Structural Transition of MnNb2O6 under Quasi-Hydrostatic Pressure

Article pubs.acs.org/JPCC

Structural Transition of MnNb2O6 under Quasi-Hydrostatic Pressure Fengxian Huang,† Qiang Zhou,†,‡ Liang Li,‡ Xiaoli Huang,† Dapeng Xu,‡ Fangfei Li,*,† and Tian Cui†,‡ †

State Key Lab of Superhard Materials and ‡College of Physics, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: High-pressure Raman scattering and X-ray diffraction studies on manganese niobate (MnNb2O6) have been carried out in a diamond anvil cell at room temperature up to 41 GPa. A pressure-induced phase transition was observed at 12 GPa accompanied by the softening of two internal vibration modes ν10(B2g) and ν8(Ag). Disappearing of most Raman vibration peaks under high pressure reveals that the NbO6 octahedra distort heavily through the transition; this is evidenced by the Xray diffraction results, but the octahedral units still exist, as the internal Nb−O strengthening vibration is observable even at the highest pressure in this study. The equation of state for MnNb2O6 in low pressure phase I was obtained with third-order Birch−Murnaghan method, yielding a zero-pressure bulk modulus B0 and its pressure derivative B0′ of 154.4 ± 3.5 GPa and 4.1, respectively. The volume dependencies of the optical lattice mode frequencies and their respective Grüneisen parameters were extracted; it is suggested that the instability of the columbite orthorhombic structure at high pressure is related to the strong deformation of the NbO6 octahedra. Decompression measurements suggest that the pressure-induced transformation is partly reversible.

1. INTRODUCTION Columbite MnNb2O6 is one of the end-member typical pegmatite minerals (Mn, Fe)(Nb, Ta)2O6,1 which is an important source for niobium extracting. Additionally, MnNb2O6 is also the prototype of oxide materials for various relevant technological applications due to its physical properties.2−5 MnNb2O6 has been widely studied under ambient pressure; the magnetic ordering (TN = 4.4K) of MnNb2O6 has been observed previously in neutron powder diffraction experiments and the measurements of the magnetoelectric and magnetic susceptibility.6,7 Nielsen et al. have determined the nuclear and magnetic structure of MnNb2O6 in their neutron powder diffraction study.8 Recently, the electrical properties of columbite MnNb2O6 have been explored experimentally with the aim of inducing mixed ionic-electronic conductivity.9,10 After that, Arroyoy de Dompablo performed a computational study on the stability and electronic properties of oxygen vacancies in MNb2O6 (M = Mn, Fe, Co, Ni, Cu); he found that the oxygen vacancy formation induces (a) a narrowing of the band gap and (b) an electronic density redistribution leading to the reduction of Nb5+ to lower oxidation states.11 Additionally, Mansurova supplemented the absence of the thermodynamic properties of MnNb2O6,12 and Husson performed Raman study on MnNb2O6 and a theory calculation about the vibration modes.13,14 At ambient condition, the columbite MnNb2O6 crystallizes in a derivative of the a-PbO2-type, in which the oxygen atoms assume a distorted HCP configuration and the cations occupy one-half of the available octahedral interstices.15 It is in orthorhombic symmetry with the Pbcn space group, and there are two distinct octahedral sites (4c and 8d). The crystal structure of columbite from different visual directions is depicted in Figure 1. The fundamental building units are © 2014 American Chemical Society

Figure 1. Crystal structure of MnNb2O6 at ambient conditions (phase I with space group Pbcn). The yellow and green color blocks represent MnO6 octahedra and NbO6 octahedra, respectively.

