Uniaxial does not necessarily mean along a crystallographic axis, since the expansion can be calculated along any direction in space. The units of the coefficients are in K −1 but because of its typical magnitudes they are rather expressed in 10 −6 K −1, or, more conveniently, in MK −1. Experimental validation using a robust protocol with data collected at more than 2 different temperatures is required to validate these cases. Moreover 127 structures have been identified which could have negative volumetric thermal expansion. Approximately 34% of these structures may have at least one orthogonal axis with negative thermal expansion, much more than generally believed. 4719 different structures, comprising 4093 different molecular compounds and 626 additional polymorphs have been analyzed on their thermal expansion properties. The anisotropy of the thermal expansion is also evaluated and found to have a very broad distribution. Normal and anomalous coefficients can be identified using these values and the associated standard deviations. The mean volumetric thermal expansion coefficient is 168.8 × 10 −6 K −1 and the mean uniaxial thermal expansion coefficient is 71.4 × 10 −6 K −1, based on 7 different observations, respectively. The thermal expansion properties of crystalline organic compounds are investigated by data mining of the Cambridge Structural Database (CSD).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |