Predicting alloy vibrational mode properties using lattice dynamics calculations, molecular dynamics simulations, and the virtual crystal approximation
Predicting alloy vibrational mode properties using lattice dynamics calculations, molecular dynamics simulations, and the virtual crystal approximation
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DOI：
10.1063/1.4812737
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发表时间：
2013-07-14
影响因子：
3.2
通讯作者：
McGaughey, Alan J. H.
中科院分区：
文献类型：
--
作者：
Larkin, Jason M.;McGaughey, Alan J. H.
通过研究两个模型合金体系：Lennard - Jones氩和Stillinger - Weber硅，对质量无序的虚晶（VC）近似进行了评估。在这两种材料体系中，完美晶体与更重质量的物质合金化，直至浓度相等。分析是通过分子动力学模拟和晶格动力学计算进行的。首先通过显式处理无序以及在VC近似下计算模式频率和寿命，在高频下的高浓度合金中发现了差异。值得注意的是，使用VC近似会低估高频模式的寿命，我们将此结果归因于在用于包含点缺陷散射的模型中忽略了高阶项。然后利用模式特性在VC近似下预测热导率。对于Lennard - Jones合金，其中高频模式对热导率有显著贡献，与不需要对热传输进行假设的Green - Kubo方法的预测相比，高频寿命的低估导致热导率被低估。基于对最小模式扩散率的观察，我们提出了一种修正方法，使VC近似下的热导率与Green - Kubo值更好地吻合。对于Stillinger - Weber合金，其中热导率由低频模式主导，高频寿命的低估并不影响热导率的预测，并且与Green - Kubo值有合理的一致性。（C）2013 AIP出版有限责任公司
The virtual crystal (VC) approximation for mass disorder is evaluated by examining two model alloy systems: Lennard-Jones argon and Stillinger-Weber silicon. In both material systems, the perfect crystal is alloyed with a heavier mass species up to equal concentration. The analysis is performed using molecular dynamics simulations and lattice dynamics calculations. Mode frequencies and lifetimes are first calculated by treating the disorder explicitly and under the VC approximation, with differences found in the high-concentration alloys at high frequencies. Notably, the lifetimes of high-frequency modes are underpredicted using the VC approximation, a result we attribute to the neglect of higher-order terms in the model used to include point-defect scattering. The mode properties are then used to predict thermal conductivity under the VC approximation. For the Lennard-Jones alloys, where high-frequency modes make a significant contribution to thermal conductivity, the high-frequency lifetime underprediction leads to an underprediction of thermal conductivity compared to predictions from the Green-Kubo method, where no assumptions about the thermal transport are required. Based on observations of a minimum mode diffusivity, we propose a correction that brings the VC approximation thermal conductivities into better agreement with the Green-Kubo values. For the Stillinger-Weber alloys, where the thermal conductivity is dominated by low-frequency modes, the high-frequency lifetime underprediction does not affect the thermal conductivity prediction and reasonable agreement is found with the Green-Kubo values. (C) 2013 AIP Publishing LLC.