How to quantify isotropic negative thermal expansion: magnitude, range, or both?

Negative thermal expansion (NTE) is the useful and counterintuitive material property of volume contraction on heating. Isotropic NTE is the rarest and most useful type, and is known to occur in a variety of different classes of materials. In this focus article we ask the simple question of how best to compare NTE behaviour amongst these different systems? We summarise the two main mechanisms for isotropic NTE, and illustrate how these favour alternatively NTE magnitude and NTE range. We argue in favour of a combined metric of NTE capacity, which balances both effects and allows unbiased identification of the most remarkable NTE materials, irrespective of the underlying microscopic mechanism at play. By organising known NTE materials according to these various metrics, we find intuitive trends in behaviour that help identify key materials for specific NTE applications.