Re-entrant phase transition of the crystalline ion conductor Ag 7P 3S 11

Abstract

A highly unusual structural evolution has been observed in temperature dependent studies of the fast ion conductor Ag 7P 3S 11, using X-ray diffraction, Raman scattering, 31P and 109Ag NMR spectroscopy, and electrical conductivity measurements. At 205 K the high-temperature γ-phase (space group C 2 / c ) undergoes a phase transition to an intermediate β-phase of different symmetry. At a temperature near 130 K another phase transition is observed resulting in the formation of an ordered low-temperature α-modification crystallizing in the same space group as the γ-phase. Restoration of the high-temperature-phase symmetry in the low-temperature phase is unambiguously confirmed by single-crystal X-ray structure determination and 31P solid state NMR peak multiplicities. The re-entrant phase behavior is further supported by temperature dependent electrical conductivity measurements, which reveal that the activation energies of the dc conductivity for the α- and γ-phases are identical and significantly lower compared to those measured in the β-phase. Although the β- to γ-phase transition is associated with a change in enthalpy, those observables reflecting silver ion dynamics show no discontinuities at the phase transition temperature. The high-temperature γ-phase crystallizes in the monoclinic system, space group C 2 / c (No. 15), a = 23.999 ( 2 ) Å , b = 6.3621 ( 3 ) Å , c = 24.909 ( 2 ) Å , β = 110.926 ( 7 ) ° , R = 0.0318 (300 K). The low-temperature α-phase is isostructural with a = 24.090 ( 1 ) Å , b = 6.3400 ( 3 ) Å , c = 24.581 ( 1 ) Å , β = 110.870 ( 6 ) ° , R = 0.0317 (120 K). Contrary to the situation in γ-Ag 7P 3S 11, all the silver atoms are well-localized in the α-phase.