A survey of thermal expansion coefficients for organic molecular crystals in the Cambridge Structural Database

Abstract

Typical ranges of thermal expansion coefficients are established for organic molecular crystals in the Cambridge Structural Database. The CSD Python API is used to extract 6201 crystal structures determined close to room temperature and at least one lower temperature down to 90 K. The data set is dominated by structure families with only two temperature points and is subject to various sources of error, including incorrect temperature reporting and missing flags for variable-pressure studies. For structure families comprising four or more temperature points in the range 90–300 K, a linear relationship between unit-cell volume and temperature is shown to be a reasonable approximation. For a selected subset of 210 structures showing an optimal linear fit, the volumetric expansion coefficient at 298 K has mean 173 p.p.m. K−1 and standard deviation 47 p.p.m. K−1. The full set of 6201 structures shows a similar distribution, which is fitted by a normal distribution with mean 161 p.p.m. K−1 and standard deviation 51 p.p.m. K−1, with excess population in the tails mainly comprising unreliable entries. The distribution of principal expansion coefficients, extracted under the assumption of a linear relationship between length and temperature, shows a positive skew and can be approximated by two half normal distributions centred on 33 p.p.m. K−1 with standard deviations 40 p.p.m. K−1 (lower side) and 56 p.p.m. K−1 (upper side). The distribution for the full structure set is comparable to that of the test subset, and the overall frequency of biaxial and uniaxial negative thermal expansion is estimated to be < 5% and ∼30%, respectively. A measure of the expansion anisotropy shows a positively skewed distribution, similar to the principal expansion coefficients themselves, and ranges based on suggested half normal distributions are shown to highlight literature cases of exceptional thermal expansion.

Development and assessment of an improved powder-diffraction-based method for molecular crystal structure similarity

“…22 The anisotropic nature of thermal expansion in molecular crystals has been discussed in recent studies and, indeed, is more commonly the norm than the exception. 27,28…”

“…22 The anisotropic nature of thermal expansion in molecular crystals has been discussed in recent studies and, indeed, is more commonly the norm than the exception. 27,28 We recently developed a new approach to improve PXRDbased comparison methods using anisotropic volume corrections. 23 The method was applied to identify candidate structures generated from first-principles crystal structure prediction (CSP) that match known experimental structures and, in the process, identified two uncredited matches from the 6th CSP blind test.…”

Molecular tiltation and supramolecular interactions induced uniaxial NTE and biaxial PTE in bis-imidazole-based co-crystals

“…14 A recent CSD survey by Bond shows that uniaxial NTE is more common than biaxial NTE in organic materials. 15 In addition, few single-component organic crystalline compounds have been reported for unusual thermal expansion properties. 3 g , r ,16…”

Colossal Linear NTE and PTE in Two-Fold Interpenetrated Network of a MOF Induced by Hinge-like Motion

“…NTE materials find possible application in various fields such as cookwares, electronics, optical devices, thermo-mechanical instruments, and aerospace engineering. ,,− A certain class of compounds are known to exhibit the NTE property, such as metal oxides, , cyano-bridged metal complexes, − and Prussian blue analogues, − and very few organic materials are known for either uniaxial or biaxial , NTE. Since thermal expansion of the materials with non-cubic lattice are anisotropic in nature due to anisotropic supramolecular interactions along different directions, linear NTE (axial NTE) has been reported for several inorganic and organic compounds. ,− NTE with large CTE is rare in inorganic materials. ,− In 2008, Goodwin and co-workers have described colossal PTE and NTE in a cyano-bridged Ag–Co complex (Ag­[Co­(CN) 6 ]) . Kepert et al have shown the same phenomenon in a nanoporous coordination framework material induced due to spin crossover …”

“…NTE materials find possible application in various fields such as cookwares, electronics, optical devices, thermomechanical instruments, and aerospace engineering. 1,2,8−10 A certain class of compounds are known to exhibit the NTE property, such as metal oxides, 11,12 cyano-bridged metal complexes, 13−16 and Prussian blue analogues, 17−19 and very few organic materials are known for either uniaxial or biaxial 20,21 NTE. Since thermal expansion of the materials with non-cubic lattice are anisotropic in nature due to anisotropic supramolecular interactions along different directions, linear NTE (axial NTE) has been reported for several inorganic and organic compounds.…”