  A "corkscrew" mechanism, that couples changes in the in-plane rotation angle
to strains along the layering axis, has been proposed previously to explain
increased compliance in certain Ruddlesden-Popper phases that facilitates
uniaxial negative thermal expansion over a wide temperature range. Following
the procedure developed to study many simple, auxetic geometries, in the
present study we derive the elastic compliances predicted by this corkscrew
mechanism assuming that the four shortest metal-anion bonds remain stiff and
changes in bond angle are modelled by a harmonic angle potential. We
subsequently analyse the limitations of this model and show that it may be
extended to An+1BnO3n+1 Ruddlesden-Popper oxide phases of general layer
thickness n.
