Nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4

Abstract

Superprotonic conduction in the LiH2PO4 system has been studied by means of high-resolution nuclear magnetic resonance measurements, which enabled us to distinguish dynamics of the two different hydrogen bonds in the structure. The protonic motion, primarily associated with the longer hydrogen bond, rather than the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system.

Impedance spectroscopy of the superprotonic conduction in LiH2PO4

“…[12][13][14] Our high-resolution nuclear magnetic resonance (NMR) study has enabled us to distinguish dynamics of the two different hydrogen bonds in the LDP system. 15 The protonic motion, primarily associated with the longer hydrogen bond, rather than with the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system. The conduction mechanism of KDP-type crystals in the paraelectric phase has been suggested to have to do with the interbond proton motion coupled with the rotational motion of the PO 4 tetrahedral group.…”

“…The activation energy of 0.37 eV obtained from the electrical conductivity is compatible with the value of 0.34 eV obtained from the 31 P NMR relaxation time measurements. 15,24,25 Figure 3 shows the temperature dependence of the dielectric constant of LDP at various frequencies, a dielectric dispersion, characteristic of superionic conduction, being more pronounced at low frequencies. [26][27][28][29] The proton motion is seen to be dynamically frozen below 210 K. The temperature of a broad peak in e 0 depending on the measuring frequency as shown by the arrows in Fig.…”

“…30 Our results thus appear to indicate that both the electrical conductivity and the dielectric dispersion arise from mobile charges, here the mobile hydrogen ions. 15 Figure 4 shows the Cole-Cole plots of the complex impedance of the LDP system obtained at several temperatures. The real (Z 0 ) and imaginary (Z 0 ) parts of the complex imped-ance were well fitted by the equivalent circuit models depending on the temperature ranges (see the insets of Fig.…”

“…Besides an additional component in the intermediate temperature region marks the emergence of an inhomogeneous local electrical environments. 15 Figure 6 shows the temperature dependence of the dielectric constants and the exponents n obtained from the fits in Fig. 4.…”

“…The anomalies of the dielectric constant and the exponent n around 240 K appear to be closely related with the dynamics of the longer hydrogen bond as revealed by motional narrowing in the 1 H NMR lineshape measurements. 15 In summary, we have employed impedance spectroscopy in order to study the nature of the superprotonic conductivity in the LiH 2 PO 4 system. The temperature-dependent dielectric constant displayed a broad peak shifting toward the high temperatures with increasing measuring frequency, a dielectric dispersion associated with room-temperature superionic conduction being manifest.…”