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In situ high-pressure x-ray diffractionof the two polymorphs of Sc2Ge2O7

Cite as: AIP Advances 10, 095209 (2020); doi: 10.1063/5.0021334Submitted: 10 July 2020 • Accepted: 21 August 2020 •Published Online: 4 September 2020

Hui Li,1,2 Shuailing Ma,2 Zhuo Yu,3 Hongyu Zhu,2 and Nana Li4,a)

AFFILIATIONS1School of Mathematics and Physics, Key Laboratory for Ionospheric Observation and Simulation, Guangxi Universityfor Nationalities, Nanning 530006, China

2State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China3School of Physics and Electronic Information, Baicheng Normal University, Baicheng 137000, China4Center for Advanced High-Pressure Science and Technology Research, Shanghai 201203, China

a)Author to whom correspondence should be addressed: nana.li@hpstar.ac.cn

ABSTRACTPyrochlore-type (P-type) and thortveitite-type (T-type) Sc2Ge2O7 were prepared through different methods. The phase stability and com-pressibility of the two polymorphs of Sc2Ge2O7 were studied using in situ synchrotron x-ray diffraction. An irreversible phase transition ofmonoclinic (C2/m) T-type Sc2Ge2O7 induced by a pressure of 4.8 GPa was observed. However, the cubic (Fd-3m) P-type Sc2Ge2O7 can resista much higher pressure of up to 30.6 GPa.

© 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0021334., s

I. INTRODUCTION

The pyrochlore structure, A2B2O7, which can be consideredto be based on the simple fluorite structure, AX2,1 has a widerange of compositions in which B is either a p-block metal ionor a transition element. Therefore, A is a trivalent rare earth.2

In recent years, pyrochlores with a 3D corner-sharing networkof tetrahedrons have attracted a significant amount of atten-tion due to their novel features, such as spin-glass,3 spin-liquid,4

and spin-ice behavior.5,6 In addition, other exotic behaviors ofthese pyrochlores have also been observed, such as the unconven-tional anomalous Hall effect,7 superconductivity,8 colossal mag-netoresistance,9 and order-by-disorder.10 Uncovering the chemicaland physical properties of these interesting pyrochlores is essen-tial for practical applications as fuel-cell materials, light sources,hosts for radioactive waste, and superconductors and for mag-netohydrodynamic power generation.8,11–14 Moreover, high-technuclear energy systems require novel materials that can with-stand extreme conditions, e.g., strong radiation fields, high tem-peratures, and high pressures. Thus, understanding the responseof pyrochlore materials under extreme conditions is essential for

condensed matter physics, nuclear engineering, materials science,and geosciences.

Recently, rare-earth-based pyrogermanates (Ln2Ge2O7) havereceived a significant amount of attention because of their poten-tial applications in high-energy laser systems, ion exchangers,humidity sensors, and deep-ultraviolet photodetectors.15 The crys-tal structures of rare-earth pyrogermanates under ambient con-ditions include the monoclinic phase for In2Ge2O7, the tetrago-nal phase for Ln = Gd–Lu, and the triclinic phase for Ln = La,Pr, or Nd–Gd. Different structural modifications of Ln2Ge2O7 canbe obtained depending on the method of synthesis. Monoclinic(T-type) and cubic (P-type) Sc2Ge2O7 can be prepared by conven-tional solid state synthesis and the high-pressure high-temperaturemethod, respectively.16 Experiments have shown that the P-typesilicate compounds Sc2Si2O7 and In2Si2O7 can be obtained at1000 ○C and 120 kbar17 and P-type Mn2V2O7 can be synthesizedat 1100 ○C and 70 kbar.18 In pyrochlore-structured materials, phasetransitions, including the order–disorder structural transformationand the crystalline–amorphous transition, have also been observedunder extreme conditions, especially high pressure.19–29 Pressurehas a significant impact on the phase stability and can lead to