Received: April 10, 2014 Revised: July 24, 2014 Published: July 28, 2014 19280

dx.doi.org/10.1021/jp503542y | J. Phys. Chem. C 2014, 118, 19280−19286

The Journal of Physical Chemistry C

Article

Table 1. Raman Vibrational Mode Assignments (cm−1) for Orthorhombic MnNb2O6 at 1 bar, and Grüneisen Parameters for Raman Lattice Modes at Room Temperature, Calculated by Linearly Fitting ln(ν) versus ln(V) Raman shift (cm−1)

symmetry

mode vibration

125

ν11(B2g)

137 178

ν11(B3g) ν11(B1g)

188

ν10(Ag)

204

ν10(B2g)

212

ν9(Ag)

243 253 261 272 286 296 313 361 383 398 489 530 559 601 620 637 814 874

ν8(Ag) ν8(B3g) ν9(B1g) ν7(Ag) ν8(B1g) ν7(B2g) ν6(Ag) ν5(B2g) ν6(B1g) ν5(Ag) ν4(B1g) ν3(Ag) ν3(B1g) ν3(B3g) ν2(B2g) ν2(Ag) ν1(B1g) ν1(Ag)

Nb−Oc stretching couple with Oc−Nb−Ob bending, Ob−Nb−Oc bending and Mn−Ob stretching Oc−Nb−Ot bending, Ob−Nb−Ob bending Nb−Oc stretching couple with Oc−Nb−Oc bending and Mn−Ot stretching Mn−Ob stretching couple with Ob−Nb−Ot bending, Oc−Nb−Ot bending, Oc−Nb−Ob bending Ob−Nb−Ot bending, Ob−Nb−Ob bending, Oc−Nb−Ot bending, Ot− Mn−Ot bending Ot−Mn-Ot bending, couple with Oc−Nb−Oc bending, Ob−Nb-Ob bending Mn−Ot stretching couple with Oc−Nb-Ob bending Mn−Ot stretching couple with O−Nb−O bending O−Nb−O bending Mn−Ot stretching couple with O−Nb−O bending Mn−Ot stretching couple with O−Nb−O bending Mn−Ot stretching couple with O−Nb−O bending Mn−Ot stretching couple with O−Nb−O bending Nb−Oc stretching couple with O−Nb−O bending Mn−Ot stretching couple with O−Nb−O bending Nb−Oc stretching couple with O−Nb−O bending Nb−Oc stretching Nb−Ob stretching vibration Nb−Ob stretching vibration Nb−Ob stretching vibration couple with Mn−O stretching vibration Nb−Ob stretching vibration Nb−Ob stretching vibration Nb−Ot stretching vibration Nb−Ot stretching vibration

Grüneisen parameters vibration of cations and anions

deformation along the chains, deformation of the interchain

0.14 0.26 1.80 1.06 −1.12 1.22

deformation in octahedrons coupled with Mn−O stretching

Nb−Ob and Nb−Oc stretching in the double chains

Nb−Ot stretching vibration

−0.55 1.35 0.17 0.50 1.79 0.91 0.34 2.13 1.77 1.12 1.30 1.18 1.54 1.53 1.02 1.11 0.75 0.49

2. EXPERIMENTAL SECTION

zigzag chains of edge-sharing octahedra that run parallel along the c axis. Individual chains are connected by corner-sharing oxygen to form a three-dimensional framework. In such structure, any modification of the octahedron may result in the deformation of neighboring octahedral chains through the shared oxygen atoms, and the large mismatch between the Mn and Nb octahedra may also play a major role in the structure change, so the structure response to pressure or temperature together with property variation is curious and needs to be understood. High pressure has been proven to be an efficient tool for improving the understanding of the main physical properties of compounds, as the distance between atoms is effectively reduced under pressure. However, there are no related highpressure studies on MnNb2O6 until now; only the natural pegmatite minerals (Mn, Fe)(Nb, Ta)2O6 and natural columbite (Mn, Fe)Nb2O6 have ever been studied using Xray diffraction (XRD) method, and the distortion of octahedron was observed.16,17 In order to further understand the structure behavior of PbO 2 -type MnNb 2 O 6 under pressure, we performed Raman scattering spectroscopy and angle dispersive XRD studies on MnNb2O6 up to 41 GPa at room temperature in a diamond anvil cell (DAC). A pressure-induced phase transition observed at pressure above 12 GPa was pointed out. The Raman spectra show that a soft-mode-driven phase transition occurs at about 12 GPa, which is in agreement with XRD studies.

