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Materials Chemistry and Physics 130 (2011) 39– 44

Contents lists available at ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

acile synthesis, thermal, magnetic, Raman characterizations of spinel structurenMn2O4

. Lia,b, B. Songc, W.J. Wanga, X.L. Chena,∗

Research & Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,hinese Academy of Sciences, PO Box 603, Beijing 100190, ChinaCollege of Chemistry and Molecular Engineering, Peking University, Beijing 100871, ChinaAcademy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China

r t i c l e i n f o

rticle history:eceived 23 August 2010eceived in revised form 15 March 2011

a b s t r a c t

Spinel structure ZnMn2O4 was synthesized by a facile method with lower reaction temperature, shorterreaction time and without regrinding starting materials compared with the traditional solid state reac-tions. A reaction mechanism was deduced according to the analysis results. The obtained ZnMn2O4 shows

ccepted 27 April 2011

eywords:xideshemical synthesishermal properties

high crystalline quality with high degree of dispersion for metals ions and the absence of phase separationor clustering. ZnMn2O4 shows a phase transition at about 600 ◦C instead of a decomposition reaction inthe temperature range of 20–1100 ◦C and an antiferromagnetic behavior with the Néel temperature ofabout 15 K. Six Raman modes are observed in the room temperature Raman spectrum of ZnMn2O4.

© 2011 Elsevier B.V. All rights reserved.

agnetic properties

. Introduction

Manganese spinel oxides have attracted much attention due toheir wide technological applications from lithium rechargeableatteries, catalysis, electrode materials for advanced batteries, sen-or, to anticorrosive coating owing to their excellent structural,hemical and physical properties [1–11]. ZnMn2O4, for example,s one of the important manganese spinel oxides with tetrahe-ral (A) and octahedral (B) voids of oxygen sublattices, centered byn2+ and Mn3+ ions, respectively (Fig. 1A). Zn2+ with O ions neigh-oring them constitute tetrahedral ZnO4 groups and Mn3+ with

ions neighboring them constitute octahedral site MnO6 groupsFig. 1A). ZnMn2O4 have attracted great attentions due to its widepplications in practical devices such as negative temperature coef-cient (NTC) thermistors [6], cathode materials of the secondaryatteries [11], excellent high-temperature materials [6] and cata-

yst [12]. Furthermore, it is an effective precursor for the synthesisf porous materials, which have attracted intense research inter-sts for potential applications in the medication, catalyst and so on13]. Consequently, lots of efforts are paid to synthesize and studyroperties of ZnMn2O4. ZnMn2O4 is traditionally synthesized by a

olid-state reaction (SSR) method usually with zinc oxide (ZnO) andanganese oxides (usually �-MnO) as starting materials [14–16].owever, SSR method has shortcomings of high reaction tempera-

∗ Corresponding author. Tel.: +86 10 8264 9036; fax: +86 10 8264 9646.E-mail address: chenx29@aphy.iphy.ac.cn (X.L. Chen).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.04.072

ture (usually above 1000 ◦C), long reaction time (from several daysto several months), and suspension during the reaction to regrindstarting materials [6]. Recently, a more facile method using oxalateprecursor to obtain ZnMn2O4 is developed [22,23]. The precipitatedoxalate needs calcinations at low temperature. The calcined pow-ders then were pressed into pellets and heated at high temperature.It is seen that this method also needs several process to obtainZnMn2O4. On the other hand, there are still many open questionsconcerning the properties of ZnMn2O4 [1,6]. For example, Chhoret al. [17] found a phase transition phenomenon of ZnMn2O4 at230 K or 271 K with strong metastability effects from the DTA curve.They reported the phase transition phenomenon had no relation-ship with any structural change. However, Driessens and Rieck [7]reported that ZnMn2O4 was stable in air below 1100 ◦C while areversible transformation from tetragonal to cubic occurred above1100 ◦C. Aiyama [18] observed a Néel transition temperature of∼200 K in ZnMn2O4, which is extremely high for a collinear spinarray of the Mn3+ ions at adjacent octahedral sites. Åsbrink et al.[1] found the disappearance of a helical spin arrangement between48 K and 80 K. Recently, Troyanchuk et al. [19] observed a weakchange in the slope of the �(T) curve in the temperature inter-val of 4.2–50 K. Consequently, new methods with lower reactiontemperature, shorter reaction time and without regrinding start-ing materials are demanded to synthesize high crystalline quality

ZnMn2O4. The properties of ZnMn2O4 are also needed to furtherstudy seriously. It is well known that three factors may influencethe properties of ZnMn2O4: the degree of dispersion for metalsions in oxide matrix, particles size and the aggregation state of the

40 H. Li et al. / Materials Chemistry and Physics 130 (2011) 39– 44

Table 1Molar ratios of Zn(OAc)2 and MnCl2·6H2O at starting materials and obtainedproducts.