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FIG. 1. Measured and computed XRD patterns, and their difference, under ambientconditions using the Rietveld refinement for (a) T-type Sc2Ge2O7 and (b) P-typeSc2Ge2O7.

pressure-induced phase translations and amorphization,19,23,27,29

which drastically changes the material properties and results in sur-prising physical and chemical phenomena. For the Gd2Ti2O7 andGd2Zr2O7 pyrochlores, the pressure-induced phase translations canbe controlled through the energetic formation of defects.28

Nevertheless, since rare-earth pyrogermanates Ln2Ge2O7 aregeometrically frustrated magnetic materials, most research hasfocused on their magnetic properties and synthesis methods. Therehas been little research on their high-pressure structural stability. Inthis work, we prepared two polymorphs of Sc2Ge2O7 using differentmethods and investigated their structural stability at high pressureusing angle-dispersive synchrotron powder x-ray diffraction (XRD).

II. EXPERIMENTAL METHODSA. Sample preparation

Using the standard solid state reaction scheme, T-typeSc2Ge2O7 was synthesized from raw materials Sc2O3 (99.99% inpurity) and GeO2 (99.99% in purity). An agate mortar was used to

mix Sc2O3 and GeO2 in a molar ratio of 1:2 for more than 4 h. Themixture was then cold-pressed into cylindrical samples, 6 mm indiameter and 3 mm in height, followed by 24 h of calcining at 1473 Kunder atmospheric conditions. The high-pressure high-temperaturemethod for synthesizing P-type Sc2Ge2O7 samples was conductedwith a cubic-anvil high-pressure high-temperature setup (SPD-6 6600) under 5.3 GPa and at 1573 K for 15 min.

B. AnalysesPowder XRD measurements of the phase purity were per-

formed under ambient conditions using an x-ray diffractometer(Rotaflex, Rigaku) with Cu-Kα radiation in the range of 10○–90○. Asynchrotron angle-dispersive x-ray source (λ = 0.6199 Å) producedby the Beijing Synchrotron Radiation Facility was used in the high-pressure XRD at the 4W2 High-Pressure Station. The samples wereloaded into a gasketed diamond anvil cell. The pressure-transmittingmedium was a methanol/ethanol/water mixture (16:3:1). The rubyluminescence method was adopted to calibrate the pressure.30 Thehighest pressure was 31.9 GPa for T-type Sc2Ge2O7 and 30.6 GPafor P-type Sc2Ge2O7 at room temperature. A CeO2 standard wasused to calibrate the sample-to-detector distance and other exper-imental parameters. To yield 1D patterns of intensity vs diffractionangle 2θ from the 2D XRD images, analyses were performed withthe software FIT2D. The software GSAS was used for the Rietveldanalyses.31

III. RESULTS AND DISCUSSIONA. X-ray diffraction patterns at ambient pressure

The ambient-pressure diffraction pattern of T-type Sc2Ge2O7was refined using the Rietveld method in the C2/m phase withcell factors of a = 6.5516(3) Å, b = 8.6898(5) Å, c = 4.8999(3) Å,β = 102.527(3)○, V = 272.33(1) Å3, and Z = 2 with GSAS.Figure 1(a) shows the refined XRD pattern, the calculated profile,and the difference. We obtained Rp = 5.80% and Rwp = 7.31% atambient pressure according to the refinement merit. In these dis-torted ScO6 octahedra, Sc3+ cations are concentrated in the centerof the crystal structure. The hexagonal arrangement of ScO6 octahe-dra on the ab planes can occur through edge-sharing mechanisms.Figure 2(a) is a schematic of the typical T-type Sc2Ge2O7 crystalstructure. The isolated Ge2O7 diorthogroups were formed by two

FIG. 2. Schematics of the phase crystalstructure of (a) T-type Sc2Ge2O7 and (b)P-type Sc2Ge2O7.

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FIG. 3. Pressure-dependent evolution of the XRD patterns of T-type Sc2Ge2O7.A high-pressure structure appeared in the quenched sample after 4.8 GPa. Newpeaks are marked with asterisks.

