Hollerweger R, Holec D, Paulitsch J, Bartosik M, Daniel R, Rachbauer R, Polcik P, Keckes J, Krywka C, Euchner H, Mayrhofer P H
Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, Vienna University of Technology, A-1040 Vienna, Austria.
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben, Austria.
Acta Mater. 2015 Jan 15;83:276-284. doi: 10.1016/j.actamat.2014.10.006.
The complex structure of TaO led to the development of various structural models. Among them, superstructures represent the most stable configurations. However, their formation requires kinetic activity and long-range ordering processes, which are hardly present during physical vapor deposition. Based on nano-beam X-ray diffraction and concomitant ab initio studies, a new metastable orthorhombic basic structure is introduced for TaO with lattice parameters = 6.425 Å, = 3.769 Å and = 7.706 Å. The unit cell containing only 14 atoms, i.e. two formula unit blocks in the direction, is characterized by periodically alternating the occupied oxygen site between two possible positions in succeeding 002-planes. This structure can be described by the space group 53 () with four Wyckoff positions, and exhibits an energy of formation of -3.209 eV atom. Among all the reported basic structures, its energy of formation is closest to those of superstructures. Furthermore, this model exhibits a 2.5 eV band gap, which is closer to experimental data than the band gap of any other basic-structure model. The sputtered TaO films develop only a superstructure if annealed at temperatures >800 °C in air or vacuum. Based on these results and the conveniently small unit cell size, it is proposed that the basic-structure model described here is an ideal candidate for both structure and electronic state descriptions of orthorhombic TaO materials.
TaO的复杂结构导致了各种结构模型的发展。其中，超结构代表了最稳定的构型。然而，它们的形成需要动力学活性和长程有序过程，而在物理气相沉积过程中这些过程几乎不存在。基于纳米束X射线衍射和伴随的从头算研究，引入了一种新的亚稳正交基本结构的TaO，其晶格参数为a = 6.425 Å、b = 3.769 Å和c = 7.706 Å。晶胞仅包含14个原子，即在c方向上有两个化学式单元块，其特征是在后续的（002）平面中两个可能位置之间的占据氧位点周期性交替。这种结构可以用空间群53（Cmc2）和四个Wyckoff位置来描述，并且表现出-3.209 eV/原子的形成能。在所有报道的基本结构中，其形成能最接近超结构的形成能。此外，该模型表现出2.5 eV的带隙，比任何其他基本结构模型的带隙更接近实验数据。溅射的TaO薄膜只有在空气或真空中于温度>800 °C退火时才会形成超结构。基于这些结果以及方便的小晶胞尺寸，本文提出这里描述的基本结构模型是正交TaO材料结构和电子态描述的理想候选者。
