Journal of Physics and Chemistry of Solids 116 (2018) 113–117

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Journal of Physics and Chemistry of Solids

journal homepage: www.elsevier.com/locate/jpcs

Studies on the structural stability of Co2P2O7 under pressure

W.P. Wang a,b, H. Pang c, M.L. Jin a,b, X. Shen a, Y. Yao a, Y.G. Wang a, Y.C. Li d, X.D. Li d,C.Q. Jin a,b, R.C. Yu a,b,*

a Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, PR Chinab School of Physical Sciences, University of Chinese Academy of Sciences, PR Chinac College of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu, 225002, PR Chinad Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100039, PR China

A R T I C L E I N F O

Keywords:Co2P2O7

High pressure synchrotron radiationStructural transformation

* Corresponding author. Beijing National Laboratory ofE-mail address: rcyu@iphy.ac.cn (R.C. Yu).

https://doi.org/10.1016/j.jpcs.2018.01.028Received 9 September 2017; Received in revised form 15Available online 23 January 20180022-3697/© 2018 Elsevier Ltd. All rights reserved.

A B S T R A C T

The crystal structural evolution of Co2P2O7 was studied by using in situ high pressure angle dispersive x-raydiffraction with synchrotron radiation. The results demonstrate that the α phase of Co2P2O7 goes through apartially irreversible structural transformation to β phase under pressure. The pressure is conductive to reduce thelongest Co–O bond length of the α phase, and then more uniform Co–O bonds and regular hexagonalarrangement of CoO6 octahedra of the β phase are favored. According to the Birch-Murnaghan equation, the fittedbulk modulus B0 is 158.1(�5.6) GPa for α phase and 276.5(�6.5) GPa for β phase. Furthermore, the first-principles calculations show that these two phases of Co2P2O7 have almost equal total energies, and also havesimilar band structures and spin-polarized density of states at their ground states. This may be the reason whythese two phases of Co2P2O7 can coexist in the pressure released state. It is found that the band gap energiesdecrease with increasing pressure for both phases.

1. Introduction

As a large family, pyrophosphate A2P2O7 (where A represents thebivalent cations, such as Fe, Co, Ni, Cu, Zn, Sr, Ba, Hg, Mg, etc.) [1–9] isknown to be of a distorted layer honeycomb structure, usually belongingto the monoclinic crystal system. Based on different synthesis conditionsand measured temperatures, A2P2O7 shows different structures. At roomtemperature, this type of compounds usually has a space group of P21/c(No. 14), which is labeled as α phase [6,9,10]. Typically, their structuresare composed of layers of double-tetrahedral P2O7 group with an intervallayer of broken-sided two-dimensional array of hexagons, which areformed by the edge sharing of AO5 hexahedra and AO6 octahedra. Whenthe temperature increases to 873 K, the α phase gradually transforms to βphase with a space group of C2/m (No. 12) [11,12]. The β phase isgenerally called the high-temperature phase and has a higher symmetrythan the α phase. The β phase belongs to the thortveitite structure [13]and is formed by the regular hexagonal arrangement of AO6 octahedrawith the insertion of double-tetrahedral P2O7 group. Besides α and βphases, Co2P2O7 has a third phase, labeled as γ [10,14]. The γ phase isanother low-temperature phase synthesized by the hydrothermalmethod, and in some reports it is called the high pressure phase [15].

Condensed Matter Physics, Institute

January 2018; Accepted 15 January

Although many studies about high-pressure properties were carriedout for phosphate APO4 [16–19], studies on A2P2O7 have rarely beenreported. Furthermore, although three different crystal structures ofCo2P2O7 have already been reported, the structural behavior of thesestructures under pressure has not been studied.

In this work, the structure transformation behavior of Co2P2O7 wasstudied using in situ high pressure angle dispersive x-ray diffraction(ADXD) with synchrotron radiation. In addition, the first principles cal-culations were applied to investigate the total energy, band structure anddensity of states (DOS) for the related phases of Co2P2O7 in order tounderstand the structural phase transformation more clearly.

