Engineering Near-Zero Thermal Expansion in Single-Phase Oxides: A TTB-Structure Platform Enabled by Entropy-Driven Doping

Achieving near-zero thermal expansion (NZTE) in single-phase materials over broad temperature ranges is crucial yet challenging for precision applications. Here, we report a new family of NZTE materials based on triple tetragonal-tungsten-bronze (TTB) oxides, including Ta 8 W 9 O 47 (TWO), (Nb 0.5 Ta 0.5 ) 8 W 9 O 47 (NTWO), and (Nb 0.5 Ta 0.5 ) 8 (Mo 0.5 W 0.5 ) 9 O 47 (NTMWO), synthesized via solid-state reaction and entropy-driven cation doping. High-temperature X-ray diffraction reveals anisotropic lattice behavior in all compositions: positive expansion along the a - and b -axes counterbalanced by negative expansion along the c -axis. This compensation yields average linear coefficients of thermal expansion (CTE) near zero ( α l < 1.30 µK −1 ) from 300 to 1673 K. Notably, Nb-doping in NTWO optimizes this balance, achieving the lowest α l of 0.656(43) µK −1 . Distinct temperature regimes highlight specific advantages: TWO maintains stable NZTE ( α dil = −0.455(1) µK −1 ) from 173–773 K; NTWO exhibits ultralow CTE (−0.079(1) µK −1 ) between 473–773 K; and NTMWO demonstrates exceptional dimensional stability at cryogenic temperatures (93–300 K, α dil = −0.096(1) µK −1 ). The bulk NZTE behavior stems from the synergy between the intrinsic anisotropic lattice expansion and microstructural features, such as rod-like grains and porosity. These results establish Ta 8 W 9 O 47 -type TTB oxides as a robust, non-ferroelectric platform for designing single-phase NZTE ceramics for precision and aerospace applications.