⇦Chinese Physics Letters, 2019, Vol. 36, No. 4, Article code 046103 High-Pressure Phase Transitions of Cubic Y$_{2}$O$_{3}$ under High Pressures by In-situ Synchrotron X-Ray Diffraction * Sheng Jiang (蒋升)1,2**, Jing Liu (刘景)3, Xiao-Dong Li (李晓东)3, Yan-Chun Li (李延春)3, Shang-Ming He (何上明)1,2, Ji-Chao Zhang (张继超)1,2 Affiliations 1Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201203 2Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210 3Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Received 6 December 2018, online 23 March 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 11775292, 11104307 and U1530134, the Natural Science Foundation of Shanghai under Grant No 18ZR1448100, and the Shanghai Sailing Program under Grant No 17YF1423600.
**Corresponding author. Email: jiangsheng@sinap.ac.cn Citation Text: Jiang S, Liu J, Li X D, Li Y C and He S M et al 2019 Chin. Phys. Lett. 36 046103    Abstract High-pressure phase transitions of cubic Y$_{2}$O$_{3}$ are investigated using in situ synchrotron x-ray diffraction in a diamond anvil cell up to 36.3 GPa. The pressure-induced phase transitions of cubic Y$_{2}$O$_{3}$, which display apparent inconsistencies in previous studies, are verified to be from a cubic phase to a monoclinic phase and further to a hexagonal phase at 11.7 and 21.6 GPa, respectively. The hexagonal Y$_{2}$O$_{3}$ displays noticeable anisotropic compressibility due to its layered structure and it is stable up to the highest pressure in the present study. A third-order Birch–Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli of 180(3), 196(7) and 177(7) GPa for cubic, monoclinic and hexagonal phases, respectively. DOI:10.1088/0256-307X/36/4/046103 PACS:61.50.-f, 61.50.Ks, 62.50.-p, 61.05.cp © 2019 Chinese Physics Society Article Text The polymorphism and structural properties of rare earth sesquioxides (Ln$_{2}$O$_{3}$, Ln=rare earth) under high pressures are of considerable interest because it provides an efficient way to act on $d$ and $f$-electron interactions.[1-4] Its applications often involve extreme pressure and temperature conditions, which could induce crystallographic and electronic changes.[5-8] According to the radius of cations, there are three known polymorphs of Ln$_{2}$O$_{3}$ under ambient conditions, which are designated as $A$, $B$, $C$, and which belong to space group (SG) $P\overline 3m1$, $C2/m$ and $Ia\overline3$, respectively. The Ln$_{2}$O$_{3}$ with larger cations (La-Nd) usually crystallize in $A$-type, and that with smaller cations (Tb-Lu, Y and Sc) adopt $C$-type. The medium Sm, Eu or Gd cations containing Ln$_{2}$O$_{3}$ are found to be stable either in $B$-type or $C$-type structure depending on the thermal history. As the temperature increases, the phase evolutions of Ln$_{2}$O$_{3}$ follow $C\to B\to A$ transition sequences.[9,10] The molecular volume of Ln$_{2}$O$_{3}$ decreases in the sequence of $C\to B\to A$, therefore pressure is also expected to give similar changes. Previous high pressure studies on these Ln$_{2}$O$_{3}$ have confirmed this speculation. The pressure-induced $C\to B$,[11-17] or the successive $C\to B\to A$ phase transitions have been investigated in $C$-type Ln$_{2}$O$_{3}$ containing small size cations.[18-21] The $C$-type Ln$_{2}$O$_{3}$ with medium size cations displays a direct pressure-induced $C\to A$ transition, such as observed in Gd$_{2}$O$_{3}$,[22,23] Eu$_{2}$O$_{3}$,[24] and Sm$_{2}$O$_{3}$,[25,26] and this phase transition sequence was also investigated in nano-Eu$_{2}$O$_{3}$.[27] Recently, a theoretical calculation on the pressure-induced $C\to A$ transformation of Ln$_{2}$O$_{3}$,[28] and also a review of structural changes in nanocrystalline Ln$_{2}$O$_{3}$ was presented.[29] The Y$_{2}$O$_{3}$ is widely used in nuclear applications, biomedicine, host materials and high dielectric constants.[30-33] The physical properties and applications of Y$_{2}$O$_{3}$ strongly depend on the grain size and crystalline phase. Many attempts have been made to understand the high pressure behaviours of Y$_{2}$O$_{3}$.[34,35] Previous reports on high pressure phase transition sequences of Y$_{2}$O$_{3}$ display noticeable inconsistencies. With cubic Y$_{2}$O$_{3}$ as the starting material, phase transition from $C\to B$ was reported at 2.5 GPa and 1273 K.[36] This transition sequence was confirmed in shock studies from 12 to 20 GPa,[37] whereas the predicated $B\to A$ phase transition was not detected. Later, the Raman spectroscopy of $C$-type Y$_{2}$O$_{3}$ revealed a successive $C\to B\to A$ structural transformation.