Nuclear magnetic resonance study of the ...

Nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4 Jin Jung Kweon, Kyu Won Lee, Cheol Eui Lee, and Kwang-Sei Lee Citation: Appl. Phys. Lett. 98, 262903 (2011); doi: 10.1063/1.3605245 View online: http://dx.doi.org/10.1063/1.3605245 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v98/i26 Published by the American Institute of Physics.

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APPLIED PHYSICS LETTERS 98, 262903 (2011)

Nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4 Jin Jung Kweon,1 Kyu Won Lee,1 Cheol Eui Lee,1,a) and Kwang-Sei Lee2 1

Department of Physics and Institute for Nano Science, Korea University, Seoul 136-713, Korea Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Gimhae 621-749, Korea 2

(Received 9 April 2011; accepted 2 June 2011; published online 28 June 2011) 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 C 2011 American Institute of Physics. high electrical conductivity of the system. V [doi:10.1063/1.3605245]

Recently, proton conduction in solids and biophysical processes has attracted considerable interest. In particular, hydrogen-bonded proton conductors, which can be used in fuel cells in the intermediate temperature range, are recently attracting great interest due to their potential as electrolytes in fuel cells.1–3 The AH2PO4-type (A ¼ Liþ, Kþ, Rbþ, Csþ, Tlþ, etc.) crystals show various physical properties depending on the ionic radius of the cation A and the hydrogen bond lengths.4 Notably, CsH2PO4 (CDP) has been studied for application of its superprotonic phase.5,6 LiH2PO4 (LDP), which belongs to the family of KH2PO4 (KDP)-type hydrogen-bonded ferroelectrics, has Li of very light mass in the cation position in its orthorhombic structure.7 Prominent electrical conductivity8 that is significantly higher than that of CDP distinguishes LDP from other KDP-family members.5,6,9 The crystal structure of LDP has been studied by x-ray diffraction, and recently by single-crystal neutron diffraction,10 which elucidated two types of hydrogen bonds with different bond lengths.4,10 Thus, distinct proton transport can take place in the LDP system possessing two different hydrogen-bond types each with a long bond length 2.677 A and a short one of 2.561 A, presumably having to do with superprotonic and normal protonic conduction, respectively.11 The well-separated high-resolution nuclear magnetic resonance (NMR) line components of the inequivalent proton sites in LDP have provided us with a unique opportunity to study the distinct proton dynamics associated with the different hydrogen-bond lengths and to reveal the origin of the high electrical conductivity of the system.12–19 In particular, unusual nuclear magnetic relaxation behaviors of the two inequivalent proton sites in LDP are addressed in this work.8,10,20–22 LDP polycrystalline powder (99%), purchased from Aldrich, was purified by recrystallization in deionized water. The 1H magic-angle spinning (MAS) NMR lineshape and spin-lattice relaxation time (T1) measurements on the LDP system were performed with a Varian Unity INFINITY plus 200 NMR spectrometer at a Larmor frequency of 200.0 MHz with a spinning rate of 11 kHz. The 7Li and 31P MAS NMR

measurements were performed at Larmor frequencies of 77.7 MHz and 80.9 MHz, respectively. The 1H, 7Li, and 31P NMR spectra were referenced by using tetramethylsilane, 1-M LiCl, and 1-M H3PO4, respectively. The electrical conductivity measurements were carried out on a powder pellet of LDP by using a conventional two-probe technique with Keithley 4200. Figure 1 shows the temperature evolution of the 1H MAS NMR spectra, which were well fitted by two separate Lorentzian line components corresponding to two crystallographically different sites.10 The line component with the smaller chemical shift can be attributed to the H1 hydrogen site of the longer hydrogen bond and that with the greater chemical shift to the H2 hydrogen site of the shorter hydrogen bond.10,23,24 Figure 2 shows the temperature dependencies of the full-width at half-maximum (FWHM) linewidths of 1H MAS NMR line components attributed to H1 and H2 and that of the 31P MAS NMR lineshape. It is interesting to note an abrupt motional narrowing of the H1 NMR linewidth around

a)

FIG. 1. (Color online) Temperature evolution of the 1H MAS NMR lineshape. Inset: temperature dependence of the electrical conductivity.

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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FIG. 2. (Color online) Temperature dependencies of the 1H and NMR FWHM linewidths.