Polycrystalline MnNb2O6 powder was synthesized using the optical floating zone method (see ref 18), then was reground finely; crystal structure was checked and verified by the XRD method. The MnNb2O6 powder together with a small chip of ruby was loaded into a DAC with diamond culet size of 400 μm. Pressure was calibrated by the ruby R1 fluorescence.19 The 4:1 mixture of methanol−ethanol was used as the pressure transmitting medium in Raman measurements, and silicon oil was used in XRD measurements. The Raman spectra were recorded in a backscattering geometry; an Acton SpectraPro 500i spectrometer with 1800 gr/mm holographic grating and a liquid-nitrogen-cooled CCD detector (Princeton Instruments, 1340*100) were used.20 The 532 nm excitation light was generated by a diode-pumped solid state Nd:Vanadate laser (Coherent Company). The laser power focused on the sample was kept below 5 mw to avoid sample heating. The Raman peaks obtained were fitted with Lorentzian functions to determine the line shape parameters. High-pressure angle dispersive synchrotron XRD measurements were performed at the 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF). Diffraction patterns were collected using a Mar-3450 charge coupled device detector with a monochromatic beam at λ = 0.6199 Å, and the focused beam size was 20 × 30 μm2. Two-dimensional XRD images were analyzed and converted to intensity versus diffraction angle 2θ patterns using the Fit2D software.21 19281



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Figure 2. (a) Representative Raman spectra of MnNb2O6 at different pressures, most vibration peaks disappear after 12.4 GPa, and newly emerging peaks are marked by stars. (b) Detailed illustration for the variation of ν1(Ag) mode under pressure; the intensity descends dramatically after 12.4 GPa (green arrow) and disappears finally. Meanwhile a novel vibration on the low frequency side emerges and grows gradually, indicated by the red arrow; the spectra in gray color (13.1 and 14.0 GPa) are inserted to demonstrate the variation of ν1(Ag) mode more clearly.

3. RESULTS AND DISCUSSION In the view of group theory analysis concerning the columbite orthorhombic structure in the Pbcn space group, as shown in Figure 1, the Mn cations occupy the Wyckoff 4c-sites of C2y symmetry, contributing the following vibrational modes (of decomposed symmetries): 1Ag + 2B1g + 1B2g + 2B3g + 1Au+ 2B1u+ 1B2u+ 2B3u. The pentavalent Nb cations and three different O ions occupy 8d positions with C1 symmetry, each of them contributing the 3Ag + 3B1g + 3B2g + 3B3g + 3Au + 3B1u+ 3B2u +3B3u modes.22 Therefore, excluding the acoustic modes (1B1u + 1B2u + 1B3u) and the silent ones (13Au), the columbite MnNb2O6 possesses 54 Raman-active modes (13Ag + 14B1g + 13B2g + 14B3g) and 38 infrared-active modes (13B1u + 12B2u + 13B3u) totally. In our Raman measurements, 24 vibrations have been observed, and the ambient Raman spectrum agrees reasonably well with those reported in literature, exhibiting all the typical Raman features of columbite orthorhombic structure.13,14 Based on a comparison with the Raman study of other isomorphous material, the assignment of the Raman modes observed in MnNb2O6 is given in a list in Table 1.13,14,23 On the basis of the vibrational nature, they are summarized and characterized in two regions, including the O−Nb−O deformation region below 400 cm−1 and the Nb−O stretching region at higher frequencies. In addition, it is important to note that there are three kinds of oxygen atoms corresponding to three kinds of Nb−O bonds in the structure: Oc (chaining oxygen) bonding to three Nb atoms, Ob (briging oxygen) bonding to two Nb atoms and one Mn atom, and the Ot

