Prediction of new ground-state crystal structure of Ta2O5

Abstract

Tantalum pentoxide (Ta 2 O 5 ) is a wide-gap semiconductor which has important technological applications. Despite the enormous efforts from both experimental and theoretical studies, the ground state crystal structure of Ta 2 O 5 is not yet uniquely determined. Based on first-principles calculations in combination with evolutionary algorithm, we identify a triclinic phase of Ta 2 O 5 , which is energetically much more stable than any phases or structural models reported previously. Characterization of the static and dynamical properties of the new phase reveals the common features shared with previous metastable phases of Ta 2 O 5 . In particular, we show that the d-spacing of ~ 3.8 Å found in the X-ray diffraction (XRD) patterns of many previous experimental works, is actually the radius of the second Ta-Ta coordination shell as defined by radial distribution functions. *Corresponding Author (wateratnanoscale@hotmail.com; yyang@theory.issp.ac.cn) Previous experimental studies have established that, Ta 2 O 5 undergoes a phase transition at T ~ 1593 K (1320 °C), from the low-temperature phase (L-Ta 2 O 5 ) to the high-temperature phase (H-Ta 2 O 5 ) [20]. In the research works that followed, many efforts are devoted to studying the crystallographic structures of both L-Ta 2 O 5 and H-Ta 2 O 5 , which still remain an issue of debate [21, 22]. Our work will focus on the atomic structures of the low-temperature phase of Ta 2 O 5 , L-Ta 2 O 5 , which is more relevant to technological applications. Due to the difficulties of growing high-quality single crystals of Ta 2 O 5 , the structural information provided by powder X-ray diffraction (XRD) is very limited. Indeed, the crystal structures are found to be critically dependent on the conditions (e.g., temperatures, pressures) of synthesis and the method of analysis [23]. A number of phases and/or structural models have been suggested for L-Ta 2 O 5 . Based on the data of powder XRD, Stephenson and Roth proposed the 11-formula-units (Z = 11, 22 Ta and 55 O atoms) orthorhombic model (referred to as L SR ) [24]. Later, the Hummel et al. proposed the T-phase (orthorhombic, space group: Pmm2) which is however nonstoichiometric in chemical components (unit cell contains 24 Ta and 62 O atoms) [21]. Grey, Mumme, and Roth

Local atomic order of the amorphous TaOx thin films in relation to their chemical resistivity

“…It should be stressed that similar DFT results were obtained for a few other Ta 2 O 5 polymorphs (e.g. for g-Ta 2 O 5 phase predicted to be more stable at DFT level, 24 but gives less convincing ts to EXAFS data), i.e. DFT predicted that O add-atom insertion leads to the formation of dumbbell defect and that O 1s binding energies for dumbbell atoms are shied by 1-2 eV toward higher values.…”

“…We also have performed EXAFS analysis using other proposed Ta 2 O 5 structures, including g-Ta 2 O 5 phase predicted to be more stable at DFT level. 24 For the b A structure the obtained bond lengths were consistently shorter in respect to the theoretical model (except for the farthest paths). While for the g structure some bonds were shorter and some were longer even within the same coordination shell introducing high distortions into the model.…”

“…ref. [22][23][24][25][26]. However, the signicance of local Ta 2 O 5 structure rather than long-range order and crystal symmetry, for the properties of this material was postulated.…”

“…There is still ongoing discussion about the crystal structure of stoichiometric Ta 2 O 5 . [21][22][23][24][25]28 To generate the scattering paths for EXAFS analysis in FEFF6 program we have considered several structures and the best agreement with experiment was obtained for the orthorhombic one, referred in literature as b A , 24,28 aer optimization at DFT level. This result is somewhat intriguing, as b A phase was predicted to be the least stable Ta 2 O 5 polymorph in DFT calculation.…”

Which phase of Ta2O5 being of the largest dielectric constant

“…Modeling of infiltration of liquids in porous media is a well-studied subject, especially in the field of hydrology where it is used to model seepage of water through porous soils and rocks. 29,30 The physics of resin flow into a mold under pressure is similar to the seepage problem and so similar approaches have been used also to model the flow of resin into a mold containing fiber mats. 23,[31][32][33][34][35][36][37][38] The models are used in these cases to determine the resin flow front and mold filling times corresponding to different infiltration conditions.…”