FIG. 4. Measured and computed XRD patterns, and their difference, for T-typeSc2Ge2O7 at 20.4 GPa. The calculated reflection positions are marked with tickmarks.

tetrahedra via a common vertex, which formed a sheet and held theoctahedral layers together.

The crystal structure data of P-type Sc2Ge2O7 were refinedbased on the powder XRD data using the Rietveld analysis.Figure 1(b) shows the measured XRD, the computed XRD, and theirdifference. P-type Sc2Ge2O7 belongs to the Fd-3m (227) space groupwith a = 9.5989(3) Å and Z = 8. We obtained Sc2Ge2O6O′ in the

TABLE I. Refined lattice parameters and atomic coordinates of the high-pressure phase of T-type Sc2Ge2O7 at 22.5 GPaand that of the atmospheric T-type and P-type Sc2Ge2O7.

High-pressureParameter T-type Sc2Ge2O7 P-type Sc2Ge2O7 phase (22.5 GPa)

Crystal system Monoclinic Cubic MonoclinicSpace group C2/m (12) Fd-3m (227) P21/c (14)a (Å) 6.5516(3) 9.5989(1) 6.7200 (6)b (Å) 8.6898(5) . . . 4.8101 (4)c (Å) 4.8999(3) . . . 3.3011 (6)β (deg) 102.527(3) . . . 106.396(3)

Atoms Wyckoff (x, y, z) Wyckoff (x, y, z) Wyckoff (x, y, z)

Sc 4g (0, 0.3073(3), 0) 16d (0.5, 0.5, 0.5) 4e (0.2187(2), 0.0315(5),0.2020(6))

Ge 4i (0.2342(8), 0, 0.4044(6)) 16c (0, 0, 0) 4e (0.2187(2), 0.0315(5),0.2020(6))

O(1) 2c (0, 0, 0.5) 8b (0.375, 0.375, 0.375) 4e (0.0705(2), 0.3327(1),0.3447(4))

O(2) 4i (0.4226(3), 0, 0.7481(7)) 48f (0.3504(8), 0.125, 0.125) 4e (0.4499(3), 0.7588(9),0.4793(5))

O(3) 8j (0.2377(5), 0.1794(7),. . . . . .0.2042(2))

Residualsa (%) Rwp = 7.31 Rwp = 9.57 Rwp = 2.34Rp = 5.80 Rp = 6.76 Rp = 1.86

aRwp and Rp as defined in GSAS.

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cubic phase, with the Ge, Sc, O, and O′ ions at 16c, 16d, 48f, and8b sites, respectively. The 16c (Ge) site coordination polyhedron isa distorted octahedron, while the 16d (Sc) site is a distorted cube,which can, in general, contain large cations. For the O atom at the48f site, the only adjustable positional parameter is x. Figure 2(b) isa schematic of P-type Sc2Ge2O7.

B. Structural phase transformation of T-typeSc2Ge2O7 under high pressure

To reveal the high-pressure behavior of T-type Sc2Ge2O7, ahigh-pressure synchrotron XRD experiment was conducted. Thepowder XRD patterns were recorded for pressures up to 31.9 GPa,and several representative XRD patterns for T-type Sc2Ge2O7 arepresented in Fig. 3. Obviously, there are some new peaks in the orig-inal phase. The intensity and multiplicity of the Bragg peaks changedsignificantly with increasing pressure, indicating that a structuralphase transition occurred at 4.8 GPa. According to the diffractionpeaks, the high-pressure structure persisted even once the pressurewas completely released. Thus, the phase transition is irreversible.Rietveld refinements of T-type Sc2Ge2O7 were performed for allpressures with GSAS. The refined results indicate that the crystalstructure of T-type Sc2Ge2O7 transformed from the C2/m to theP21/c phase (Z = 1 and S.G. 14). Figure 4 shows the refined pat-tern at 20.4 GPa with Rp = 0.83% and Rwp = 1.69% for the mon-oclinic (P21/c) structure. The same high-pressure phase has beenreported for Gd2Zr2O7.32 Table I lists the refined lattice param-eters and atomic coordinates at 22.5 GPa. For comparison, datafor the atmospheric structure of two types of Sc2Ge2O7 are alsopresented.

