Previous High-pressure Studies Research Articles

Pressure-Induced Structural Phase Transition of Co-Doped SnO2 Nanocrystals

Co-doped SnO2 nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P42/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO2 undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl2-type phase (space group Pnnm), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO2. The compressibility of different phases will be discussed.

Shear stress induced phase transitions of cubic Eu2O3 under non-hydrostatic pressures

Pressure-induced phase transitions in cubic Eu2O3 subjected to non-hydrostatic conditions have been studied by in situ high-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements up to 30.1 and 43.8 GPa, respectively. Both x-ray diffraction and Raman spectroscopy results indicate that the pressure-induced transition routines of cubic Eu2O3 depend on the nature of stress loading. In contrast to our previous high-pressure studies of cubic Eu2O3 under hydrostatic pressure, where cubic Eu2O3 transforms directly into a hexagonal structure, the x-ray diffraction data show that cubic Eu2O3 begins to transform into the monoclinic phase at a non-hydrostatic pressure of about 4.3 GPa, while the monoclinic to hexagonal phase transition is initiated at about 6.4 GPa. These phase transitions have also been confirmed by Raman spectroscopy; the hexagonal phase is stable up to at least 43.8 GPa; and the material decompressed from high pressures is composed of a monoclinic phase, showing that the cubic Eu2O3 to monoclinic phase transition is irreversible due to the constructive nature. Pressure coefficients of Raman peaks and Grüneisen mode parameters of cubic, monoclinic, and hexagonal phases followed under pressure were determined. Furthermore, this study provides evidence for the shear stress-induced cubic to monoclinic phase transition in cubic Eu2O3 and the corresponding mechanism.

Crystal structure solution of a high-pressure polymorph of scintillating MgMoO4 and its electronic structure

The structure of the potentially scintillating high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ ($\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$) has been solved by means of high-pressure single-crystal x-ray diffraction. The phase transition occurs above 1.5 GPa and involves an increase of the Mo coordination from fourfold to sixfold accommodated by a rotation of the polyhedra and a concommitant bond stretching resulting in an enlargement of the $c$ axis. A previous high-pressure Raman study had proposed such changes with a symmetry change to space group $P2/c$. Here it has been found that the phase transition is isosymmetrical ($C2/m\phantom{\rule{4pt}{0ex}}\ensuremath{\longrightarrow}\phantom{\rule{4pt}{0ex}}C2/m$). The bulk moduli and the compressibilities of the crystal axes of both the low- and the high-pressure phase, have been obtained from equation of state fits to the pressure evolution of the unit-cell parameters which were obtained from powder x-ray diffraction up to 12 GPa. The compaction of the crystal structure at the phase transition involves a doubling of the bulk modulus ${B}_{0}$ changing from 60.3(1) to 123.7(8) GPa and a change of the most compressible crystal axis from the (0, $b$, 0) direction in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ to the ($0.9a$, 0, $0.5a$) direction in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$. The lattice dynamical calculations performed here on $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ served to explain the Raman spectra observed for the high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ in a previous work demonstrating that the use of internal modes arguments in which the ${\mathrm{MoO}}_{n}$ polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ was also calculated and compared with the electronic structures of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ and ${\mathrm{MgWO}}_{4}$ shedding some light on why ${\mathrm{MgWO}}_{4}$ is a much better scintillator than any of the phases of ${\mathrm{MgMoO}}_{4}$. These calculations yielded for $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ a ${Y}_{2}\mathrm{\ensuremath{\Gamma}}\ensuremath{\longrightarrow}\mathrm{\ensuremath{\Gamma}}$ indirect band gap of 3.01 eV in contrast to the direct bandgaps of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ (3.58 eV at $\mathrm{\ensuremath{\Gamma}}$) and ${\mathrm{MgWO}}_{4}$ (3.32 eV at $Z$).

Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs

Physical separation of immiscible carbonatites from CO2-bearing alkaline silicate melts is frequently invoked to explain the common presence of carbonatites in alkaline rock suites. We present new isobaric 1.0 GPa crystallization and liquid immiscibility experiments between 1250 and 925°C on chemically complex CO2-bearing subvolcanic basanitic, nephelinitic and nepheline-syenitic compositions from the intrusive complex on Brava Island, Cape Verdes. In the experiments, carbonatites exsolve from silicate melt along the basanite–nephelinite melt fractionation pathway throughout an unmixing temperature window from 1100 to 950°C. Liquid immiscibility is suppressed by 950°C by the onset of alkali-feldspathoid crystallization, which deflects the residual silicate melt away from the two-liquid field. While evolving along the two-liquid field boundary, silicate melt increases in SiO2 from 46 to 51 wt% and decreases in CaO / (Na2O + K2O) (by weight) from 0.83 to 0.12, reproducing the observed compositional array of alkaline silicate subvolcanic rocks from Brava. The conjugate carbonatite melts are moderately alkaline Ca-carbonatites with 5 to 9 wt% SiO2 and CaO / (Na2O + K2O) that decreases from 5.64 at 1100°C to 1.60 at 950°C. These coexisting silicate and carbonatite melt compositions show that previous high-pressure experimental studies have not yielded truly conjugate melt pairs, mainly due to the use of modified starting materials resulting in melt compositions too enriched in alkalis to match the natural alkaline silicate rock record.

Variable pressure JSR study of low temperature oxidation chemistry of n-heptane by synchrotron photoionization mass spectrometry

Low temperature oxidation chemistry is crucial in the auto-ignition process of internal combustion engines. The development of laboratory based reactors and diagnostic systems promotes understanding of the low temperature oxidation mechanism. This work develops a variable pressure jet-stirred reactor (VP-JSR) platform working from one to ten bar. Compared to previous high pressure studies which routinely use gas chromatography to analyze low temperature oxidation products, the difference in this work is the possibility of probing intermediates by synchrotron vacuum ultraviolet photoionization mass spectrometry via molecular beam sampling. The setup was validated by repeating n-heptane low temperature oxidation experiments at one and ten bar, respectively. Good agreement was observed between the data in this work and the data in the literature. A much more detailed species pool was probed compared to the literature study, including H2O2, carbonyl acids, alkylhydroperoxides, and keto-hydroperoxides. As a first demonstration of the usefulness of this design, the low temperature oxidation of n-heptane with initial fuel mole fraction of 0.005, residence time of 2 s, and equivalence ratio of 1.0 was studied at one, five, and ten bar. The preliminary results, including reactants, final products, and the initial low temperature oxidation intermediates, are discussed. The VP-JSR system developed in this work is valuable for study of the fuel chemistry from one to ten bar, to develop and examine chemical kinetic models, and to guide the development of the reaction system at even higher pressures, such as those in engine function and supercritical conditions.

Understanding the Mechanism of Ferroelectric Phase Transition in RbHSO4: A High-Pressure Raman Investigation.

Previous high-pressure dielectric and diffraction studies on rubidium hydrogen sulfate (RbHSO4) observed ferroelectric phase transition below 1 GPa pressure. We have performed high-pressure Raman spectroscopy studies on RbHSO4 up to a maximum pressure of 5.15 GPa and at ambient temperature to understand the microscopic origin and mechanism of ferroelectric transition. On the basis of the pressure dependence of Raman mode frequencies and their full-width at half-maxima, we observed a transition around a pressure of 0.3 GPa, similar to the ferroelectric transition discovered in dielectric measurements, followed by another transition around 2.4 GPa. These phase transitions are evident from the appearance/disappearance of Raman-active modes and the change in the slope of frequencies with pressures. From the pressure dependence of the S-O and S-OH frequencies, we deduce that HSO4- ion ordering results in ferroelectric phase transition around 0.3 GPa. Further, the transition around 2.4 GPa pressure is associated with significant changes in the stretching and bending vibrational frequencies and indicates a structural phase transition with possible lowering of the crystal symmetry. Interestingly, no significant changes are observed in the Raman spectrum around 1 GPa, at which a phase transition was noticed in earlier X-ray and dielectric studies.