[20] However, Wang et al. observed that the phase transitions in pour cubic Y$_{2}$O$_{3}$ followed a $C\to A$ sequence,[38] and Y$_{2}$O$_{3}$:Eu$^{3+}$ undergoes a $C\to B\to A$ transition.[38,39] Yusa et al. found that the pressure-induced $C\to A$ phase transition occurred in cubic Y$_{2}$O$_{3}$, and the $B$ phase was found to coexist throughout the transition process $C\to A$.[40] With $B$-type Y$_{2}$O$_{3}$ as the initial phase, a phase transition of $B\to A$ was observed at 23.5 GPa,[41] which is larger than the theoretical result of 18.0 GPa based on the density functional theory (DFT).[42] In view of this description, it can be seen that the phase transformations in Y$_{2}$O$_{3}$ from previous reports have been inconsistent and pressure-induced $C\to B$,[36,37] $C\to A$,[38] and $C\to B\to A$[20] transition sequences have been reported. To clarify the pressure-dependent structural evolution of Y$_{2}$O$_{3}$, and to further give an insight into the effect of applied pressure on the structure of Ln$_{2}$O$_{3}$, in situ angular dispersive synchrotron x-ray diffraction (ADXD) experiments were carried out up to 36.3 GPa. It was confirmed that the $C$-type Y$_{2}$O$_{3}$ underwent a successive $C\to B\to A$ phase transition, the structure evolution of Y$_{2}$O$_{3}$ under high pressure was discussed, and the bulk moduli for the $C$, $B$ and $A$ phases were determined. The Y$_{2}$O$_{3}$ sample is a polycrystalline in powder form with purity of 99.99%. The starting material was heated at 900$^{\circ}\!$C for 6 h to eliminate the possible hydroxide and adsorptive water. High pressure XRD measurements were carried out in a symmetric diamond anvil cell (DAC) with a pair of diamond anvils (culet sizes of 400 m). The T301 stainless steel gasket was pre-indented to 40 µm and served as gasket. The heat-treated sample with a small amount of platinum (Pt) was loaded into the sample hole with a diameter of 120 m, and silicone oil was selected as the pressure-transmitting medium. The pressures were determined from the measured unit cell parameters of Pt.[43] The synchrotron ADXD measurements were carried out at 16-IDB station of the advanced photon source (APS), Argonne National Laboratory (ANL), using a beam wavelength 0.3928 Å. CeO$_{2}$ powder was used to calibrate the distance and orientation of the detector. The collected images were integrated into one-dimensional diffractions patterns by Fit2D software package.[44]
cpl-36-4-046103-fig1.png
Fig. 1. The SEM image of initial $C$-type Y$_{2}$O$_{3}$.
The initial crystalline size of the sample in the present study was analyzed using a scanning electron microscope (SEM). Figure 1 displays a typical SEM image of initial cubic Y$_{2}$O$_{3}$. The micrograph shows an uniform polycrystalline aspect of randomly oriented crystals with a grain size in 100–300 nm. Typical XRD patterns upon compression are demonstrated in Fig. 2(a). The diffraction patterns at ambient pressure can be indexed into the cubic structure. The lattice parameter was determined to be $a=10.609(1)$ Å, which is in excellent agreement with the reported results.[38,40] The diffraction peaks were marked with the corresponding ($hkl$) values. As the pressure increases, the diffraction peaks shift toward higher 2$\theta $ angles accompanied by a change of the relative intensity. Splitting and broadening of the diffraction peaks are observed at 11.7 GPa along with the appearance of some new diffraction peaks belonging to a monoclinic phase. These results indicate that a $C\to B$ phase transition occurred at around 11.7 GPa. The cubic phase coexists with the monoclinic phase up to 21.6 GPa. Above 21.6 GPa, some other new diffraction peaks emerge, which would be indexed into an $A$-type using the Le Bail refinement.[45] It is indicated that the $B$-type further transforms to the $A$-type above 21.6 GPa. This value is in the same level as that of Zhang's work with $B$-type Y$_{2}$O$_{3}$ as the starting material.[41] When compressed to 34.8 GPa, the $B$-type fully converted to the $A$-type and remained stable to the highest pressure. The ADXD measurements demonstrate that the cubic Y$_{2}$O$_{3}$ experienced $C\to B\to A$ structural evolutions under high pressures, and this structural transformation sequence is in agreement with the previous Raman experiments[20] and energy dispersive x-ray diffraction (EDXD) studies.[46] Meanwhile, it is contrary to Wang's ADXD measurements,[38] which reported a $C\to A$ phase transition in pour Y$_{2}$O$_{3}$, and a $C\to B\to A$ structural evolutions in Y$_{2}$O$_{3}$ doped with 1% Eu$^{3+}$.