Rietveld refinements were conducted on all XRD data. Figure 5shows the relation between the pressure and the lattice parametersfor T-type Sc2Ge2O7. The lattice constant decreased by 46.3% and27.8% for the b- and c-axes, respectively, during the structural phasetransition. In contrast, there was a small increase for the a-axis. Amuch larger change was observed along the b-axis compared with

FIG. 5. Lattice parameters vs pressure for T-type Sc2Ge2O7.

FIG. 6. Schematics of the C2/m to P21/c phase transition of T-type Sc2Ge2O7 inthe ab and ac planes.

the decrease along the a- and c-axes. Consequently, the phase tran-sition is accomplished by the reconstruction of the anion and cationsublattices, and the schematic structural transformation (in ab andac planes) is shown in Fig. 6. The transformation process of the

FIG. 7. Observed volume vs pressure variation and the B–M fit of T-typeSc2Ge2O7, and its high-pressure phase.

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FIG. 8. XRD results of P-type Sc2Ge2O7 for various pressures. It was stable up to30.6 GPa. The rhombuses represent the peak of the gasket.

T-type Sc2Ge2O7 from the C2/m to P21/c phase is illustrated fromleft to right in this figure. In the process of phase transition, the coor-dination number is also changed from 6 to 7 for Sc3+ cations andfrom 4 to 7 for Ge4+ cations. Ge4+ cations are more likely to move tothe empty space in the arrow direction in the ab plane, as shown inthe upper part of Fig. 6, and then drive In1 and In4 ions much closerto each other upon compression, which indicates that the compress-ibility along the In1–In4 direction is larger than the other directions.In addition, in the lower part of Fig. 6, we can see that the layered

structure becomes much denser after the phase transition in the acplane. It can also explain the observed much larger change in theb-axis.

Figure 7 illustrates the relation between pressure and unit vol-ume for T-type Sc2Ge2O7 up to 31.9 GPa. The cell volume reducedby ∼13.7% in the phase transformation. The state function can befitted using the third-order Birch–Murnaghan (B–M) equation,

P =3.02Bo[(

V0

V)

73− (

V0

V)

53]

×(1 +3.04(B′o − 4) × [(

V0

V)

23− 1]), (1)

where B0 and B′0 are the equilibrium bulk modulus and the corre-sponding derivative for volume V0. By fitting, B0 and B′0 were calcu-lated to be 244(7) GPa and 4 (fixed), respectively, with V0 = 135.5(1)Å3 for the C2/m phase. B0 was also calculated to be 280(7) GPaand B′0 = 4 (fixed) with V0 = 119.2(5) Å3 for the P21/c phase. Thehigh-pressure phase exhibited a much larger bulk modulus com-pared with the T-type phase, and the considerable discrepancy canbe contributed to the increase in the near neighbor coordinations.

C. Structural stability of P-type Sc2Ge2O7under high pressure

Figure 8 shows a few representative XRD patterns of P-typeSc2Ge2O7 at pressures of up to 30.6 GPa. There are no obvious dif-ferences between the diffraction patterns, suggesting that the crystalstructure of P-type Sc2Ge2O7 is more stable than that of T-typeSc2Ge2O7 up to the highest pressure. Except for the O48f atom,all atoms are in special sites in this ordered pyrochlore structure.Hence, by changing the x positional parameter of the O48f atom,

FIG. 9. Observed volume vs pressure fit-ted with the third-order B–M equation ofstate for P-type Sc2Ge2O7. Parameter xvs pressure is plotted in the inset.