(terminal oxygen) atom bonding to one Nb atom and two Mn atoms. Accordingly, Nb−Oc bonds link the double chains of NbO6 octahedra; Nb−Ob bonds connect two NbO6 units in chains; Nb−Ot bonds connect the double chains and MnO6 octahedra chains, which point toward the Mn atoms. The Raman spectra are dominated by the shortest Nb−O t symmetric stretching vibration of the NbO6 unit at about 874 cm−1 in ambient conditions. Figure 2 presents selected Raman spectra of MnNb2O6 up to 40.7 GPa at room temperature. In these spectra, most Raman peaks in the O−Nb−O deformation vibration region between the 100 and 400 cm−1 shift toward higher frequencies (blue shift) with intensities decreased upon compression, while there are two vibrations at 204 and 243 cm−1 that shift toward lower frequencies (blue shift), and it will be discussed in detail later. When the pressure increases up to 12.4 GPa, three additional weak Raman peaks appear, and their intensities grow stronger with the application of higher pressure. After this pressure, most other modes vanished, indicating a pressure-induced structural transition. The new vibration modes show a blue shift to higher frequencies with increasing intensities under further compression. At around 14.6 GPa, only the three newly emerged vibration modes in the O−Nb−O deformation vibration region could be clearly observed. However, with further increasing pressure up to 25.0 GPa, no vibration signal can be detected in this region. In the Nb−O stretching region, all the vibration modes presented pressure-induced blue shifts at different variation 19282



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rates, consistent with the idea that the bond distance decreases and vibration becomes stiffened upon compression. The dominant ν1(Ag) mode located at 874 cm−1 at ambient pressure drastically lost its intensity upon compression after 12.4 GPa, as shown in Figure 2b; it became a weak shoulder of the new one that emerged around 890 cm−1. Meanwhile, the vibrations at 530, 559, 601, 620, and 637 cm−1 are assigned to Nb−Ob (bridging oxygen) stretching modes, and the vibrations at 398 and 489 cm−1 are assigned as Nb−Oc (chaining oxygen) stretching modes; all of them vanished at 14.6 GPa. The distinct changes in the pressure dependence as well as the appearance/disappearance of the stretching Raman modes also indicate a pressure-induced structural transition consistently. Notably, all the Raman vibrations lose intensity upon increasing pressure, and only one broad weak ν1(Ag) mode is observable at 41 GPa, as shown in Figure 2. Figure 3a shows the pressure dependence of the vibration frequencies in the Nb−O stretching vibration region between 900 and 400 cm−1. It is clearly seen that vibrations of 874 and 821 cm−1 disappear; meanwhile another two adjacent vibrations appear for the new structure, and two emergent vibrations show blue shift under pressure with a smaller pressure dependence above 14.6 GPa. These vibrations are assigned to Nb−Ot (terminal oxygen) stretching modes corresponding to the symmetric and antisymmetric stretching vibrations of the NbO6 unit in the double chains. In the bridging Nb−Ob stretching vibration region between 500 and 700 cm−1, similarly, all vibrations of the phase I vanish, and the appearance of two novel vibrations is observed. In addition, the 489 cm−1 vibration attributed to chaining Nb−Oc stretching vibration disappears after 13.5 GPa, where the oxygen atoms play an important role in interchain bonding. Obviously, the 874 and 821 cm−1 vibrations are affected by pressure much less than others, suggesting the NbO6 unit still exist under high pressure phase, however, they may be densely packed or twisted and deformed. As the terminal oxygen Ot connects two Mn atoms, which is more firm in the MnO6 octahedron, while the chaining Oc atoms is shared by neighboring NbO6 units to form double NbO6 chains in MnNb2O6, all the chaining Nb−Oc stretching and most bridging Nb−Ob stretching vibrations disappear in the new phase, suggesting possible deformation or relative rotation of the NbO6 octahedra belonging to different zigzag chains, and this dense packing or deforming in the double octahedral chains is supposed to be perpendicular to the a-axis. Figure 3b is the pressure dependence of the vibration frequencies between the 100 and 400 cm−1 region, in which the O−Nb−O bending vibrations are predominant. They contribute to the deformation of the octahedra and chains, and also contribute to the interchain vibrations. All vibration modes show a blue shift to higher frequencies accompanying decreased intensities upon compression, this we expect that the 204 and 243 cm−1 vibrations show a softening trend. The 243 cm−1 vibration is hard to be discerned at atmospheric pressures, but it is easily observable at high pressures above 0.8 GPa. It is worth noting that the frequency of the low-wavenumber mode 204 cm−1 decreased to 189 cm−1 at 11.8 GPa (in Figure 3); this is attributed to the bending vibration of Ob−Nb−Ot bands, Ob−Nb−Ob bands, Oc−Nb−Ot bands, and Ot−Mn−Ot bands, corresponding to deformations along the chains and deformations of the interchains, with further increasing pressure, 204 cm−1 mode vanished above 12.4 GPa. On the other hand, the soft mode 243 cm−1 increased to 247 cm−1 at 1.8 GPa, while it decreased to 236 cm−1 at 10.0 GPa (in Figure

