• Co-doping YTaO4 with Ti, Zr, or Hf significantly reduces thermal conductivity to 1.23 W·m−1·K−1 at 900°C, outperforming conventional YSZ.
• The medium-entropy ceramics achieve high thermal expansion coefficients (10.4 × 10−6 K−1 at 1400°C), matching nickel-based superalloys.
• Fracture toughness is enhanced to 4.1 ± 0.5 MPa·m1/2 via ferroelastic domain switching and microcrack toughening.
• The simultaneous improvement in thermal and mechanical properties makes these MECs promising candidates for next-generation TBC top coats.