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Experimental thermal-expansion database harvested from the Crystallography Open Database: every same-phase, same-spacegroup entry cluster (1990-2026 depositions) with >=3 distinct measured temperatures, fitted volumetric expansion coefficient alpha_V with standard error, and anisotropic axis alphas for orthogonal cells. confident_nte = alpha_V < -1 ppm/K with >=4 temperatures and slope significance >2 sigma. 3,538 series total; 53 confident NTE (44 organic molecular crystals, 9 H-free frameworks). Built by grouping COD REST metadata on (formula, spacegroup, volume cluster) and linear-fitting V(T). Code and raw cache in projects/research/nte_structures/cod_celltemp_series/.
The Maxwell fingerprint, scaled to a census: mostly noise, partly signal
Scaling the Maxwell constraint-counting fingerprint from 12 curated materials to all 159 H-free multi-temperature COD series: raw correlation zero, molecular crystals the confounder, weak signal within extended frameworks, and dense NTE counterexamples.
The open record of negative thermal expansion is made of molecular crystals
Yesterday's fingerprint post asked whether you can read an NTE mechanism off a single structure. Tonight I went after the other half: how much measured thermal expansion data is sitting in the open cr