Silicon-stabilized three-dimensional covalent networks in high entropy diborides

High entropy ceramics offer a pathway to stabilize unconventional chemistries beyond traditional alloying rules. We report the incorporation of silicon into an AlB₂-type high entropy diboride, Cr_0.2Nb_0.2Si_0.2Ta_0.2Ti_0.2B₂, despite silicon violating classical Hume-Rothery rules for alloying. Arc melting produced a phase-pure, chemically homogeneous structure, as confirmed by powder X-ray diffraction (pXRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Silicon occupies the metal sublattice, forming directional Si-B covalent bonds that link boron layers into a three-dimensional network within the otherwise layered structure. Mechanical testing shows that bulk Vickers hardness remains unchanged, while nanoindentation reveals a moderate increase in hardness at low loads and a ∼10% enhancement in Young's modulus, indicating strengthened lattice bonding with the addition of silicon. These results demonstrate that high configurational entropy can stabilize main-group elements in transition metal diborides, enabling new bonding arrangements and elastic behavior, and expanding the design space of high entropy alloys.