We use a symmetry-motivated approach to analyze neutron pair distribution function data to investigate the character of the soft phonon modes in negative thermal expansion (NTE) material ${\mathrm{ReO}}_3$. This analysis shows that its local structure is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring-type deformations than in the related NTE compound ${\mathrm{ScF}}_3$. The lack of flexibility in ${\mathrm{ReO}}_3$ restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. These results support the idea that structural flexibility is an important factor in NTE materials. Surprisingly, our results show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the initial pressure-induced phase transition in ${\mathrm{ReO}}_3$. The result indicates that the dynamic motions associated with rigid unit modes are highly anisotropic in these systems.

中文翻译：

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负热膨胀材料ReO3中的软模式各向异性

我们使用对称驱动的方法来分析中子对分布函数数据，以研究负热膨胀 (NTE) 材料中软声子模式的特征 ${\mathrm{氧化还原}}_3$. 该分析表明，其局部结构以同相八面体倾斜模式为主，并且八面体单元对剪刀型变形的灵活性远低于相关的 NTE 化合物${\mathrm{SF}}_3$. 缺乏灵活性${\mathrm{氧化还原}}_3$将 NTE 驱动声子限制在互易空间的较小区域，从而限制了 NTE 的幅度和温度范围。这些结果支持结构柔韧性是 NTE 材料的一个重要因素的观点。令人惊讶的是，我们的结果表明，即使在升高的温度下，局部波动也符合初始压力诱导相变的对称性和有序参数方向${\mathrm{氧化还原}}_3$. 结果表明，在这些系统中，与刚性单元模式相关的动态运动是高度各向异性的。