Microstructural and ionic transport studies of hydrothermally synthesized lanthanum fluoride nanoparticles

Fast fluoride ion conducting materials exhibit high value of ionic conductivity with negligible electronic conductivity at ambient temperature in comparison to oxygen ion conducting materials and thus find many potential electrochemical device applications.^1–3 Fluoride materials in nano-crystalline form often exhibit higher conductivity in comparison to its micro-crystalline forms due to presence of the larger fraction of interface regions, providing fast migration paths for fluoride ions.^4–6 Earlier review on the synthesis, transport behavior and application aspects of various fast fluoride ion conducting materials has shown that LaF₃ is a potential electrolyte material for a variety of solid state ionic device applications including batteries, chemical sensors and selective ion electrodes to name only a few.² Various reports are known discussing mainly the use of LaF₃ as a solid electrolyte material in different chemical sensors for the detection of fluorine,^7,8 oxygen⁹ and humidity.^10,11 In addition, LaF₃ is also known to be an efficient host matrix for luminescent centers and thus gained a great potential to be used in different optoelectronic devices.^12,13 The structure of most of the fluoride ion conductors are derived from either fluorite or tysonite type. LaF₃ belongs to the family of the rare earth fluorides of type RF₃ (R: La-Sm) exhibiting tysonite structure. In the present work, nanocrystalline LaF₃ is synthesized by hydrothermal method. This synthesis route is widely used for the preparation of various nanoscale materials because of its simple method of operation and furthermore, this synthesis route also overcomes the use of inert gas condition and expensive metal containers.² The structure and formation of the material is confirmed by powder X-Ray Diffraction. The surface morphology of the particles was observed by high resolution scanning electron microscopy technique. The sample homogeneity was affirmed by EDX (Energy Dispersive X-Ray analysis) and Raman spectroscopy techniques. The particle size was calculated by both XRD and high resolution transmission electron microscopy (HRTEM) techniques. Furthermore, the electrical properties of the present material have been investigated by impedance spectroscopy. This technique has been extensively used as a versatile tool in conductivity measurements which tracks the ion motion through the electrical response of the material resulting from the movement of mobile ions and is often used to separate out bulk and interfacial contribution from the ac response of the material.¹⁴ 

Nanocrystalline LaF₃ material was synthesized by hydrothermal method. LaCl₃ and NH₄F were used to serve as the source respectively for lanthanum and fluorine. 1 M LaCl₃ was dissolved in 30 ml of deionized (DI) water and mixed thoroughly on stirring with a magnetic stirrer to get a clear solution. Similarly 3M NH₄F was dissolved in 30 ml of DI water and mixed well in a separate beaker. The above solutions were mixed on stirring to form a well-mixed solution. Then, the suspension solution was transferred to 100 ml Teflon-lined autoclaves for hydrothermal reaction. The autoclaves were sealed and maintained at 423 K for 24hrs in the heating oven. After the reaction was completed, the autoclave was cooled down to room temperature. Further the precipitate was centrifuged and washed several times with DI water and finally the material was dried in at 353 K for few hours.

XRD measurements were carried out using X-ray diffractometer (XRD, Rigaku, D/MAX-IIIC X-ray diffractometer, Tokyo, Japan) with CuK_α radiation (λ = 1.5406 Å). The Rietveld refinement was carried out using GSAS (General Structure Analysis System) program.¹⁵ The grain size and morphology of the particles were characterized by employing a field emission scanning electron microscope (FE-SEM Philips XL30 FEG, Eindhoven, Netherland) and furthermore, the particle size was measured using transmission electron microscope (JEOL 2010 F HRTEM, Japan) with a 200 kV operating voltage. Raman spectra of powders were recorded at room temperature employing a HR 800 Raman spectrophotometer (Jobin Yvon- Horiba, France) using monochromatic He-Ne LASER (632.8 nm), operating at 20 mW. Spectral peaks arising due to different modes were resolved by deconvoluting the spectra using the PeakFit (Version 4.12) software. The Voigt line shape which is a linear combination of Gaussian and Lorentzian line-shapes was used to fit the Raman peaks after background subtraction. The best fit was obtained by using minimum number of peaks with R² value greater than 0.99. Conductivity measurements were performed using impedance spectroscopy (SI1260 + SI1296, Solartron, UK) over the temperature range of 323 K-423 K. For the above measurement, a pellet form of the sample with a diameter of 12 mm had been employed. Both the faces of the pellet serving as the electrodes were smeared with silver powder.

