On how differently the quasi-harmonic approximation works for two isostructural crystals: Thermal properties of periclase and lime
On how differently the quasi-harmonic approximation works for two isostructural crystals: Thermal properties of periclase and lime
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DOI：
10.1063/1.4906422
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发表时间：
2015-01-28
影响因子：
4.4
通讯作者：
Dovesi, R.
中科院分区：
文献类型：
--
作者：
Erba, A.;Shahrokhi, M.;Dovesi, R.
采用基于密度泛函理论的周期性从头算方法计算了两种同质结构简单氧化物（方镁石MgO和石灰CaO）的谐和准谐热性质。发现钙相对于镁阳离子的更易极化的特性显著影响准谐波近似的有效域，对于热结构性质（例如体积的温度依赖性V（T）、体积模量K（T）和热膨胀系数α（T）），其从MgO的[0 K-1000 K]降低到CaO的仅[0 K-100 K]。相反，热力学性质（如熵S（T）和定容比热CV（T））至少在2000 K以下才能可靠地描述，而准谐波恒压比热C-P（T）在两种情况下都能可靠地描述到1000 K左右。通过考虑三个不同类别（局部密度近似，广义梯度近似和混合动力）的五种不同的表达式，在这里明确地研究了所采用的近似DFT的交换相关泛函的效果。计算的谐波热力学性质被发现是几乎独立的所采用的功能，而准谐波的结构特性更受影响的功能的选择，与差异，增加系统变得更软。(C)2015年AIP Publishing LLC。
Harmonic and quasi-harmonic thermal properties of two isostructural simple oxides (periclase, MgO, and lime, CaO) are computed with ab initio periodic simulations based on the density-functional-theory (DFT). The more polarizable character of calcium with respect to magnesium cations is found to dramatically affect the validity domain of the quasi-harmonic approximation that, for thermal structural properties (such as temperature dependence of volume, V(T), bulk modulus, K(T), and thermal expansion coefficient, alpha(T)), reduces from [0 K-1000 K] for MgO to just [0 K-100 K] for CaO. On the contrary, thermodynamic properties (such as entropy, S(T), and constant-volume specific heat, CV(T)) are described reliably at least up to 2000 K and quasi-harmonic constant-pressure specific heat, C-P(T), up to about 1000 K in both cases. The effect of the adopted approximation to the exchange-correlation functional of the DFT is here explicitly investigated by considering five different expressions of three different classes (local-density approximation, generalized-gradient approximation, and hybrids). Computed harmonic thermodynamic properties are found to be almost independent of the adopted functional, whereas quasi-harmonic structural properties are more affected by the choice of the functional, with differences that increase as the system becomes softer. (C) 2015 AIP Publishing LLC.