Small bipolarons and extended states of guest Na and Rb atoms in quasi-two-dimensional disorderedM7.8−δAl7.8Si8.2O32.0(M=Na, Rb)

Abstract

The evolution of the electronic properties of guest Na and Rb atoms in a disordered deformable lattice is investigated for a series of guest-atom densities $n$. The quasi-two-dimensional host ${M}_{7.8\ensuremath{-}\ensuremath{\delta}}{\mathrm{Al}}_{7.8}{\mathrm{Si}}_{8.2}{\mathrm{O}}_{32.0}$ ($M$=Na, Rb), known as zeolite P, is used. The Na system is a stubborn bipolaronic insulator to the maximum $n$ of 1.03. In contrast, the Rb system exhibits a crossover from a bipolaronic insulator to a conducting phase analogous to a disordered metal at $n$ = 0.89. A critical region undergoing polaronic melting appears in the vicinity of the crossover on the insulating side, evidenced by a reduction in the small bipolaron absorption band and a drop in the activation energy. Transition to the conducting phase coincides with the appearance of a midinfrared band and an increase in the charge-carrier decay length, suggesting the polaronic and extended nature of the carriers. These findings constitute rare examples of electron-lattice coupling opening (or closing) a mobility gap and scaling the continuity (or discontinuity) of a conducting transition in the face of disorder.