Figure 3. Raman shifts as functions of pressure for MnNb2O6.

3), it is attributed to the Oc−Nb−Ob bending vibration coupled with Mn−Ot stretching vibration, corresponding to deformations of edges-sharing NbO6 units in the chains couples with stretching of Mn−O bonds. We examined the structural similarity between MnNb2O6 and other perovskites to identify the soft modes. In LaAlO3 perovskite, the high pressure structure transitions from rhombohedral to cubic accompanied by a red shift of the A1g mode around 123 cm−1 with increasing pressure, and this transition is induced by the rotation of the AlO6 octahedra around the hexagonal (001) direction.24−26 In columbite orthorhombic structure, the NbO6 unit and MnO6 unit share edges (Ob,Oc) and (Ot, Ot), forming zigzag chains along the c axis, respectively. Individual NbO6 chains are connected by corner-sharing oxygen (Oc) to form double chains, which are connected to MnO6 chains by corner-sharing oxygen (Ot, Ob). Thus, modification of each octahedron results in deformation of the neighboring octahedral chains through 19283



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gradient when silicon oil is used as the pressure media; with argon serving as the pressure transmitting media, no amorphization trend is found. This indicates that the pressure medium has a measurable impact on the structure of sample under high pressure. After releasing pressure, the ambient structure is partially resumed with only a few strong diffractions being detected. The volumes of MnNb2O6 in low pressure orthorhombic phase (Pbcn space group) below 12.6 GPa was calculated from XRD measurements. The P−V data was used to get the equation of state (EOS) using a third-order Birch−Murnaghan (BM) method.29

the sharing of oxygen atoms. We think this phase transition would probably be due to the deformations of edge-sharing NbO6 units. The variations of internal vibration energies indicate that the Nb−O bond length and O−Nb−O bond angle changes and the chains deform under high pressure. In order to check the existence of the phase transition in MnNb2O6 under high pressures, XRD measurements were performed up to 37 GPa, which is believed to offer straightforward evidence for phase transitions. Representative XRD patterns of MnNb2O6 are shown in Figure 4a. At about

−7/3 ⎡ ⎛ V ⎞−5/3⎤ 3B0 ⎢⎛ V ⎞ ⎥ P= −⎜ ⎟ ⎜ ⎟ ⎥⎦ 2 ⎢⎣⎝ V0 ⎠ ⎝ V0 ⎠

⎧ ⎡⎛ ⎞−2/3 ⎤⎫ ⎪ ⎪ V 3 ⎢ ⎨1 + (B0′ − 4) ⎜ ⎟ − 1⎥⎬ ⎢ ⎥ ⎪ 4 ⎣⎝ V0 ⎠ ⎦⎪ ⎩ ⎭

(1)

where B0 is the bulk modulus and B′0 is the pressure derivative. The fitting result for the orthorhombic phase gives a bulk modulus B0 = 154.4 ± 3.5 GPa at a fixed B0′ = 4.1; this value is in good agreement with that of (Mn, Fe)Nb2O6, whose B0′ is 149(1) to 153(1) GPa and B′0 is from 4.1(2) to 4.8(3).16 The EOS is presented in Figure 5 together with the P−V data from

Figure 4. (a) High-pressure XRD patterns of MnNb2O6 at room temperature. (b) D-spacing change of MnNb2O6 as a function of pressure.

