Preparation, characterization and dielectric tunability of La2NiMnO6 ceramics

Preparation,characterization and dielectric tunability of La 2NiMnO 6ceramicsChangnian Li a ,Bangxu Liu b ,Yanyan He a ,Chao Lv a ,Hua He a ,Yebin Xu a ,⇑a School of Optics and Electronic Information,Huazhong University of Science and Technology,Wuhan,Hubei 430074,China bSchool of Electrical and Electronic Engineering,Huazhong University of Science and Technology,Wuhan,Hubei 430074,Chinaa r t i c l e i n f o Article history:Received 26October 2013Received in revised form 17December 2013Accepted 18December 2013Available online 28December 2013Keywords:CeramicsOxide materials Sol–gel processes Dielectric responsea b s t r a c tLa 2NiMnO 6powders were prepared by using a polyvinyl alcohol (PVA)sol–gel route.Differential scanning calorimetry (DSC),thermogravimetric (TG)analysis,Fourier transform infrared spectroscopy (FT-IR),X-ray diffraction (XRD)and field emission scanning electron microscopy (FESEM)were used to characterize the precursors and the derived oxide powders.Single-phase La 2NiMnO 6of about 40–50nm diameters was synthesized at 600°C for 2h,without the formation of any intermediate 2NiMnO 6ceramics sintered at 1000°C had tunability of 28%and 20%at 8V/mm at 100kHz and 1MHz,respectively.The electric field dependence of dielectric constant of the ceramics was well described by using the ‘‘multi-polarization mechanism’’model.Ó2013Elsevier B.V.All rights reserved.1.IntroductionMagnetodielectric materials,which have a dielectric constant that is modulated by applying magnetic field,are expected to lead to novel devices,including capacitive magnetic field sensors and tunable high-frequency filters [1–3].Double-perovskite La 2NiMnO 6,a magnetic semiconductor,has received considerable attention recently because the material has a relatively high Curie temperature (T c )of 280K [4]and a colossal magnetodielectric coupling (up to $20%)over a wide temperature range across room temperature [5,6].A giant tunability of dielectric permittivity was also reported in bulk La 2NiMnO 6[7].Unlike conventional ferroelectrics,such as barium strontium titanate [8,9],La 2NiMnO 6shows low voltage tunability.The structure of La 2NiMnO 6is rhom-bohedral at high temperatures and transforms to monoclinic or orthorhombic [5,10–12]at low temperatures.Both high tempera-ture rhombohedral and low temperature monoclinic/orthorhombic phases coexist over a significantly wide temperature range,across room temperature [5,11,12].Generally,La 2NiMnO 6was prepared by using solid-state reactions at high temperatures [5,7,13–15].Several wet chemical methods,including Pechini method [4,6,11,16,17],glycine–nitrate method [12,18],ethylene glycol gel method [19]and nitrate decomposition method [10],have been used to synthesize La 2NiMnO 6and La 2NiMnO 6nanoparticles [16,17,19].It is reported that synthetic conditions affected thecrystallographic forms of La 2NiMnO 6[4,12,16–19].Samples prepared by the glycine–nitrate method at temperatures of less than 500°C in air are orthorhombic;annealing the samples at 1300°C produces La 2NiMnO 6phase with rhombohedral symmetry [18].La 2NiMnO 6synthesized by using the Pechini method in air at 550–800°C is rhombohedral;samples heated to T >800°C in air were biphasic:monoclinic and rhombohedral [4].La 2NiMnO 6nanoparticles prepared by the same Pechini method showed orthorhombic symmetry [18,19].Chandrasekhar et al.[19]prepared La 2NiMnO 6nanoparticles at 600°C by ethylene glycol gel method,with coexistence of rhombohedral and orthorhombic phases.More recently,Mao et al.