - Te4+ is unusually large due to lone pair effects and so could favour the DPv structure. In situ XRD experiments on ID06-LVP for CaMnMnReO6 and an analogue, CaMnCoReO6 which forms both DDPv and DPv polymorphs, are requested to observe the structure under HPHT synthesis conditions to test this hypothesis and to discover whether DDPv or intermediate structures are formed during synthesis.