Large atomic displacements act as dominators for reducing thermal conductivity in Y3-x<i>A</i>xNbO7 (<i>A</i> = Ca, In, Mg, Al; x = 0.05, 0.1) fluorites

Y3NbO7 is regarded as a prospective material for utilization in thermal barrier coating applications, given its low thermal conductivity, high thermal expansion coefficients (TECs), and low oxygen ion conductivity. This study focuses on the dominant mechanisms of thermal properties of Y3-xAxNbO7 (A = Ca, In, Mg, Al; x = 0.05, 0.1) through the introduction of different types of cations. The oxygen ion conductivity of Y3-xAxNbO7 is as low as 2.16 × 10−5 S·cm−1 at 900 °C, which is dominated by contents of oxygen vacancies and electronegativity disorder. Three factors can contribute to reduction in thermal conductivity, including the phonon scattering caused by point defects, disorders in at. wt., ionic radius, and electronegativity, as well as atomic displacement parameters (ADPs). A comparison of the correlation between lattice point defects and thermal conductivity reveals that they are not the primary factor contributing to reduction in thermal conductivity. Conversely, doping elements with larger ADPs have been shown to significantly enhance the intensity of an-harmonic vibrations, thereby increasing TECs to 11.93 × 10−6 K−1 at 1400 °C and reducing thermal conductivity to 1.25 W m−1 K−1 at 25 °C. The effects of ADPs on reducing thermal conductivity overweigh that of lattice point defects in Y3NbO7 ceramics, and both TECs and thermal conductivity are optimized at the same time. This work proposes that large atomic displacements can act as a means of reducing thermal conductivity and increasing TECs for various ceramics.