The molarratios ofZn(Ac)2 toMnCl2·6H2O

T (◦C) Obtained products

9:1 (A) 800 ZnO + Mn3−xZnxO4

8:2 (B) 800 ZnO + Mn3−xZnxO4

7:3 (C) 800 ZnO + Mn3−xZnxO4

6:4 (D) 800 ZnMn2O4 + Mn3−xZnxO4

5:5 (E) 800 ZnMn2O4

4:6 (F) 800 ZnMn2O4

3:7 (G) 800 ZnMn2O4 + impurityphase

4:6 (H) 700 ZnMn2O4

5:5 (I) 700 ZnMn2O4

4:6 (J) 600 ZnMn2O4

5:5 (K) 600 ZnMn2O4

4:6 (L) 500 ZnMn2O4 + Mn3O4

prisdotatipt

2

2

pwaawLa

2

aa(map(CfMd

with the lattice spacing of (1 1 2) planes of ZnMn O .

5:5 (M) 500 ZnMn2O4 + Mn3O4

articles. Here, we develop a facile effective method with lowereaction temperatures, shorter reaction time and without pressingnto pellets, regrinding the starting materials during reaction toynthesize high crystalline quality ZnMn2O4 with high dispersionegree of the two metal ions and the absence of phase separationr clustering. A reaction mechanism is proposed for the forma-ion of ZnMn2O4 based on our experimental results. The thermalnd magnetic properties of ZnMn2O4 are investigated. It is foundhat the sample is stable in the temperature range of 20–1100 ◦Cn air and magnetic properties measurements indicate the sam-le shows antiferromagnetism (AFM) order with a Néel transitionemperature about 15 K.

. Experimental

.1. Preparation of ZnMn2O4

High purity Zn(CH3COO)2·2H2O [Zn(OAc)2] and MnCl2·6H2O without furtherurification were used as starting materials. In a typical run, Zn(OAc)2 and MnCl2eighted at predetermined molar ratio (summarized in Table 1) were successively

dded into a beaker containing 100 ml distilled water. Then, the beaker was heatedt 100 ◦C until the distilled water was consumed away. Then, the obtained mixtureas grinded uniformly in an agate mortar, followed by loaded into an Al2O3 crucible.

astly, the crucible was heated to the desired temperature (500–850 ◦C) for 20 h in muffle furnace. Finally, black-brown powders were obtained and characterized.

.2. Characterizations

Phase analysis was characterized by an X-ray diffraction (XRD) performed on MAC-M18XHF diffractometer with Cu K� (� = 0.154178 nm) radiation at 50 kVnd 200 mA. Transmission electron microscopy (TEM) and high-resolution TEMHRTEM) (Philips CM 200 FEG) observations were performed to obtain the detailed

orphologies and structures information. The element compositions were char-cterized by energy dispersive X-ray spectroscopy (EDS) and inductively coupledlasma-atomic emission spectrometry (ICP-AES). X-ray photoelectron spectroscopyXPS, VG ESCAlab MKII) was engaged using Al K� radiation (h� = 1486.6 eV). A

P-G high-temperature differential thermal instrument was employed to per-orm thermal difference analysis (DTA) and thermo gravimetric analysis (TGA).

agnetic properties were measured on a superconducting quantum interferenceevice (SQUID, MPMS-7) magnetometer. Raman spectrum was collected at room

Fig. 1. (A) Crystal structure of ZnMn2O4 and (B) XRD patterns of samples D, E and F.

temperature by a multichannel modular triple Raman system (JY-64000) using the532 nm line of a solid-state laser as the excitation source.

