Highly pure High-Entropy Boride ceramics (HEB) were produced by two-step spark plasma sintering, consisting of boro/carbothermal reduction of oxides mixtures and pressure-assisted sintering. This work focused on the HEB materials with non-equimolar compositions of transition metals (Ti-Zr-Hf-Nb-Ta), demonstrating a reliable and cost-effective process for producing high-entropy diboride ceramics with a single-phase solid solution. High-entropy borides may have a wide composition range, but (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ is one of the most investigated compositions. Based on the (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ structure, a group of high-entropy boride ceramics with non-equal transition metal atom ratios were theoretically predicted and experimentally investigated. The theoretical predictions were performed using the Density Function Theory implemented in the VASP program. The Special Quasirandom Structures were used to disorder materials. All structures with non-equimolar ratios had a single phase according to XRD analysis. Nanoindentation was performed to measure both the hardness and Young's modulus of the sintered materials. This study investigated the effect of different concentrations of metal elements on the processing and mechanical properties of high-entropy diboride ceramics.

Acknowledgment
This work was supported by the Slovak Research and Development Agency under contract no. APVV-21-0402, APVV-SK-CZ-RD-21-0089, and APVV-19-0497, and the project VEGA 2/0116/22. This study was performed as part of the implementation of the project “Building-up Centre for advanced materials application of the Slovak Academy of Sciences”, ITMS project code 313021T081, supported by the Research & Innovation Operational Programme funded by the ERDFT.