Hygroscopicity, phase transition and thermal expansion in Yb2-Ga W3O12 system

Negative and Near-Zero Thermal Expansion in A2M3O12 and Related Ceramic Families: A Review

“…Most of the studies on A 2 M 3 O 12 phases and their solid solutions are driven by the vast chemical flexibility of this family while preserving the Sc 2 W 3 O 12 -type structure, and the consequent possibility to change, control and even finely tune the CTE through chemical substitutions, especially of the cations at the crystallographically unique octahedral site (Wu et al, 2006;Wu et al, 2007a;Wu et al, 2007b;Xiao et al, 2008b;Wu et al, 2009a;Miller et al, 2012b;Prisco et al, 2016a;Muller et al, 2019;Machado et al, 2021).…”

“…Water molecules likely interact with the framework through hydrogen bonding and it has been proposed that the oxygen atoms from water molecules intact with A 3+ or M 6+ , while hydrogen atoms from water molecules interact with the framework 2-fold oxygens (Wu et al, 2016). These interactions between water molecules and the framework can cause amorphization in molybdates and tungstates (Marinkovic et al, 2005;Yu et al, 2008;Li et al, 2012) or even change the crystal symmetry in some tungstates (Kol'tsova, 2001;Sleight, 2003;Cao et al, 2016;Pontón et al, 2017;Machado et al, 2021). Symmetry lowering by transformation to the monoclinic phase, or the presence of water molecules in channels, will inhibit, or attenuate, the asymmetrical framework librations of the quasi-rigid polyhedra, due to freezing of polyhedra rocking motions or due to steric effects, respectively.…”

“…Molybdates become amorphous when hygroscopic and lose water at temperatures higher than 403 K (Sumithra and Umarji, 2004;Sumithra and Umarji, 2006). Tungstates, on the other hand, lose water more easily, at temperatures not higher than 373 K and do not necessary become amorphous (Yu et al, 2008;Muller et al, 2019;Machado et al, 2021). It has been proposed that differences in hygroscopicity between tungstates and molybdates are due to lower absorption energies of water molecules into the crystal framework in tungstates (Wu et al, 2016).…”

“…There are two main ways to reduce or inhibit hygroscopicity in tungstates and molybdates. One is to partially substitute larger A 3+ for smaller cations, such as in the cases of Er 2-x Fe x Mo 3 O 12 and Yb 2-x Ga x W 3 O 12 (Cheng et al, 2007;Machado et al, 2021). Suppression of hygroscopic behavior also has been achieved by coating the hygroscopic A 2 M 3 O 12 material with a hydrophobic material, such as C 3 N 4 (Liu et al, 2017).…”

“…In addition, because of their low stiffness, it is expected that adding these ceramics would decrease the Young's modulus of conventional ceramic and metal matrices especially at high filler loadings, although this does not present a problem for more compliant polymer matrices (Pontón et al, 2019b). Lastly, the full release of structural water molecules that are accommodated within the crystal structure of some A 2 M 3 O 12 phases, such as Y 2 Mo 3 O 12 , Y 2 W 3 O 12 and Yb 2 W 3 O 12 , is mandatory for the preparation of reliable composites (Machado et al, 2021). In fact, a drying step prior to the composite manufacturing has become a common strategy (Yanase et al, 2009;Zhou et al, 2014;Zhou et al, 2015;Zhou et al, 2017;Pontón et al, 2018;Pontón et al, 2019b;Liu et al, 2020).…”