Effect of Grain Size on Pressure-Induced Structural Transition in

ARTICLE pubs.acs.org/JPCC

Effect of Grain Size on Pressure-Induced Structural Transition in Mn3O4 Hang Lv,† Mingguang Yao,† Quanjun Li,† Zepeng Li,†,^ Bo Liu,† Ran Liu,† Shuangchen Lu,† Dongmei Li,† Jun Mao,‡ Xiangling Ji,‡ Jing Liu,§ Zhiqiang Chen,|| Bo Zou,† Tian Cui,† and Bingbing Liu*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China State Key Laboratory Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130012, China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China GeoScience Department, Stony Brook University, Stony Brook, New York 11794, United States ^ College of Science, Civil Aviation University of China, Tianjin 300300, China

)

‡

ABSTRACT: The size effect on structural transitions of Mn3O4 has been investigated under pressures by in situ synchrotron X-ray diffraction and Raman technique in a diamond anvil cell. Compared with bulk Mn3O4, Mn3O4 nanoparticles show an obvious elevation of phase transition pressure and different phase transformation routines with the occurrence of a new high-pressure phase at 14.5
23.5 GPa. The new phase most probably has an orthorhombic CaTi2O4-type structure, which is regarded as a metastable phase transforming to the higher pressure marokite-like structure. By the return to ambient pressure, the marokite phase is quenchable in bulk Mn3O4, whereas the coexistence of hausmannite and marokite phase is observed in the recovered Mn3O4 nanoparticles. It is proposed that the unique atomic conformation in Mn3O4 spinel structures, the cation distribution, and the higher surface energy together with the size-induced effect of nanocrystalline Mn3O4 probably play crucial roles in the high-pressure behavior of Mn3O4 nanoparticles.

’ INTRODUCTION High-pressure researches on nanostructural material have been of considerable interest because of the appearance of many novel high-pressure behaviors in the nanomaterials.1
4 Previous highpressure studies on nanocrystalline materials show that the grain size, shape, and structure of the nanocrystals have significant effects on the phase transition pressure, compressibility, and even phase transition routines,5,6 in which, the size effect of nanomaterials has been found to be the most important factor all along and has attracted great enthusiasm. Many typical nanomaterials have been studied, such as ZnO,7 TiO2,8 CeO2,9 ZnS,10 Si,11 and so on, and the size effects are found to influence their high-pressure behaviors, including the mechanical properties and transformations under pressure. Swamy et al.12 studied nanocrystalline anatase TiO2 using the high-pressure method and found a size-dependent phase selectivity of anatase at high pressure. All of these results show that nanomaterials give birth to distinct, usually enhanced, properties compared with conventional bulk polycrystalline materials. Therefore performing high-pressure studies on nanomaterials is important not only for exploring the new structures and properties in materials but also for fundamental scientific contribution, for example, understanding the factors that lead to phase transitions in materials. r 2011 American Chemical Society

Spinels with AB2O4 formula are binary oxides, which have important technological applications, including use as magnetic materials,13 superhard materials,14 and high-temperature ceramics.15 Various spinel structures result from the different cation distributions in the A (tetrahedral) and B (octahedral) sites.16 Highpressure studies on this family (mainly on bulk materials) have found three proposed orthorhombic phases of CaMn2O4-, CaTi2O4-, and CaFe2O4-type structures as high-pressure polymorphs of spinels.17 Recently, Mao et al. compared the compressional behaviors of bulk and nanorod LiMn2O4 and revealed that nanostructured materials can accommodate more stress compared with their bulk counterparts and make the nanomaterials possibly have enhanced application in battery. To our knowledge, this is the first high-pressure study on AB2O4 formula nanomaterials. Therefore it is also expected the nanosize effect could alter compressibility, transition pressures, and even phase transformation routines for other members in the AB2O4-type Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 14, 2011 Revised: October 11, 2011 Published: November 02, 2011 2165

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Figure 1. (a) TEM image of Mn3O4 nanoparticles. (b) XRD patterns of the as-synthesized Mn3O4 nanoparticles and bulk Mn3O4.

