structure in ball-and-stick format: green atoms are Y, grey atoms are W and red atoms are O. (b) Shows the structure in terms of its corner-sharing YO$_6$ and WO$_4$ coordination polyhedra: green octahedra are YO$_6$ units, grey tetrahedra are WO$_4$ units. The unit cell is shown as the dashed black line.[]{data-label="Y2W3O12picture"}](figure_1.pdf){width="100.00000%"}

Y$_2$W$_3$O$_{12}$: an NTE material with no RUMs
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Y$_2$W$_3$O$_{12}$ [@Forster_Sleight_1999_IntJInorgMat_Y2W3O12NTEExpt] is a member of the **A**$_2$**M**$_3$O$_{12}$ family of NTE materials, where **A** is a trivalent cation in octahedral coordination, and **M** is either W or Mo with tetrahedral coordination [@Evans_etal_1997_JSolStatChem_NTEinA2M3O12Family; @Mary_1999]. **A**$_2$**M**$_3$O$_{12}$ materials have an orthorhombic structure—Y$_2$W$_3$O$_{12}$ is shown in —and thus, whilst they can (and in the case of this family, do) have volumetric NTE, their expansion is anisotropic.

The **A**$_2$**M**$_3$O$_{12}$ family is of interest because of its customisability: the **A** site can withstand a wide range of substitutions, from ions as small as Al$^{3+}$ to ions as large as Ho$^{3+}$ [@Lind_2012_Mater_NTEReview], providing the opportunity to tune thermal expansion behaviour. In general, the smaller the **A** cation, the smaller the NTE; *in extremis* Al$_3$W$_2$O$_{12}$ actually exhibits weak PTE [@Evans_etal_1997_JSolStatChem_NTEinA2M3O12Family].

Unlike most **A**$_2$**M**$_3$O$_{12}$ compounds, Y$_2$W$_3$O$_{12}$ shows NTE along all three axes [@Forster_Sleight_1999_IntJInorgMat_Y2W3O12NTEExpt]. This is unusual for a non-cubic NTE 