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 as an intermediate 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 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.