In our previous work, anisotropic chemical bonding, low shear deformation resistance, damage tolerance ability, low thermal conductivity, and moderate thermal expansion coefficient of Y₄Al₂O₉ (YAM) were predicted. In this work, phase-pure YAM powders were synthesized by solid-state reaction between Y₂O₃ and Al₂O₃ and bulk YAM ceramics were prepared by hot-pressing method. Lattice parameters and a new set of X-ray powder diffraction data were obtained by Rietveld refinement. The mechanical and thermal properties of dense YAM ceramics were investigated. The measured elastic moduli are close to the theoretical predicted values and the stiffness can be maintained up to 1400 ℃. The flexural strength and fracture toughness are 252.1±7.3 MPa and 3.36±0.20 MPa·m^1/2, respectively. Damage tolerance of YAM was also experimentally proved. The measured average linear thermal expansion coefficient (TEC) of YAM is 7.37×10^-6 K^-1, which is very close to the theoretical predicted value. Using high-temperature X-ray diffraction (XRD) analysis, volumetric TEC is determined (23.37±1.61)×10^-6 K^-1 and the anisotropic TEC are α_a = 7.34×10^-6 K^-1, α_b = 7.54×10^-6 K^-1, and α_c = 7.61×10^-6 K^-1.

The mechanical and thermal properties of Y₄Al₂O₉ were predicted using a combination of first-principles and chemical bond theory (CBT) calculations. Density functional theory (DFT) computations were performed for the structural, mechanical, and thermal properties, and the results were confirmed by chemical bond theory. Based on the calculated equilibrium crystal structure, heterogeneous bonding nature has been revealed, i.e., Al–O bonds are stronger than Y–O bonds. Low second-order elastic constants c₄₄, c₅₅, and c₆₆ demonstrate the low shear deformation resistance. Low G/B ratio suggests that Y₄Al₂O₉ is a damage tolerant ceramic. Y₄Al₂O₉ shows anisotropy in elastic behavior based on the discussion of direction dependence of Young’s modulus. The hardness is predicted to be 10.2 GPa from calculated elastic moduli. The thermal expansion coefficient (TEC) calculated by chemical bond theory is 7.51×10^-6 K^-1. In addition, the minimum thermal conductivity of Y₄Al₂O₉ is estimated to be 1.13 W·m^-1·K^-1, and the thermal conductivity decreases with temperature as 1305.6/T.