LATTICE-DYNAMICS OF BERLINITE (ALPO4) - A COMPARATIVE-STUDY WITH QUARTZ (SIO2)

We present model calculations for the lattice dynamics of berlinite (A lPO4). The short-range potentials used for the calculation are based o n the ones determined for quartz (SiO2). The polarizability of the oxy gen ion is taken into account by an adiabatically moving shell. A good description both of the experimental frequencies and the high-frequen cy dielectric constants is obtained. Estimates of the ionic as well as effective charges of the constituent ions are given. The deduced char ges in conjunction with the obtained polarizability indicate a similar charge state of the oxygen ion in quartz and berlinite. The ionic cha rges of phosphorus and aluminium differ strongly. The same observation is made for the short-range forces between the cations and oxygen. On the basis of these results the picture of an oxygen framework crystal is discarded. Special effort is devoted to the investigation of equil ibrium conditions. The stability of the respective tetrahedra turns ou t to be of central importance for the dynamics of both quartz and berl inite. Consequences for the alpha- to beta-phase transition are discus sed.