Fig. 1 shows both the experimentally observed and calculated XRD patterns of hydrothermally synthesized LaF₃ after the Rietveld refinement. The value of the weight refinement parameter and goodness of fitting (χ²) are found to be 0.085 and 1.54% respectively. The XRD data is in consistent with the standard JCPDS plot (JCPDS card number 00-032-0483) with no additional reflections confirming the formation of the material with the hexagonal structure (space group: $P 3 ¯ C1$⁠). The lattice parameters of LaF₃ after the refinement is found to be a = b = 7.1885(8) Å, c = 7.3546(7) Å and the results are in agreement with the earlier reported values.¹⁶ The refined atomic positions are given in Table I. The mean crystallite sizes and lattice strain were calculated from the extent of peak broadening of the most intense peak (1 1 1) using Scherrer’s semi-empirical formula after subtracting the instrumental broadening determined from the mono-crystalline silicon diffraction line and respectively found to be 35 nm and 0.55 %. The crystal structure of hexagonal LaF₃ is drawn using crystal maker after considering the Rietvled refinement parameters such as lattice parameters, space group of $P 3 ¯ C1$ and the atomic positions of respective elements of the present material (Table I). LaF₃ crystal exhibits hexagonal structure consisting of three fluorine sub-lattices in the ratio 12:4:2 per unit cell and the immobile lanthanum cations are located on the layers perpendicular to the c-axis.¹⁷ 

Fig. 2 shows the SEM micrograph of hydrothermally synthesized LaF₃ indicating uniform distribution of nearly spherical shape of the particles with grain size of about 37 (±4) nm. The presence of elements La and F in the investigated material is confirmed by EDX as shown in Fig. 2(b). The elemental mapping image of LaF₃ is also shown as an inset of Fig. 2(b). The images leveled La and F are those obtained for the respective elements in the particles showing the chemical homogeneity.

Fig. 3(a) to Figure 3(c) show the high resolution TEM images of the LaF₃ nanoparticles at different magnifications. Particles are seen to be nearly spherical and non-agglomerated with the average size of 35 nm, agreeing well with the calculations from XRD pattern. Debye-Scherrer ring pattern of LaF₃ nanoparticles is shown in Fig. 3(d). Reflections from the (002), (110), (111), (300), (113), (302) and (223) lattice planes are clearly seen, confirming the formation of high pure LaF₃ nanoparticles with hexagonal structure. The inter-planar d-spacing for the (002), (111), (300), (302) and (223) planes are found to be 3.55 Å, 3.14 Å, 1.98 Å, 1.74 Å and 1.41 Å respectively, matching well with the hexagonal LaF₃ structure.

Fig. 4 shows the Raman spectra of hydrothermally synthesized nanocrystalline LaF₃ recorded at room temperature. The vibrational (Raman and IR) active modes for the hexagonal structure of LaF₃ single crystal with point group $D 3 d 4$ $( P 3 ¯ C 1 )$⁠, determined using symmetry adapted modes (SAM) program available on the Bilbao crystallographic server,¹⁸ consists of 17 Raman modes (5A_1g +12_Eg) and 19 IR modes (7A_2u +12_Eu). It is not so surprising that we see only half or one-third of the possible modes unless we do polarization Raman measurements on oriented single crystals. Observed Raman modes corresponding to the wave numbers 225.6 cm⁻¹ and 387.3 cm⁻¹ have A_1g symmetry while the other modes observed at 199 cm⁻¹, 290.8 cm⁻¹, 311 cm⁻¹ and 363.2 cm⁻¹ have E_g symmetry. The well matching observed Raman modes with the characteristic Raman modes of LaF₃ is the further evidence for the phase purity of the LaF₃ nanoparticles.^19,20