Effects of Composition, Pressure, and Temperature on the Elastic Properties of SiO2–TiO2 Glasses: An Integrated Ultrasonic and Brillouin Study

We have systematically investigated the elastic properties (ρ, VP, VS, K, μ and σ) of eight SiO2–TiO2 glasses, varying in composition from 1.3 to 14.7 wt% TiO2, as a function of pressure up to 0.5 GPa by the pulse superposition (PSP) ultrasonic technique, and two compositions (1.3 and 9.4 wt% TiO2) up to ~5.7 GPa by Brillouin scattering in a diamond anvil cell. The parameters were also measured after annealing to 1020 °C. Composition–elasticity relationships, except for K and σ, are more or less linear; the annealing simply makes the relationships more uniform (less scatter). There is excellent agreement between the ultrasonic and Brillouin measurements at ambient and high pressure. The pressure-induced anomalous elastic behavior (negative dVP/dP and dK/dP) becomes more negative (more compressible) with the increasing TiO2 content. Correspondingly, the acoustic Grüneisen parameters become more negative with increases in the TiO2 content, reaching a minimum near ~8–10 wt% TiO2. The comparison of the low- and high-pressure ultrasonic and Brillouin VP and VS in two glasses (1.3 and 9.4 wt% TiO2) shows excellent agreement, defining the reversible elastic behavior at low pressures and irreversible behavior at higher pressures (≥5.7 GPa) well. This result is consistent with our previous high-pressure Raman study showing an irreversible structural change in a similar pressure range.

Structural investigation and compression of a co-crystal of indomethacin and saccharin

Indomethacin : saccharin cocrystal has been studied under high pressure conditions and the amide interactions compared with previous high pressure studies.

Stability and flexibility of heterometallic formate perovskites with the dimethylammonium cation: pressure-induced phase transitions.

We report the high-pressure properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) templated by dimethylammonium (NH2(CH3)2, DMA+) with the general formula [DMA]MI0.5CrIII0.5(HCOO)3, where MI = Na+ (DMANaCr) and K+ (DMAKCr). The high-pressure Raman scattering studies show crystal instabilities in the 4.0-4.4 GPa and 2.0-2.5 GPa ranges for DMANaCr and DMAKCr, respectively. The mechanism is similar in the two compounds and involves strong deformation of the metal-formate framework, especially pronounced for the subnetwork of CrO6 octahedra, accompanied by substantial compressibility of the DMA+ cations. Comparison with previous high-pressure Raman studies of sodium-chromium heterometallic MOFs show that the stability depends on the templated cation and increases as follows: ammonium < imidazolium < DMA+. Density functional theory (DFT) calculations are performed to get a better understanding of the structural properties leading to the existence of phase transitions. We calculate the energy of the hydrogen bonds (HBs) between the DMA+ cation and the metal formate cage, revealing a stronger interaction in the DMAKCr compound due to a HB arrangement that primarily involves the energetically preferred bonding to KO6 octahedra. This material however also has a smaller structural tolerance factor (TF) and a higher vibrational entropy than DMANaCr. This indicates a more flexible crystal structure, explaining the lower phase transition pressure, as well as the previously observed phase transition at 190 K, which is absent in the DMANaCr compound. The DFT high-pressure simulations show the largest contraction to be along the trigonal axis, leading to a minimal distortion of the HBs formed between the DMA+ cations and the metal-formate sublattice.

Striking isotope effect on the metallization phase lines of liquid hydrogen and deuterium

Liquid atomic metallic hydrogen is the simplest, lightest, and most abundant of all liquid metals. The role of nucleon motions or ion dynamics has been somewhat ignored in relation to the dissociative insulator-metal transition. Almost all previous experimental high-pressure studies have treated the fluid isotopes, hydrogen and deuterium, with no distinction. Studying both hydrogen and deuterium at the same density, most crucially at the phase transition line, can experimentally reveal the importance of ion dynamics. We use static compression to study the optical properties of dense deuterium in the pressure region of 1.2-1.7 Mbar and measured temperatures up to ~3000 K. We observe an abrupt increase in reflectance, consistent with dissociation-induced metallization, at the transition. Here we show that at the same pressure (density) for the two isotopes, the phase line of this transition reveals a prominent isotopic shift, ~700 K. This shift is lower than the isotopic difference in the free-molecule dissociation energies, but it is still large considering the high density of the liquid and the complex many-body effects. Our work reveals the importance of quantum nuclear effects in describing the metallization transition and conduction properties in dense hydrogen systems at conditions of giant planetary interiors, and provides an invaluable benchmark for ab-initio calculations.

