Adams Mark A, Refson Keith, Gabrys Barbara J
ISIS Facility, CCLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK.
Phys Chem Chem Phys. 2005 Nov 7;7(21):3685-92. doi: 10.1039/b505543a. Epub 2005 Sep 15.
The measured high resolution (deltaE/E approximately 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T(d). Because of LO-TO splitting the ammonium ion asymmetric stretch, nu3, has components calculated to be at higher frequencies than those of the symmetric stretch, nu1, which contradicts the assignment scheme produced from optical data.
本文给出了氟化铵的高分辨率（ΔE/E约为2 - 3%）非相干非弹性中子散射测量光谱，并结合现有光谱进行了讨论。此外，通过从头算晶格动力学计算得到了一整套色散曲线，并用于对光谱进行严格解释。铵离子的振动模式出现在560 cm⁻¹处，这些模式的非谐性估计为4%，约为其他卤化铵中观察到的一半。非谐性的降低归因于更强的氢键作用和更深的势阱。除了振动模式从测量值向上偏移约40 cm⁻¹外，计算结果与观察到的光谱吻合良好。在这个系统中，色散和LO - TO分裂很重要，模式频率变化高达135 cm⁻¹。光学模式区域计算得到的LO - TO分裂性质表明这是一个伪立方体系，证实了铵离子的点群对称性非常接近Td。由于LO - TO分裂，铵离子的不对称伸缩振动ν3的计算分量频率高于对称伸缩振动ν1，这与光学数据得出的归属方案相矛盾。
