Substitution of Ti3+ and Ti4+ in hibonite (CaAl12O19)
Substitution of Ti3+ and Ti4+ in hibonite (CaAl12O19)
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黑铝矿 (CaAl12O19) 中 Ti3 和 Ti4 的替代品
DOI：
10.2138/am.2014.4532
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发表时间：
2014
影响因子：
3.1
通讯作者：
K. Knight
中科院分区：
文献类型：
--
作者：
P. M. Doyle;P. Schofield;A. Berry;A. Walker;K. Knight
摘要 通过中子粉衍射数据的 Rietveld 精修确定了八种合成黑铝石样品的结构，这些样品具有不同的 Ti 氧化态和 Ti 浓度（2.4-15.9 wt% TiO2），跨越了陨石中天然黑铝石报道的范围。发现 Ti3+ 只占据八面体共享面 M4 位点，无论 Ti4+ 是否存在。 Ti4+ 位于三角双锥体 M2 位点和 M4 位点之间。所有样品的比率（M2 上的 Ti4+）：（M4 上的 Ti4+）似乎都是恒定的，无论 Ti3+ 和 Ti4+ 的浓度如何，平均值为 0.18(2)。根据密度泛函理论的计算，这些取代位点被证明是黑铝矿中 Ti 最稳定的构型，尽管预测的 Ti4+ 对 M4 的偏好并不像观察到的那么强。这归因于实验和计算结构的不同 Ti 含量，并表明 Ti 位点占用可能在这些浓度之间发生变化。此外，结果表明，Ti 优先占据邻近的 M4 位点，使得 Ti3+-Ti3+ 的稳定能为 83 kJ/mol，Ti4+-Ti4+ 的稳定能至少为 15 kJ/mol，从而发生 Ti-Ti 相互作用。用于推断 Ti3+/Ti4+ 的陨石和合成黑铝石的光谱特征和电子自旋共振数据实际上源自这些 Ti-Ti 相互作用。如果独立确定 STi，则黑铝矿中 Ti4+ 的量可以根据晶胞参数确定。黑碳沸石中的Ti3+/Ti4+可能记录了早期太阳星云的氧逸度（fO2），然而，在用Ti价态和fO2解释光谱数据时，需要考虑Ti3+-Ti3+和Ti4+-Ti4+相互作用的存在以及Ti4+-Ti3+相互作用的可能性。 Hibonite 作为单矿物氧压计必须谨慎使用，因为晶体化学（包括 Ti-Ti 相互作用）具有独立于 fO2 稳定 Ti 氧化态的潜在作用。
Abstract The structures of eight synthetic samples of hibonite, with variable Ti oxidation state and Ti concentration (2.4-15.9 wt% TiO2) that span the range reported for natural hibonite found in meteorites, were determined by Rietveld refinements of neutron powder diffraction data. Ti3+ was found to exclusively occupy the octahedral face-sharing M4 site irrespective of the presence or absence of Ti4+. Ti4+ partitions between the trigonal bipyramidal M2 site and the M4 site. The ratio (Ti4+ on M2):(Ti4+ on M4) appears to be constant for all the samples, with an average of 0.18(2) irrespective of the concentrations of Ti3+ and Ti4+. These substitutional sites were shown to be the most stable configurations for Ti in hibonite from calculations using density functional theory, although the predicted preference of Ti4+ for M4 over M2 is not as strong as is observed. This is attributed to the different Ti contents of the experimental and calculated structures and suggests that the Ti site occupancies might change between these concentrations. Furthermore, it is shown that Ti has a preference to occupy neighboring M4 sites such that Ti-Ti interactions occur with stabilization energies of 83 kJ/mol for Ti3+-Ti3+ and at least 15 kJ/mol for Ti4+-Ti4+. Features in optical spectroscopy and electron spin resonance data from meteoritic and synthetic hibonites that have been used to infer Ti3+/Ti4+ are shown to actually derive from these Ti-Ti interactions. The amount of Ti4+ in hibonite can be determined from the unit-cell parameters if STi is determined independently. Ti3+/Ti4+ in hibonite may record the oxygen fugacity (fO2) of the early solar nebula, however, the existence of Ti3+-Ti3+ and Ti4+-Ti4+ interactions and the potential for Ti4+-Ti3+ interactions need to be considered when interpreting spectroscopic data in terms of Ti valence state and fO2. Hibonite as a single-mineral oxybarometer must be used with caution due to the potential role of crystal chemistry (including Ti-Ti interactions) to stabilize Ti oxidation states independently of fO2.
影响因子：
3.9
作者：
A. Berry;A. Walker;J. Hermann;H. O’Neill;G. Foran;J. Gale
通讯作者：
A. Berry;A. Walker;J. Hermann;H. O’Neill;G. Foran;J. Gale
影响因子：
64.8
作者：
Berry, Andrew J.;Danyushevsky, Leonid V.;Sutton, Stephen R.
通讯作者：
Sutton, Stephen R.
影响因子：
5
作者：
N. Tailby;A. Walker;A. Berry;J. Hermann;K. Evans;J. Mavrogenes;H. O’Neill;I. S. Rodina;A. Soldatov;D. Rubatto;S. Sutton
通讯作者：
N. Tailby;A. Walker;A. Berry;J. Hermann;K. Evans;J. Mavrogenes;H. O’Neill;I. S. Rodina;A. Soldatov;D. Rubatto;S. Sutton