Evolution of Interatomic and Intermolecular Interactions and Polymorphism of Melamine at High Pressure

Melamine (C3H6N6; 1,3,5-triazine-2,4,6-triamine) is an aromatic substituted s-triazine, with carbon and nitrogen atoms forming the ring body, and amino groups bonded to each carbon. Melamine is widely used to produce laminate products, adhesives, and flame retardants, but is also similar chemically and structurally to many energetic materials, including TATB (2,4,6-triamino-1,3,5- trinitrobenzene) and RDX (1,3,5-trinitroperhydro-1,3,5-triazine). Additionally, melamine may be a precursor in the synthesis of superhard carbon-nitrides, such as β-C3N4. In the crystalline state melamine forms corrugated sheets of individual molecules, which are stacked on top of one another, and linked by intra- and inter-plane N-H hydrogen bonds. Several previous high-pressure X-ray diffraction and Raman spectroscopy studies have claimed that melamine undergoes two or more phase transformations below 25 GPa. Our results show no indication of previously reported low pressure polymorphism up to approximately 30 GPa. High-pressure crystal structure refinements demonstrate that the individual molecular units of melamine are remarkably rigid, and their geometry changes very little despite volume decrease by almost a factor of two at 30 GPa and major re-arrangements of the intermolecular interactions, as seen through the Hirshfeld surface analysis. A symmetry change from monoclinic to triclinic, indicated by both dramatic changes in diffraction pattern, as well as discontinuities in the vibration mode behavior, was observed above approximately 36 GPa in helium and 30 GPa in neon pressure media. Examination of the hydrogen bonding behavior in melamine’s structure will allow its improved utilization as a chemical feedstock and analog for related energetic compounds.

A study of tantalum pentoxide Ta2O5 structures up to 28 GPa

Tantalum pentoxide Ta2O5 with the orthorhombic L-Ta2O5 structure has been experimentally studied up to 28.3 GPa (at ambient temperature) using synchrotron angle-dispersive powder X-ray diffraction (XRD). The ambient pressure phase remains stable up to 25 GPa where with increased pressure a crystalline to amorphous phase transition occurs. A detailed equation of state (EOS), including pressure dependent lattice parameters, is reported. The results of this study were compared with a previous high-pressure XRD study by Li et al. A clear discrepancy between the ambient-pressure crystal structures and, consequently, the reported EOSs between the two studies was revealed. The origin of this discrepancy is attributed to two different crystal structures used to index the XRD patterns.

High-pressure Raman spectroscopy on low albite

The pressure dependence of the Raman spectrum of low albite, NaAlSi3O8, has been investigated from 0.0001 to 10.4 GPa, at room temperature, on a single crystal compressed hydrostatically in a diamond anvil cell. The Raman vibrational features move to higher wavenumbers υ i with increasing pressure, due to the decrease in the unit-cell volume corresponding to a drastic shrinkage of the framework. The slopes Δυ i/ΔP of the four investigated bending modes (i.e. at 478, 507, 578 and 815 cm−1, at 0.0001 GPa) show evident changes at ~6.5 and ~8.5 GPa. This behaviour may be ascribed, in the absence of phase transitions, to the evolution of the compressional mechanisms at the atomic scale found in previous high-pressure studies on albite (mainly by X-ray diffraction), through a model based on tilts of rigid tetrahedra. The Raman data of this study allowed also to bracket the pressure range in which the occurrence of the first change in the compressional behaviour was found by X-ray diffraction.

