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9Short cite this: Magn. Reson. Solids 21, 19602 (2019)
doi: 10.26907/mrsej-19602
Thermally activated mobility of lithium ions in Li1-xCuSbO4 as
studied by 7Li NMR
D. Gafurov1'2, M.-I. Sturza3, E. Vavilova1'*
1Zavoisky Physical-Technical Institute, FRC KSC of RAS, 420029 Kazan, Russia
2Kazan Federal University, 420008 Kazan, Russia
3Leibniz Institute for Solid State and Materials Research IFW, 01069 Dresden, Germany
* E-mail: jenia.vavilova@gmail.com
(Received December 13, 2019; accepted December 16, 2019; published December 21, 2019)
The Li diffusion in Li1_xCuSbO4 with lithium deficiency x = 0.02,0.05, 0.07 and 0.1 was studied by investigation of motional narrowing of the central 7Li NMR line in the temperature range from 290 K to 470 K. The activation energy of hopping of lithium ions was determined from the temperature dependence of the NMR linewidth. The dependence of the obtained activation energy on the lithium content indicates the opening of motion channels in the crystals at x close to 0.1 due to the formation of free vacancies at the lithium position, which favors the thermally activated hopping of ions.
PACS: 76.60.-k, 76.60.Es, 66.30.H-, 82.47.Aa, 82.56.Lz, 82.56.Na. Keywords: NMR, lithium, diffusion, cathode material, LiCuSbO4.
1. Introduction
Intensive studies of low-dimensional transition metal oxides containing alkali metal ions, such as lithium and sodium, are currently very active, due to the prospects of their practical use as electrode materials for the manufacture of lithium-ion and sodium-ion batteries. Complex mixed oxides of alkali and transition metals exhibit a wide range of unusual physical properties, demonstrating the close connection of their crystal structure and magnetic, lattice and charge subsystems. Low dimensionality, in particular, is favorable to high ionic mobility due to the presence of channels for the movement of ions in such crystal structures [1]. NMR is a powerful method for studying internal structures, structural transformations, ion mobility and interfacial interactions in energy storage and conversion systems. NMR spectroscopy is distinguished from other research methods because it does not require direct contact with the test sample and does not include the use of aggressive media that destroy the test material. In addition, this local method allows one to selectively study the conductivity of interfaces and bulk materials, in particular, in powder samples; it provides data on the properties of the material, and not on grain boundaries. There are many different methods for studying ion mobility using nuclear magnetic resonance [2]. However, when working with transition metal oxides containing paramagnetic ions, it is very difficult to use such popular techniques as relaxometry or magic angle spinning due to the short relaxation time and large inhomogeneous spectral width caused by dipole interaction with paramagnetic ions. In this situation, the study of the temperature evolution of the width and shape of the NMR line becomes an effective method of characterization of ionic mobility [3]. With growth of temperature, the increasing hopping rate and, correspondingly, decreasing correlation time leads to the phenomenon of the so-called motion narrowing of the line. The study of the temperature dependence of the NMR line width allows one to obtain a number
fThis paper was selected at the International Conference "Magnetic Resonance - Current State and Future Perspectives" (EPR-75) , Kazan, September 23-27, 2019. The MRSej Editor, Prof. M.S.Tagirov is responsible for the publication.
Thermally activated mobility of lithium ions in Lil-xCuSbO4 as studied by 7Li NMR
O
Figure 1. The crystal structure of LiCuSbO4 [4].
of parameters characterizing the diffusion of ions in the crystal. The LiCuSbO4 compound is interesting primarily for its low dimensional magnetism [4,5]. Due to the presence of strong Cu-spin frustrations, a successive change of nontrivial correlated phases with increasing magnetic field was found in it. The introduction of defects of various types is expected to have a significant effect on its magnetic and lattice characteristics. Moreover, such a type of defects as vacancies at the lithium position can also change the activation energy and ion mobility, as was observed in a number of other compounds [6-8]. In this work, ionic mobility in LiCuSbO4 compound with various levels of lithium deficiency was investigated by nuclear magnetic resonance.
2. Materials and methods
The 7Li nucleus has a spin S = 3/2 and has a quadrupole moment 7Q = -0.04 barn. The 7Li NMR experiments were performed using a Redstone broadband pulsed spectrometer from Tecmag in the temperature range 290 - 470 K and in a magnetic field 0.725 T. A phase-cycling solid-echo pulse sequence consisting of two 90-degree pulses separated by short delay time was used for collecting the echo signal. Spectra were obtained by the Fourier transformation of the echo signal obtained at a frequency of 11.985 MHz. At relatively low temperatures (close to room temperature), when the line width was large enough, the shape of the spectrum was obtained by the spectrum was obtained as a combination of sections of lines measured at different frequency points with a sequential change in frequency. Polycrystalline sample of Li1_xCuSbO4 (x = 0.02,0.05,0.07 and 0.1) were prepared through solid-state reaction using stoichiometric mixture of dried Li2CO3, CuO and Sb2O5 [4]. In order to confirm the formation of the desired phase, all the samples were characterized by means of conventional powder X-ray diffraction.
