Metal-Organic Precursor Synthesis, Structural Characterization, and Multiferroic Properties of GdFeO3 Nanoparticles.

GdFeO3 nanoparticles were fabricated by a facile metal-organic precursor method using citric acid as a complexing agent. The phase purity and structural analysis by powder X-ray diffraction and FTIR studies indicates that the material is highly crystalline with an orthorhombic structure. Electron microscopic (TEM and SEM) studies of rare earth ferrites reveal worm-shaped nanoparticles with an average grain size of 95 nm. The high-resolution TEM study provides an insightful image, which shows an interplanar spacing of approximately 0.12 nm that corresponds to the (112) crystalline plane. A high surface area of 231.5 m2 g-1 has been achieved with a mesoporous texture, which in turn gives a high dielectric constant. Well-defined hysteresis is obtained with a saturation magnetization of 17.5 emu g-1, remanent magnetization of 3.9 emu g-1, and coercive field of -446 Oe. Room-temperature ferroelectricity in GdFeO3 nanoparticles has been found for the first time with no leaky current and hence may be used in multistate memory devices.

Introduction

Ternary perovskite-type oxides have gained growing interest at the nanoscale due to their special properties and applications such as a high dielectric constant and surface area, photocatalysis, electrocatalysis, optical properties, and magnetic and ferroelectric interactions.[1−4] Multiferroic materials exhibit multifunctional properties and depict enormous applications, such as memories, hydrogen evolution, sensors, and actuators, as a result of the coexistence of at least two ferroic orders.[5−8] These ferroic properties may be ferrotoroidicity, ferroelasticity, or, most commonly, ferroelectricity and ferromagnetism.[9] The main focus of technological devices demands the fabrication of innovative materials at the nanoscale so that their multiferroic properties must allow the fine-tuning of new applications.[8] Particular attention has been given to the synthesis of nanocrystalline orthoferrite oxides due to the huge diversity of their functional properties, which make them ideal candidates for electrical and magnetic properties.[8,10] Earlier reports have shown strong magnetoelectric coupling by the single crystals of DyFeO3 and GdFeO3, but the multiferroic nature appears only at very low temperatures.[11] The synthesis strategy of multiferroic GdFeO3 has attracted the attention of researchers in order to obtain the ultrapure powders using low-temperature chemical routes with room-temperature ferroelectricity. The low-temperature method includes solvothermal,[12,13] hydrothermal,[14] reverse micelle,[15,16] polymeric citrate precursor,[17] and sonochemical[18] methods used for the fabrication of various metal oxide nanoparticles. Though several reports are available on hydrothermal, sol–gel, colloidal, co-precipitation, and combustion synthesis of GdFeO3,[19−22] there are less reports that discuss the perceptible room-temperature ferroelectricity in GdFeO3 nanoparticles. GdFeO3 has a perovskite distorted ABO3-type structure, and there exists tilting of the rigid FeO6 octahedral symmetry. When the A-site cation is too small for its 12-coordinate cavity in the cubic perovskite structure, it breaks the symmetry of GdFeO3 and through the exchange interaction between Gd and Fe, spin ferroelectric polarization is generated in GdFeO3 nanoparticles. Recently, room-temperature ferroelectricity was observed in nanocrystalline YMnO3 and YCrO3 prepared by an organic precursor route.[23,24] In earlier reports, room-temperature ferroelectricity was reported in GdFeO3 nanoparticles prepared by a chemical co-precipitation method but a large leakage current was a major drawback.[25] While in the single crystal of GdFeO3, the ferroelectric property was found at 2 K.[26] In this paper, we report the synthesis of pure nanocrystalline GdFeO3 by a low-temperature facile polymeric citrate precursor method based on complexation between ethylene glycol and citric acid. The structure, morphology, electrical, and magnetic properties of the as-synthesized samples were characterized by the XRD, SEM, BET, LCR meter, P–E loop tracer, and SQUID magnetometer techniques. In addition to significant ferromagnetic characteristics in GdFeO3 nanoparticles, room-temperature ferroelectricity with better results was achieved.

