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Stereo-chemically active lone pairs are typically described as an importantnon-bonding effect, and large interest has centered on understanding thederived effect of lone pair expression on physical properties such as thethermal conductivity. To manipulate such properties, it is essential tounderstand the conditions that lead to lone pair expression and to provide aquantitative chemical description. Here we first use density functional theorycalculations to establish the presence of stereo-chemically active lone pairson antimony in $\text{MnSb}_{2}\text{O}_{4}$. The lone pairs are formed througha similar mechanism to those in binary post-transition metal compounds in anoxidation state of two less than their main group number, where the degree oforbital interaction determines the expression of the lone pair. In$\text{MnSb}_{2}\text{O}_{4}$ the Sb lone pairs interact through a void spacein the crystal structure, and they minimize their mutual repulsion byintroducing a deflection angle. This angle increases significantly withdecreasing Sb-Sb distance, thus showing the highly destabilizing nature of thelone pair interactions. Analysis of the chemical bonding in the structure showsthat it is dominated by polar covalent interactions. A database search ofrelated ternary chalcogenide structures shows that for structures with a lonepair the degree of lone pair expression is largely determined by whether theantimony-chalcogen units are connected or not, suggesting a cooperative effect.Isolated $\text{SbX}_3$ units have larger X-Sb-X bond angles, and thereforeweaker lone pair expression than connected units. Since increased lone pairexpression is equivalent to an increased orbital interaction (covalentbonding), which typically leads to increased heat conduction, this can explainthe previously established correlation between larger bond angles and lowerthermal conductivity.

Expression and interactions of stereo-chemically active lone pairs and their relation to structural distortions and thermal conductivity