3. Results and discussion

The molar ratios of the starting materials, reaction temperature,and phase compositions of the products are summarized in Table 1.Fig. 1B shows the typical XRD pattern of samples D, E, and F. Allthe diffraction peaks are well indexed based on a tetragonal spinelZnMn2O4 cell (ICDD-PDF: 24-1133, space group: I41/amd, the struc-ture is shown in Fig. 1A, indicating that high purity tetragonalstructured ZnMn2O4 with space group of I41/amd was synthesized.Lattice constants of the obtained ZnMn2O4 of samples D, E and Fare a = 5.7152 A and c = 9.2380 A, a = 5.7153 A and c = 9.2422 A, anda = 5.7160 A and c = 9.2546 A calculated using Dicvol program [20].The lattice constants increase with the increase of MnCl2·6H2O inthe starting materials. The c/a is 1.61, 1.62, 1.62 at room tempera-ture, respectively, consistent with the value of spinel ZnMn2O4 [21],suggesting a high spin state of Mn3+ in the ZnMn2O4. Only Zn, Mn,and O elements are detected by EDX (Fig. 2B) in samples E and F. ICP-AES analysis further reveals the atomic molar ratio of Zn:Mn:O is1:2:4, well consistent with the predetermined ratio, implying highdispersion degree of the two metal ions in the obtained sample.

Fig. 2A shows a typical low magnification TEM image ofZnMn2O4 of sample E. The obtained ZnMn2O4 has irregular shapewith an average size of ∼600–700 nm. Corresponding spot pat-terns of selected area electron diffraction (SAED) and HRTEM latticefringes, as shown in panels C and D of Fig. 2, reveal the single crys-talline nature of the as-prepared ZnMn2O4 and the absence of phaseseparation or clustering in the obtained sample. The lattice spacingcalculated from the HRTEM (Fig. 2D) is 0.3033 nm, well agreement

2 4The precursors with various molar ratios of Zn(OAc)2 to

MnCl2·6H2O were selected as the starting materials. The obtainedproducts were then characterized by XRD. It is found that the

H. Li et al. / Materials Chemistry and Physics 130 (2011) 39– 44 41

F 4, (C)

Z image

pcmsi3(pwmommgmoZt

tZa5wTmm

Zb

wl

ig. 2. (A) Low-magnification TEM image of ZnMn2O4 particle. (B) EDX of ZnMn2OnMn2O4 particle, the left top inset shows the HRTEM of the area circled of HRTEM

roducts are strongly dependent on the molar ratio of the pre-ursor. To study the relationship between the products and theolar ratio of the starting materials, the reaction temperature was

et to 800 ◦C. When the molar ratio of Zn(OAc)2 and MnCl2·6H2Oncreased into 9:1, 8:2, and 7:3, the main phase is ZnO (ICDD-PDF:6-1451) coexisted with a minority of Mn3−xZnxO4 (1.1 < x < 1.7)samples A, B and C) (Supporting Information, Fig. S1). In sam-les A, B, and C, the Mn3−xZnxO4 (1.1 < x < 1.7) amount increasesith the increase of the molar ratio of MnCl2·6H2O in the startingaterials, determined from the XRD patterns. With the molar ratio

f Zn(OAc)2 and MnCl2·6H2O increased into 6:4 (sample D), theain phase becomes tetragonal ZnMn2O4 phase coexisted with ainority cubic Mn3−xZnxO4 (1.1 < x < 1.7) spinel phase [22,31]. Sin-

le phase ZnMn2O4 (samples E and F) is only obtained with theolar ratio of Zn(OAc)2 to MnCl2·6H2O is 4:6 and 5:5. Thus, in

rder to obtain high purity ZnMn2O4, the proper molar ratio ofn(OAc)2 and MnCl2·6H2O is 5:5–4:6 when the reaction tempera-ure is 800 ◦C.

The relationship between phase compositions of products andemperature is also investigated, as summarized in Table 1. SinglenMn2O4 powder is obtained when the reaction temperature isbove 600 ◦C with the molar ratio of the starting materials is 4:6 and:5 (samples H, I, J, K). However, Mn3O4 coexisted with ZnMn2O4hen the reaction temperature is below 600 ◦C (samples L and M).

herefore, to obtain high purity ZnMn2O4, the reaction temperatureust be higher than 600 ◦C when the molar ratio of the startingaterials is 4:6 and 5:5.The reaction mechanism and the reaction process for the

nMn2O4 formation is easily deduced from the relationship

etween the product and the molar ratio of the starting materials.