Experimental Section

The chemical reagents used in the work were Gd(NO3)3·xH2O (Alfa Aesar, 99.9%), Fe(NO3)3·9H2O (Rankem, 98%), citric acid (Spectrochem, 99%), and ethylene glycol (SD Fine-Chem Ltd., 99%). All the chemicals were of analytical grade purity and used without any further purification. For the synthesis of GdFeO3, an as-prepared 0.1 M solution of ferric salt was added in the beaker containing 1.4 mL of ethylene glycol followed by addition of 21 g of citric acid. Then, the mixture was left for 2 h under stirring at room temperature to obtain a clear solution. Further, 25 mL of 0.1 M aqueous solution of Gd3+ salt was added followed by continuous stirring for 2 h. The solution was further heated at 70 °C for 2 h to accelerate the polyesterification reaction between ethylene glycol and citric acid. The formed viscous gel was heated in muffle furnace at 135 °C for 20 h to evaporate the excess of solvent. The resin was then charred at 300 °C for 2 h in the same muffle furnace wherein it turned into a black mass precursor, which was frivolously ground to finely ground powder using a Teflon stick. Based on the existence of crystallization peaks and weight loss, the GdFeO3 was calcined at 900 °C in a programmable furnace for 12 h in order to achieve a good crystalline structure. Powder GdFeO3 thus obtained was brownish red in color. The stabilization of nanoparticles happens at the expense of a decrease in availability of the grain size area. Since the volume to surface area ratio decreases, the particle size increases and thus the surface energy also decreases. The increase in the sintering temperature increases the stability of nanoparticles at the cost of a decrease in the specific surface area.[25] A schematic representation of the reaction between ethylene glycol, citric acid, and metal ions (Gd3+ and Fe3+) is shown in Figure .

Figure 1

Schematic representation of the condensation process between metal cations (Fe3+ and Gd3+ ions), citric acid, and ethylene glycol in the polymeric precursor method.

Powder X-ray diffraction studies have been carried out using a Bruker D8 Advance X-ray diffractometer, which was fitted with Ni-filtered Cu Kα radiation of wavelength 1.54056 Å. The diffraction data was recorded in the 2θ range of 20° to 70° with a step size of 0.05° at an interval of 1 s. The Kα2 reflections were removed by a stripping procedure to obtain accurate lattice constants. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer spectrometer of Model 1750. Transmission electron microscopic (TEM) studies were carried out using an FEI TEM 200 kV (model Tecnai G2 20) electron microscope equipped with digital imaging and a 35 mm photography system. TEM specimens have been prepared by using a small amount of the powder sample and dispersed in ethanol on an ultrasonicator (UP-500, Ultrasonic Processor) and sonicated for 30 min at the optimized intensity. An approximately 100 μL volume of the sonicated sample was placed on a carbon-coated copper grid. Scanning electron microscopic (SEM) measurements have been carried out on a new versatile series ZEISS EVO 50 equipped with 0.2 to 30 kV acceleration voltages having a magnification range from 15× to 200,000× and a resolution of 5 nanometers.

The surface area of GdFeO3 nanoparticles was determined at 77 K using a BET surface area analyzer (model: Nova 2000e, Quantachrome Instruments Limited, USA) with the help of a multipoint BET method. Nearly 0.08 g of the sample was placed in the cell and allowed to degas in vacuum at 250 °C for 3 h, which removes the contaminants such as moisture and adsorbed gases from the sample. The degassed powder was then analyzed for data collection by admitting the known quantities of the adsorbed dry N2 gas into the sample cell containing the solid adsorbate. For dielectric and ferroelectric studies, a pellet of 8 mm in diameter was prepared by mixing 100 mg of GdFeO3 nanoparticles with 2–3 drops of 5% aqueous solution of polyvinyl alcohol. The slurry was allowed to dry and the dried material was then reground and pressed with an applied 5 tons of pressure. The pellet was then sintered in air at 1000 °C for 8 h for further measurements. Silver paste (Ted Pella, Inc.) was applied to both faces before drying the pellet at 100 °C in an oven. Dielectric properties were measured on the as-prepared sample using a high-frequency LCR meter (model: 6505P, Wayne Kerr, UK) in a 100 kHz to 1 MHz frequency range and a temperature range of 25–430 °C. The room temperature ferroelectric measurements were carried out on the sintered pellet at a 50 kHz frequency and varying applied voltages on the P–E loop tracer (M/S Radiant Instruments, USA). Magnetic properties have been studied using MPMS with a SQUID magnetometer at an external magnetic field of ±60 kOe in the temperature range of 5–300 K.