1.0 GPa, the diffraction pattern is indexed in orthorhombic symmetry. All diffraction peaks shift toward higher 2θ angles, indicating a decrease of interplanar distance of crystal planes under compression. The orthorhombic phase remains stable up to 10.3 GPa, as shown in Figure 4b, consistent with the Raman results. A new diffraction peak for the high-pressure phase appears after 12.6 GPa, indicating the structure transition. Two additional diffraction peaks can be observed up to 15.1 GPa, and the diffraction peaks owing to the orthorhombic phase become weak and disappear finally. As most diffraction peaks get weak and broaden with pressure, only a few broad and weak diffraction peaks can be obtained for the high pressure phase, and these few peaks restrain any accurate Rietveld refinement to resolve the exact structure with accurate atomic positions in a unit cell.27,28 At 37 GPa, the full width of half-maximum (fwhm) of major peak is about 5 times the width at low pressure, suggesting a probable trend of pressure-induced amorphous MnNb2O6; however, the high pressure gradient surroundings could also cause the peak weakening and broadening, especially for the silicon oil. Its hydrostaticity became worse at about 40 GPa, so we performed a double check with argon as the pressure transmitting media to get a better hydrostatic condition. The results confirmed that the peak weakening and broadening are caused by the pressure

Figure 5. Pressure dependence of volume per formula unit for the lowpressure phases of MnNb2O6. Note that Fe/(Fe + Mn) ratios (0.80 for sample RAO no. 17 and RAO no. 15; 0.15 for sample KRA no. 11).16 The solid curves are third-order BM EOS fits of the data.

ref 16 for comparison. In ref 16, Pistorino et al. indicated that the volume per formula unit increases when the Fe/(Fe + Mn) ratio descends; for pure MnNb2O6 it should have the biggest volume at ambient pressure, which is consistent with our results. The knowledge of the volume dependencies of pressure allows us to estimate the Grüneisen parameters associated with them. For each vibration mode in the Brillouin zone, a single mode Grüneisen parameter is defined30,31 as γ=−

∂ ln(v) ∂ ln(V )

(2)

Figure 6 shows the plot of ln(ν) vs ln(V) for the Raman frequencies. The Grüneisen parameters resulting from a linear 19284



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I was obtained with third-order BM method, and the bulk modulus of the columbite is determined as 154.4 ± 3.5 GPa, with a pressure derivative of 4.1. This study provides significant information for better understanding of the physical properties of MnNb2O6 under extreme conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-431-85168881. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Xiaodong Li, Yanchun Li, and Lun Xiong for their help during the experimental research. Angle dispersive XRD experiments of this work were performed at 4W2 HPStation, BSRF assistance in the synchrotron measurement. Portions of this work were performed at X17C beamline, NSLS, Brookhaven National Laboratory and HPCAT, APS, Argonne National Laboratory. This work was supported by the National Basic Research Program of China (No. 2011CB808200), the National Natural Science Foundation of China (Nos. 11274137, 91014004, 11074090, 11004074), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20100061120093) and National Found for Fostering Talents of Basic Science (No. J1103202).

