Thermal Expansion and Stability of Layer Structures in Ionic AB Compounds
- 1 May 1956
- journal article
- research article
- Published by AIP Publishing in The Journal of Chemical Physics
- Vol. 24 (5) , 1037-1040
- https://doi.org/10.1063/1.1742674
Abstract
Thermal expansion in PbO, SnO, or LiOH, in which single layers of O2— or Li+ alternate with double layers of Pb2+, Sn2+, or (OH)—, respectively, is relatively small normal to the layers (c‐axis). This ``anomaly'' is related to asymmetrical dilatation of the polarizable Pb2+, Sn2+ in the field of O2— anions, or, respectively, to elongation of (OH)— in the field of Li+: When the cation‐anion distance increases with temperature, the resulting decrease in these deformations reduces the spacing between the layers of like polarizable ions and the effective thermal expansion. Structures of this type are not stable for AB compounds if the polarized ion is the anion, but are if it is the cation: As cation polarizability increases with polarization, asymmetric, one‐sided coordination of polarizing anions occurs, the energy being further lowered by dispersion forces acting between dilated cations. On the contrary, anions tightened by cations approach the behavior of rigid ions in coordinating cations more symmetrically in three‐, rather than two‐dimensional networks.Keywords
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