Results and Discussion

The X-ray diffraction pattern of the GdFeO3 sample is shown in Figure . All the reflections could be satisfactorily indexed in an orthorhombic structure of GdFeO3 (JCPDS: 74-1900). Note that no extra peak due to any other impurity phase such as α-Fe2O3 and Gd2O3 has been found that attributes to the monophasic formation of GdFeO3 at 900 °C. The significantly high intensity and appearance of distinct reflections may be associated to the good crystallinity in as-prepared biferroic nanoparticles. The average particle size of GdFeO3 nanopowder was calculated from the diffraction pattern using Scherrer’s formula[27] given bywhere D, λ = 1.5418 Å, β, K = 0.91, and θ are the crystallite size, wavelength of X-ray, full width at half-maximum (FWHM) of the diffraction pattern, Scherrer’s constant, and the diffraction angle, respectively. The values of β are extracted from fitting the peak to Gaussian distribution. The calculated average grain size is found to be 92 nm.

Figure 2

Powder X-ray diffraction pattern of GdFeO3 nanoparticles.

FTIR investigation further supports the formation of the as-prepared GdFeO3. Figure a,b shows the FTIR spectra of the GdFeO3 precursor after charring and calcination at 300 and 900 °C, respectively. Before calcination, the vibrations at 1623 and 1383 cm–1 are seen, which are attributed to the carbonyl stretching peak (esterification between citric acid, ethylene glycol, and metal ions).[28,29] On the other hand, the characteristic band at 3437.8 cm–1 is associated to the atmospheric water vapor vibrations, while the band at 2354 cm–1 is attributed to the carbon dioxide stretching mode. After calcination at 900 °C, the carbonyl peak diminishes, indicating the removal of organic species. In Figure b of the FTIR spectra after calcination, the bands obtained in the range of 560–590 cm–1 are assigned to υ(Fe–O) and υ(Gd–O) in Fe–O–Fe and Gd–O–Fe systems, respectively, confirming the formation of GdFeO3. The band at 592.4 cm–1 could be assigned to this characteristic stretching.[30,31] The appearance of the IR band at 395 cm–1 corresponds to the O–Fe–O deformation vibration. At a lower wave number, the prominent bands are characteristic of oxygen–metal stretching modes that confirm the formation of GdFeO3 after 900 °C calcination.

Figure 3

FTIR (a) before and (b) after calcination of GdFeO3.

SEM, TEM, and HRTEM studies have been investigated to examine the morphology, size, and lattice spacing of the as-prepared samples. SEM and TEM images at different resolutions showed the formation of a roughly undefined morphology of nanoparticles having an average particle size of 95 nm with a narrow size distribution as shown in Figure a,b. The average TEM particle size is found to be slightly larger than the XRD crystallite size of 92 nm as obtained by the Scherrer’s studies. The GdFeO3 nanoparticles are highly magnetized and hence could not well disperse; thus, the particles were in an agglomerated form as shown in the TEM/SEM micrographs. The high-resolution TEM image of GdFeO3 nanoparticles shows the presence of lattice fringes (interlayer fringes ≈ 12 Å) as shown in Figure c. This lattice fringe spacing matches with the interplanar ‘d’ spacing of GdFeO3 nanoparticles in the [112] direction. The corresponding selected area electron diffraction (SAED) image of GdFeO3 nanoparticles is shown in Figure d. The appearance of sharp diffraction spots in a circular pattern confirms the nanocrystalline nature and indexes with orthorhombic GdFeO3. The elemental mapping of the GdFeO3 nanoparticles is shown in Figure e, which confirms the presence of Gd, Fe, and O in the as-prepared sample. Note that the experimentally loaded composition was found to have a very close agreement with the EDAX spectra as shown in Figure f. The proportions of the constituents obtained from the weight percentages as seen in the table of the EDAX spectrum indicates that the molar ratio of two metal ions (Gd3+:Fe3+) are approximately close to a 1:1 ratio and they have good stoichiometry and pure chemical compositions. The table further displays the experimental composition, which shows the 1:1 mole ratio, confirming the Gd:Fe nearly equal mole ratio.

