Thermal Expansion Case Study

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5.1 Preamble
5.1.1 Thermal expansion
Most of the materials exhibit a thermal expansion upon heating, which can be explained by the inter-atomic potential of a diatomic molecule. With an increase in energy of the system, the vibrations increase affecting the bond length. In those cases whereby the interatomic potential is harmonic, thermal expansion wouldn’t be realised, as the average interatomic distance remains the same across the temperature region. In contrast, normally the inter-atomic potential is anharmonic, in which an increase in the average bond length is observed with a rise in temperature [252,259].
Thermal expansion in real terms is more complicated, in the case of solids, as expansion corresponds to all the bonds that contribute to a volumetric expansion, for which a simple diatomic picture does not account for [260]. Thermal expansion is expressed by its coefficient of thermal expansion, αV . For a volumetric expansion, it is defined as (5.1)
In case expansion occurs along a specific direction, the linear coefficient of
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As no single crystal neutron data is available for CoGF, the structural model based on single-crystal X-ray diffraction reported by Hu et al. [30] is used as a starting model.
The deposited structures in Cambridge Crystallographic Data Centre, CCDC numbers: 709780-3,5 (Mn, Fe, Co, Ni, Zn), based on reference [30], has one of the hydrogen sites split to H21 and H22, with an occupancy of 0.5 each. It is a common practice that split-sites are used when two sites are not related by symmetry. However, such a usage, in this instance, is irrational when the sites are symmetrically related. For this reason, H22 is invalid and thus forfended. Whereas H21 is incremented to a full occupancy. This rectification is employed for MnGF and CoGF, throughout this

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