中子衍射和EPSR模拟研究硅酸钠玻璃中铁位的性质和分布
Nature and distribution of iron sites in a sodium silicate glass investigated by neutron diffraction and EPSR simulation
摘要：
采用高分辨中子衍射和经验势结构精细化（EPSR）方法研究了NaFeSi 2 O 6（NFS）玻璃的短程和中程结构。大部分（~60%）的Fe是4-配位的（[4]Fe），仅对应于三价铁，Fe 3+，距离d[4] Fe 3 +-O=1.87 $\pm$ 0.01 π。这与通过玻璃化学计量预测的3D结构不同。大多数[4] Fe 3+位点的存在说明了与晶体NaFeSi 2 O 6的结构不同的玻璃结构，晶体NaFeSi 2 O 6仅含有八面体Fe 3+。玻璃结构的EPSR模拟表明，[4] Fe 3+随机分布在硅酸盐网络中，与硅酸盐四面体共角。大多数[4] Fe 3+位置的存在不同于相应晶相的结构，其仅包含八面体Fe 3+。[4] Fe 3+的网络形成行为，加上Na+离子作为电荷补偿剂的存在，是含铁玻璃的特殊物理性质的起源，例如随着铁浓度的增加，硅酸钠玻璃的弹性模量增加。我们的数据还提供了5配位Fe的直接证据，平均距离d[5]Fe-O=2.01 $\pm$ 0.01 Å。该第二Fe群体涉及Fe 2+和Fe 3+两者。5-配位的Fe原子倾向于通过主要共享边缘而分离。三价铁在NFS玻璃中的双重作用的直接结构证据为理解NFS玻璃的特殊性质提供了支持，例如磁性，光学，电子或热力学性质。
英文摘要：
The short and medium range structure of a NaFeSi2O6 (NFS) glass has been investigated by high-resolution neutron diffraction with Fe isotopic substitution, combined with Empirical Potential Structure Refinement (EPSR) simulations. The majority (~60%) of Fe is 4-coordinated ([4]Fe) and corresponds only to ferric iron, Fe3+, with a distance d[4]Fe3+-O=1.87$\pm$0.01Å . This is at variance with the 3D-structure predicted by glass stoichiometry. The existence of a majority of [4]Fe3+ sites illustrates a glass structure that differs from the structure of crystalline NaFeSi2O6, which contains only octahedral Fe3+. The EPSR modeling of glass structure shows that [4]Fe3+ is randomly distributed in the silicate network and shares corner with silicate tetrahedra. The existence of a majority of [4]Fe3+ sites differs from the structure of the corresponding crystalline phase, which contains only octahedral Fe3+. The network-forming behavior of [4]Fe3+, coupled with the presence of Na+ ions acting as charge-compensators, is at the origin of peculiar physical properties of Fe-bearing glasses, such as the increase of the elastic modulus of sodium silicate glasses with increasing Fe-concentration. Our data provide also direct evidence for 5-coordinated Fe, with an average distance d[5]Fe-O=2.01$\pm$0.01Å. This second Fe population concerns both Fe2+ and Fe3+. 5-coordinated Fe atoms tend to segregate by sharing mainly edges. The direct structural evidence of the dual role of ferric iron in NFS glass provides support for understanding the peculiar properties of NFS glass, such as magnetic, optical, electronic or thermodynamic properties.
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