Light-induced changes in molecular arsenic sulfides: State of the art and new evidence by single-crystal X-ray diffraction
Light-induced changes in molecular arsenic sulfides: State of the art and new evidence by single-crystal X-ray diffraction
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DOI：
10.2138/am.2006.2165
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发表时间：
2006-08-01
影响因子：
3.1
通讯作者：
Menchetti, Silvio
中科院分区：
文献类型：
--
作者：
Bonazzi, Paola;Bindi, Luca;Menchetti, Silvio
通过测定晶胞尺寸，逐步监测了β-As4S4-As8S9系列单晶和合成β-As4S3单晶的光诱导结构变化。除化学计量比钙长石(As8S9)外，所有β-As4S4-As8S9系列晶体的晶胞体积均随曝光时间的延长而显著增大。合成的β-As4S3晶体在长时间的光照下没有明显的变化。在光诱导过程的选定步骤中，对不同组成的晶体进行了晶体结构的细化。结构结果清楚地表明，当晶体暴露在光下时，As4S5分子在结构中的百分比增加。因此，光照引起的晶胞体积的增加似乎与结构中As4S5随机取代As4S4有关，反应是5As(4)S(4)+3O(2)->；4As(4)S(5)+2As(2)O(3)。本研究的结果结合对先前发表的数据的批判性综述表明，As4S4分子能够在光照下结合硫转化为As4S5，而As4S3或As4S5分子不会发生任何修饰。硫的掺入程度似乎受到分子堆积类型和分子类型的严格控制。最后，只有纯的β-As4S4才观察到最终完全转化为仲藻酸，而最初含有可变数量的β-As4S4微区的非化学计量比的As8S9-x晶体在光照下只部分转化为仲藻酸。
Light-induced structural changes in single crystals belonging to the beta-As4S4-As8S9 series and in a crystal of synthetic beta-As4S3 were monitored step by step by determining the unit-cell dimensions. A marked increase of unit-cell volume as a function of exposure time was observed for all the crystals belonging to the beta-As4S4-As8S9 series except for stoichiometric alacranite (As8S9). No significant change upon long exposures to light was observed for the synthetic beta-As4S3 crystal. Crystal structure refinements were carried out for crystals with different composition at selected steps of the light-induced process. The structural results clearly showed that the percentage of the As4S5 molecule in the structure increases when a crystal is exposed to light. Therefore, the increment of the unit-cell volume induced by light exposure appears to be related to a random replacement of As4S5 for As4S4 in the structure according to the reaction 5As(4)S(4) + 3O(2) -> 4As(4)S(5) + 2As(2)O(3). The results obtained in the present study combined with a critical review of data previously published indicate that the As4S4 molecule is able to incorporate sulfur to convert to As4S5 upon exposure to light, whereas either As4S3 or As4S5 molecules do not go undergo any modification. It appears that the extent of sulfur incorporation is strictly controlled by the type of molecular packing as well as by the kind of molecule. A final, complete conversion to pararealgar was observed only for pure beta-As4S4, whereas non-stoichiometric As8S9-x crystals initially containing variable amounts of beta-As4S4 microdomains convert only partially to pararealgar upon light exposure.