First, the residual solid (precursor) after the evaporation ofater which is used to mix Zn(OAc)2 and MnCl2·6H2O were col-

ected and characterized. Fig. 3 shows the XRD patterns of the

SAED pattern of ZnMn2O4 particle and (D) HRTEM image of the fringe of the same.

precursor of samples D, E, and F. It is seen that the Zn(OAc)2hydrolyzed into Zn(OH)2 (ICDD-PDF: 48-1066). Partly Zn(OH)2decomposed into ZnO (ICDD-PDF: 36-1451) and H2O accordingto Eq. (1) upon decomposition of the zinc acetate. MnCl2·6H2Ofirstly hydrolyzed into MnCl2·2H2O (ICDD-PDF: 25-1043), then intoMnCl2·H2O (ICDD-PDF: 01-0184), finally into MnCl2, consistentwith the reported results [24]. Then, the undecompensed Zn(OH)2decomposed into ZnO and H2O according to Eq. (1) and MnCl2·6H2Odecomposed into Mn metal and Cl2 gas according to Eq. (2) whenthe mixture of the starting materials are heated in the muffle fur-nace [24].

Zn(OH)2�−→ZnO + H2O (1)

MnCl2�−→Mn + Cl2 ↑ (2)

Then, the resultant Mn metal coming from the decompositionof MnCl2 react with O2 coming from air according to Eq. (3), leadingto the formation of MnO2.

Mn + O2�−→MnO2 (3)

If MnO2 is insufficient, all resultant MnO2 react with a fraction ofZnO according to Eq. (4), resulting in the formation of ZnxMn3−xO4(1.1 < x < 1.7). As a result, ZnO and ZnxMn3−xO4 (1.1 < x < 1.7) mix-ture will be obtained.

2xZnO + 2(3 − x)MnO2 = 2Mn3−xZnxO4 + (2 − x)O2 (4)

If MnO2 is enough, the redundant MnO2 will react with a fraction

of Mn3−xZnxO4 (1.1 < x < 1.7) according to Eq. (5), resulting in theformation of ZnMn2O4.

Mn3−xZnxO4 + 3(x − 1)MnO2�−→xZnMn2O4 + (x − 1)O2 (5)

42 H. Li et al. / Materials Chemistry and Physics 130 (2011) 39– 44

F mpleF

4

EtrTrtpcr[

FZ

ig. 3. XRD patterns of precursors of the samples. (A) XRD pattern of precursor of sa.

When the molar ratio of the starting materials is proper (5:5 or:6), high purity ZnMn2O4 will be obtained.

The thermal stability of the obtained ZnMn2O4 of sample was studied by DTA and TGA, performed using CP-G high-emperature differential thermal instrument in the temperatureange of 20–1100 ◦C at 5 ◦C min−1. No weight loss is observed in theGA curve (Fig. 4, black line) in the whole measuring temperatureange, indicating no decomposition reaction occurred during theemperature range of 20–1100 ◦C. However, at 600 ◦C, exothermic

henomenon is observed in the DTA curve (Fig. 4, red line). The twoharacteristics suggest a phase transition occurred at 600 ◦C. Theesult here is quite different from the results reported by Chhor et al.17]. They found a phase transition occurred at 230 or 271 K with

ig. 4. TGA and DTA spectra measured in the temperature range of 20–1200 ◦C ofnMn2O4.

D. (B) XRD pattern of precursor of sample E. (C) XRD pattern of precursor of sample

strong metastability effects, had no relationship with any structuralchange.

XPS spectrum for ZnMn2O4 of sample E were recorded to obtaininformation on the valence of 3d cations, as shown in Fig. 5. Thecharge-shifted spectra were corrected using the adventitious C 1sphotoelectron signal at 285 eV. The Mn spectrum shows two peaks,the Mn L2 and L3 ionization edge, corresponding to the excita-tion of the Mn 2p1/2 and 2p3/2 electrons into the narrow band ofunoccupied 3d energy states. The two peaks are fitted using XPS-

PEAK41 software. The Gaussian fitted curve is shown in Fig. 5. Thecenter position for the Mn L2 and L3 core level binding energiesis estimated to be about 652.2 and 640.7 eV, respectively. This isconsistent with that for the ZnMn2O4 [1]. The full width at half

Fig. 5. XPS spectra of Mn 2p3/2 and Mn 2p1/2 in ZnMn2O4 and the Gaussian fittedXPS spectra.

H. Li et al. / Materials Chemistry and Physics 130 (2011) 39– 44 43

Fa

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l

�

wCbi�

ig. 6. (A) Temperature dependence of magnetizations of ZnMn2O4. (B) M–H curvest 10 K and 100 K for ZnMn2O4.

aximum (FWHM) for the Mn L2 and L3 peaks is 4.12 eV and.41 eV. The energy difference between the Mn L2 and L3 peaks is1.8 eV for the ZnMn2O4. The intensity ratio of the L3/L2 for Mn innMn2O4 is 1.96 calculated from the 4 eV wide integration windowentered at maximum values of the corresponding Gaussian fittedeaks.

