Hidden Structural Order in OrthorhombicTa2O5

Abstract

We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta2O5. Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy highsymmetry structural model where all Ta and O atoms have correct formal oxidation states of +5 and −2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta2O3 plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of Ta2O5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive collective relaxation of the O sublattice.Tantalum pentoxide (Ta 2 O 5 ) is one of the most extensively studied transition metal oxides due to its potential for technological applications such as anti-reflection coatings, photocatalysis, and high-k dielectrics for highdensity transistors [1][2][3]. It has recently attracted interest as the material of choice for resistance-change memory and the related memristor [4][5][6], however, its crystalline structure, which is critical to its unique structural and electronic properties, is still unclear.Ta 2 O 5 has an orthorhombic phase up to ∼ 1350 • C, above which a transition to a tetragonal phase occurs [7,8]. Although the low-temperature orthorhombic phase is of technological relevance, it has proven difficult to characterize structurally. Early X-ray diffraction studies [9,10] found that the orthorhombic Ta 2 O 5 phase consists of two-dimensional (2D) layers of Ta 2 O 3 in the ab plane, and two-fold coordinated O atoms that connect the Ta atoms in adjacent layers (forming linear chains of -Ta-O-Ta-O-Ta-) along the c direction. In the Ta 2 O 3 plane, it was understood that the Ta atoms form a triangular arrangement, but the arrangement of O atoms was unresolved, with many weak superstructure lines [10]. Later, Roth and coworkers reported that the Ta 2 O 3 plane consists of a disordered array of Ta atoms in octahedral and pentagonal bipyramidal arrangements, with an 11-fold increase of the lattice parameters in the b direction [11]. The superstructure periodicity was shown to change when aliovalent cations, such as W or Li, were incorporated in small amounts to stabilize the phase [12,13]. The superstructures consist of a set of small-unit basic structures and invoked the concept of infinitely adaptive structures [14] for nonstoichiometric Ta 2 O 5−x thatwithin certain composition limits-every composition orders into a different superstructure.Extensive theoretical studies have been made with regards to the bulk properties, dielectric response, and oxygen vacancies (and their diffusion) of Ta 2 O 5 [15][16][17][18][19][20][21][22][23]. In these studies, the low-temperature Ta 2 O 5 pha...

Comparison of oxygen vacancy defects in crystalline and amorphous Ta2O5

“…The lattice parameters and formation energy from our calculation could be found in our previous work [40]. sX gives similar values to the GGA [31] for these parameters. The inter layer 2-fold O sites is 40%, intra-layer 2-fold site 20%, and intra-layer 3-fold site 40%.…”

“…Various simplified higher symmetry models have been proposed to represent the bonding within the layers, such as the orthorhombic b model (space group Pmmm), and the hexagonal d model (space group P6/mmm) [20,21]. Recently Lee et al [31] proposed a novel orthorhombic k phase model with Pbam symmetry, which is a simplification of the 11 formula unit (FU) model. It has a much lower total energy (1.7 eV per FU) than the b and d models and it was lower even than a relaxed 11 FU model.…”

Oxygen vacancy defects in Ta2O5 showing long-range atomic re-arrangements

“…Recently Lee et al 31 proposed a novel orthorhombic k phase model with Pbam symmetry, which is a simplification of the 11 FU model. It has a much lower total energy (1.7 eV per FU) than the b and d models and it was lower even than a relaxed 11 FU model.…”

Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

“…1; for the sake of comparison, respective sX-LDA band structures are also shown. Crystalline Ta2O5 has a complex layered structure and has only recently been explained in a simple model [44], known as the λ phase. Stability with respect to the amorphous phase has been predicted [44].…”

“…2(a) is built from respective crystalline phases, which significantly reduces the modelling complexity. Although the amorphous structure could be more useful in experiment, it is still advantageous to use the λ Ta2O5, which has adaptive structure and predicted stability over the amorphous phase [44], to understand the key structures and the energetic features of the interface. The interface is chosen to be perpendicular to both of the (001) direction of Ta2O5 and the (110) direction of TaO2, which minimizes the lattice and bond mismatch.…”