An unexpectedly stable Y2B5 compound with the fractional stoichiometry under ambient pressure

Boron-rich yttrium borides are an exceptional group of compounds not only with excellent mechanical properties, but also with particular superconducting and thermoelectric properties. Although the Y–B compounds with integral components have been extensively investigated experimentally and theoretically, the yttrium borides with the fractional stoichiometries are rarely observed. Herein, utilizing a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we made an investigation on a broad range of stoichiometries of yttrium borides. An extraordinary stable Y₂B₅ compound possessing the fractional stoichiometry with the monoclinic P12₁/c1 phase is firstly uncovered. Structurally, the P12₁/c1-Y₂B₅ crystalline consists of the distorted B₆ octahedrons and seven-member B rings. Remarkably, the B–B covalent network following the increment of the boron content in six concerned yttrium borides undergoes an increasing dimension, quasi one-dimensional chain → two-dimensional B ring → a combination of two-dimensional B ring and three-dimensional B₆ octahedron → three-dimensional B₂₄ cage. According to a microscopic hardness model, P12₁/c1–Y₂B₅ is considered as an incompressible and hard material with the hardness of 18.83 GPa. More importantly, Fm-3 m-YB₁₂ can be classified into an ultra-incompressible material with the appreciable Vickers hardness of 33.16 GPa. The present consequences can provide important insights for understanding the complex crystal structures of boron-rich yttrium borides and stimulate further experimental synthesis of novel multifunctional materials with the fractional compositions.