Unexpected Trend Deviation in Isoelectronic Transition Metal Borides A₃T₅B₂ (A = group 4, T = group 9): Ti₃Co₅B₂- vs. Perovskite-Type Studied by Experiments and DFT Calculations

We present the first bulk synthesis of Ti₃Co₅B₂, which was realized by arc-melting a Co-rich elemental mixture. Our X-ray diffraction studies revealed a Ti/Co mixed occupancy on one of two possible Ti sites suggesting a phase width with x = 0–0.52 in Ti_3–xCo_5+xB₂. Moreover, we studied experimentally and theoretically the isoelectronic substitution of Ti and Co by their respective higher homologues (Zr/Rh and Hf/Ir). Surprisingly, Ti₃Co₅B₂-type phase was obtained only for the Hf/Ir combination (single crystal analysis of Hf₃Ir₅B₂), whereas for Zr/Rh a perovskite-like phase (ZrRh₃B_x) was discovered instead. We found that small but crucial differences (atomic radius ratio and electronegativity difference) between elements of the same group in the periodic Table are responsible for the unexpected trend deviation. This finding is supported by DFT calculations of the free energy of formation.