Free energy minimization calculations of negative thermal expansion in AlPO4-17 Original Research Article

The free energy minimization approach has been applied to hexagonal AlPO4-17 to calculate its thermal expansion. Strong negative thermal expansion (NTE) is calculated for both the a and c cell edges in agreement with experiment. This structure allows rocking motions of rigid polyhedra; however, our calculated phonon density of states indicates that vibrations of this type are not important for NTE in this compound. Our calculations confirm that the NTE is due to apparent decreases in bond distances rather than a decrease in Al–O–P angles. This in turn is related to the transverse thermal motion of oxygen in the Al–O–P linkages.