cpl-36-4-046103-fig2.png
Fig. 2. Representative angle dispersive x-ray diffraction profiles from Y$_{2}$O$_{3}$ samples under selected pressures: (a) compression and (b) decompression. The diffraction peaks marked with $\blacklozenge$, $\star $ and • are contributed by phases of $B$, $A$ and Pt, respectively.
It is worthwhile to mention that the phase $B$ transforms to $A$ in a wide pressure range from 21.6 to 34.8 GPa. This phenomenon is attributed to the fact that the phases $B$ and $A$ have similar lattice energy.[22] The XRD patterns of the material upon decompression (Fig. 2(b)) reveal that the $B\to A$ phase transition is reversible, and the $C\to B$ structural transformation is irreversible. The $B$-type structure appears at 20.2 GPa during decompression process, which exhibits a hysteresis of 1.4 GPa compared to that of $B\to A$ phase transition on compression (21.6 GPa). The $A$-type transforms to $B$-type completely at around 9.1 GPa. The irreversible $C\to B$ phase transition could be explained by the high kinetic barrier due to its structural reconstructive nature.
cpl-36-4-046103-fig3.png
Fig. 3. Experimental data together with Le Bail refinements of the XRD pattern at the listed pressures: (a) at 0.1 MPa, (b) at 18.1 GPa and (c) at 36.3 GPa. Solid line, symbols and solid line at the bottom represent observed and calculated patterns and their differences, respectively. The rows of vertical bars indicate the diffraction positions. The diffraction peaks marked with • is contributed by Pt.
cpl-36-4-046103-fig4.png
Fig. 4. Le Bail refinement profile for the monoclinic phase of Y$_{2}$O$_{3}$ after pressure release. The diffraction peaks marked with • is contributed by Pt.
To determine the unit cell parameters of Y$_{2}$O$_{3}$, all of the measured XRD patterns were refined using the GSAS+EXPGUI program based on the Le Bail method.[45] The representative refined diffraction patterns for the three different pressures are depicted in Figs. 3(a)–3(c), respectively. As seen in Fig. 3(a), the Bragg peaks in the pattern at ambient pressure could be well indexed by a cubic structure. The diffraction pattern after pressure release is also refined, which indicates that the final sample is a pure monoclinic phase, as shown in Fig. 4, the lattice parameters for $B$-type Y$_{2}$O$_{3}$ obtained from the refinement are $a=13.803(2)$ Å, $b=3.513(1)$ Å, and $c=8.647(2)$ Å, consistent with the previous XRD experiment.[41] Figure 5 shows the pressure dependence of normalized lattice parameters. It is found that the $A$-type Y$_{2}$O$_{3}$ upon compression is highly anisotropic, and the $a$-axis is reduced less than 1%, while the $c$-axis is reduced with 3.30% from 21.6 to 36.3 GPa. It is concluded that the axial compression sequence is correlated to the axial length sequence. This anisotropic compressibility is attributed to its layered structures, which have weak van der Waals bonds between the atoms in neighboring layers.[22] In addition, the $c$-axis varies almost linearly with pressure, whereas the $a$-axis exhibits a strong nonlinear dependence on pressure.
cpl-36-4-046103-fig5.png
Fig. 5. Pressure dependence of normalized lattice parameters of Y$_{2}$O$_{3}$.
