Reversible Structural Phase Transition in ZnV2O6 at High Pressures

Article pubs.acs.org/JPCC

Reversible Structural Phase Transition in ZnV2O6 at High Pressures Ruilian Tang,† Yan Li,*,†,‡ Nana Li,† Dandan Han,† Hui Li,† Yongsheng Zhao,† Chunxiao Gao,† Pinwen Zhu,† and Xin Wang*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, Jilin, China College of Physics, Jilin University, Changchun 130012, Jilin, China

‡

ABSTRACT: The structural and electrical properties of ZnV2O6 under high pressure have been studied using Raman spectroscopy, in situ angle dispersive X-ray diffraction (ADXRD), and alternating current (AC) impedance spectroscopy. The results of Raman spectra indicate that ZnV2O6 undergoes a reversible structural change around 16.6 GPa, as evidenced by the appearance of new peaks. The results of Rietveld refinements from in situ ADXRD data indicate that the monoclinic symmetry (C2/m) is retained up to 16.0 GPa and the C2 phase comes to coexist between 16.0 and 16.9 GPa. Above 16.9 GPa, the high-pressure phase can be distinguished only as the C2 structure. The transformation process from the C2/m phase to the C2 phase is mainly caused by the more distorted ZnO6 octahedra and VO6 octahedra at higher pressures. The equal bond distances Zn−O2 and V−O3 in the C2/m phase become unequal in the C2 phase. Furthermore, the measurements of the AC impedance spectroscopy of ZnV2O6 reveal obvious changes in its electrical transport properties at 14.1 GPa which could correspond to the observed phase transition in the Raman and ADXRD measurements. The combined analyses of experimental results suggest the occurrence of a reversible structural phase transition of ZnV2O6 around 16.0 GPa.

1. INTRODUCTION 3d transition-metal vanadates have been extensively investigated in view of their importances to basic or fundamental science as well as catalysis, optoelectronics, and electrochemical devices field.1−9 During the last few decades, researchers have been interested in exploring the influence of high pressure and high temperature on the structures of binary and ternary transition-metal vanadium oxides. Many of the binary V−O system compounds show metal− insulator (MI) transitions as a function of temperature. For example, the MI transition in V4O7 was accompanied by charge separation and charge ordering (CO).10−12 In β-A0.33V2O5 (A = Na and K), the CO phase collapsed under relatively high pressure (≥7 GPa) in all A+ compounds, and then a pressure-induced superconducting phase appeared.13,14 In divalent A2+ compounds, β-A0.33V2O5 (A = Ca and Sr) did not show any superconductivity although the CO phases were suppressed under relatively low pressures for β-Ca0.33V2O5 at 3.5 GPa and β-Sr0.33V2O5 at 1.5 GPa, respectively.15 In this context, metavanadate MV2O6 (M = divalent ions, such as Mg, Ca, Zn, Cd) compounds are interesting candidates for further investigations. In the studies of vanadium oxides, a reversible phase transition as a function of temperature was found in many of MV2O6 compounds.16,17 In the low-temperature structure (brannerite-type), vanadium ions had the distorted octahedral 5 + 1 coordination, whereas in the high-temperature structure (columbite-type), vanadium ions with the nearest five oxygen atoms formed a well-defined trigonal bipyramid. The structure of ZnV2O6 is a typical representative of MV2O6 compounds with the brannerite (ThTi2O6) structure.16,18 As shown in Figure 1a−c, the Zn2+ ions are octahedrally coordinated and the V5+ ions have an irregular octahedral coordination. The © 2014 American Chemical Society

Figure 1. Structure of ZnV2O6 at ambient conditions; (a) the unit cell, (b) ZnO6 octahedron, and (c) VO6 octahedron.

structure can be characterized by a distorted cubic close-packed oxygen network. The VO6 octahedra linked by edges can form the infinite anionic layers parallel to (001), and the ZnO6 octahedra joined by opposite edges can form the infinite rows along the b axis and link the anionic layers. So far, vanadium oxides exhibit rich behaviors by varying pressures and temperatures. Nevertheless, to our knowledge in Received: November 17, 2013 Revised: April 29, 2014 Published: May 5, 2014 10560

dx.doi.org/10.1021/jp411283m | J. Phys. Chem. C 2014, 118, 10560−10566

The Journal of Physical Chemistry C

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Figure 2. (a) Selected Raman spectra of ZnV2O6 under high pressure. Two new peaks that appeared above 16.6 GPa are marked with asterisks. (b) Pressure dependences of Raman shifts of ZnV2O6. Empty symbols represent the first run, and solid symbols the second run. The vertical dashed line represents the pressure point of phase transition.

