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.

ABO₃钙钛矿结构（ABO₃ perovskite structure）的A位或B位阳离子有序可形成AA’B₂O₆或A₂BB’O₆型双钙钛矿（double perovskites, DPvs）；若两个位点均发生1:1阳离子有序，则可得到AA’BB’O₆型双双钙钛矿（double double perovskites, DDPvs）。在我们针对锰基双钙钛矿与双双钙钛矿开展的高压高温（high-P,T, HPHT）合成研究中，我们发现了一项反常实验结果：CaMnMnTeO₆仅生成了(Ca₀.₅Mn₀.₅)₂MnTeO₆结构的双钙钛矿，而CaMnMnReO₆（其中Re的离子半径略小于Te）与CaMnMnWO₆（其中W的离子半径略大于Te）均生成了双双钙钛矿。我们提出假设：在高压高温合成条件下会大量生成Te⁴+——由于孤对电子效应，Te⁴+的离子半径异常偏大，因此更倾向于稳定双钙钛矿结构。我们请求在ID06-LVP束线对CaMnMnReO₆以及可同时生成双双钙钛矿与双钙钛矿多晶型的类似物CaMnCoReO₆开展原位X射线衍射（in situ XRD）实验，以在高压高温合成条件下观测其结构，验证上述假设，并探究合成过程中是否会形成双双钙钛矿或中间结构。