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.

Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions

“…Re-entrant structural transitions such as that we find in Ca 2 Co 2 O 5 as a function of temperature in the absence of external stimulus (e.g., pressure, composition, magnetic field, etc.) are commonly found in liquid crystals; 49 52 , Ag 7 P 3 S 11 (C2/c→ β-Ag 7 P 3 S 11 , unknown structure→ C2/c) 53 , and BaFe 2 (PO 4 ) 2 (R-3→ P-1→ R-3) 54 .…”

Complex Superstructures of Mo2P4O15

“…In Pn, the equivalent number is 63. This contrasts with many frameworks where high-temperature structures are considerably more symmetric than low, though this is not a thermodynamic requirement (for example, systems with re-entrant phase transitions such as Rochelle's salt, 68,69 Ag 7 P 3 S 11 , 70 and malononitrile 71 involve a high-to-low symmetry transition on warming). In fact, under ambient conditions, Mo 2 P 4 O 15 decomposes before it transforms to the expected simple P2 1 /c subcell structure.…”

NMR Multi-Time Correlation Functions of Ion Dynamics in Solids

“…In detail, (i) ions that occupy the same or periodic sites at the beginning and at the end of the mixing time do contribute since, within experimental resolution, the frequencies are identical at equivalent sites, (ii) ions that reside at different sites, on average, do not contribute, and (iii) rearrangements of neighboring ions have no effect, i.e., single-particle correlations functions are measured. In 109 Ag NMR work on glassy and crystalline silver ion conductors, Δω i typically exceeds Δω d by about 3 orders of magnitude so that the above condition can be met [20][21][22][23][24][25][26][27]. Specifically, all 109 Ag NMR MT-CF of the present contribution were measured using an evolution time t p = 100μs.…”