using the 532 nm line of an argon ion laser (run 2). Pressureinduced shifts of overlapping Raman bands were analyzed by fitting the peaks to Lorentzian functions to determine the line shape parameters in both runs. In situ ADXRD experimental runs were carried out at the beamline X17C of the National Synchrotron Light Source at Brookhaven with a monochromic wavelength of 0.4112 Å and Mar CCD detector. Two-dimensional XRD images were analyzed using the FIT2D software, yielding one-dimensional intensity versus diffraction angle 2θ patterns. Rietveld analyses were performed with the software GSAS.22 Van der Pauw electrodes were integrated on one facet of DAC for the electric properties measurement under high pressure. The fabrication process of the detecting microcircuit was reported in our previous publications.23,24 The measurements of AC impedance spectroscopy in the frequency range of 1 Hz to 10 MHz at pressures up to 25 GPa were carried out by using a Solartron 1260 impedance analyzer equipped with a Solartron 1296 dielectric interface. The applied AC voltage was 0.1 V.

situ pressure-dependent structural information on MV2O6 compounds is limited. We present comprehensive investigations on the pressure-induced phase transition in ZnV2O6 at room temperature. Using Raman spectroscopy, in situ angle dispersive X-ray diffraction (ADXRD), and alternating current (AC) impedance spectroscopy, we show that ZnV2O6 transforms from C2/m phase to C2 phase because of the distortion cooperation of the ZnO6 octahedra and VO6 octahedra. The structural phase transition in ZnV2O6 is also accompanied by the change in its electric conductance. The phase transition is reversible upon decompression.

2. EXPERIMENTAL DETAILS The sample of ZnV2O6 was synthesized by a sol−gel method. Analytical grade V2O5, Zn(NO3)2·6H2O, and oxalic acid (H2C2O4) were used as the starting materials. V2O5 and oxalic acid in a molar ratio of 1:3 were dissolved in distilled water followed by constant stirring to form a transparent blue solution. Afterward, a 0.1 mol/L Zn(NO3)2 solution was slowly titrated into the solution until a cationic molar ratio of Zn/V = 1:10 was reached. The above solution was continuously stirred at 338 K and then heat treated at 373 K to get a precursor. This precursor was pressed into a pellet and then heat treated to obtain the final products. The reaction temperature was chosen to be 873 K in air over a period of 10 h. X-ray diffraction (XRD, Rigaku) with Cu Kα radiation (λ = 1.5418 Å) was used to determine the crystal structure. A diamond anvil cell (DAC) was utilized to generate high pressure with the T301 stainless steel as the gasket, which was preindented to a 50 μm thickness. The powder samples were loaded into the DAC along with chips of ruby for measuring the sample pressure.19 A 16:3:1 methanol/ethanol/water mixture was chosen as pressure medium to provide hydrostatic conditions in Raman spectroscopy measurements, and argon20,21 was chosen in in situ ADXRD measurements. Two Raman spectroscopy runs were carried out on Renishaw in Via Raman microscope using the 514.5 nm line of an argon ion laser (run 1) and on Jobin Yvon T64000 Raman microscope

3. RESULTS AND DISCUSSION 3.1. Raman Spectra at High Pressures. According to the factor group analysis based on C2/m (space group 12), the structure of ZnV2O6 yields the irreducible representation at the Brillouin zone center as the following: Γopt = 8Ag (R ) + 4Bg (R ) + 4A u (IR ) + 8Bu (IR )

(1) 25

to which the acoustic modes (Au+2Bu) should be added. Figure 2a shows the selected Raman spectra of ZnV2O6 under high pressure at room temperature. There are 11 phonon modes in total clearly observed in the Raman spectra at ambient conditions. The assignments of Raman bands are taken from the previous report by Baran, and the results are listed in Table 1.26−30 At lower frequencies, the peaks at 143 and 168 cm−1 represent (V2O2)n (generated by the edge-sharing between pairs of VO6 octahedral) stretching mode. Two bands appearing at 267 and 348 cm−1 are generally associated with lattice modes and ZnO6 10561



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Table 1. Observed Raman Frequencies and Band Assignments for ZnV2O6 at Ambient Conditions

a

Raman frequency (cm−1)

assignmenta

914 784 716 514 431 348 300 267 218 168 143

ν(VO)str νas(VOV)str ν(V2O2)n+ν(V3O)str νs(VOV)str ZnO6 modes ν(V3O)str ZnO6 modes lattice modes ν(V2O2)n ν(V2O2)n

str, stretch; as, antisymmetric stretch; s, symmetric stretch. Figure 3. Pressure dependence of ADXRD patterns of ZnV2O6 up to 26.5 GPa.

