High-pressure spectroscopic study of siderite (FeCO3) with a focus on spin crossover
High-pressure spectroscopic study of siderite (FeCO3) with a focus on spin crossover
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DOI：
10.2138/am-2015-5319
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发表时间：
2015-11-01
影响因子：
3.1
通讯作者：
Dubrovinsky, Leonid
中科院分区：
文献类型：
--
作者：
Cerantola, Valerio;McCammon, Catherine;Dubrovinsky, Leonid
含铁碳酸盐已被认为可能是地球内部碳的候选寄主矿物，因此它们的光谱性质可以限制深层的碳循环。在这里，我们使用穆斯堡尔谱、拉曼光谱和X射线吸收近边结构光谱相结合的方法研究了合成FeCO3(菱铁矿)中的高压自旋交叉。这些对短程原子环境敏感的技术表明，在室温和准静水条件下，菱铁矿中的自旋跨越发生在一个很宽的压力范围内，在40到47 Gpa之间，而不是以前的X射线衍射数据，它描述为大约43 GPa时的急剧体积崩塌。基于这些观察，我们认为菱铁矿中的电子自旋配对是一个动态过程，其中Fe原子在交叉区可以是高自旋也可以是低自旋。从自旋交叉前后的拉曼光谱中提取的模格吕内森参数表明，在相变后，Fe-O八面体的硬度发生了剧烈的变化，变得更加致密，因此更难压缩。在菱铁矿单晶和粉末样品上进行的穆斯堡尔实验证明了差应力对菱铁矿铁原子在金刚石砧腔中的局域结构的影响。粉末和单晶的四极分裂值的差异表明，粉末样品中Fe位的局部扭曲导致自旋越界在较高的压力下开始，并拓宽了自旋越界压力范围。
Fe-bearing carbonates have been proposed as possible candidate host minerals for carbon inside the Earth's interior and hence their spectroscopic properties can provide constraints on the deep carbon cycle. Here we investigate high-pressure spin crossover in synthetic FeCO3 (siderite) using a combination of Mossbauer, Raman, and X-ray absorption near edge structure spectroscopy in diamond-anvil cells. These techniques sensitive to the short-range atomic environment show that at room temperature and under quasi-hydrostatic conditions, spin crossover in siderite takes place over a broad pressure range, between 40 and 47 GPa, in contrast to previous X-ray diffraction data that described the transition as a sharp volume collapse at approximately 43 GPa. Based on these observations we consider electron spin pairing in siderite to be a dynamic process, where Fe atoms can be either high spin or low spin in the crossover region. Mode Gruneisen parameters extracted from Raman spectra collected at pressures below and above spin crossover show a drastic change in stiffness of the Fe-O octahedra after the transition, where they become more compact and hence less compressible. Mossbauer experiments performed on siderite single crystals as well as powder samples demonstrate the effect of differential stress on the local structure of siderite Fe atoms in a diamond-anvil cell. Differences in quadrupole splitting values between powder and single crystals show that local distortions of the Fe site in powder samples cause spin crossover to start at higher pressure and broaden the spin crossover pressure range.