2. Experiments

Co2P2O7 was successfully synthesized by calcinations of NH4Co-PO4⋅H2O, which were performed under the chemical precipitationmethod. Detailed synthesis processes are described elsewhere [20]. Theprepared Co2P2O7 compound has an α phase structure and a nano/-microstructural morphology.

The ADXD experiments were conducted at the 4W2 synchrotron beamline of the Beijing Synchrotron Radiation Facility (BSRF). The

of Physics, Chinese Academy of Sciences, Beijing, 100190, PR China.

2018

W.P. Wang et al. Journal of Physics and Chemistry of Solids 116 (2018) 113–117

synchrotron radiation X-ray wavelength used was 0.6199 Å, and thefocused beam size was about 25 μm, which was defined as the full widthat half-maximum (FWHM) in both horizontal and vertical directions. Thehigh-pressure apparatus was the diamond anvil cell (DAC) with 300 μmculets, which can generate high pressures on the powder samples andalso be transparent to X-ray radiation. Co2P2O7 powders and a small rubygrain were pressed into a T301 stainless steel gasket, in which the real-time pressure can be obtained from the fluorescence spectra of theloaded ruby [21,22]. In our experiments for Co2P2O7, no pressuretransmitting medium was used, indicating a non-hydrostatic condition inthe sample chamber [23–25].

The method of spin-polarized Generalized Gradient Approximation(GGA) in Cambridge Serial Total Energy Package (CASTEP) was used todo the first-principles calculations. The Perdew-Burke-Ernzerhof (PBE)prescription was used to describe the exchange correlation energy withan energy cutoff of 450 eV.

3. Results and discussion

Fig. 1 shows the ADXD patterns of Co2P2O7 under different pressuresup to 47.3 GPa at room temperature. The standard XRD patterns of α andβ phases are also represented with the bar graphs in pink and dark yellowcolors, respectively, at the bottom of the diagram. The primary synthe-sized Co2P2O7 nanoparticles are of the α phase at ambient pressure [20].With increasing pressure, the diffraction peaks of the sample graduallyshift to the right (noted by the black dotted lines), indicating thecompression of the unit cell. When the pressure is increased to 9.4 GPa,one new notable peak (the peak at 2θ¼ 12.45–12.99�, indicated by thered dotted line) emerges. So we consider that the phase transition startsat about 9.0 GPa. Because of the peak broadening induced by pressureand the peak overlap caused by the low symmetry of the monoclinicstructure of Co2P2O7 as well as the close diffraction peak positions for thethree reported Co2P2O7 crystal structures, it is difficult to determine thestructural evolution. To identify the structural evolution, the XRD patternof the released state that has less peak broadening was selected foranalysis. According to the database of crystal structures and careful an-alyses, it can be confirmed that the released state is composed of α and βphases (the dark cyan circles and rhombuses for α and β phases,respectively).

In order to demonstrate that Co2P2O7 consists of α and β phases underpressure, we carried out first-principles calculations to optimize thecrystal structures using the lattice parameters obtained from the XRDpatterns at different pressures and simulated the powder diffractionpatterns to check the agreements. We found that the experimentally

Fig. 1. The ADXD patterns of Co2P2O7 compound under different pressures.

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obtained ADXD patterns of Co2P2O7 under different pressures can befitted well by the simulated ones. As an example, Fig. 2 shows the ADXDpattern of Co2P2O7 under 26.0 GPa and the simulated powder diffractionbar graphs of α and β phases. We also present the lattice parameters of αand β phases in Table 1.

With increasing pressure, the intensity of the peaks of β phase (forexample, the peaks at 2θ¼ 12.45–12.99�) also increases, indicating thatα phase converts gradually to β phase. It should be noted that themaximum pressure of 47.3 GPa in our experiments is not enough to finishthe phase transformation from α phase to β phase. After releasing pres-sure to ambient pressure, the peaks of both phases are maintained in theXRD pattern, implying the coexistence of α and β phases. The resultsshow that the phase transition is at least partially irreversible.