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we can determine the structural ordering degree. Sharp changes inx have been reported in previous research on the pressure-inducedphase transitions of pyrochlore oxides. For example, a rapid declinewith increasing pressure for Gd2Zr2O7 implied a phase transition.32

Moreover, a dramatic x-coordinate increase was observed after thetransition for Sm2Zr2O7.33 From our refined results, the x parame-ter for the O48f atom as a function of pressure does not exhibit anyanomaly up to the highest pressure indicating the structural stabilityin the P-type Sc2Ge2O7, as shown in the inset of Fig. 9, which indi-cates that P-type Sc2Ge2O7 has good structural stability. Obviously,the two polymorphs of Sc2Ge2O7 exhibit different responses to highpressure. This is because although the chemical proportions of thetwo samples are identical, their atomic arrangements are completelydifferent.

The study of pressure on pyrochlores A2B2O7 has become morecommon in recent years, especially zirconates and titanates. Experi-mental and theoretical studies have shown that zirconates are unsta-ble around 20 GPa–30 GPa20,32,33 or at low pressure.34 For example,a complete transformation to the distorted defect-fluorite structurehas been observed above 18 GPa for Sm2Zr2O7

32 and above 15 GPafor Gd2Zr2O7.33 Gd2Ti2O7 undergoes a subtle distortion of the lat-tice at 9 GPa at room temperature.35 Germanate, which also has apyrochlore structure, has a stable structure under high pressure.36,37

Previous studies have shown that we can define the stability lim-its for pyrochlore phases in terms of minimal and maximal ionicradius ratios, A3+/B4+.2 For B ions with very small radii, such asMn4+and Ge4+, the pyrochlore phase can be prepared only usinghigh-pressure methods. Zirconate and titanate compounds weresynthesized almost by the standard solid state reaction method. Itmay be that the differences in composition methods lead to the dif-ferences in high-pressure behavior. In addition, to observe the phasetransition of P-type Sc2Ge2O7, perhaps increasing the experimentalpressure would be an effective means.

Figure 9 presents the relation between pressure and the unit cellvolume of P-type Sc2Ge2O7. The data were fitted as done for T-typeSc2Ge2O7. The obtained bulk modulus is B0 = 328(7) GPa, which isquite high compared with the other ordered pyrochlore oxides. Thebulk modulus for Gd2Zr2O7 was 186(12) GPa,32 for example, andthe values for A2Ti2O7 (A = Sm, Tb, Y, and Ho) were 164.8(1.5)GPa, 199(1) GPa, 204(3) GPa, and 213(2) GPa, respectively.38,39

The cubic phase of Sc2Ge2O7 was harder compared with the mon-oclinic phase. The structural differences and cohesive energy dis-crepancies in the germanates may contribute to the different bulkmoduli.

IV. CONCLUSIONSThe structural stability of the T-type and P-type phases of

Sc2Ge2O7 was studied under high pressures. The monoclinic T-type Sc2Ge2O7 transformed from a C2/m to a P21/c structure above4.8 GPa, and during this process, the atomic volume reduced signif-icantly and the cation coordination number increased. This phasetransition was irreversible and sluggish. The bulk moduli B0 = 244(7)GPa and 280(7) GPa were measured for the high-pressure mon-oclinic T-type Sc2Ge2O7. However, the bulk modulus for P-typeSc2Ge2O7 was B0 = 328(7) GPa, and it was mainly stable up to30.6 GPa.

ACKNOWLEDGMENTSThis research was supported by the Guangxi Natural Science

Foundation (Grant Nos. 2018GXNSFBA050034 and 2019GXNSFBA185001), the Middle-Aged and Young Teachers’ Basic Ability Pro-motion Project of Guangxi (Grant No. 2020KY04022), the ChinesePostdoctoral Science Foundation (Grant No. 2019M653493), andthe Open Project of State Key Laboratory of Superhard Materials(Jilin University) (Grant No. 201909).

DATA AVAILABILITY

The data that support the findings of this study are availablefrom the corresponding author upon reasonable request.

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