Pressure-driven crystal structure evolution in RbB₂C₄ compounds

As an extreme physical condition, high pressure serves as a potent means to substantially modify the interatomic distances and bonding patterns within condensed matter, thereby enabling the macroscopic manipulation of material properties. We employed the CALYPSO method to predict the stable structures of RbB₂C₄ across the pressure range from 0 GPa to 100 GPa and investigated its physical properties through first-principles calculations. Specially, we found four novel structures, namely, P6₃/mcm-, Amm2-, P1-, and I4/mmm-RbB₂C₄. Under pressure conditions, electronic structure calculations reveal that all of them exhibit metallic characteristics. The calculation results of formation enthalpy show that the P6₃/mcm structure can be synthesized within the pressure range of 0–40 GPa. Specially, the Amm2, P1, and I4/mmm structures can be synthesized above 4 GPa, 6 GPa, 10 GPa, respectively. Moreover, the estimated Vickers hardness value of I4/mmm-RbB₂C₄ compound is 47 GPa, suggesting that it is a superhard material. Interestingly, this study uncovers the continuous transformation of the crystal structure of RbB₂C₄ from a layered configuration to folded and tubular forms, ultimately attaining a stabilized cage-like structure under the pressure span of 0–100 GPa. The application of pressure offers a formidable impetus for the advancement and innovation in condensed matter physics, facilitating the exploration of novel states and functions of matter.