spinels family. This motivated us to study further the highpressure-induced phase transitions for other nanocrystalline spinels. Here we take Mn3O4 as a case study. In particular, Mn3O4 occurs in nature as hausmannite, a tetragonally distorted spinel in which the different states of Mn, Mn2+, and Mn3+, occupy the tetrahedral and octahedral sites, resepectively. Because of the Jahn
Teller instability of the Mn3+ (d4) ions, MnO6 octahedra are elongated along [001] direction, which causes the tetragonal symmetry (I41/amd; Z = 4).18 Bulk hausmannite Mn3O4 has been reported to transform into an orthorhombic marokite-like phase at ∼10 GPa, which keeps stable up to 38.7 GPa19,20 (the highest studied pressure in all the existing reports). In addition, as a representative spinel with excellent physical properties, the structure stability of the nanocrystalline Mn3O4 under pressure is of interest for an understanding of the structure transition of AB2O4-type spinels, which is relevant to many research areas, including understanding the phase transition mechanism and engineering materials with enhanced properties. In this study, we report a high-pressure study on Mn3O4 nanoparticles using in situ X-ray diffraction and Raman spectroscopy in a diamond anvil cell (DAC). We also performed the same high-pressure measurements on bulk Mn3O4 up to 47.3 GPa, well beyond the existing studied pressure on bulk Mn3O4. Compared with the bulk, nanocrystalline Mn3O4 shows quite different high-pressure behaviors, including the elevation of phase transition pressure and phase transformation routines. A new highpressure phase has been observed at 14.5
23.5 GPa in Mn3O4 nanoparticles. The unusual high-pressure behaviors observed in Mn3O4 nanoparticles have been discussed in the framework of atomic conformation in Mn3O4 spinel structure, the cation distribution and the higher surface energy together with the size-induced effect of nanocrystalline Mn3O4.

’ EXPERIMENTAL SECTION The Mn3O4 nanoparticle samples used in this study were synthesized by an easily reproducible two-phase approach reported elsewhere.21 Bulk Mn3O4 with microsize was synthesized by calcining the Mn2O3 precursor powders. The samples were characterized using TEM (200 KV, HITACHI, H-8100) and X-ray diffraction (XRD, D8 DISCOVER GADDS) with Cu Kα radiation. Both samples were then placed in a DAC with a culet size of 400 μm for high-pressure study. The T301 stainless-steel

gasket was preindented by the diamonds to an initial thickness of 40 μm, and then a center hole of 110 μm diameter was drilled as the sample chamber. A small amount of either bulk Mn3O4 or nanoparticle powders and a tiny ruby chip were placed inside the gasket hole and filled with a 4:1 methanol
ethanol mixture as the pressure medium. The pressure was determined from the frequency shift of the ruby R1 fluorescence line. In situ angle-dispersive X-ray diffraction (ADXD) experiments were performed at 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF) and X17C beamline of Brookhaven National Laboratory. The diffraction data were collected using MAR165 CCD detector. The Bragg diffraction rings were recorded with an imaging plate detector, and the 2D X-ray diffraction (XRD) images were analyzed using the FIT2D software, yielding 1D intensity versus diffraction angle 2θ patterns. The average acquisition time was 300 s. The sample
detector distance and geometric parameters were calibrated using a CeO2 standard from NIST. High-pressure Raman spectra were recorded on a Renishaw inVia Raman Microscope in the backscattering geometry using 514.5 nm line of an argon ion laser, provided with a CCD detector system. The Raman bands were analyzed by fitting spectra with Lorentzian functions to determine the line shape parameters.

’ RESULTS AND DISCUSSION Figure 1a shows the representative transmission electron microscopy (TEM) image of the typical Mn3O4 nanocrystals. It reveals that all nanoparticles have a highly uniform size distribution with an average diameter of ∼10 nm. The XRD patterns of Mn3O4 nanoparticles and bulk Mn3O4 under ambient condition are shown in Figure 1b. It can be seen that both samples have a crystalline tetragonal-spinel structure with no impurity phase (JCPDS 80-0382). The diffracted peaks of Mn3O4 nanoparticles are significantly broadened compared with its bulk counterpart due to the well-known finite nanosize effect of the crystallites. Figure 2a shows the high-pressure XRD patterns of bulk Mn3O4. From this figure, it is observed that a phase transition from hausmannite phase (I41/amd; Z = 4) to orthorhombic marokitelike phase (Pbcm; Z = 4) takes place at 11.5 GPa, and the highpressure phase can be preserved up to the highest studied pressure of 37.8 GPa. This is in good agreement with the previous reported phase transition in bulk Mn3O4.19,22 Diffraction patterns recorded on our Mn3O4 nanoparticles are shown in Figure 2b. 2166

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Figure 2. XRD patterns of bulk Mn3O4 (a) and Mn3O4 nanoparticles (b) with increasing pressure at room temperature. Peaks marked by the solid circles are from the new high-pressure phase. The asterisk (*) denotes the diffraction from the stainless-steel gasket in the experiment.