Figure 4

(a) SEM, (b) TEM, (c) HRTEM, (d) SAED and (e) elemental mapping of Gd, Fe, and O, and (f) EDAX spectra; the inset shows table of elemental compositions of GdFeO3 nanoparticles.

The nitrogen adsorption isotherm and the BET plot of GdFeO3 nanoparticles (Figure a,b) shows the prominent hysteresis loop of the type-IV isotherm, which could be associated with the capillary condensation in mesopores.[32] The specific surface area of GdFeO3 nanoparticles is found to be 231.5 m2 g–1, which has been found to be comparatively much higher than earlier reports.[19,33] The pore size calculation of the nanoparticles for the mesopore size distribution has been carried out by the Barrett–Joyner–Halenda (BJH) method. Figure c,d shows the Barrett–Joyner–Halenda (BJH) and Dubinin–Astakhov (DA) plots of GdFeO3 nanoparticles with the later displayed pore radius of 15 Å. As shown in Figure c, the BJH pore size distribution curve confirmed the predominance of mesopores of a radius of 18.3 Å for GdFeO3 nanoparticles. The positive BET constant value from the BET surface area studies of GdFeO3 was found to be as high as 3.34. This shows the high affinity of solid GdFeO3 with the adsorbate (the N2 molecules), which may lead to the high heat of adsorption.

Figure 5

(a) Nitrogen adsorption isotherm, (b) BET plot, and (c) BJH and (d) DA plots for the pore size distribution of as-prepared GdFeO3 nanoparticles.

The variation of dielectric constant (ε) and dielectric loss (D) of GdFeO3 nanoparticles with the frequency and temperature is shown in Figure . The dielectric constant and dielectric loss were found to decrease with the increase in frequency (Figure a), which might be associated to the failure of electric dipoles to follow the alternating applied field, and this can be justified on the basis of Maxwell–Wagner interfacial polarization.[34,35] The temperature dependence of the dielectric constant shows a slight increase below 300 °C; however, beyond which, a rapid increase of the dielectric constant with the temperature could be seen as shown in Figure b and was corroborated to the reports of other ferrites.[36] This may be attributed to the fact that the temperature enhances the orientation of dipoles; however a low-frequency dielectric constant is strongly dependent due to the interfacial and dipolar polarizations. The accumulation of charges on the grain boundaries and more charge carriers get excitation from their trapping centers, which contribute to the polarization carrier and lead to the high dielectric constant. The dielectric loss decreases with the increase in frequency and achieve saturation in the higher frequency range. The high dielectric loss of GdFeO3 nanoparticles originates from electron hopping and local structural distortions. With the increase in frequency, the electron hopping decreases and thus the dielectric loss decreases at a higher frequency range. The high dielectric constant and low dielectric loss of as-prepared nanoparticles could be attributed to large amounts of nano-size porosities with distinct grain and grain boundary structures. Dielectric properties increase at high temperatures, which is due to increase in thermally activated drift mobility of electric charges.[36] The dc conductivity (σdc) is plotted as a function of reciprocal temperature (Figure c) and it well obeys the Arrhenius relation, , where σo is the pre-exponential term and Econd is abbreviated for the conduction activation energy. Econd was calculated from the slope of the straight line of logσdc versus the 1000/T plot and was found to be 0.28 eV, which shows that the grain boundary defects and ionic charges are the main carriers of thermal conductivity in as-prepared GdFeO3 nanoparticles.[37]

Figure 6

Variation of ε and D with the (a) frequency and (b) temperature and (c) showing the temperature dependence of dc conductivity of GdFeO3 nanoparticles.