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REFERENCES

(1) Dos Santos, C. A.; Zawislak, L. I.; Kinast, E. J. Crystal Chemistry and Structure of the Orthorhombic (Fe, Mn)(Ta, Nb)2O6 Family of Compounds. Braz. J. Phys. 2001, 31, 616−631. (2) Lee, H. J.; Kim, I. T.; Hong, K. S. Dielectric Properties of AB2O6 Compounds at Microwave Frequencies (A = Ca, Mg, Mn, Co, Ni, Zn, and B = Nb, Ta). Jpn. J. Appl. Phys. 1997, 36, 1318−1320. (3) Pullar, R. C. The Synthesis, Properties, and Applications of Columbite Niobates (M2+Nb2O6): A Critical Review. J. Am. Ceram. Soc. 2009, 92, 563−577. (4) Scharf, W.; Weitzel, H.; Maartense, I.; Yaeger, I.; Wanklyn, B. M. Magnetic Structures of CoNb2O6. Magnetism Magnetic Mater. 1979, 13, 121−124. (5) Prabhakaran, D.; Wondre, F. R.; Boothroyd, A. T. Preparation of Large Single Crystals of ANb2O6 (A = Ni, Co, Fe, Mn) by the Floating-Zone Method. J. Cryst. Growth 2003, 250, 72−76. (6) Weitzel, H. Magnetische Struktur von Columbit, FeNb2O6. Z. Anorg. Allg. Chem. 1971, 380, 119−127. (7) Holmes, L. M.; Ballman, A. A.; Hecker, R. R. Antiferromagnetic Ordering in MnNb2O6 Studied by Magnetoelectric and Magnetic Susceptibility Measurements. Solid State Commun. 1972, 11, 409−413. (8) Nielsen, O. V.; Lebech, B.; Larsen, F. K.; Holmes, L. M.; Ballman, A. A. A Neutron Diffraction Study of The Nuclear and Magnetic Structure of MnNb2O6. J. Phys. C: Solid State Phys. 1976, 9, 2401− 2410. (9) García-Alvarado, F.; Orera, A.; Canales-Vazquez, J. On the Electrical Properties of Synthetic Manganocolumbite MnNb2O6−δ. Chem. Mater. 2006, 18, 3827−3834. (10) Orera, A.; García-Alvarado, F.; Irvine, J. T. S. Effect of TiSubstitution on The Electrical Properties of MnNb2O6−δ. Chem. Mater. 2007, 19, 2310−2315. (11) Arroyo y de Dompablo, M. E.; Lee, Y.-L.; Morgan, D. First Principles Investigation of Oxygen Vacancies in Columbite MNb2O6 (M = Mn, Fe, Co, Ni, Cu). Chem. Mater. 2010, 22, 906−913. (12) Mansurova, A. N.; Gulyaeva, R. I.; Chumarev, V. M.; Mar’evich, V. P. Thermochemical Properties of MnNb2O6. J. Therm. Anal. Calorim. 2010, 101, 45−47. (13) Husson, E.; Repelin, Y.; Dao, N. Q.; Brusset, H. Etude Par Spectrophotométries D’absorption Infrarouge et de Diffusion Raman

Figure 6. Plot of ln ν versus ln V for the Raman frequencies.

fit of ln(ν) as a function of ln(V) at room temperature are listed for all the modes in Table 1. Grüneisen parameters indicate the vibration frequencies dependence of the volume; it can reflect the anharmonic effects, indicating how sensitive the vibrational eigenfrequencies are to the change of volume. In addition, the negative values of γ indicate that the vibrations corresponding to the soft modes are considerably unstable. On the other hand, except in the very low frequency region, one is tempted to write “γ ≈ const × S”. S is the mode bond-stretching vibrational entropy. When S is close to 1, a mode is mostly bond stretching vibration, while S near 0 indicates bond-bending modes.32 So γ is much bigger in the bond-stretching part of the interatomic interaction than in the bond bending part, which agrees well with our Raman results (Table 1). The knowledge is essential not only for thermodynamic relation, but also for the interpretation of the internal friction and sound attenuation experiments.33−35

4. CONCLUSIONS In conclusion, the structural behavior of MnNb2O6 was investigated by Raman scattering and angle dispersive synchrotron XRD measurements up to 40 GPa at room temperature. The pressure-induced phase transition at about 12 GPa was observed. Raman scattering results reveal two softening modes of the internal ν10(B2g) and ν8(Ag) vibration, implying the distortion of the NbO6 octahedra under high pressure. Grüneisen parameters show that instability of the columbite orthorhombic structure at high pressure is caused by the strong deformation of the NbO6 octahedra, suggesting a possible soft-mode-driven phase transition. According to the XRD data, a trend of pressure induced amorphous transition can be observed. The EOS for MnNb2O6 in low pressure phase 19285