Figure 3. Profile fitting of the X-ray diffraction pattern of Mn3O4 nanoparticles at 36.7 GPa in the regions of (a) 12.2
16 and (b) 15.5
20.

Figure 4. XRD patterns of Mn3O4 collected on pressure release to atmospheric pressure at room temperature.

In sharp contrast with the bulk sample, two pressure-induced structural transformations can be clearly observed upon compression. The first one takes place at ∼14.5 GPa, at which pressure the diffraction peaks from the initial hausmannite phase (tetragonal structure) change obviously into an previously unreported phase in Mn3O4 (see later discussion). With pressure increased

further up to 28.4 GPa, a second phase transition starts to appear, at which pressure a new peak at ∼17.7 (d = 2.01 Å) occurs, combined with corresponding changes in other diffraction peaks. All diffraction peaks assigned to this high-pressure phase arise remarkably with pressure increased. We notice that the patterns from the second high-pressure phase show very similar features to that of the high-pressure orthorhombic marokite-like phase observed in bulk material, and thus an orthorhombic structure (Pbcm; Z = 4) is used to fit the diffraction pattern. (See Figure 3.) The fitting gives relatively satisfied results to the experimental curve although, there is slightly difference from the diffraction patterns, which is usually due to the nanosize effect. The lattice constants are determined to be about a = 2.915 Å, b = 9.884 Å, and c = 9.211 Å and the cell volume V = 265.38 Å3 for the pattern obtained at 31.1 GPa. We also tried to deduce the fine structure information on this phase but it is not successful because the observed X-ray reflections from the nanocrystals are too broad. In Figure 4 are shown the ambient pressure XRD patterns for bulk Mn3O4 and Mn3O4 nanoparticles samples released from high pressures. In the case of bulk Mn3O4, the high-pressure marokite structure can be quenchable and stable to room pressure, with no trace of the initial hausmannite phase, whereas for Mn3O4 nanoparticles, the coexistence of both initial hausmannite phase and the high-pressure marokite phase are observed to ambient pressure. 2167

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Figure 5. Raman spectra of Mn3O4 at elevation of pressure to (a) 47.3 GPa for bulk and (b) 44 GPa for nanoparticles.

Figure 6. Pressure shifts of the observed Raman modes of Mn3O4.

It is clear that Mn3O4 nanoparticles have a different phasetransition process compared with its bulk material. To confirm these results, we also carried out high-pressure Raman spectra on both bulk Mn3O4 and Mn3O4 nanoparticles in Figure 5, and their pressure dependences are shown in Figure 6. For bulk Mn3O4, the Raman scattering spectrum at 1.85 GPa shows a strong peak at 665 cm
1 along with two weak peaks at 378 and 325 cm
1, which are well consistent with the previously reported spectrum on bulk hausmannite Mn3O4.23 The Raman band at 665 cm
1 can be assigned to the A1g mode, which is from the Mn2+-O stretching vibration of the tetrahedral MnO4. With increasing pressure to 12.8 GPa, the intensity of A1g mode for the hausmannite phase decreases remarkably, and the combination of the presence of a series of new Raman modes indicates the occurrence of a phase transition at this pressure, which is consistent with our XRD results. However, besides the strongest mode at ∼650 cm
1 reported at 13 GPa,20 the other modes at lower frequency for the marokite-like Mn3O4 shown in our work have not been reported in previous literature, and their origin is still unknown. The Raman modes of the marokite phase can be preserved up to the highest studied pressure of 47.3 GPa and upon decompression to ambient pressure. Different from the bulk Mn3O4, the Raman modes of Mn3O4 nanoparticles under pressure shown in Figure 5b and Figure 6 display two phase transitions. At 15.5 GPa, the A1g mode starts to show an asymmetric shape with a new shoulder at

