A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre. In this centre, Fe(III) replaces Si(IV) in the tetrahedral site, whereas H+ is coupled to Fe(III) to maintain the charge balance. The spectroscopic investigations, mainly the interpretation of the Electron Spin Echo Envelope Modulation, allowed a detailed localisation of the proton to be obtained. H+ occurs in the channels crossing the crystal parallel to the crystallographic c axis, in a largely eccentric position. The Fe(III)-H+ distance, evaluated in 2.70 A, is found associated with a non-negligible isotropic hyperfine coupling, which can be linked to the relative stability of the S2 centre in natural amethyst.

中文翻译：

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天然紫水晶中 Fe(III):H+ 缺陷中心的表征

通过连续波和脉冲电子顺磁共振 (EPR) 研究了紫水晶的天然单晶，目的是在结构上表征替代 S2 Fe(III):H+ 中心。在该中心，Fe(III) 取代了四面体位置的 Si(IV)，而 H+ 与 Fe(III) 偶联以保持电荷平衡。光谱研究，主要是对电子自旋回波包络调制的解释，允许获得质子的详细定位。H+ 出现在平行于晶体学 c 轴穿过晶体的通道中，在很大程度上偏心位置。在 2.70 A 中评估的 Fe(III)-H+ 距离与不可忽略的各向同性超精细耦合相关，这可能与天然紫水晶中 S2 中心的相对稳定性有关。