The unit cell volume data as a function of pressure is plotted in Fig. 6. The volume decreases by 7.98% when the $C\to B$ transformation occurs, which is at the same level as that of Er$_{2}$O$_{3}$[11] and Yb$_{2}$O$_{3}$.[13] The relative volume change for $B\to A$ phase transition is 1.88%. The $C\to B$ phase transformation is reconstructive and the volume change is large, while the transition of $B$ to $A$ is displacive and involves only a slight deformation.[10] The pressure-volume data for the parent and high pressure phases were fitted into the third-order Birch–Murnaghan equation of state (EOS),[47] $$\begin{alignat}{1} P(V)=\,&\frac{3B_{0}}{2}\Big[\Big(\frac{V_{0} }{V}\Big)^{7/3}-\Big(\frac{V_{0} }{V}\Big)^{5/3}\Big]\\ &\cdot\Big\{1+\frac{3}{4}(B'_{0} -4)\Big[\Big(\frac{V_{0} }{V}\Big)^{2/3}-1\Big]\Big\},~~ \tag {1} \end{alignat} $$ where $B_{0}$ and $B'_{0}$ correspond to bulk modulus and its pressure derivative. The least square fitting yields the bulk modulus 180(3), 196(7) and 177(7) GPa for cubic, monoclinic and hexagonal phases, respectively, when $B'_{0}$ is fixed at 4. The similar values of bulk moduli reveal that the high pressure phases have similar compressibility with that of the cubic phase, as reported in Gd$_{2}$O$_{3}$[22] and Sm$_{2}$O$_{3}$.[25] The current experimental EOS parameters of Y$_{2}$O$_{3}$ are listed in Table 1. For a comparative purpose, the previous experimental and theoretical results available are also included in Table 1. It is worthwhile to mention that the equilibrium unit-cell volume $V_{0}$ obtained from the present XRD experiment is in agreement with the previous ADXD measurement,[41] whereas they are both larger than the values determined by DFT with the local density approximation (LDA),[40] which commonly underestimates the value of $V_{0}$. Furthermore, the isothermal bulk modulus of $B$-type Y$_{2}$O$_{3}$ determined experimentally in this study 196(7) GPa is consistent with Halevy's EDXD result, where $B_{0}$ is 192(10) GPa,[46] and is slightly larger than other previous theoretical calculations and experimental values.[40-42] However, it should be noted that the bulk moduli of $C$-type and $A$-type Y$_{2}$O$_{3}$ have not been determined in Halevy's work,[46] which are expected to have similar values with that of $B$-type Y$_{2}$O$_{3}$, and thus would be consistent with the present study. In addition, the bulk modulus of Ln$_{2}$O$_{3}$ generally increases with the decrease of the Ln$^{3+}$-cation radius.[42] Lonappan et al. reported that the bulk modulus of the $C$-type Ho$_{2}$O$_{3}$ is 178 GPa,[12] Y$_{2}$O$_{3}$ display similar cation size with that of Ho$_{2}$O$_{3}$, and is expected to have similar bulk modulus.
cpl-36-4-046103-fig6.png
Fig. 6. Experimental pressure-volume data for Y$_{2}$O$_{3}$. The solid lines correspond to the third-order Birch–Murnaghan equation of state fitting to the $P$–$V$ data. The estimated error bars lie within the size of the symbols.
Table 1. Equation-of-state parameters ($B_{0}$ and $B'_{0}$) and unit-cell volumes ($V_{0}$) for the three phases of Y$_{2}$O$_{3}$.
Phase $V_{0}$ (Å$^{3}$) $B'_{0}$ $B_{0}$ (GPa) Method
74.49(7) 4 (fixed) 147(2) ADXD Ref.  [40]
$C$ 71.58 3.98 163.5 DFT-LDA Ref.  [40]
74.633(3) 4 (fixed) 180(3) ADXD This study
$B$ 68.55(12) 4 (fixed) 155(4) ADXD Ref.  [40]
66.16 4.83 117.9 DFT-LDA Ref.  [40]
70.78 3.5 (fixed) 138 DFT-PAW Ref.  [42]
68.99(10) 4 (fixed) 159(3) ADXD Ref.  [41]
71.42 4 (fixed) 192(10) EDXD Ref.  [46]
68.5(2) 4 (fixed) 196(7) ADXD This study
66.78(53) 4(fixed) 159(15) ADXD Ref.  [40]
64.63 4.37 144.6 DFT-LDA Ref.  [40]
$A$ 69.58 3.5 (fixed) 136 DFT-PAW Ref.  [42]
67.79 4 (fixed) 156(3) ADXD Ref.  [41]
67.9(2) 4 (fixed) 177(7) ADXD This study
