In view of device failure , it is important to consider why the like - species of oxygen vacancies and interstitials can cluster together as observed . We propose a schematic picture to enable a better qualitative understanding of the microphysics underlying the observed oxygen migration into rings , and the subsequent formation of clusters of oxygen vacancies and interstitials and the associated band diagram ( Figure 4 ) . Initially , incipient oxygen defects are uniformly spread throughout the as - grown TaOx film ( Figure 4a ) , which then laterally migrate due to thermallydriven forces upon formation of the conducting channel ( Figure 4b ) , as discussed above . Local changes in the electrical potential due to the clustering of negative interstitials and positive vacancies can cause significant bending of the tantalum oxide conduction and valence bands that can partially neutralize the charge on each species , [ 38 ] as indirectly evidenced by significant band - shifts in Figure 2c . This bandbending can decrease repulsions among likecharged species and also enable them to agglomerate , as the cohesion energy for oxygen vacancies can be quite low , and is likely to stabilize the ring [ 7,34,39,40 ] ( Figure 4c ) . We also calculated an approximate potential profile across the ring ( Figure S16 ) that look qualitatively similar to the one proposed in this cartoon . Subsequent lateral forces followed by clustering of vacancies and interstitials follows the initial bending of bands , as shown in Figures 4b-4c . Due to continued supply of energy through cycling and the large surface - area - to - volume ratio of the rings , they break apart to form clusters of oxygen interstitials and vacancies ( Figure 4d ) , associated with the fading of the ring observed in Figure 2b . Additionally , we observe that most of the bright regions in Figures 1b-1c are in proximity or contact with a dark region , and vice versa ( pointed out in Figure S3 ) . This suggests that there are significant vacancy - vacancy and interstitial - interstitial attractive forces or there are significant vacancy - interstitial barriers . The attractive forces likely originate from the strong clustering , as mentioned above . The barrier could originate from the oppositely charged defects in the bright and dark regions behaving as dopants , creating an electric field at the interface of these regions to prevent complete neutralization of the charged defects , much like oppositely charged dopants in a p - n junction . Direct observation of clustering of like - species is an important observation that shines light on a prominent failure mechanism of such devices . We specifically point to the fact that many real world devices are smaller than the ring features observed here and are operated at much lower power levels . Similar experiments utilizing low - power operations on identical devices have yielded strikingly different results , with the device endurance being much higher ( > 10 8 ) and no rings were observed . [ 17 ] As our measurements show , in - operando x - ray absorption spectromicroscopy is a powerful tool for studying chemical and electronic structure in oxide materials , including device evolution and failure with electrical     cycling and inhomogeneous localized phenomena . With in - operando , high - voltage electrical cycling of tantalum oxide devices , we observed the development of submicrometer features with a ring of oxygen interstitials and an inner core of oxygen vacancies , which could be reproduced using thermally - driven lateral forces . A key observation here is that a significant amount of displaced oxygen moved radially outward from the conduction channel and was stored as interstitials , with a unique spectral signature , in the tantalum oxide film rather than in to the adjacent tantalum metal electrode . [ 41,42 ] These results provide experimental data that help in understanding previous models regarding oxygen ion migration , [ 3,[5][6][7][8]17,26,32 ] metastable cohesion of oxygen defects , [ 7,34,40 ] role and sign of thermophoresis , [ 12][13][14]43 ] the composition and structure of conduction channels that tend towards failure , [ 5,6,8,31 ] and the localization of resistance switching . [ 14,37,43 ] Most importantly , we directly observed a failure mechanism caused by clustering of like - species of oxygen .