[20]synthesized La 2BMnO 6(B =Ni and Co)nanoparticles using a molten-salt reaction at 700°C in air.Kumar and Kaur [21]reported the structural and magnetic properties of La 2NiMnO 6thin films deposited using the pulsed laser deposition technique.The aim of this paper is to synthesize La 2NiMnO 6nanoparticles by using PVA sol–gel method and study the dielectric tunability of La 2NiMnO 6ceramics.PVA route has been used successfully to syn-thesize various monophase,fine and pure mixed-oxide powders [22–27],but no literature currently reported the synthesis of La 2NiMnO 6.On the other hand,although dielectric tunable properties of La 2NiMnO 6has been reported,the relationship between dielec-tric constant and electric field has not well studied.In the present paper,PVA sol–gel method was used to prepare nanocrystalline La 2NiMnO 6powder.Single phase La 2NiMnO 6was prepared at 600°C.Electric field dependence of the dielectric constant of La 2NiMnO 6ceramics was well described by using the ‘‘multi-polarization mechanism’’model.0925-8388/$-see front matter Ó2013Elsevier B.V.All rights reserved./10.1016/j.jallcom.2013.12.170Corresponding author.Tel.:+862787543855;fax:+862787543427.E-mail address:xuyebin@ (Y.Xu).2.Experimental proceduresLa2NiMnO6nanoparticles were synthesized by using a PVA sol–gel route.The similar method has been described previously[24–27].High purity La2O3,MnCO3 and Ni(NO3)2Á6H2O were used as the starting ingredients.5%PVA solution was made by dissolving PVA(MW=79,000)in deionized water at80°2O3and MnCO3powders were dissolved in nitric acid and mixed with the PVA solution.Fi-nally,stoichiometric Ni(NO3)2Á6H2O was added to the solution.The molar ratio of hydroxyl groups of PVA to metal ions(OH/M)was1.5.With continuous heating at80°C under constant stirring to evaporate superfluous water,the volume of the solution decreased and the solution viscosity increased continuously.A colloid was formed with the evolution of NO x gas resulting from decomposition of the ni-trate ions.Throughout the process,no precipitation was observed.Then,the sample was heated in an oven at250°C for2h.The colloid swelled and self-ignited with a large volume of gases.The auto-ignition resulted in a voluminous La2NiMnO6pre-cursor powder.The as prepared powder was calcined at400–800°C for2h in air.Simultaneous differential scanning calorimetry(STA449F3,Netzsch,Germany) was used to monitor decomposition and pyrolysis of the precursor at a heating rate of10°C/min in static air.Fourier transform infrared spectroscopy(FT-IR,Vertex70, Bruker Optik GmbH,Ettlingen,Germany)was used to determine chemical bonding of the La2NiMnO6precursor and powders.Crystalline phases were identified by using powder X-ray diffraction(XRD)with Cu K a radiation(XRD-7000S,Shimadzu Corp.,Kyoto,Japan).Field-emission scanning electron microscopy(FESEM,JSM-7600F,JEOL,Kyoto,Japan)was used to observe grain size and the morphology of La2NiMnO6powders.The La2NiMnO6powders calcined at800°C for2h were pressed into disks of 12mm in diameter and1–2mm in thickness.The disks were sintered in air at 1000°C for3h and then electroded with silver paint.Dielectric constant was mea-sured by using an Agilent4294A precision impedance analyzer(Agilent Technolo-gies,Santa Clara,CA)at25°C.XRD and SEM were used to characterize phase compositions and microstructure of the sintered samples.3.Results and discussionFig.1shows DSC–TG curve of the La2NiMnO6precursor.The weight loss in the temperature range between room temperature and600°C is12.3%.Thefirst broad endothermic peak between 70°C and230°C in the DSC curve,accompanied by a mass loss of about4.3%in the TG curve,is due to dehydration of the precur-sor.Thefirst exothermal peak at about400°C in the DSC curve in-volves the decomposition of nitrates and the carbonization and volatilization of PVA.The second small exothermal peak around 600°C,accompanied by a sharp mass loss of about3.8%in the TG curve,is associated with oxidation of residual organics and the crystallization of La2NiMnO6.Fig.2shows FT-IR spectra of the La2NiMnO6precursor and pow-ders calcined at400–600°C.For the La2NiMnO6precursor,the broad absorption band at3448cmÀ1can be assigned to O–H stretching.The absorption band at about1489and1388cmÀ1 can be assigned to the splitting