Temperature and magnetic field dependent dc magnetization measurements were carried out using a superconducting quantum interference device magnetometer under an external magnetic field of ±60 kOe at temperatures ranging from 5 to 300 K (Figure ). The susceptibility of GdFeO3 nanoparticles was studied as a function of temperature at a constant magnetic field of 1 kOe as shown in Figure a. The molar magnetic susceptibility (χM) decreases with the increasing temperature; however, the susceptibility anomaly could be seen at 13.5 K, which is associated to the magnetic ordering in the nanoparticles. The result is found to be consistent with Curie–Weiss law in the paramagnetic region. According to the Curie–Weiss law, the temperature dependence of the inverse susceptibility (χ–1) was fitted with a positive extrapolated Weiss temperature (θp ≈ 175 K) as shown in Figure a, which implies the ferromagnetic interaction. The ferromagnetic hysteresis loop at 5 K of GdFeO3 nanoparticles is shown in Figure b, which has a coercive field (Hc) of approximately −446 Oe and saturation (Ms) and remanent (Mr) magnetizations of ∼17.5 and 3.9 emu g–1, respectively. The weak ferromagnetic behavior shown by GdFeO3 may be due to the distortion from the ideal perovskite structure, resulting in the alignment of Fe3+ ions slightly canted, which gives the small net magnetization. The total magnetization in the temperature range 5–300 K corresponds to the sum of iron containing sublattices and the paramagnetic gadolinium sub lattices.[38] The enhanced magnetic properties were observed in this study as compared to the previous reports.[19−22,32,39] Hence, we conclude that this method is able to prepare nanocrystalline orthoferrite powders with improved ferromagnetic characteristics.

Figure 7

(a) Temperature dependence of molar and inverse molar susceptibility and (b) M-H curve at 5 K of as-prepared GdFeO3 nanoparticles. Inset a shows a closer look of magnetic ordering and inset of b is a magnified version of the M-H graph.

Polarization–electric field (P–E) hysteresis measurements of GdFeO3 nanoparticles at different applied electric fields are shown in Figure at a 50 kHz frequency. The appearance of prominent hysteresis at room temperature with a remanent polarization (Pr) of 0.014 μC/cm2, saturation polarization (Ps) of 0.052 μC/cm2, and coercive field (Ec) of −2.001 kV/cm is attributed to the ferroelectric characteristics in GdFeO3 nanoparticles. The ferroelectric loop of GdFeO3 nanoparticles is found to be dependent on the applied electric field, where as the applied field increases, the area of loop increases. Both the saturated and remanent ferroelectric polarizations increase by the increase in the applied electric field, and this observation could be associated to the potential application of as-prepared GdFeO3 nanoparticles in nonvolatile multistate memory devices. The ferroelectric property may be elaborated by the canted antiferromagnetic ordering with two non-equivalent spin pairs of GdFeO3, which led the room-temperature ferroelectric hysteresis to be comparable to that of the SmFeO3-type compound.[40] The ferroelectricity was reported earlier in the single crystal of GdFeO3 at a low temperature of 2 K; however, a large leakage current at room temperature was reported.[25,26] In the case of bulk orthoferrite, YFeO3-type compounds have been found to be non-ferroelectric nature, but as the particle size is decreased, ferroelectric polarization is observed at room temperature. In the recently reported paper, the effect of cation stoichiometry on the ferroelectricity by growing a sample using a Y3Fe5O12 (yttrium iron garnet, YIG) were investigated in that results of an antisite defect mechanism for room-temperature ferroelectricity but a large leakage current were observed, which could be a major drawback in spintronics applications.[41] In another recent study, in orthorhombic RMnO3 (R = rare-earth cation) perovskites, specifically SmMnO3 that is reported have room-temperature ferroelectricity prepared by nano-engineering room temperature, ferroelectricity has been found due to a spin-driven mechanism.[42,43] We report the room-temperature ferroelectricity with a negligible leakage current in GdFeO3 nanoparticles for the first time to the best our knowledge, which possess wide applications.

Figure 8

(a) Full and (b) closer view of P-E hysteresis loops of GdFeO3 nanoparticles at different electric fields and measured at 50 kHz.

Conclusions

Monophasic and nanocrystalline GdFeO3 (95 nm) has been prepared with a high surface area (231.5 m2 g–1) using a metal–organic precursor route. The nanoparticles were further extensively investigated by means of HRTEM, SAED, and EDAX studies. The appearance of well-established hysteresis in electrical and magnetic properties with enhanced parameters sets its multiferroic characteristics. The room-temperature ferroelectricity was found for the first time, for which we believe it may have potential applications in multistate memory devices.