Crystal growth and negative thermal expansion properties of a Y₂Mo₄O₁₅ single crystal

A bulk-size single crystal of Y₂Mo₄O₁₅ with 20 × 11 × 8 mm³ was successfully grown by the top-seed solution growth (TSSG) method. The full-width at half maximum of (100) and (010) crystal faces is 37 and 27 arcsec, respectively. The thermal conductivity coefficients κ ₁₁, κ ₂₂, κ ₃₃, and κ ₁₃ are determined to be 1.519, 2.097, 0.445, and 0.997 W m^-1 K^-1, respectively. It is worth noting that the Y₂Mo₄O₁₅ crystal shows significant anisotropy thermal expansion properties, which exhibits a negative thermal expansion along the b-axis (α ₂₂ = -5.11 × 10^-6 K^-1). The crystal structure analysis shows that the shrinking of Mo-O bond lengths along the b-axis with the increasing temperature would be the main origin of the negative thermal expansion properties for Y₂Mo₄O₁₅ crystal, which does not comply with the current mechanism.