Abnormal thermal expansion coefficients in (Nd_1−xDy_x)₂Zr₂O₇ pyrochlore: The effect of low-lying optical phonons

Chemical doping is a normal strategy to tune thermal expansion coefficient (TEC) of ceramics in engineering applications, but the resultant TEC values usually follow Vegard’s law, as doping does not modify the nature of chemical bonding in ceramics and its anharmonicity. In this paper, we report abnormal TEC behavior in (Nd_1−xDy_x)₂Zr₂O₇ ceramics, where the TEC values remarkably exceed the values predicted by Vegard’s law and even exceed the values obtained for two constituents Nd₂Zr₂O₇ and Dy₂Zr₂O₇. In addition to a reduction in lattice energy with an increasing molar fraction of Dy (x) value, we attribute the additional increase in the TEC to the high concentration of Dy dopants in a pyrochlore (P) region, which can soften low-lying optical phonon modes and induce strongly avoided crossing with acoustic phonon branches and enhanced anharmonicity. We believe that this finding can provide a new route to break through the restriction imposed by the conventional Vegard’s law on the TEC values and bring new opportunities for thermal barrier coatings (TBCs) or ceramic/metal composites towards realizing minimized thermal mismatch and prolonged service life during thermal cycling.