Anomalous thermal expansion behaviour of several open frame-work compounds has been extensively investigated using the techniques of inelastic neutron scattering and lattice dynamics. These compounds involve increasing level of structural complexity and flexibility, which leads to increased values of thermal expansion coefficients approaching colossal values. In several compounds, neutron inelastic scattering experiments have produced quantitative estimates of the anharmonicity of phonons over a range of low energies, and thereby explained the observed thermal expansion quantitatively. The anharmonicity is found to be an order of magnitude larger than that in usual materials. Lattice dynamical calculations have correctly predicted the observed anharmonicity in the neutron experiments and revealed the overall nature of phonons involved. In compounds showing negative thermal expansion, the phonons responsible have rather low energies up to 10 meV. In most compounds, the anharmonic phonons span all over the Brillouin zone, while in some cases the specific phonons are limited to certain wave-vectors. The nature of specific phonons responsible for anomalous behavior is found to be different in all these compounds. These phonons generally involve transverse vibrations, librations and internal distortions of the polyhedral units. The paper reviews recent advances in the understanding of anomalous thermal expansion behaviour.

中文翻译：

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晶体固体中的声子和异常热膨胀行为

已经使用非弹性中子散射和晶格动力学技术广泛研究了几种开放框架化合物的异常热膨胀行为。这些化合物涉及越来越高的结构复杂性和灵活性，这导致热膨胀系数值增加，接近巨大值。在几种化合物中，中子非弹性散射实验已经对低能量范围内声子的非谐性进行了定量估计，从而定量地解释了观察到的热膨胀。发现非谐性比普通材料大一个数量级。晶格动力学计算正确预测了中子实验中观察到的非谐性，并揭示了所涉及声子的整体性质。在表现出负热膨胀的化合物中，负责的声子具有相当低的能量，高达 10 meV。在大多数化合物中，非谐声子跨越整个布里渊区，而在某些情况下，特定声子仅限于某些波矢量。发现负责异常行为的特定声子的性质在所有这些化合物中都不同。这些声子通常涉及多面体单元的横向振动、振动和内部变形。该论文回顾了在理解异常热膨胀行为方面的最新进展。发现负责异常行为的特定声子的性质在所有这些化合物中都不同。这些声子通常涉及多面体单元的横向振动、振动和内部变形。该论文回顾了在理解异常热膨胀行为方面的最新进展。发现负责异常行为的特定声子的性质在所有这些化合物中都不同。这些声子通常涉及多面体单元的横向振动、振动和内部变形。该论文回顾了在理解异常热膨胀行为方面的最新进展。