modes, respectively. The peak located at 300 cm−1 can be assigned to VO3 mode (originating from the sharing of VO6 octahedra between neighboring double chains). For the stretching vibrations of the V−O−V bonds running along the VO6 octahedra parallel to the b axis, two Raman lines at 431 and 784 cm−1 are assigned. The first line is assigned to a symmetric stretching mode, and the other to the corresponding antisymmetric vibration. In the region of 500−750 cm−1, the Raman lines are assigned to the stretching motions of the (V2O2)n and V3O. The most intense Raman line at 914 cm−1 corresponds to the stretching vibrations of V−O bonds. The frequency shifts of all the observed Raman modes with pressure in two experimental runs are shown in Figure 2b. With increasing pressures, all the Raman bands are shifted to the higher frequencies up to 16.6 GPa. Dramatic spectral changes occur at 16.6 GPa, as two new peaks appear, which are marked by asterisks in Figure 2a. These two Raman modes are located at 714 and 904 cm−1 and are involved mainly with the V−O stretching and V−O−V bending motions. Accordingly, the subsequent changes result from the rearrangements of V−O bonds during the compression. With further increasing pressures, the Raman spectra become somewhat deteriorated at 30.6 GPa, mostly because of the enhanced electronic screening effect leading to the enhancements of the coupling of phonons to electronic states, which was confirmed by the increasing conductivity as discussed below. When the pressure is released, the spectrum of the starting monoclinic phase was recovered. From our high-pressure Raman measurements, the structure of ZnV2O6 was stable below 16.6 GPa at room temperature. Above 16.6 GPa, the Raman spectra exhibited clear evidence for a change of structural symmetry, and the transition process was reversible. The deteriorated Raman spectra above 30 GPa could be interpreted as a tendency for insulator-to-metal transition; similar behaviors have been reported in many vanadium oxides.31,32 In order to further understand the structural and electronic changes of ZnV2O6, high-pressure in situ ADXRD and AC impedance spectra were measured. 3.2. In Situ ADXRD Measurement at High Pressures. Belonging to the series of MV 2O 6 compounds, ZnV2 O6 crystallizes in space group C2/m with a structure derived from a pseudohexagonal close-packed arrangement of oxygen atoms.18 At ambient pressure, the following lattice parameters have been obtained for ZnV2O6: a = 9.2450(2) Å, b = 3.5331(5) Å, c = 6.5791(7) Å, and β = 111.7(9)°. In situ ADXRD patterns of ZnV2O6 are collected up to 26.5 GPa at room temperature, and the representative patterns are displayed in Figure 3.

The diffraction patterns appear similar at each pressure point except for peaks shifting and broadening. ZnV2O6 had been refined in space group C2 by earlier work, but subsequent studies showed that the structure should be in space group C2/m.18,33 So the space group C2/m was chosen from the beginning during the Rietveld refinements, and this phase can be refined quite well up to 16.0 GPa. However, continuing to fit the structure within the C2/m symmetry will cause an anomalous increase of unit cell volume at 16.0 GPa. Therefore, a possible structural transformation should be taken into consideration. Because of the resemblances in structure, the C2 phase is chosen as the candidate for the new high-pressure phase. Here, we have taken a different strategy to refine the structure above 16.0 GPa, and the structure is refined in separate space group C2/m, C2, and in both C2/m and C2 at 16.0, 16.9, and 18.4 GPa, respectively. As can be seen from Table 2, the smallest R value is obtained for the Table 2. Results of the Merits of the Refined Structure in Separate Space Group C2/m, C2, and in both C2/m and C2 at 16.0, 16.9, and 18.4 GPa space group C2/m C2 C2/m + C2

R factors

16.0 GPa

16.9 GPa

18.4 GPa

Rp Rwp Rp Rwp Rp Rwp

0.61% 0.84% 0.77% 1.12% 0.59% 0.79%

0.57% 0.80% 0.72% 1.13% 0.56% 0.74%

0.54% 0.79% 0.52% 0.73% 0.74% 0.96%

mixed phases of C2/m and C2 at 16.0 and 16.9 GPa and for the C2 phase at 18.4 GPa. Figure 4 shows the Rietveld refinement results of ZnV2O6 at 0.8, 16.9, and 18.4 GPa, respectively. We should note the small difference among these R values, which indicates these two structures are closely related. Indeed, the C2 structure resembles the C2/m structure with the reduced symmetry because of a Jahn−Teller distortion of the ZnO6 octahedra.18 When the results of Raman spectra are combined, a subtle structural phase transition from C2/m phase to C2 phase should happen above 16.0 GPa. To have a clear structural point of view, we have plotted the crystal structures of the ZnV2O6-C2/m phase at 14.5 GPa and the ZnV2O6-C2 phase at 21.4 GPa from the refined data (Figure 5a−f). 10562



