Rattling effect in YbaCabSi7.5Al4.5O1.5N14.5 α-SiAlON ceramics

When a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called “rattling,” has been reported in pyrochlores and some cage compounds, but not in α-SiAlON. In this study, we reveal that in (Yb+Ca) co-doped α-SiAlON ceramics (Yb_aCa_bSi_7.5Al_4.5O_1.5N_14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3…, 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser-flash method. We found that substituting Ca²⁺ with smaller Yb³⁺ cations in Ca-α-SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb³⁺ with larger Ca²⁺ cations in Yb-α-SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α-SiAlON, well-known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La₂Zr₂O₇ and 8YSZ.