Study examines the superconducting transition induced by pressure in electrides

"Given the many potential applications of superconducting materials, the understanding of high temperature superconductors is a key scientific challenge in condensed matter physics," Aitor Bergara and Guochun Yang, two of the researchers who led the study, told Phys.org Study email.

Electrodes are ionic compounds in which most electrons reside in the interstitial regions of the crystal and behave like anions. Because of their structural peculiarity, these compounds possess interesting physical properties. For example, the magnitude and distribution of their interstitial electrons can be efficiently modulated, either by adjusting their chemical composition or in external conditions, such as pressure.

Overall, electrides are very bad superconductors. For example, the electrically observed superconducting transition temperature of a canonical electride[California[It[Californie[Ca₂₄al₂₈O₆₄]⁴⁺ (4th^–)₄ It is now well known that, under high pressure, alkali metals can easily lose their outer orbital electrons and form electrics.

"Interestingly, the lithium electrolyte (Li) induced by the pressure is metallic," said Bergara and Yang. "In addition, phosphorus (P) has moderate electronegativity, so they can trap some electrons in Li-rich Li-P compounds, while the remaining electrons can remain in the interstitial regions. the predictions, these works It is possible to adjust the morphology of interstitial electrons by modifying the ratio Li / P and thus to obtain compounds with new electronic properties. For example, according to our calculations, the Li 6P's electrolyte should have a superconducting transition temperature of 39.3K, breaking the existing record among electrides. "

Predicting the atomic structure of materials from first principles (based solely on their composition) is an extremely difficult task. This usually involves classifying a very large number of energy minima on a multidimensional surface network. In recent years, researchers have introduced several computational methods to speed up this process, one of them calling CALYPSO.

"In our study, we used the Calypso program developed by Yanming Ma and his colleagues at Jilin University, which implements an algorithm for optimizing particle swarms to determine preferred crystal structures, simply fixing the Li: P ratios and pressure as the only starting inputs, "Bergara and Yang explained. "Once the most stable structures were identified, we characterized their physical properties, for example, we explored their superconducting properties in the McMillan-Allen-Dynes approximation."

In their study, Bergara, Yang and their colleagues reported that a stable pressure-induced binder₆The electrolyte P can become a superconductor with a predicted superconducting transition temperature of 39.3 K; the highest planned up here in the known electrides. They discovered that the interstitial electrons of the compound, with connected dumbbell electrolyte states, play a dominant role in this superconducting transition.

"Our prediction not only breaks the superconducting transition temperature record in electrides, but also allows for a better understanding of these materials," said Bergara and Yang.

According to researchers' forecasts, other Li-rich phosphures, such as Li₅P, Li₁₁P₂, Li₁5P₂and Li₈P, could also be superconducting electrics, but their_c should be lower. This recent study by Bergara, Yang and their colleagues could pave the way for new research on high temperature superconductivity in similar binary compounds.

"We believe that research on superconducting electrodes is just beginning," said Bergara and Yang. "Much remains to be explored, for example, the analysis of the superconducting mechanism in new electrolyte compounds, especially under high pressure, as we have shown in this article, an effective way to design such materials. Superconductors consist of exploring the metallic electride compounds formed between weak electronegative elements and strong electropositive elements. "

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More information:
Ziyuan Zhao et al. Predicted superconducting transition induced by pressure in Li6P electrolyte, Letters of physical examination (2019). DOI: 10.1103 / PhysRevLett.122.097002

Yanchao Wang et al. Prediction of crystalline structure via particle-swarm optimization, Physical examination B (2010). DOI: 10.1103 / PhysRevB.82.094116