of the m3asymmetric stretching of metal carbonates[28],while those at about1068and 850cmÀ1are attributed to the m1and m2modes of the carbonate ions,respectively[29].For the precursor calcined at400°C,the intensities of bands related to carbonate decreased.After calcining at500°C,the absorption due to carbonate decreased greatly and a well-established strong absorption band at606cmÀ1indicates the formation of La2NiMnO6.For perovskite ABO3,the B–O bond stretching vibration of the BO6octahedra was observed in the 500–700cmÀ1[30].The606cmÀ1band is due to the joint action of the Ni–O and Mn–O stretching vibrations.After calcining at 600°C,all the peaks of carbonate disappeared.Fig.3shows XRD patterns of the La2NiMnO6precursor calcined at various temperatures for2h.The precursor is XRD amorphous, as characterized by the broad continuum.After heating the precur-sor at400°C for2h,the powder is still amorphous.At500°C,all the reflections correspond to perovskite structure,indicating the formation of La2NiMnO6.According to the FT-IR spectra,there ex-ists a small amount of amorphous carbonate.After calcining at 600–800°C,the diffraction peaks become stronger and sharper, reflecting higher crystallization.Therefore,we can concludethat542 C.Li et al./Journal of Alloys and Compounds590(2014)541–545single-phase La2NiMnO6was formed at600°C.The phase has rhombohedral structure[4].Lattice parameters calculated by using a least-squares refinement program[31]are a=0.551003nm,and=1.323343nm,which are comparable to those reported previously[11,12].This low-temperature rhombohedral phase is different from the orthorhombic phase reported by Joseph et al.[18]in air at T<500°C.The main function of PVA is to provide a polymeric network to hinder cation mobility,allowing local stoichiometry to be main-tained and minimize precipitation of unwanted phases.In aqueous PVA solutions,many metals can be stabilized by the polymer through interactions with OH groups[23].Pure La2NiMnO6that can be obtained at a low temperature is attributed to the molecular level mixing of the cations in the solution.Fig.4shows FESEM micrographs of the La2NiMnO6precursor calcined at600°C and800°C 2NiMnO6nanoparticles with a diameter of40–50nm were obtained at600°C.The sample calcined at800°C showed larger particle size(80–90nm).On the other hand,the sample was partially sintered,so that neck forma-tion between particles is clearly observed.Fig.5shows XRD pattern of the La2NiMnO6ceramics sintered at 1000°C for2h.It is similar to that of the powder calcined at 800°C.SEM photograph of the La2NiMnO6ceramics is shown in Fig.6.The sample is porous due to the low sintering temperature.Fig.7shows dc bias electricfield dependence of dielectric con-stant of the La2NiMnO6ceramics at100kHz and1MHz at room temperature.The dielectric constant decreases from749at 100kHz to320at1MHz,showing obvious dielectric dispersion. Loss tangent of the sample at1MHz is about9,which is compara-ble with that reported in Ref.[7].The high loss tangent is due to the high porosity.A non-linearity is observed,with a tendency towards saturation at$5V/mm.Tunability is calculated byt¼ðeð0ÞÀeðEÞÞ=eð0Þ¼ðCð0ÞÀCðEÞÞ=Cð0Þwhere C is the capacitance and E is the applied dc electricfield.At a given appliedfield E=8V/mm,tunability of the La2NiMnO6ceram-ics at100kHz and1MHz are28%and20%,respectively.These values are lower than that reported by Tang et al.[7].Their sample was at1400°C.However,we found that the La2NiMnO6ceramics sintered at1300°C had too low resistance,and dielectric constant cannot be measured.This difference can be attribute to the differ-ence in synthesis methods.The lower tunability of our samples as compared with the reported data in Ref.