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des Niobates de Structure Columbite. Spectrochim. Acta 1977, 33, 995−1001. (14) Husson, E.; Repelin, Y.; Dao, N. Q.; Brusset, H. J. Normal Coordinate Analysis of The MNb2O6 Series of Columbite Structure (M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd). Chem. Phys. 1977, 67, 1157−1163. (15) Sturdivant, J. H. The Crystal Structure of Columbite. Z. Kristallogr. 1930, 75, 88−105. (16) Pistorino, M.; Nestola, F.; Ballaran, T. B.; Domeneghetti, M. C. The Effect of Composition and Cation Ordering on The Compressibility of Columbites up to 7 GPa. Phys. Chem. Miner. 2006, 33, 593−600. (17) Tarantino, Serena C.; Zema, M.; Ballaran, T. B. Crystal Structure of Columbite under High Pressure. Phys. Chem. Miner. 2010, 37, 769−778. (18) Li, L.; Feng, G. L.; Wang, D. J.; Yang, H. Optical Floating Zone Method Growth and Photoluminescence Property of MgNb2O6 Crystal. J. Alloys. Compd. 2011, 509, 263−266. (19) Mao, H. K.; Xu, J.; Bell, P. M. Calibration of The Ruby Pressure Gauge to 800 kbar under Quasi-Hydrostatic Conditions. J. Geophys. Res. 1986, 91, 4673−4676. (20) Huang, X. L.; Duan, D. F.; Wang, K. Structural and Electronic Changes of SnBr4 under High Pressure. J. Phys. Chem. C 2013, 117, 8381−8387. (21) Hammersley, A. P.; Svensson, S. O.; Hanfland, M. TwoDimensional Detector Software: from Real Detector to Idealised Image or Two-Theta Scan. High Pressure Res. 1996, 14, 235−248. (22) Rousseau, D. L.; Bauman, R. P.; Porto, S. P. S. Normal Mode Determination in Crystals. J. Raman Spectrosc. 1981, 10, 253−258. (23) Husson, E.; Repelin, Y.; Dao, N. Q.; Brusset, H. Normal Coordinate Analysis for CaNb2O6 of Columbite Structure. J. Chem. Phys. 1977, 66, 5173−5180. (24) Bouvier, P.; Kreisel, J. Pressure-Induced Phase Transition in LaAlO3. J. Phys.: Condens. Matter 2002, 14, 3981−3991. (25) Zhao, J.; Ross, N. L.; Angel, R. J. Polyhedral Control of The Rhombohedral to Cubic Phase Transition in LaAlO3 Perovskite. J. Phys.: Condens. Matter 2004, 16, 8763−8773. (26) Itie, J. P.; Couzinet, B. Pressure-Induced Disappearance of The Local Rhombohedral Distortion in BaTiO3. Europhys. Lett. 2006, 74, 706−711. (27) Mishra, A. K.; Murli, C.; Garg, N.; Chitra, R.; Sharma, S. M. Pressure-Induced Structural Transformations in Bis(glycinium)oxalate. J. Phys. Chem. B 2010, 114, 17084−17091. (28) Pravica, M.; Shen, Y.; Quine, Z.; Romano, E.; Hartnett, D. HighPressure Studies of Cyclohexane to 40 GPa. J. Phys. Chem. B 2007, 111, 4103−4108. (29) Birch, F. Finite Elastic Strain of Cubic Crystals. Phys. Rev. 1947, 71, 809−824. (30) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Saunders College: Philadelphia, PA, 1976, 462−465. (31) Holzapfel, W. B. Equations of State for Solids under Strong Compression. Z. Kristallogr. 2001, 216, 473−488. (32) Fabian, J.; Allen, P. B. Thermal Expansion and Grüneisen Parameters of Amorphous Silicon: A Realistic Model Calculation. Phys. Rev. B 1997, 79, 1885−1888. (33) Vacher, R.; Pelous, J.; Plicque, F.; Zarembowitch, A. Ultrasonic and Brillouin Scattering Study of The Elastic Properties of Vitreous Silica between 10 and 300 K. J. Non-Cryst. Solids 1981, 45, 397−410. (34) Morath, C. J.; Maris, H. J. Phonon Attenuation in Amorphous Solids Studied by Picosecond Ultrasonics. Phys. Rev. B 1996, 54, 203− 213. (35) White, B.; Pohl, R. O. Elastic Properties of Amorphous Solids below 100 K. Z. Phys. B: Condens. Matter 1996, 100, 401−408.

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Structural Transition of MnNb2O6 under Quasi-Hydrostatic Pressure

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