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Table 3. Zn−O and V−O Bond Lengths of ZnO6 Octahedra and VO6 Octahedra in the ZnV2O6-C2/m Phase at 14.5 GPa and the ZnV2O6-C2 Phase at 21.4 GPa ZnV2O6

Figure 4. Observed (solid circles) and calculated (solid lines) diffraction patterns of ZnV2O6 at 0.8, 16.9, and 18.4 GPa. The difference curve and the tick marks for the calculated reflection positions are plotted at the bottom of each panel.

Zn−O distance (Å)

figure

V−O distance (Å)

figure

C2/m

Zn−O1 Zn−O1 Zn−O2 Zn−O2 Zn−O2 Zn−O2

1.8416 1.8416 2.1966 2.1966 2.1966 2.1966

Figure 5b V−O1(i) V−O1(ii) V−O2 V−O3(i) V−O3(ii) V−O3(ii)

1.7671 2.1251 1.6676 2.0731 1.8997 1.8997

Figure 5c

C2

Zn−O1 Zn−O1 Zn−O2(i) Zn−O2(i) Zn−O2(ii) Zn−O2(ii)

1.8998 1.8998 1.8801 1.8801 2.6998 2.6998

Figure 5e V−O1(i) V−O1(ii) V−O2 V−O3(i) V−O3(ii) V−O3(iii)

1.6319 2.3628 1.6621 2.2391 2.0342 1.5341

Figure 5f

pressure−volume data of the two phase of ZnV2O6 were fitted to a second-order Birch−Murnaghan equation of state (B′ = 4):34

The information on bond distances of ZnO6 octahedra and VO6 octahedra are also listed in Table 3. After the structural phase transition, the equal bond distances Zn−O2 and V−O3 in the C2/m phase become unequal in the C2 phase. The transformation process from the C2/m phase to the C2 phase is mainly caused by the more distorted ZnO6 octahedra and VO6 octahedra at higher pressures. A detailed analysis of cell parameters as a function of pressure is shown in Figure 6a−d. The lattice parameters a, b, and c decrease consistently with pressure, and the monoclinic angle β increases upon compression. A small but discernible change in compressibility at 16.0 GPa is observed from the ZnV2O6-C2/m to the ZnV2O6-C2 phase transition, where remarkable changes were observed from our high-pressure Raman measurements. These discontinuities of lattice parameters with pressure are also reflected in the volume reduction, as shown in Figure 7. To determine the bulk modulus B0, its pressure derivative B′, and the molar volume at ambient conditions V0, the experimental

P=

−7/3 ⎡ ⎛ V ⎞−5/3⎤ ⎛ 3 ⎞ ⎢⎛ V ⎞ ⎥ ⎜ ⎟B − ⎜ ⎟ ⎜ ⎟ ⎝ 2 ⎠ 0 ⎢⎝ V0 ⎠ ⎥⎦ V ⎝ ⎠ 0 ⎣

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

(2)

The bulk modulus is determined to be B0 = 147(2) GPa for the ZnV2O6-C2/m phase and B0 = 237(1) GPa for the ZnV2O6-C2 phase. 3.3. AC Impedance Spectroscopy Measurement at High Pressures. It has been demonstrated in many cases that the subtle change of structure consequentially brings some influence on the transport coefficient, such as the anomaly in c/a for Zn accompanied by a small variation in the resistance,35 and an obvious change in the impedance spectroscopy of CdS reflecting the structural phase transition.36 Hence, the anomaly

Figure 5. Schematic comparison between the ZnV2O6-C2/m phase at 14.5 GPa ((a) the unit cell, (b) ZnO6 octahedron, and (c) VO6 octahedron) and the ZnV2O6-C2 phase at 21.4 GPa ((d) the unit cell, (e) ZnO6 octahedron, and (f) VO6 octahedron). 10563



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Figure 6. Variations of lattice parameters with pressure.

observed in ZnV2O6 by both Raman spectroscopy and X-ray diffraction was likely to cause some subtle changes in its electrical transport properties. On the basis of this judgment, we measured the AC impedance spectroscopy of ZnV2O6 under pressure in order to extract more information on pressure effects on the sample. Typical Nyquist plots of ZnV2O6 at different pressures are displayed in Figure 8. Two overlapped semicircles can be seen clearly: the arcs that represent the contribution of grain boundary to the total resistance are plotted on the right side of the Nyquist plots, and on the left side it shows the grain contributions. With increasing pressures, all the arcs shift from right to left with reduced areas, which is due to the associated decrease of the impedances. At higher pressures, the arc corresponding to the grain boundary becomes smaller than that of the grain, and the situation is reversed at lower pressures. When the pressure is above 14.1 GPa, the arcs corresponding to the grain boundary almost disappear.

