Ionic Mobility, Phase Transitions, and Superionic Conduction in Solid Solutions (100 − x)PbF2-xZrF4 and Crystals K2ZrF6, (NH4)2ZrF6, KSnZrF7, and M(NH4)6Zr4F23 (M = Li, Na)

Abstract

Methods of ( 19 F, 1 H) NMR and impedance spectroscopy are used to investigate the internal mobility and ionic conduction in solid solutions arising in the system PbF 2 -ZrF 4 and polycrystals KSnZrF 7 , Li ( Na )( NH 4 ) 6 Zr 4 F 23 , and M 2 ZrF 6 (M = K, NH 4 ). Factors responsible for the form of ionic motions and their energetics at 170-550 K are considered. It is established that the phase transitions in these compounds are connected with the crystal transition to a superionic state and that the high ionic (superionic) conductivity of beta phases is due to the diffusion of fluoride ions, ammonium cations, and possibly alkali metal cations. The obtained data testify to a substantial role of chainlike aggregation of anionic groupings and a variableness of structural mechanisms of formation of such chains in fluorozirconates for the development of translational diffusion in these compounds.

1H and 19F NMR relaxation time studies in (NH4)2ZrF6 superionic conductor

“…Above 370 K, the 1 H NMR line width starts decreasing and finally reaches a very small value (0.15 G) around 400 K similar to that of a liquid. Narrowing of the NMR signal and the corresponding decrease in T 1 indicates the diffusion of both NH 4 and ZrF 6 ions through the lattice and the high ionic conductivity observed by Kavun et al [11] above 400 K also supports our observation. The 1 H NMR line width above 400 K measured by Kavun et al [11] reaches a limit of the modulation width indicating the diffusion of ammonium ions.…”

“…Narrowing of the NMR signal and the corresponding decrease in T 1 indicates the diffusion of both NH 4 and ZrF 6 ions through the lattice and the high ionic conductivity observed by Kavun et al [11] above 400 K also supports our observation. The 1 H NMR line width above 400 K measured by Kavun et al [11] reaches a limit of the modulation width indicating the diffusion of ammonium ions. The observed T 1 maximum at 240 K may be attributed to the competition between the diffusion and the reorientational motion of the NH 4 ions.…”

“…We have recently [35] observed a similar cross relaxation between 1 H and 19 F in (CH 3 ) 4 NPF 6 . Indeed, in the present compound, Kavun et al [11] have also observed that the 19 F second moment reaches the rigid lattice limit at a much higher temperature of about 250 K. Figure 4 shows fitting of Eq. (2) to the 1 H T 1 data and the simulated graph of the BPP curve for the 19 F T 1 data in the same temperature range.…”

“…Both decrease in T 1 and narrowing of the NMR signal may be attributed to the diffusion of both ammonium and fluorine ions through the lattice. High ionic conductivity above 400 K observed by Kavun et al [11] supports this assumption and is attributed to the sublattice movement of F ions. The observed maximum in fluorine T 1 in the present study around room temperature may be attributed to the competition between the diffusion and the reorientational motion.…”

“…Infrared and Raman active NH 4 -vibrations in AHFZ [4] are found to occur at relatively high wavenumbers, which are characterized by moderate to weak hydrogen bonds [14]. Kavun et al [11] have estimated the activation energy for the translational diffusion of ions. Lalowicz et al [3] have carried out a proton NMR line shape calculation and analysis for AHFZ at 4.2 K and attributed their results to the tunneling rotations of the ammonium ion.…”