3. Results and discussion
The crystal structure of the studied compound LiCuSbO4 is shown in Fig. 1. Cu2+ ions are covalently bonded to four O2- ions. The resulting non-planar CuO4 plaquettes form chains oriented along the a-axis. The chains are interconnected by SbO6 octahedra. The crystal structure contains 2 positions of lithium ions. The NMR response of each of their S = 3/2 nuclei consists of the main line and quadrupole satellites. Due to powder averaging and mainly due to inhomogeneous broadening, the structure of the spectrum is unresolved and it is recorded as one line with a low intensity pedestal. At room temperature, the line has a rather Gaussian shape; with increasing temperature, it acquires the Lorentzian form for all four samples.
Figure 2. Normalized 7Li NMR spectra of Li1_xCuSbO4 with x = 0.02 (left panel) and x = 0.1 (right panel) obtained at frequency 11.985 MHz at three characteristic temperatures 300, 350 and 450 K. Solid lines are drawn to guide the eye.
The line shape transformation for samples with the smallest and greatest lithium deficiency from this series is shown in Fig. 2. To conduct the analysis most correctly, we used further the line width at half maximum as a measured parameter. The temperature dependences of the line width for four samples shown in Fig. 3 demonstrate that at temperature above approximately 350K, the line width decreases sharply and reaches a plateau at T & 420K.
In the compounds with a high concentration of paramagnetic ions, the inhomogeneous line width, which reflects the distribution of local fields generated by the magnetic moments of these ions at the position of the nucleus, is usually determined by the temperature dependence of the magnetic susceptibility of ions. As can be seen from the data on the lash shift in the undoped compound, the bulk and local (Knight shift) susceptibility increases with decreasing temperature below 300 K. Our measurements of all four samples demonstrate that above 300 K the position of the line changed very weakly, almost within the measurement error. Since, as noted above, the line shift reflects the local susceptibility of the electronic subsystem at the position of the nucleus, we can conclude that at the temperatures of our experiments this contribution to the line width should not change significantly. Thus, the line width at a temperature of approximately 320 K is the so-called line width of the rigid lattice. Further narrowing of the line with increasing temperature is associated with an increase in the motion of Li. In this way, if the frequency of the
Thermally activated mobility of lithium ions in Lii-xCuSbO4 as studied by Li NMR
x= 0.02
Temperature (K)
_i___I___I_,_I_
12 ■
10 ■
72. 8 ■
6 ■
x =0.05
Temperature (K)
250 300 350 400 450 500
x= 0.10
Temperature (K)
250 300 350 400 450 500
250 300 350 400 450 500
Figure 3. Temperature dependences of the NMR line width at half maximum for samples with various lithium deficiencies. Solid line is approximation according to Eq. (2).
ion jumps exceeds the inhomogeneous broadening caused by the interaction with paramagnetic ions and other lithium nuclei, the local fields in the nuclei position become more and more averaged. This leads to a narrowing of the line width down to « 5 _ 6 kHz at T reaching 450 K. The line width at these temperatures is determined by processes that are not averaged by the diffusion of ions, primarily the inhomogeneity of the external magnetic field on the sample length. The narrowing process is accompanied by a change in the shape line from Gaussian to Lorentzian.
The ion hopping frequency can be characterized by the correlation time, which varies with temperature in an activation manner according to the Arrhenius law
Thop = To exp(Ea/kBT), (1)
where Ea is the activation energy, kB is the Boltzmann constant, t0 is determined by the characteristic frequencies of local jumps of the ion at high temperatures. The temperature dependence of the line width, thus, allows one to estimate the activation energy of Li diffusion. There are several ways to determine the value of activation energy [9-11]. For a very rough estimation of the activation energy, one can use the empirical Waugh-Fedin formula Ea(eV) = 1.617 ■ 10-3 ■ Tc(K), where Tc is the onset temperature of motional narrowing. Assuming Tc = 320 K we get Ea = 0.517 eV. For more precise determination of activation energy of Li-ion motion in samples with different lithium content we use the function given in ref. [10]:
AvR
Av (T) =
1 + f ^ - l)exp(
+ D,
(2)
V B
kB T
where Av(T) is the line width at half maximum, Avr is the line width for rigid lattice, Ea
is the activation energy, B and D relate respectively to the linewidth of nuclei of thermally activated ions and temperature-independent line width caused, e.g., by the inhomogeneity
