Prediction of crystalline Ta4O9 phase using first principles-based cluster expansion calculations

Christian Søndergaard Pedersen,Juan Maria Garcia Lastra,Yang Li,Jin Hyun Chang,Nini Pryds

doi:10.1063/5.0027018

Christian Søndergaard Pedersen, Juan Maria Garcia Lastra + Show 3 more

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https://doi.org/10.1063/5.0027018

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Journal: APL Materials	Publication Date: Dec 1, 2020
Citations: 2	License type: cc-by

Affiliation: Technical University of Denmark

Abstract

Tantalum is the only element of Group 5 in the periodic table that lacks any experimental reports on the existence of reduced crystalline oxide between the pentoxide (Ta2O5) and the dioxide (TaO2). We computationally predict the existence of a novel tantalum oxide with Ta4O9 stoichiometry, which lies at the midpoint between Ta2O5 and TaO2. The ground-state Ta4O9 structure was found through simulated annealing based on a cluster expansion model, which is trained using 186 density functional theory calculations. The newfound Ta4O9 material has space group number 10 (P2/m), and it can be viewed as an oxygen-deficient λ-Ta2O5 structure in which oxygen vacancies aggregate pair-wise in nearest-neighbor sites. Tad–Tad bonds fill the spatial void of the oxygen vacancies, keeping the system non-magnetic and non-metallic. The synthesis of the new Ta4O9 crystal is deemed feasible through a controlled reduction of λ-Ta2O5. The reported Ta4O9 has the potential to open new avenues in catalysis and resistive switching device applications where the reduced tantalum oxides are broadly employed.

Highlights

Great attention has been focused on tantalum oxide since the turn of the millennium
Various phases of tantalum oxide have been investigated for use in photocatalysis,1 in electrocatalysis,2 as coating in biomedical applications,3 and as the resistive layer in Resistive Random Access Memory (ReRAM) devices
Using density functional theory (DFT) calculations combined with cluster expansion (CE), we have found a new ground state of the TaOx system with the chemical formula Ta4O9, which is stable relative to phase separation of Ta2O5 and TaO2

Summary

Introduction

Great attention has been focused on tantalum oxide since the turn of the millennium. Various phases of tantalum oxide have been investigated for use in photocatalysis, in electrocatalysis, as coating in biomedical applications, and as the resistive layer in Resistive Random Access Memory (ReRAM) devices. The most stable stoichiometry of the tantalum-oxide system is Ta2O5. Various phases of tantalum oxide have been investigated for use in photocatalysis, in electrocatalysis, as coating in biomedical applications, and as the resistive layer in Resistive Random Access Memory (ReRAM) devices.. The most stable stoichiometry of the tantalum-oxide system is Ta2O5. Ta2O5 fabricated at room temperature is amorphous, samples are known to crystallize when fabricated at temperatures above ∼400 ○C.8. Likewise, annealing samples at temperatures between 500 ○C and 700 ○C leads to crystallization.. Ta2O5 crystallites were observed in an electroformed nanofilament of a ReRAM device.. Ta2O5 crystallites were observed in an electroformed nanofilament of a ReRAM device.12 This finding raises a need for further investigation of crystalline tantalum oxide even in resistive switching applications where devices are usually reported as nonstoichiometric. Ta2O5 fabricated at room temperature is amorphous, samples are known to crystallize when fabricated at temperatures above ∼400 ○C.8 Likewise, annealing samples at temperatures between 500 ○C and 700 ○C leads to crystallization. Recently, Ta2O5 crystallites were observed in an electroformed nanofilament of a ReRAM device. This finding raises a need for further investigation of crystalline tantalum oxide even in resistive switching applications where devices are usually reported as nonstoichiometric.

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