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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >~(31)P and ~(23)Na Solid-State NMR Studies of Cation Dynamics in HT-Sodium Orthophosphate and the Solid Solutions (Na_2SO_4)_x-(Na_3PO_4)_(1-x)
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~(31)P and ~(23)Na Solid-State NMR Studies of Cation Dynamics in HT-Sodium Orthophosphate and the Solid Solutions (Na_2SO_4)_x-(Na_3PO_4)_(1-x)

机译：HT-正磷酸钠和固溶体（Na_2SO_4）_x-（Na_3PO_4）_（1-x）中阳离子动力学的〜（31）P和〜（23）Na固态NMR研究

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摘要

The high-temperature phases of sodium orthophosphate, HT-Na_3PO_4, and of the solid solutions (Na_2SO_4)_x-(Na_3PO_4)_(1-x) are characterized by their plastic crystalline state with dynamically disordered PO_4~(3-) and SO_4~(2-) anions and a remarkably high cation conductivity. Since HT-Na_3PO_4 possesses a fully occupied cation sublattice (no vacancies), it has been proposed that cation transport and anion reorientations are dynamically coupled ("paddle-wheel mechanism"). However, no direct evidence for this coupling has been reported. In the present study, the validity of this mechanism is investigated on the basis of ~(23)Na and ~(31)P nuclear magnetic resonance (NMR) experiments. Temperature-dependent measurements of the static ~(31)P linewidth indicate that in the solid solutions with 0.04 <= x <= 0.25 the acceleration of sodium ionic mobility is closely correlated with the acceleration of phosphate rotational motion, associated with a second-order phase transition near 400 K. Temperature-dependent measurements of the ~(23)Na longitudinal and transverse relaxation times have been analyzed using the theory of quadrupolar relaxation under nonextreme narrowing conditions. Consistent with theoretical predictions sizeable dynamic frequency shifts are detected. All of the data are consistently analyzed quantitatively in terms of two distinct motional processes. A low-temperature process, whose relaxation strength is independent of sample composition, is clearly accelerated by the onset of fast anion rotation occurring at the second-order phase transition temperature. In addition, a high-temperature process, which is almost absent in HT-Na_3PO_4 but whose importance increases with increasing sulfate content, signifies vacancy hopping. This dependence on composition is easily understood because the substitution of PO_4~(3-) by SO_4~2-) generates cation vacancies. The activation energies of both processes are near 0.45 eV, and the corresponding timescales grown increasingly similar with increasing sodium sulfate content. Altogether, the results give strong evidence for a dynamic coupling between anionic reorientation and cation diffusion, supporting the concept of a paddle-wheel mechanism.
机译：正磷酸钠HT-Na_3PO_4和固溶体（Na_2SO_4）_x-（Na_3PO_4）_（1-x）的高温相具有动态无序PO_4〜（3-）和SO_4的塑性结晶状态〜（2-）阴离子和极高的阳离子电导率。由于HT-Na_3PO_4具有完全占据的阳离子亚晶格（无空位），因此提出了阳离子传输和阴离子重新定向是动态耦合的（“桨轮机制”）。但是，没有直接证据表明这种耦合。在本研究中，基于〜（23）Na和〜（31）P核磁共振（NMR）实验研究了该机制的有效性。静态〜（31）P线宽的温度相关测量结果表明，在固含量为0.04 <= x <= 0.25的固溶体中，钠离子迁移率的加速与磷酸盐旋转运动的加速密切相关，与二阶相关400 K附近的相变。〜（23）Na纵向和横向弛豫时间的温度相关测量值已使用非极窄条件下的四极弛豫理论进行了分析。与理论预测一致，检测到较大的动态频率偏移。所有数据均根据两个不同的运动过程进行了定量分析。松弛过程的强度与样品组成无关的低温过程明显地由于在二阶相变温度下发生的快速阴离子旋转而加速了。另外，HT-Na_3PO_4中几乎不存在但其重要性随着硫酸盐含量的增加而增加的高温过程表示空位跳跃。这种对组成的依赖性很容易理解，因为用SO_4〜2-）取代PO_4〜（3-）会产生阳离子空位。这两个过程的活化能都接近0.45 eV，并且相应的时间尺度随着硫酸钠含量的增加而越来越相似。总之，这些结果为阴离子再取向与阳离子扩散之间的动态耦合提供了有力的证据，从而支持了叶轮机构的概念。

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