Acoustic softening dominates over Klemens–Abeles disorder in governing the lattice thermal conductivity of (Gd_1−xSm_x)TaO₄ ceramics

Low thermal conductivity oxide ceramics are usually designed around point-defect scattering. Cation mass and ionic-radius mismatch, the Klemens–Abeles descriptors, serve as the primary screening variables. Here we show that single-phase fergusonite (Gd_1-xSm_x)TaO₄ ceramics (x = 0–1) reveal a second, distinct route to low conductivity. This route involves acoustic softening coupled to unit-cell dilation. Replacing Gd with Sm expands the rare-earth (RE)–O coordination shell while leaving the TaO₆ octahedron nearly unchanged. The unit cell grows monotonically with x, and the lattice softens. The mean sound velocity and Debye temperature both drop by about 20% from x = 0 to x = 0.8. The room-temperature lattice thermal conductivity falls by 32%, from 5.11 to 3.48 W m^-1 K^-1. A Cahill minimum-conductivity estimate is built from sound velocity and atomic number density alone. It reproduces most of this decrease. This result shows that acoustic softening, rather than disorder, sets the baseline trend. Disorder is still active but offset. The Klemens–Abeles scattering parameter peaks at x = 0.2–0.6, away from the conductivity minimum at x = 0.8. The end member SmTaO₄ provides a decisive test. Its sound velocity matches that of the x = 0.8 composition, but its conductivity rebounds. This isolates residual rare-earth-sublattice disorder as the extra channel that deepens the minimum. Acoustic softening and unit-cell volume therefore deserve a place alongside mass and radius mismatch when screening complex oxides for low thermal conductivity.