Materials exhibiting a low thermal conductivity (k) are key components in several devices, e.g. gas engines where they act as thermal barrier coatings and thermoelectric [GN1] coolers. Recently, Liu et al. [1], predicted ultra-low thermal conductivity in scheelites (ABO4). For instance, BaMoO4 and BaWO4 thermal conductivity are predicted to reach values below 1 Wm−1K−1. In this work[GN2] , we have synthesized BaMoO4 and BaWO4 ceramics and measured their thermal conductivity, which we find to be of 0.8 and 1.2 Wm-1K-1 at 400 K, respectively. This work confirms thus the low thermal conductivity of scheelite structure [2].
     To reach even lower thermal conductivity, we took inspiration from[GN3]  perovskites (ABO3) where it has been shown that A-site deficiency leads to significantly lowered thermal conductivity values [3]. Therefore, we have synthesized A-deficient scheelite La2Mo3O12, Ce2W3O12 and La2W3O12 ceramics and measured ultra-low thermal conductivity values (≈ 0.7 Wm−1K−1, 0.4-0.2 Wm−1K−1 and 0.2[GN4]  Wm−1K−1, respectively in the range of 400–900 K) [2].
     The impact of vacancies on the reduction in thermal conductivity has been then evaluated in BiVO4 -Bi2/3MoO4 solid solution. Indeed, this solid solution allows to control the amount of vacancies in the structure through the value of x in ?i1-(?/3)?x/3????1-??4 (? denotes vacancies). We find that the thermal conductivity decreases with increasing vacancies content from 1.8 Wm-1K-1 for x=0 down to 1.5, 1.3 and 1.2 Wm-1K-1 for x=0.05, 0.2 and 0.4, respectively at 300 K.
 
References 
[1] Y. Liu, D. Jia, Y. Zhou, Y. Zhou, J. Zhao, Q. Li, B. Liu, J. Materiomics. 6 (2020) 702.
[2] E. Bsaibess, F. Delorme, I. Monot-Laffez, F. Giovannelli, Scripta Materialia 201 (2021) 113950.
[3] A.V. Kovalevsky, A. A. Yaremchenko, S. Populoh, A. Weidenkaff, J.R. Frade, J. Phys. Chem. C. 118 (2014) 4596.