Integrating abnormal thermal expansion and ultralow thermal conductivity into (Cd_,Ni)₂Re₂O₇ via synergy of local structure distortion and soft acoustic phonons

Abnormal thermal expansion denotes intriguing negative and zero thermal expansion (NTE and ZTE). The integration of abnormal thermal expansion with low thermal conductivity is highly desirable for high-end devices. However, the coexistence of seemingly contradictory features presents a grand challenge for abnormal-thermal-expansion materials. Herein, a unique NTE, for the first time, is reported in Cd₂Re₂O₇ with a volumetric coefficient of thermal expansion, α_V of -5.0 × 10⁻⁶/K (5 - 210 K). Moreover, a remarkable ZTE is attained in Cd_1.95Ni_0.05Re₂O₇ with α_V of 1.1 × 10⁻⁶/K (5 - 180 K). Concomitantly, the attained ultralow thermal conductivities of 0.38 and 0.56 W/mK at 300 K of the two compositions are also highly competitive, exceeding prevailing abnormal-thermal-expansion materials. Concurrently, Cd₂Re₂O₇-based oxides demonstrate orders of magnitude higher electrical conductivity than representative NTE materials. By a combined investigation of synchrotron X-ray techniques, aberration-corrected scanning transmission electron microscope, and phonon calculations, the synergy of local structure distortion highlighted by ordered Re-Re dimer and soft acoustic phonons is integral to the simultaneous accomplishment of distinctive thermal expansion and thermal conductivity, which is totally distinct from predominant optical-phonon driven NTE. This integration of intriguing thermal properties effectively resolves thermal issues and permits excellent size and temperature stability. This work opens up an unusual strategy to delicately control thermal expansion and thermal conductivity simultaneously.