∼683 cm
1 and the intensity of this new peak increases quickly while the initial A1g peak disappears with pressure increases, indicating that a structural phase transition takes place. At 29.5 GPa, the broad peaks centered at about 250 and 690 cm
1 along with the weak peak at 560 cm
1 start to rise. Such Raman modes show the similar Raman features as those of bulk marokite-like structured Mn3O4, and it suggests the occurrence of the second high-pressure phase in nanocrystalline Mn3O4. Therefore, this phase can be regarded as the marokite-like phase, in agreement with our XRD results. The marokite phase is stable up to 44 GPa and can be quenchable to atmospheric pressure. Upon decompression to ambient pressure, Raman spectrum of nanocrystalline Mn3O4 exhibits an additional peak at 655 cm
1 besides the peaks from the marokite phase, indicating the coexistence of hausmannite and marokite phase in our sample. All of these results are consistent with the high-pressure XRD results. Therefore, Mn3O4 nanoparticles show an elevation of phase transition pressure (14.5 GPa) and different phase transformation sequences compared with the bulk. The structure transition from hausmannite phase to marokite-like phase in nanoparticles takes place at 28.4 GPa, much higher than that for the bulk (11.5 GPa). More important, a new phase transition at 14.5
23.5 GPa has been observed in our nanostructural Mn3O4. At >14.5 GPa, Mn3O4 nanoparticles show the several new diffraction peaks besides the tetragonal-spinel structure (hausmannite), indicating the phase transition starts to emerge. At 17.3 GPa, most diffraction peaks from the starting hausmannite phase disappear, and nine diffraction peaks from the high-pressure phase are observed, which appear at 7.5 (d = 4.75 Å), 11.2 (d = 3.19 Å), 13.1 (d = 2.72 Å), 13.9 (d = 2.56 Å), 15.1 (d = 2.36 Å), 16.5 (d = 2.16 Å), 20.1 (d = 1.78 Å), 21.7 (d = 1.65 Å), and 22.5 (d = 1.59 Å), respectively. Such transformations in the diffraction peaks have not been observed before in Mn3O4, indicating that nanosize effect leads to this new high-pressure phase transition in the nanoparticle sample. In general, high-pressure studies on the spinel compounds within the formula AB2O4 show the routine phase transition to a orthorhombic structure, including the CaMn2O4type structure (Pbcm, Pmab), CaTi2O4-type structure (Bbmm, Cmcm), CaFe2O4-type structure (Pnma), and ZnCa2O4-type structure (Pbcm).17,24
26 We attempted to fit the diffraction peaks with all of these possible structures besides the orthorhombic marokite-like structure (the reported high-pressure phase of bulk Mn3O4). The most satisfactory fitting was provided by an 2168

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Table 1. Cell Parameters of Tetragonal Mn3O4 bulk Mn3O4 P (GPa)

a (Å)

c (Å)

Mn3O4 nanoparticles V (Å3)

V/V0

P (GPa)

a (Å)

c (Å)

V (Å3)

V/V0

0

5.772(13)

9.490(5)

316.2

1

0

5.744(76)

9.452(3)

311.9

1

1.9 4

5.742(21) 5.730(71)

9.407(54) 9.356(58)

310.19 307.28

0.981 0.972

1.7 3.4

5.736(6) 5.730(15)

9.385(44) 9.338(28)

308.9 306.6

0.990 0.983

6.1

5.713(8)

9.342(77)

305.02

0.965

8.6

5.691(4)

9.251(48)

299.7

0.961

8.5

5.692(89)

9.294(18)

301.21

0.953

10.7

5.700(91)

9.208(14)

299.3

0.957

11.9

5.710(12)

9.203(39)

300.1

0.954

Figure 7. Unit-cell volume as a function of pressure determined for the tetragonal Mn3O4: (a) bulksize and (b) nanosize.

orthorhombic CaTi2O4-type structure with space group Bbmm, resulting in the lattice parameters a = 10.300 Å, b = 9.500 Å, c = 3.390 Å, and V = 331.76 Å3. This new phase was transformed to the marokite-like phase at higher pressure and was not observed during decompression. Therefore, the defined crystal structure of the new high-pressure phase to be an orthorhombic CaTi2O4-type structure is reasonable, which can be taken as a metastable phase transforming to the higher pressure orthorhombic marokite-like structure. To understand the observed difference between bulk and nanosize Mn3O4, we give the crystallographic lattice constants and the cell volumes of the two Mn3O4 samples (with tetragonal structure) at several selected pressures in Table 1. Between P = 0 and 8.5 GPa, the data of bulk Mn3O4 show a variation of 1.4% (1.8% in ref 19) for the a cell parameter, and c varies by 2.1% (2.9% in ref 19), which reveals a considerable compressional anisotropy with the c axis being more compressible than the a axis. The decrease in volume of 4.7% in our work (6.2% in ref 19) indicates a rather low compressibility in this pressure range. In the case of Mn3O4 nanoparticles, the data show a variation of 0.9% for the a cell parameter, which is much smaller than that for bulk Mn3O4 (1.4%) in the same pressure range, whereas c varies by 2.1%, the same as that of bulk sample. The volume shows a 3.9% contract in the pressure range from ambient to 8.6 GPa, which is lower than that of bulk Mn3O4 obtained from our work. Comparing with bulk Mn3O4, the c axis is much more compressible than a or b axes, resulting in an increased compressional anisotropy in nanoparticles. The tetragonal distortion (c/21/2a) has been used to describe the Jahn
Teller distortion in Mn3O4 and supposed to be an important driven force for the phase transition of the material.22 In our studies, this parameter decreases