In order to investigate the process of phase transition under pressure,the changes in the lattice parameters of α and β phases, including a, b, c,angle β and relative volume V/V0, are presented in Figs. 3 and 4. It can beseen that a, b and c become smaller for both α and β phases withincreasing pressure. The difference is that angle β of α phase decreaseswith increasing pressure, whereas β phase displays a contrary tendency.As a result, the relative volume V/V0 of the unit cell decreases withpressure, as shown in Figs. 3c and 4c. The experimental data are fittedusing the Birch-Murnaghan (BM) equation [26]:

P ¼ 32B0

"�VV0

��7=3

��VV0

��5=3#�(1� 3

4

�4� B'

0

�"�VV0

��2=3

� 1

#);

where V and V0 are the unit cell volumes of Co2P2O7 at measured pres-sure P and ambient pressure, respectively; B0 and  B0

0  represent the bulkmodulus and its first-order derivative, respectively. According to the BMequation, the fitted B0 and  B0

0  for α phase are 158.1(�5.6) GPa and3.0(�0.3), respectively, and B0¼ 276.5(�6.5) GPa and  B0

0 ¼ 2.9(�0.4)for β phase. The B0 value of α phase is very close to that of YbPO4, ErPO4

and GdPO4, while the B0 value of β phase is close to that of YPO4 [17].The crystal structures of these two phases along the [001] direction

are shown in Fig. 5a and b, respectively. The tetrahedra of PO4 andpolyhedra of CoOx (x¼ 5 and 6, respectively) are presented in green andgray colors, respectively, while the chemical bonds are also presented inthe corresponding colors. In this honeycomb structure, the cobalt cationsaround a double tetrahedral P2O7 are linked by the blue lines, forming ahexagon. The six nearest O anions around one cobalt cation in the CoOxpolyhedron are linked by the red lines for CoO6 octahedra and the yellowlines for CoO5 hexahedra. When comparing the α and β phases ofCo2P2O7, the bond length of P–O and the bond angle of O–P–O arealmost the same in PO4 tetrahedra, while the P–O–P angles in the

Fig. 2. The ADXD pattern of Co2P2O7 under 26.0 GPa and the simulateddiffraction bar graphs of α and β phases.

Fig. 3. (a, b, c) Lattice parameters a, b, c, angle β and relative volume V/V0 of α phase Co2P2O7 versus pressure.

Fig. 4. (a, b, c) Lattice parameters a, b, c, angle β and relative volume V/V0 of β phase Co2P2O7 versus pressure.

Fig. 5. (a, b). The schematic structure models of α and β phases of Co2P2O7

along the [001] direction.

Table 1The lattice parameters of Co2P2O7 under pressure.

Pressure (GPa) Lattice parameters (Ǻ)

α phase (P21/c) β phase (A2/m)

a b c Angle a b c Angle

0.0 7.0025 8.3633 9.0118 113.60 4.5101 8.5337 6.6454 102.402.7 6.9990 8.2857 9.0020 113.485.5 6.9955 8.2098 8.9904 113.369.4 6.9915 8.1364 8.9804 113.25 4.5047 8.3857 6.4619 103.1612.4 6.9881 8.0636 8.9689 113.14 4.5030 8.3388 6.4025 103.4315.2 6.9851 7.9922 8.9594 113.01 4.5015 8.2902 6.3459 103.6718.5 6.9809 7.9248 8.9495 112.89 4.4997 8.2444 6.2901 103.9220.4 6.9790 7.8537 8.9384 112.77 4.4987 8.1984 6.2350 104.1823.6 6.9761 7.7864 8.9278 112.65 4.4967 8.1525 6.1829 104.4126.0 6.9734 7.7213 8.9178 112.52 4.4958 8.1076 6.1304 104.6633.0 6.9671 7.5936 8.8995 112.26 4.4942 8.0628 6.0800 104.9036.2 6.9650 7.5312 8.8882 112.13 4.4933 8.019 6.0309 105.1539.6 6.9626 7.4700 8.8793 112.00 4.4922 7.9766 5.9818 105.4043.2 6.9610 7.4099 8.8687 111.87 4.4910 7.9326 5.9353 105.6247.3 6.9588 7.3510 8.8596 111.74 4.4898 7.8900 5.8883 105.86

