Superlattice ordering transitions driven by short-range structure in barium calcium carbonates
Superlattice ordering transitions driven by short-range structure in barium calcium carbonates
复制标题
碳酸钡钙中短程结构驱动的超晶格有序转变
DOI：
10.1039/d1fd00086a
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发表时间：
2022
影响因子：
3.4
通讯作者：
Joester, Derk
中科院分区：
文献类型：
--
作者：
Whittaker, Michael. L.;Pri-gal, Efrat;Schmidt, Asher;Joester, Derk
方解石 (BaxCa1−xCO3) 是方解石和白云石等菱形碳酸盐矿物的合成类似物，在阳离子和阴离子亚晶格上都是无序的。在这里，我们展示了多种奇异的超晶格结构，包括我们称为方铝石的白云石类似物，可以在高温下由方解石形成。在 150-600 °C 的温度下，二阶方解石到方解石的转化是由钡和钙对不同氧配位数的偏好驱动的，并通过局部碳酸盐重新取向来促进。在升高的压力下，所有三个维度的阳离子偏析都会产生进一步的超晶格有序，产生与方解石具有相同 Rm 对称性但大 6 倍的超晶胞。当回到环境条件时，这种高度有序的结构会松弛回到方铝矿结构。 Ba0.5Ca0.5CO3 的三种天然多晶型物（方长石、斜长石、重晶石）都不是由方解石形成的，尽管被认为受到大力青睐。相反，在通过二阶过程短暂转变为类方长石结构之后，镁铝石又通过一阶过程转变为类方长石结构。总之，这些结果表明，碳酸钡钙体系中结构之间的高温转变路径可以通过粗化来驱动，并通过短程有序的相似性来促进，在概念上类似于之前描述的低温转变。许多奇异的高温碳酸盐结构是不稳定的，但可能参与自然观察到的亚稳矿物相之间的转化途径，这表明短暂相在塑造过去和当前矿物分布方面发挥着重要作用。
Balcite (BaxCa1−xCO3) is a synthetic analog of rhombohedral carbonate minerals like calcite and dolomite that is disordered on both the cation and anion sublattices. Here, we show that multiple exotic superlattice structures, including a dolomite analog that we call balcomite, can form from balcite at elevated temperatures. The second-order balcite-to-balcomite conversion at temperatures between 150–600 °C is driven by the preference of barium and calcium for different oxygen coordination numbers and facilitated by local carbonate reorientation. At elevated pressure, further superlattice order arises from cation segregation in all three dimensions, producing a supercell with the same Rm symmetry as balcite but 6× larger. This highly ordered structure relaxes back to the balcomite structure upon returning to ambient conditions. None of the three naturally occurring polymorphs of Ba0.5Ca0.5CO3 (alstonite, paralstonite, barytocalcite) formed from balcite despite being putatively energetically favored. Instead, alstonite transformed to a balcomite-like structure via a first-order process after transiently converting to a paralstonite-like structure via a second-order process. Together, these results show that high temperature transformation pathways between structures in the barium calcium carbonate system can be driven by coarsening and are facilitated by similarity in short-range order, conceptually analogous to previously described low-temperature transformations. Many of the exotic high temperature carbonate structures are unstable, but may participate in transformation pathways between naturally observed metastable mineral phases, suggesting important roles for ephemeral phases in shaping past and current mineral distributions.