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P MAS

230 K, where an abrupt increase in the electrical conductivity takes place (see inset of Fig. 1). Thus, the H1 proton motion can be taken to be directly coupled to the electrical conductivity of LDP.25 In comparison to the case of H1, motional narrowing of phosphorus takes place at lower temperatures and that of H2 at higher temperatures. It is to be noted that interbond proton hopping is generally believed to occur in conjunction with the reorientational motion of the PO4 tetrahedra.26 The 1H NMR spin-lattice relaxation for both H1 and H2, marked by a discontinuity at TD ¼ 215 K, was well fitted by a single-exponential function below TD, whereas stretchedexponential fitting, with M(t) ¼ M(0) exp[-(t/T1)1n], was necessary above TD. The exponent n represents the degree of random distribution of the correlation time and thus that of the inhomogeneity of the microscopic environment.27 The spin-lattice relaxation for H1 was well fitted with a fixed value of n ¼ 0.35 for the stretched-exponential fitting at all temperatures above TD. On the other hand, temperature-dependent values of n were necessary for the stretched-exponential fitting of H2 between TD and 350 K, above which temperature, the spin-lattice relaxation was well fitted by a single-exponential function (inset of Fig. 3). The spin-lattice relaxation of 31P NMR, which was well fitted by a single-exponential function, consisted mostly of a component with a very long time constant (T1 longer than 600 s at 300 K) and a small portion of just a few percent, relaxing with a much shorter time constant (0.1 s at room temperature). Figure 3 shows the temperature dependencies of the 1H NMR spin-lattice relaxation rate (1/T1) measurements, together with that of the fast-relaxing component of the 31P NMR spin-lattice relaxation rate. While a discontinuity is manifest at TD in Fig. 3, no anomaly was seen in the heat capacity or in the crystal structure.10 Thus, the abrupt change taking place in 1H NMR appears to have to do with proton dynamics rather than with a structural phase transition. The 1 H NMR spin-lattice relaxation in LDP is in fact quite intriguing; while nearly identical spin-lattice relaxation is observed below TD for H1 and H2 as would usually be expected,28 quite distinct behaviors in the spin-lattice relaxa-

FIG. 3. (Color online) Temperature dependencies of the 1H and 31P NMR spin-lattice relaxation rates (1/T1). The solid lines represent fits to the BPP model. Inset: temperature dependence of the exponent n for H2 of the stretched-exponential spin-lattice relaxation.

tion take place above TD, which may be attributed to the very different dynamics of H1 and H2 as revealed also by the lineshape measurements (Figs. 1 and 2).29,30 The 1H and 31P NMR spin-lattice relaxation rates in the high-temperature motional narrowing regime were well fitted by the Bloembergen-Purcell-Pound (BPP) model,31 from which activation energies of 0.14 eV for H1 and 0.27 eV for H2, respectively, were obtained. In the case of 31P NMR, an activation energy of 0.34 eV was obtained. The 7Li NMR linewidth showed little temperature dependence, with only a slight motional narrowing taking place above TD (Fig. 4). Besides, the very long 7Li NMR T1 [440 s at 300 K], especially in view of the quite short proton NMR T1’s above TD, indicates that the lithium ion is quite immobile,32 in excellent agreement with our neutron diffraction study.10 Unlike in superionic conductors in which the lithium ionic motion dictates the electrical conductivity, the immobile lithium in LDP makes little contribution to the electrical conductivity, whereas the H1 proton motion of the longer hydrogen bond is found to be dominant. Thus, the LDP

FIG. 4. Temperature dependence of the 7Li MAS NMR FWHM linewidth.


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system that exhibits outstanding electrical conductivity turns out to be a superprotonic conductor rather than a Li ionic conductor as may generally be supposed. In summary, we have studied the microscopic dynamics in the LiH2PO4 system with two distinct hydrogen bond lengths, exhibiting outstanding electrical conductivity, by means of high-resolution NMR techniques. As a result, the nature of the superprotonic conduction in the system was revealed in a comprehensive manner; An onset of an abrupt increase in the electrical conductivity was found to be accompanied by motional narrowing of the NMR lineshapes and a discontinuity in the proton NMR spin-lattice relaxation. Besides, unique proton nuclear magnetic relaxation behaviors, arising from the distinct proton dynamics of the two crystallographically inequivalent hydrogen sites, were identified. This work was supported by the National Research Foundation of Korea (Cyclotron Users Program 20110018400 and NRL Program 2011-0018628). The measurements at the Korean Basic Science Institute (KBSI) are gratefully acknowledged. 1

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