Monopolar polarization of crystals

[Stanley514]

This one article mention such strange issue as monopole polarization
in highly asymmetric crystals.

http://prb.aps.org/abstract/PRB/v32/i10/p6832_1

(I did not read entire article).
I always thought that polarization could be only dipolar because you
need to get charge separation.How monopole thing could exist in a
crystal?

 

[DrDu]

As far as I can tell, he models the elementary cell as an arrangement of both point charges and dipoles, both polarizable. Hence there is also an indirect interaction of the point charges via the polarized lattice. This is not in contradiction with the multipole expansion of an elementary cell not containing a monopole contribution as he does not consider a multipole series around a single point but several points inside one cell.