Fig. 6A presents the temperature dependence of magnetizationM–T) of ZnMn2O4 of sample E from 5 K to 300 K in the zero-field-ooled (ZFC, labeled as red line and red �) and field-cooled (FC,abeled as black line and ©) process with an applied field of 50 Oe.rom 300 K to ∼38 K, the ZFC curve almost overlaps with the FCurve while the obvious discrepancies between FC and ZFC arebserved below 38 K. Below 38 K, a magnetization cusp located at15 K appears in the ZFC curve while the magnetization descendsith the increase of temperature in the FC curve, suggesting a

pin-glass (SG) or AFM order of the sample. The ac magnetizationsere measured from 5 K to 50 K under 10, 500, 1 kHz, respectively

Supporting Information, Fig. S2). No SG behavior characteristic, i.e.,eal part �′ of complex susceptibility is frequency independent, isbserved. Thus, it is concluded that the sample is AFM order withhe Néel temperature is ∼15 K.

The �–T curve above the Néel temperature fits the Curie–Weissaw [25].

= C

T + �(6)

here C is the Curie constant, which can be calculated by

= �NA(g�B)2S(S + 1)/3kB, considering only the nearest neigh-or interactions and NA is the Avogadro constant. The �eff

s the effective magnetic moment, can be derived by usingeff = g�B(S(S + 1))1/2, where the Landé factor g = 2 [25]. The Curie

Fig. 7. Room temperature Raman scattering spectrum of ZnMn2O4.

constant obtained from the �−1–T curve is 0.01425 emu Oe K mol−1

and the � is about −333 K. Thus, the �eff can be derived tobe 4.9 �B/Mn evaluated from the experimental data, in goodagreement with the magnetic moment based on a high-spin con-figuration of Mn3+ with three t2g and one eg electrons �eff = 4.9[21], calculated by �eff = g�B(S(S + 1))1/2. Fig. 5B shows M–H curvesmeasured at 10 K and 100 K, respectively. Clear hysteresis at 10 Kwith the coercive force of 93 Oe is observed while only paramag-netic (PM) behavior is observed in the M–H curve at 100 K.

Raman spectrum is a good undestroyed tool to characterizesamples. However, up to now, there is rare Raman characteri-zation of ZnMn2O4. Here, the first order Raman modes for theZnMn2O4 is collected at room temperature. ZnMn2O4 with spacegroup of I41/amd shows a tetragonal distortion with c/a = 1.62.Factor group analysis yields 10 Raman modes, represented by2A1g + 3B1g + B2g + 4Eg [26], which can be compared with the fiveRaman active modes (A1g + Eg + 3T2g) of the cubic spinel [27,28]. Inthis study, only six Raman modes, located at 171, 321, 384, 476,635, 677 cm−1, as shown in Fig. 7, are observed from the tetrag-onal ZnMn2O4, well consistent with that of ZnMn2O4 reportedby Samanta et al. [29,30]. In the cubic spinel oxides, the modesabove 600 cm−1 are usually correspond to the motion of oxygenin the tetrahedral AO4 group [28], so the two peaks at 635 and677 cm−1 are considered to represent A1g symmetry. The other low-frequency modes are characteristics of the octahedral site (BO6).However, the vibrate modes are still needed to further study.

4. Conclusions

To summarize, spinel structure ZnMn2O4 with high crystallinequality was synthesized by a facile method. This method has meritof lower reaction temperature, shorter reaction time and withoutregrinding starting materials compared with the traditional solidstate reactions. A reaction mechanism is proposed for the formationof spinel structure ZnMn2O4. The synthesized products were char-acterized by XRD, TEM, DTA and TGA, SQUID and Raman spectrum.The obtained ZnMn2O4 shows a phase transition at about 600 ◦Cinstead of a decomposition reaction in the temperature range of20–1100 ◦C and an antiferromagnetic behavior with the Néel tem-perature of about 15 K. Six Raman modes are observed in the roomtemperature Raman spectrum of ZnMn2O4.

Acknowledgement

This work was financially supported by 2009 Ludo FrevelCrystallography Scholarship Award (The International Centre forDiffraction Data, ICDD, USA).

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ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.matchemphys.2011.04.072.

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