with pressure from 1.163 at ambient pressure to 1.155 at 8.5 GPa for bulk Mn3O4, which suggests the suppressed Jahn
Teller distortion of MnO6 octahedra.22 For our Mn3O4 nanocrystalline, this tetragonal distortion decreases from 1.164 to 1.150 with pressure increased from ambient to 8.6 GPa and becomes 1.142 at 10.7 GPa. The decrease in the value of this parameter before the phase transition is much larger than that observed in bulk, which suggests an obvious suppression in Jahn
Teller distortion of MnO6 octahedra in Mn3O4 nanocrystals. The volume variation of hausmannite phase Mn3O4 as a function of pressure is plotted in Figure 7. From 10.7 to 11.9 GPa, a discontinuity is observed in nanocrystalline Mn3O4, with the nanoparticles becoming less compressible, which probably portends the observed phase transition to the new orthorhombic phase. The Birch
Murnaghan equation of state, using an assumed value of 4 for the pressure derivation of the bulk modulus (B00 ), has been used to fit the experimental data from 0 to 8.6 GPa, giving a bulk modulus (B0) of 202 ( 6 GPa for Mn3O4 nanoparticles. (If B00 = 5, then the bulk modulus becomes 198 ( 5 GPa.) For bulk Mn3O4 crystals, the bulk modulus obtained from 0 to 8.5 GPa is 166 ( 18.3 GPa (assuming B00 = 4) or 163 ( 17.8 GPa (assuming B00 = 5), which is close to that reported in ref 18 (137 ( 4 GPa, B00 = 4 or 134 ( 4 GPa, B00 = 5). It is obvious that the bulk modulus obtained from the initial hausmannite phase of Mn3O4 nanoparticles is significantly larger than that of the bulk counterparts.19 Numerous studies on nanosized materials suggest that the decrease in particle size results in obvious elevations of phase transition pressure and bulk modulus compared with the bulk, which has been explained by the higher surface energy contribution in nanomaterials.27,28 For our Mn3O4 nanoparticles, their surface 2169

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The Journal of Physical Chemistry C energy is also expected to be higher than that of the bulk, and thus the enhancements in both transition pressure and bulk modulus can be easily understood. Besides the significant enhancements of bulk modulus and phase transition pressure, the Mn3O4 nanoparticles also exhibit a different phase transition routine compared with its bulk counterparts. The phase transition sequences of Mn3O4 nanoparticles is hausmannite phase (∼14.5 GPa) f the new high-pressure phase (28.4 GPa) f orthorhombic marokitelike phase, in contrast with the phase transition of hausmannite phase (11.5 GPa) f orthorhombic marokite-like phase in bulk Mn3O4. This unusual behavior is different from nanosize-effectinduced phase transitions observed in some other typical nanomaterials. For example, ZnS nanorods and TiO2 nanoparticles can directly transform to a second high-pressure phase (rock salt phase in ZnS and baddeleyite structure in TiO2) or amorphous phase, for which the transition to the first high-pressure phase (zinc blende phase in ZnS and α-PbO2 structure in TiO2) in their bulk conterparts is absent.29,12 The authors proposed that the shape of ZnS nanorods and the varying surface energy contributions to the total energy of each phase in TiO2 as a function of crystal size played the crucial roles in the special pressure behaviors of phase absence in the processes of structural transitions, respectively. Here we discuss the possible reason for the presence of the additional new high-pressure phase in nanocrystalline Mn3O4. Previous research on Mn3O4 has shown that the volume reduction and the structure deformation combined with the suppressed Jahn
Teller distortion can be the two main driving forces for the pressure-induced structural transformation.22 In our work, compared with bulk Mn3O4, nanocrystalline Mn3O4 shows an increased compressional anisotropy in the tetragonal structure and a reduced Jahn
Teller distortion, lower than that for the bulk. Therefore, we suggest that these two factors play a crucial role in the unusual phase transition process in Mn3O4 nanoparticles. Furthermore, Mn3O4 is a type of spinel with AB2O4 formula, in which the different states of Mn, Mn2+ and Mn3+, occupy the tetrahedral and octahedral sites. Previous research on hausmannite Mn3O4 gave access to a Mn3+/Mn2+ ratio of 1.5 in 10
20 nm sized Mn3O4 particles, slightly lower than the value of 2.05 to 3.05 for the bulk,30,31 indicating a different cation distribution in nanoparticles compared with the bulk. In addition, it is reported the local stresses within the nanodomain structure and the cation inversion may reduce the Jahn
Teller distortion in the compounds with AB2O4 formula.32 Therefore, this unique atomic configuration in Mn3O4 spinel structures and the different cation distribution in nanoparticles probably can be related to the different structure deformation combined with the suppressed Jahn
Teller distortion observed in Mn3O4 nanocrystals. As we know, for bulk Mn3O4, the volume reduction under pressure is another driving force for its structural transformation. However, the nanoparticles become obviously less compressible at 10.7 to 11.9 GPa (before the structure transition). The similar compressibility decreases with the decrease in the particle size has also been observed in other nanomaterials because it is suggested that the nanomaterials can accommodate more stress and strain than the bulk.33,34 Therefore, the observed unusual volume change, that is, very low compressibility in Mn3O4 nanoparticles at 10.7 to 11.9 GPa, could be another reason that Mn3O4 nanoparticles does not transform into the marokite phase, as observed in the high-pressure phase transition of bulk Mn3O4 but undergoes a new CaTi2O4-type orthorhombic structure transition before the transformation into marokite phase, a different transition