W.P. Wang et al. Journal of Physics and Chemistry of Solids 116 (2018) 113–117

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double-tetrahedral P2O7 are 180� and less than 180� for β and α phases,respectively. Another difference is that the CoOx polyhedra around P2O7contain CoO6 octahedra and CoO5 hexahedra in α phase, whereas onlyCoO6 octahedra are found in β phase. In another way of thinking, theCoO5 hexahedra can be considered pseudo CoO6 octahedra in whichthere is one Co–O bond length longer than the other five, as indicated bythe dark dotted lines. According to the analyses, the pressure isconductive to reduce the length of the longest Co–O bond to get moreuniform Co–O bonds and form regular hexagon arrangement of CoO6octahedra in the phase transformation from α to β phase, causing theP–O–P angle to increase to 180� in β phase.

To understand the irreversibility of the phase transformation from αto β phase, we performed the first-principles calculations for Co2P2O7.We obtained final total energies of �5506.0206 eV/f.u. and�5506.0462 eV/f.u. for α and β phases, respectively. We think that thesealmost equal total energies are the possible reason why these two phasescoexist after releasing pressure. The total energies for α and β phases at

Fig. 8. The band gap energy versus pressure for α and β phases.

W.P. Wang et al. Journal of Physics and Chemistry of Solids 116 (2018) 113–117

47.3 GPa were also calculated as �5502.1382 eV/f.u. and�5502.5724 eV/f.u. for α and β phases, respectively. The slightly lowerenergy of β phase (compared to that of α phase) may be a driving force forthe gradual phase transition from α to β phase as well as the coexistenceof both phases. The small difference between the total energies of the twophases causes a very slow kinetics of phase transformation, giving rise tothe coexistence of both phases at high and normal pressures.

The band structure and spin-polarized DOS around the Femi level EFfor both phases of Co2P2O7 are shown in Figs. 6 and 7, respectively. Wecan see that α and β phases have similar band structures as well as spin-polarized DOS. The α phase has a band gap of 2.079 eV and the β phasehas a band gap of 2.025 eV, which is slightly lower than the experimentaldata [27]. This is consistent with the viewpoint that band structure cal-culations usually underestimate the value of the band gap energy [28,29]. The states in the conduction band near the Fermi level of α phase arecomposed of the major Co 3d spin down states and minor O 2p states. Inaddition, the Co 3d (spin up and down), O 2p and P 2p constitute thestates of the valence band near the Fermi level for α phase. The β phasehas similar characteristics. The pressure dependence of band gap energyfor α and β phases were also calculated and the results are shown in Fig. 8.It can be found that the band gap energies decrease with pressure for bothphases. The pressure coefficient is �0.01097 (�0.00036) eV/GPa and�0.01203 (�0.00061) eV/GPa for α and β phases, respectively.

Fig. 6. (a, b). The band structure and spin-polarized DOS of α phaseof Co2P2O7.

Fig. 7. (a, b). The band structure and spin-polarized DOS of β phaseof Co2P2O7.

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4. Conclusions

The structure stability of Co2P2O7 was investigated using in situ highpressure ADXD with synchrotron radiation. The results indicate that thelattice parameters a, b and c become smaller for both α and β phases withincreasing pressure. The difference is that angle β of α phase decreaseswith increasing pressure, while β phase exhibits a contrary tendency. Thepressure induces the CoO6 octahedra and CoO5 hexahedra of α phase toform a more uniform hexagon of CoO6 octahedra. Finally, the α phase ofCo2P2O7 transforms irreversibly to the β phase under pressure. Thealmost similar total energies for the two phases may account for thecoexistence of the two phases after releasing pressure. It is found that theband gap energies decrease with increasing pressure for both phases.

Acknowledgements

This work was supported by the National Key Research Program ofChina (Grant No. 2016YFA0300701, 2017YFA0206200) and the Na-tional Natural Science Foundation of China (Grant Nos. 11174336,11374343, 11574376).

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