Figure 7. Isothermal compression curve of ZnV2O6 at ambient temperature. The solid line is a second-order Birch−Murnaghan fit to the experimental data.

Figure 8. Nyquist plots of impedance spectroscopy of ZnV2O6 at high pressures. 10564



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dominant role in the high-pressure phase. If Rt designates the total measured resistance, namely Rt = Rg + Rgb, the variation of Rt as a function of pressure is shown in Figure 11. The value of

To deeply analyze the impedance spectroscopy, the obtained data are usually modeled by an equivalent circuit consisting of resistor R and constant phase element (CPE).37 The CPE is expressed by the following equation and defined by two values, T and P. Z = 1/[T (jω)P ]

(3)

where T is expressed in units of capacitance component; ω and j are the frequency and the imaginary unit, respectively. The CPE is identical to a capacitance component when the exponent P = 1, to a resistance component when P = 0, and to a Warburg element when P = 0.5. When a CPE is placed in parallel to a resistance, a Cole element (depressed semicircle) is produced. The values of circuit parameters are estimated by nonlinear least-squares fitting with the aid of the program Zview (Scribner Associates, Inc., Southern Pines, NC). On the basis of the impedance results, two sets of parallel components in the series as illustrated in Figure 9 have been Figure 11. Fitting result of the total resistance of ZnV2O6 versus pressure.

Rt decreases remarkably with increasing pressure, and an abnormal change can be clearly seen at 14.1 GPa. In theory, the electronic transport behavior was closely related to the crystal structure, and the pressure-induced phase transition in a conductive phase could be identified by the abrupt changes in its transport properties.38−40 Therefore, deducing from the changes of the total resistances, the phase transition in ZnV2O6 occurred at 14.1 GPa. This change was consistent with the results of Raman spectroscopy and ADXRD under high pressure, and it provided a further proof that the ZnV2O6 underwent a structural phase transition under pressure. The difference of pressure points for the phase transition between the optical and electric measurements could be attributed to the nonhydrostatic conditions of the sample.

Figure 9. Equivalent circuit used to represent the electrical properties of grain and grain boundary effects in ZnV2O6 at 12.9 GPa.

employed to model the impedance spectroscopy. The Rg and Rgb represent the resistance of grain and grain boundary, respectively. The values of Rg and Rgb are directly obtained from the fitting results, as shown in Figure 10.

4. CONCLUSIONS In this work we have presented detailed vibrational, structural, and electrical studies of ZnV2O6 under high pressure by means of Raman spectroscopy, in situ ADXRD, and AC impedance spectroscopy measurements. We have observed abrupt changes of the Raman spectra around 16.6 GPa. Moreover, ADXRD measurements evidenced that ZnV2O6 underwent a phase transition starting at 16.0 GPa from the C2/m phase to the C2 phase because of the enhanced distortions of ZnO6 octahedra and VO6 octahedra under high pressure. Meanwhile, this structural phase transition has also been observed to be accompanied by the changing of the total resistance of ZnV2O6 under high pressure.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-431-85168881. Fax: +86-431-85168881. *E-mail: [email protected]. Phone: +86-431-85168881. Fax: +86-431-85168881.

Figure 10. Pressure dependence of resistances for grain and grain boundary.

The values of Rg and Rgb decrease remarkably with increasing pressure, and this results in the decrease of total resistance. Two obvious discontinuities can be seen clearly at 14.1 GPa. The resistances of grain boundaries become much smaller than those of grains above 14.1 GPa, indicating that the carrier dispersion effect of grain boundaries becomes weaker with increasing pressures. On the other side, the grain resistances begin to play a

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Grant 51172091; the 10565



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Program for New Century Excellent Talents in University, China (NCET-12-0240); Jilin Province Science and Technology Development Program, China (20130101023JC); and the National Fund for Fostering Talents of Basic Science, China (Grant J1103202). We acknowledge Dr. Chen Zhi-Qiang and Dr. Hong Xing-Guo for technical support with the high-pressure experiments at the X17C beamline of the NSLS synchrotron.

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Reversible Structural Phase Transition in ZnV2O6 at High Pressures

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