Roles of Mass, Structure, and Bond Strength in the Phonon Properties and Lattice Anharmonicity of Single-Layer Mo and W Dichalcogenides,The Journal of Physical Chemistry C

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Roles of Mass, Structure, and Bond Strength in the Phonon Properties and Lattice Anharmonicity of Single-Layer Mo and W Dichalcogenides
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2015-08-04 00:00:00 , DOI: 10.1021/acs.jpcc.5b04669
Liang-Feng Huang _{1,

2} , Zhi Zeng _{1,

3}

Affiliation

A hierarchical first-principles study has been performed to reveal the roles of mass, structure, and atomic bond strength in phonon spectra, phonon anharmonicity, thermal expansion, and thermomechanics of single-layer Mo and W dichalcogenides (MX₂, X = S, Se, and Te). The strength of the M–X bond is determined by the competition between ionicity and covalency and increases (decreases) with increasing the cation (anion) nucleon number. The total mass and cation–anion mass ratio isotopically influence phonon frequencies. The twofold lattice dimensionality renders the bending ZA mode with parabolic dispersion and negative Grüneisen constant (γ). While nonorthogonal bonds lead to interdirection vibrational hybridizations, which increases γ_ZA but decreases γ_TA and γ_LA. The minima of γ_TA and γ_LA decrease with decreasing bond strength and become negative in MTe₂. MX₂ always has a negative thermal expansion at low temperatures (T < 50 K) due to the advanced excitation of those low negative-γ ZA modes. At higher temperatures, the excitation of other positive-γ modes results in positive thermal expansion. Additionally, thermal expansion is determined jointly by lattice stiffness, phonon excitation, and phonon anharmonicity. The contributions of these involved factors are quantitatively disentangled here, and their relationships with mass, structure, and bond strength are revealed. The softening of the bulk modulus of MX₂ under heating is mainly caused by thermal expansion, which is partially canceled by the stiffening effect from phonon excitation. Both bulk modulus and its thermal softening rate decrease with anion nucleon number, due to the decrease in bond strength and bond-strength anharmonicity, respectively.

中文翻译：

质量，结构和键强度在单层Mo和W二硫属元素化物的声子性质和晶格非谐性中的作用

进行了分层的第一性原理研究，以揭示质量，结构和原子键强度在单层Mo和W二硫代双氰化物的声子谱，声子非谐性，热膨胀和热力学中的作用（MX ₂，X = S， Se和Te）。M–X键的强度取决于离子性和共价性之间的竞争，并且随着阳离子（阴离子）核子数的增加而增加（减少）。总质量和阳离子-阴离子质量比会同位素影响声子频率。双重晶格维数使弯曲ZA模式具有抛物线色散和负Grüneisen常数（γ）。非正交键会导致相互间的振动杂交，从而增加_γZA但减小_γTA和_γLA。的γ的最小值_TA和γ _LA减少与MTE降低粘合强度和成为负₂。MX ₂在低温（T <50 K）下始终具有负的热膨胀性，这是由于那些低的负γZA模式的高级激发所致。在更高的温度下，其他正γ模态的激发导致正热膨胀。另外，热膨胀是由晶格刚度，声子激发和声子非谐性共同决定的。这些涉及的因素的贡献在这里定量地解开了，并揭示了它们与质量，结构和结合强度的关系。MX ₂的体积模量的软化加热下的热膨胀主要是由热膨胀引起的，而热膨胀又被声子激发的加强作用所抵消。体积模量及其热软化速率均随阴离子核子数的增加而降低，这分别是由于键强度和键强度非谐性降低所致。

更新日期：2015-08-04

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