[7]is due to its porosity and the different crystal structure.Fig.4.FESEM micrographs of La2NiMnO6precursor calcined at(a)600°C and(b)800°C for2h.Fig.5.XRD pattern of La2NiMnO6ceramics sintered at1000°C for2h.Fig.6.The SEM photograph of La2NiMnO6ceramics sintered at1000°C for2h.The tunability of ferroelectrics is generally explained by using the phenomenological theory of Devonshire,as described by the Johnson equation[32]:e E e E0¼1ð1þa e30e3E0E2Þ1ð1Þwhere e0is the vacuum dielectric constant,E is the external dc bias field,e E0is the dielectric constant at zero bias,e E is the dielectric constant at E,and a is the anharmonic coefficient that is supposed to be an order parameter of the anharmonic interactions.In order to further understand the dielectric nonlinear behavior of the La2NiMnO6,the dielectric constants under bias electricfield was fitted to the phenomenological Johnson equation,but poorfitted results were obtained.In this case,a more sophisticated model should be considered,by taking into account extrinsic polarization contribution.Chen and Zhi[33]introduced a‘‘reorientational polarization’’to describe the‘‘extrinsic’’contribution to the dielectric constant and a multi-polarization-mechanism model was suggested,with the following equatione E¼e E0ð1þa e30e3E0E2Þ1þP0xe0½coshðExÞÀ2ð2ÞIt was adopted to describe thefield dependence of the dielectric constant,where x=P0V/k B T,and P0is the polarization of a polar nanoregion,V is the volume of a nanoregion,k B is the Boltzmann constant and T is the temperature[33].Thefirst term of the equa-tion,i.e.the Johnson term,represents the intrinsic contribution, while the second term,ngevin term,represents the extrinsic reorientational polarization[33].Using Eq.(2),the dielectric con-stant-electricfield data of the La2NiMnO6ceramics were wellfitted (Fig.5)at100kHz and1MHz to be:e E¼574:4ð1þ0:00601E2Þ13þ165:0½coshð0:45398EÞ À2ð3Þe E¼265:2ð1þ0:00183E2Þ13þ55:2½coshð0:43833EÞ À2ð4ÞAccording to thefitted Johnson contribution,the calculated ‘‘intrinsic’’tunability at8V/mm was10%,at both100kHz and 1MHz,indicating that the intrinsic contribution to tunability is independent on frequency.Therefore,the electricfield dependence of dielectric constant of the La2NiMnO6ceramics are well described by including the additional‘‘extrinsic’’contribution,instead of the Johnson’s approximation.Tang et al.[7]argued that the tunability of La2NiMnO6ceramics was intrinsically related to electronic ferroelectricity due to the charge ordering of Ni2+and Mn4+.Our present work shows that extrinsic reorientational polarization has contribution to the tun-ability.The‘‘extrinsic’’tunability of the La2NiMnO6ceramics at 100kHz at8V/mm is18%,exceeding the‘‘intrinsic’’tunability of 10%.The‘‘reorientational polarization’’is more likely arisen from polar nanoregion[17,33],i.e.,mesoscopic regions with local polar-ization ascribed to the most well known relaxor ferroelectrics.The inhomogeneous distribution of the B-site cations(due to the coex-istence of two B-site cations Ni2+and Mn4+)with different charges and chemical bonding characteristics in the double perovskite structure is responsible for the quenched random electricfield (originating from charged compositionalfluctuations in the nano-particles)and hence polarized nano-regions[17,34].4.ConclusionsPVA sol–gel method has been used to synthesize La2NiMnO6. Phase pure rhombohedral phase La2NiMnO6powders of 40–50nm in diameter were obtained at600°C for2h,without the formation of any intermediate 2NiMnO6ceramics have tunability of28%and20%at8V/mm at100kHz and1MHz, respectively.The electricfield dependence of dielectric constant can be well described by using the‘‘multi-polarization mecha-nism’’model.The‘‘extrinsic’’polarization due to‘‘reorientation of the polar nanoregions’’has contribution to the tunability of the La2NiMnO6ceramics.AcknowledgmentsThis work was supported by the Natural Science Foundation of China under Grant Nos.10975055and60771021.The authors wish to acknowledge the Analytical and Testing Center in Huazhong University of Science and Technology for FT-IR analysis. 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