  In this paper, we investigate analytically the experimental observations of
structural collapse and Tc enhancement in the rare earth-doped 122-type
iron-based pnictide superconductors [S. R. Saha et.al. Phys. Rev. B 85, 024525
(2012), arXiv:1105.4798 (2011); B. Lv et al. PNAS 108, 15705 (2011).]. Based on
the real-space effective c-axis lattice constant theory of superconductivity
[X. Q. Huang, arXiv:1001.5067], it is shown that the abrupt c-axis reduction of
the superconductors is due to the structural phase transition (an
ultra-short-range symmetry breaking) of the charge stripe lattice. The
existence of this phase transition corresponds to the change from the
full-doped superconducting planes to the half-doped superconducting planes in
the studied superconductors. It is estimated that this phase transition may
help to promote the superconducting transition temperature of the 122 family up
to as high as 80 K.
