Department of Physics, Chulalongkorn University, Bangkok 10330, Thailand.
J Phys Condens Matter. 2012 Mar 7;24(9):095802. doi: 10.1088/0953-8984/24/9/095802. Epub 2012 Feb 9.
The effects of Na atoms on high pressure structural phase transitions of CuIn(0.5)Ga(0.5)Se(2) (CIGS) were studied by an ab initio method using density functional theory. At ambient pressure, CIGS is known to have chalcopyrite (I42d) structure. The high pressure phase transitions of CIGS were proposed to be the same as the order in the CuInSe(2) phase transitions which are I42d → Fm3m → Cmcm structures. By using the mixture atoms method, the Na concentration in CIGS was studied at 0.1, 1.0 and 6.25%. The positive mixing enthalpy of Na at In/Ga sites (Na(InGa)) is higher than that of Na at Cu sites (Na(Cu)). It confirmed previous studies that Na preferably substitutes on the Cu sites more than the (In, Ga) sites. From the energy-volume curves, we found that the effect of the Na substitutes is to reduce the hardness of CIGS under high pressure. The most significant effects occur at 6.25% Na. We also found that the electronic density of states of CIGS near the valence band maximum is increased noticeably in the chalcopyrite phase. The band gap is close in the cubic and orthorhombic phases. Also, the Na(Cu)-Se bond length in the chalcopyrite phase is significantly reduced at 6.25% Na, compared with the pure Cu-Se bond length. Consequently, the energy band gap in this phase is wider than in pure CIGS, and the gap increased at the rate of 31 meV GPa(-1) under pressure. The Na has a small effect on the transition pressure. The path of transformation from the cubic to orthorhombic phase was derived. The Cu-Se plane in the cubic phase displaced relatively parallel to the (In, Ga)-Se plane by 18% in order to transform to the Cmcm phase. The enthalpy barrier is 0.020 eV/atom, which is equivalent to a thermal energy of 248 K. We predicted that Fm3m and Cmcm can coexist in some pressure range.
采用基于密度泛函理论的第一性原理方法研究了 Na 原子对 CuIn(0.5)Ga(0.5)Se(2)（CIGS）高压结构相变的影响。在常压下，CIGS 被认为具有黄铜矿（I42d）结构。CIGS 的高压相变被认为与 CuInSe(2) 相变的顺序相同，即 I42d→Fm3m→Cmcm 结构。通过使用混合原子方法，研究了 CIGS 中 0.1、1.0 和 6.25%的 Na 浓度。Na 在 In/Ga 位（Na(InGa)）的混合焓为正，高于 Na 在 Cu 位（Na(Cu)）的混合焓。这证实了之前的研究结果，即在 Cu 位上，Na 比（In，Ga）位更倾向于取代。从能量-体积曲线可以看出，Na 取代的影响是在高压下降低 CIGS 的硬度。6.25%Na 时的影响最为显著。我们还发现，CIGS 近价带最大值的电子态密度在黄铜矿相中明显增加。在立方相和正交相，带隙接近。此外，在 6.25%Na 的黄铜矿相中，Na(Cu)-Se 键长比纯 Cu-Se 键长显著缩短。因此，该相的能带隙比纯 CIGS 宽，在压力下以 31meV/GPa(-1)的速率增加。Na 对相变压力的影响很小。推导了从立方相向正交相转变的途径。为了转变到 Cmcm 相，立方相中 Cu-Se 平面相对于（In，Ga）-Se 平面平行移动 18%。焓垒为 0.020eV/原子，相当于 248K 的热能。我们预测 Fm3m 和 Cmcm 可以在一定压力范围内共存。