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routine compared with the bulk. Therefore, the unique atomic conformation in Mn3O4 spinel structures, the cation distribution, and the higher surface energy together with the size-induced effect of nanocrystalline Mn3O4 are supposed to play crucial roles in the high-pressure behavior of Mn3O4 nanoparticles.

’ CONCLUSIONS In summary, the structural transformations of bulk Mn3O4 and Mn3O4 nanoparticles have been studied by in situ XRD and Raman measurements under pressure. Bulk Mn3O4 with micrometer size exhibits the similar high-pressure behaviors as to previous reports, and its high-pressure orthorhombic marokite-like phase has been found to be stable even up to 47.3 GPa. Compared with the bulk, nanocrystalline Mn3O4 shows an elevation of phase transition pressure and different phase transformation routines. Under pressure, bulk Mn3O4 transformed directly from hausmannite phase to marokite phase, whereas nanocrystalline Mn3O4 undergoes two phase transitions: from hausmannite phase to the new high-pressure phase and then to orthorhombic marokite-like phase. The new high-pressure phase has been recognized to be an orthorhombic CaTi2O4-type structure, which occurs at 14.5
23.5 GPa. In addition, the nanosized hausmannite phase has an enhanced bulk modulus. On decompression to ambient pressure, the marokite phase is quenchable in bulk Mn3O4, whereas for Mn3O4 nanoparticles, the coexistence of hausmannite and the marokite phase has been observed in the recovered samples. It is proposed that the unique atomic conformation in Mn3O4 spinel structures, the cation distribution, and the higher surface energy together with the nanosize effect play crucial roles in the unusual high-pressure behavior of Mn3O4 nanoparticles. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +00 86 431 85168256. Fax: +00 86 431 85168256.

’ ACKNOWLEDGMENT This work was supported financially by the NSFC (10979001, 51025206, 51032001, 11074090, 21073071, 11004075), the National Basic Research Program of China (2011CB808200), the Cheung Kong Scholars Program of China, and the National Fund for Fostering Talents of Basic Science (J0730311). This research was partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 10-43050. Portions of this work were performed at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (grant no. KJCX2-SW-N20, KJCX2-SW-N03). ’ REFERENCES (1) Wang, Z. W.; Zhao, Y.; Schiferl, D.; Qian, J.; Downs, R. T.; Mao, H. K.; Sekine, T. J. Phys. Chem. B 2003, 107, 14151–14153. (2) Shen, L. H.; Li, X. F.; Ma, Y. M.; Yang, K. F.; Lei, W. W.; Cui, Q. L.; Zou, G. T. Appl. Phys. Lett. 2006, 89, 141903-1–141903-3. (3) Jiang, J. Z.; Olsen, J. S.; Gerward, L.; Morup, S. Europhys. Lett. 1998, 44, 620–626. (4) Wang, Z. W.; Saxena, S. K.; Pischedda, V.; Liermann, H. P.; Zha, C. S. Phys. Rev. B 2001, 64, 012102-1–012102-4. 2170

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