Re‐Entrant Phase Transition of the Crystalline Ion Conductor Ag₇P₃S₁₁.

Abstract

A highly unusual structural evolution has been observed in temperature dependent studies of the fast ion conductor Ag 7 P 3 S 11 , using X-ray diffraction, Raman scattering, 31 P and 109 Ag NMR spectroscopy, and electrical conductivity measurements. At 205 K the high-temperature γ -phase (space group C2/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 31 P 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 C2/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 7 P 3 S 11 , all the silver atoms are well-localized in the α-phase.