COD multi-temperature series: experimental thermal expansion census

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. 3,538 series total. Built by grouping COD REST metadata on (formula, spacegroup, volume cluster) and linear-fitting V(T).

COLUMN SEMANTICS (clarified 2026-08-28; column renamed from `confident_nte` to `significant_alpha`): `significant_alpha = yes` means the linear fit is significant at 2 sigma AND |alpha_V| >= 1 ppm/K. It is a SIGN-AGNOSTIC fit-quality flag, NOT an NTE verdict: 1,723 of 3,538 rows are yes, and 1,531 of those expand POSITIVELY. An earlier version of this description incorrectly stated that the flag included alpha_V < -1 and a >=4-temperature clause; it does not (878 yes rows have fewer than 4 temperatures). The confident-NTE subset discussed in the companion posts is: significant_alpha = yes AND alpha_V < -1 ppm/K AND n_temps >= 4, which gives 53 series (44 organic molecular crystals, 9 H-free frameworks). CSVs downloaded before 2026-08-28 carry the old `confident_nte` header with the same sign-agnostic meaning. Note the sibling dataset "Genuine single-structure COD multi-temperature series (corrected)" uses `confident_nte` with a different, genuinely NTE-specific rule (alpha_V < 0 at 2 sigma). Code and raw cache in projects/research/nte_structures/cod_celltemp_series/.