In conclusion, our high-pressure in situ ADXD experiment infers that the cubic Y$_{2}$O$_{3}$ experiences $C\to B\to A$ structural evolutions at 11.7 and 21.6 GPa, respectively. After pressure release, Y$_{2}$O$_{3}$ exhibits a pour monoclinic phase at ambient pressure. The compressibility of $A$-type Y$_{2}$O$_{3}$ is highly anisotropic, with the $c$-axis much more compressible than the $a$-axis. The zero pressure bulk moduli of 180(3), 196(7), and 177(7) GPa for cubic, monoclinic and hexagonal phases have been determined. Portions of this work were carried out at the BL15U1 beamline, Shanghai Synchrotron Radiation Facility (SSRF) and 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF). References Density functional study of the phase stability and Raman spectra of Yb 2 O 3 , Yb 2 SiO 5 and Yb 2 Si 2 O 7 under pressureUnified understanding of the valence transition in the rare-earth monochalcogenides under pressureStress induced phase transition in Gd2O3 films by ion beam assisted reactive electron beam-physical vapor deposition (EB-PVD)POLYMORPHISM OF THE RARE EARTH SESQUIOXIDES 1High-Pressure Phase Transition to the Gd 2 S 3 Structure in Sc 2 O 3 : A New Trend in Dense Structures in SesquioxidesExperimental Determinations of the High-Pressure Crystal Structures of Ca 3 N 2High-Pressure Studies on CeO 2 Nano-Octahedrons with a (111)-Terminated SurfaceAnomalous compression behaviour in Nd 2 O 3 studied by x-ray diffraction and Raman spectroscopyThermodynamics of rare earth sesquioxidesPressure-Induced Cubic to Monoclinic Phase Transformation in Erbium Sesquioxide Er 2 O 3High-pressure phase transition in Ho2O3Mössbauer and energy-dispersive x-ray-diffraction studies of the pressure-induced crystallographic phase transition in C -type Yb 2 O 3 Pressure-induced phase transition in cubic Lu2O3Phase transitions in under high pressureHigh-Pressure Structural Transitions of Sc 2 O 3 by X-ray Diffraction, Raman Spectra, and Ab Initio CalculationsStructural and vibrational properties of single crystals of Scandia, Sc 2 O 3 under high pressureStructural transformations in cubic Dy2O3 at high pressuresPhase transformation of Ho 2 O 3 at high pressurePhase transitions in yttrium oxide at high pressure studied by Raman spectroscopyPhotoluminescence and phase transition in Er2O3 under high pressureStructural phase transitions of cubic Gd 2 O 3 at high pressuresPressure-induced phase transformations in cubic Gd2O3The Phase Transition of Eu 2 O 3 under High PressuresNanocrystalline titanium dioxide films: Influence of ambient conditions on surface- and volume-related photoluminescencePhase transformation in Sm2O3 at high pressure: In situ synchrotron X-ray diffraction study and ab initio DFT calculationX-ray diffraction and spectroscopy study of nano-Eu2O3 structural transformation under high pressureStructural properties and the pressure-induced C → A phase transition of lanthanide sesquioxides from DFT and DFT + U calculationsPressure-Induced Structural Transition Trends in Nanocrystalline Rare-Earth Sesquioxides: A Raman InvestigationSPIE ProceedingsRadiation sensitive scintillator/optical fibre system for radiation dosimetry in burning plasma machineLow-voltage ZnO thin-film transistors based on Y2O3 and Al2O3 high-k dielectrics deposited by spray pyrolysis in airHexagonally Ordered KLaF 4 Host: Phase-Controlled Synthesis and Luminescence StudiesPhase transitions and thermodynamic properties of yttria, Y 2 O 3 : Inelastic neutron scattering shell model and first-principles calculationsSize-Dependent Amorphization of Nanoscale Y 2 O 3 at High PressureHigh-Pressure B-Type Polymorphs of Some Rare-Earth SesquioxidesShock-induced phase transition of M2O3 (M = Sc, Y, Sm, Gd, and In)-type compoundsHigh-pressure induced phase transitions of Y2O3 and Y2O3:Eu3+Photoluminescence studies of Y 2 O 3 :Eu 3+ under high pressureDense Yttria Phase Eclipsing the A-Type Sesquioxide Structure: High-Pressure Experiments and ab initio CalculationsPressure-induced phase transition of B-type Y 2 O 3Ab initio study on structure and phase transition of A- and B-type rare-earth sesquioxides Ln2O3 (Ln=La–Lu, Y, and Sc) based on density function theoryReduction of shock-wave data with mean-field potential approachTwo-dimensional detector software: From real detector to idealised image or two-theta scanEXPGUI , a graphical user interface for GSASPressure-induced structural phase transitions in Y 2 O 3 sesquioxideExperimental compressions for normal hydrogen and normal deuterium to 25 kbar at 4.2 K
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