Fig. 5 (inset) shows typical impedance plots of the LaF₃ at different temperatures. The complex impedance plots corresponding to the materials having both grain and grain boundaries contributions generally consist of two semicircles followed by a spike at lower frequencies. The resistance and capacitance hence time constant is larger for grain boundaries in comparison to grains.²¹ At lower frequencies, the mobile ions cannot penetrate to the blocking electrodes but are accumulated and depleted at each electrode surface resulting in the appearance of a spike at lower frequency region of the impedance plots. In the present system, the impedance plot at a typical temperature comprises of a single depressed semicircle characterizing the intrinsic bulk grain behavior of the ion transport.²² The absence of an inclined straight line (spike) in the lower frequency regions indicates that the impedance contributions from the electrode-electrolyte interface do not seem to have much significance on the charge of transport of the present material. The depressed semicircles make their centers on a line below the real axis indicating departure from ideal-Debye behavior. The radius of the semicircle decreases with increasing temperature due to increase in conductivity of the material. Such depression may originate from the presence of the distribution of relaxation times within the bulk response. Ideally the impedance assembly related to the process of charge transport in the bulk specimen (ideal Debye case) is represented by parallel combination of R and C where R represents the bulk resistance and capacitance C arises due to the electric relaxation process. The equivalent circuit (inset in Fig. 5) appropriate to the present observed impedance plots consists of the parallel combination of bulk resistance (R_b) and constant phase element (CPE), where CPE is generally considered as a leaky capacitor (i.e. hybrid between a resistor and a capacitor) and its impedance is given by $Z * = 1 B ( j ω ) n$⁠, (0<n<1). At n = 0 and 1, it is respectively, considered to be a pure resistor and a capacitor. In comparison to the equivalent circuit considered for the ideal Debye case, we have replaced C by CPE, which accounts for the observed depression of the semicircle and also the non-ideal electrolyte symmetry.²³ At each temperature, the dc resistance is obtained by analyzing the impedance spectra with Z-view fitting program. The temperature variation of dc conductivity exhibiting Arrhenius relation $σ= σ 0 exp − E a k B T$ is also shown in Fig. 5. Here σ₀ is the pre-exponential factor, E_a is the activation energy, T is the temperature, and k_B is the Boltzmann constant. The activation energy is found to be about 0.50eV, which is close to the value 0.45 ±0.03eV reported by Roos et al. in the same range of temperature. It is generally assumed that the intrinsic point defects in LaF₃ are thermally generated by Schottky mechanism.²⁴ 

Fig. 6 shows the frequency dependence of the conductivity of LaF₃ nanoparticles at various temperatures. At all the temperatures, the curve consists of a frequency independent plateau region corresponding to the dc conductivity. At higher frequencies, the conductivity spectra exhibits dispersion in a power law fashion and as temperature increases, the frequency at which the dispersion becomes prominent shifts to the higher frequency region. The observed behavior can be explained through Almond and West formalism as²⁵ 

Here n is the power law exponent, whose value generally lies between 0 < n < 1 and f_p is the cross over frequency from dc to the dispersive region representing the hopping frequency (f_h) of the mobile ions.²⁶ Both dc conductivity (σ_dc) and hopping frequency (f_h) at each temperature are obtained after fitting the frequency dependent conductivity plots with Equation (1). In the present case, the value of power constant n is around 0.94 to 0.95. The dc conductivity values at different temperatures after the fitting give similar values to those obtained from the complex impedance plots. The dashed line through the solid circles in Fig. 6 shows the log(2σ_dc) vs. log(f_h) plot with a slope of unity (σ_dc ∝ f_h) indicating that the characteristic onset frequency is thermally activated with the same activation energy as dc conductivity. This shows that the activation energy calculated from dc conductivity studies is due to the hopping process. Furthermore, it also indicates that the carrier concentrations of mobile fluoride ions are almost independent of temperature.

There has been renewed interest in the scaling of ac conductivity data for both ion conducting materials and glasses.^26–30 Scaling is nothing but the ability to scale the different data sets into a master curve in order to get common underlying behavior. The ac conductivity isotherms obtained at different temperatures follow a scaling law of the form $σ σ d c =F f f 0$⁠. Here f₀ is the characteristic frequency and F is the scaling function independent of temperature. Fig. 7(a) shows the conductivity master plot using the scaling parameter hopping frequency (f₀ = f_h) for the frequency axis. This type of scaling parameter has been initially employed for the lithium telluride glasses within the composition range 0.1 ≤ x ≤ 0.3.²⁵ It is seen from the Fig. 7(a) that the conductivity spectra at different temperatures have successfully collapsed into a single master curve indicating that the dynamic processes occurring at different frequencies has the same activation energy and time-temperature superposition is fulfilled. Master plot for the complex impedance is shown Fig. 7(b), which further supports temperature independent relaxation behavior of the mobile fluoride ions in LaF₃. The bulk resistance R_b, and $Z max ″$ are used as the scaling parameters for the real and imaginary axes of complex impedance plots.^31,32

Nanocrystalline LaF₃ with average crystallite size of 35nm was successfully synthesized by hydrothermal method after taking the precursors LaCl₃ and NH₄F as the source for lanthanum and fluorine respectively. Both XRD and Raman spectroscopy techniques confirmed the phase formation of the material LaF₃. The crystallite size of nanocrystalline LaF₃ has been calculated by using Scherrer’s semi-empirical formula which was in agreement with those calculated from the HRTEM studies. The high resolution SEM studies confirmed the uniform shape and size of the particles. The plot of log(2σ_dc) vs log(f_h) shows a straight line behavior with slope unity indicated that the conductivity arises due to hopping motions of the charge carriers. Master plots of complex impedance isotherms and frequency dependence conductivity have been successfully obtained indicating temperature independent relaxation behavior.