Size disorder as a descriptor for predicting reduced thermal conductivity in medium- and high-entropy pyrochlore oxides

A step forward from high-entropy ceramics to compositionally complex ceramics: a new perspective

“…2). Also, ordered MECs with two sublattices include Gd2(Sn1/4Ti1/4Hf1/4Zr1/4)2O7 and (Sm1/2Gd1/2)2(Ti1/3Hf1/3Zr1/3)2O7 [14].…”

“…Here, we typically distinguish HECs and MECs based on high-or medium-entropy mixing on one of the cation sublattices (typically according to the one with the highest ideal mixing entropy). For HECs with two cation sublattices, high-entropy mixing can occur at one cation sublattice, e.g., in (La1/5Ce1/5Nd1/5Sm1/5Eu1/5)2Zr2O7 [14,34] and (Ba1/2Sr1/5)(Zr1/5Sn1/5Ti1/5Hf1/5Nb1/5)O3 [26], or on both cationic sublattices, e.g., in (Gd1/5La1/5Nd1/5Sm1/5Y1/5)(Co1/5Cr1/5Fe1/5Mn1/5Ni1/5)O3 [25] ( Fig. 2).…”

“…Other oxide systems of significant interests include those with the fluorite [13,23,75,76,84], spinel [29][30][31][32]96], pyrochlore [14,[33][34][35][36][37], and perovskite [24][25][26][27][28] structures.…”

“…In Fig. 3, we have further added our data points for three Compositionally Complex fluorite-based oxides: (1) "YSZ-like" non-equimolar Hf0.284Zr0.284Ce0.284Y0.074Yb0.074O2-δ [13] and (2) (YDyErNb1/2Ta1/2)O7 rare earth niobate/tantalate [Wright, Wang, Chen and Luo, unpublished results], both of which are in the disordered fluorite structure, as well as (3) (Sm1/4Eu1/4Gd1/4Yb1/4)2(Ti1/2Hf1/4Zr1/4)2O7 in the ordered pyrochlore structure [14], in addition to rocksalt ESOs reported by Braun et al [78]. Fig.…”

“…For example, size disorder (see definition in Ref. [14]) has been proposed to be a more effective descriptor (than ideal mixing entropy itself) in describing thermal conductivity in medium-and high-entropy pyrochlore oxides ( Fig. 4(a) and 4(b)) [14].…”