Pressure-induced noble gas insertion into Linde-type A zeolite and its incompressible behaviors at high pressure

High-pressure behaviors of Linde-type A (LTA) zeolite were investigated using a quasi-hydrostatic pressure medium of noble gases (helium or argon). High-pressure in situ X-ray diffraction measurements combined with the use of a diamond anvil cell did not observe any structural phase transition for LTA zeolites up to a pressure of approximately 12GPa at room temperature. However, it was found that the compressibility strongly depended not only on the pressure medium but also on the pressurization process. LTA zeolites showed significant incompressible behaviors at pressures between 2 and ∼7GPa when it was compressed in helium. Based on careful analyses of the data together with the results of previous high-pressure studies, the incompressibility of LTA zeolites was induced by the penetration of a large amount of noble gas atoms into the cages of LTA zeolites under high pressures. The present results offer interesting and important information on the atomic diffusion process for open-structured materials under high pressures and thus have great implications for the development of new noble-gas-atom filled materials, as well as geochemical implications.

Compression studies of face-to-face π-stacking interaction in sodium squarate salts: Na2C4O4 and Na2C4O4•3H2O

High-pressure Raman scattering and synchrotron X-ray diffraction measurements of sodium squarate (Na(2)C(4)O(4), SS) are performed in a diamond anvil cell. SS possesses a rare, but typical structure, which can show the effect of face-to-face π-stacking without interference of other interactions. At ~11 GPa, it undergoes a phase transition, identified as a symmetry transformation from P2(1)/c to P2(1). From high-pressure Raman patterns and the calculated model of SS, it can be proved that the phase transition results from the distorted squarate rings. We infer it is the enhancement of π-stacking that dominates the distortion. For comparison, high-pressure Raman spectra of sodium squarate trihydrate (Na(2)C(4)O(4)●3H(2)O, SST) are also investigated. The structure of SST is determined by both face-to-face π-stacking and hydrogen bonding. SST can be regarded as a deformation of SS. A phase transition, with the similar mechanism as SS, is observed at ~10.3 GPa. Our results can be well supported by the previous high-pressure studies of ammonium squarate ((NH(4))(2)C(4)O(4), AS), and vice versa. High-pressure behaviors of the noncovalent interactions in SS, SST, and AS are compared to show the impacts of hydrogen bonding and the role of electrostatic interaction in releasing process.

Re-investigation of the crystal structure of enstatite under high-pressure conditions

A synthetic single crystal of pure orthoenstatite (MgSiO3, space group Pbca ) has been investigated at high pressure for structural determinations by in situ single-crystal X-ray diffraction using a diamond-anvil cell. Ten complete intensity data collections were performed up to 9.36 GPa. This study significantly improved the accuracy of structural parameters in comparison to a previous high-pressure structural study, allowing a more detailed examination of structural behavior of orthoenstatite at high pressures and a comparison to other more recent structural studies performed on orthopyroxenes with different compositions. The structural evolution determined in this work confirms the high-pressure evolution found previously for other orthopyroxenes and removes some ambiguities originating from the less accurate published data on the MgSiO3 structure at high pressure. The structural compression is mostly governed by significant volume decrease of the Mg1 and Mg2 octahedra, affecting in turn the kink of the tetrahedral chains, especially the TB chain of larger SiO4 tetrahedra. The Mg2 polyhedron undergoes the largest volume variation, 8.7%, due especially to the strong contraction of the longest bond distance (Mg2-O3B), whereas Mg1 polyhedral volume decreases by about 7.4%. The compressional behavior of the tetrahedral sites is quite different from previously published data. The TA and TB tetrahedral volumes decrease by about 2.8 and 1.8%, respectively, and no discontinuities can be observed in the pressure range investigated. Using the data on the pure orthoenstatite as reference, we can confirm the basic influences of element substitutions on the evolution of the crystal structure with pressure.

Bonding and structural changes in siderite at high pressure

Understanding the physical and chemical properties of carbonate minerals at extreme conditions is important for modeling the deep carbon cycle, because they represent likely hosts for carbon in the lower mantle. Previous high-pressure studies have identified a structural and electronic phase transition in siderite using X-ray diffraction and X-ray emission spectroscopy. The Fe end-member of the carbonate group, siderite (FeCO3), exhibits unique high-pressure behavior that we investigated using a combination of in situ Raman spectroscopy, synchrotron X-ray diffraction, and theoretical methods. In this Raman spectroscopy study, we observed the appearance of a new CO3 symmetric stretching mode at 20 cm−1 lower frequency beginning at approximately 46 GPa. This softening is due to the lengthening of the C-O bonds as a result of a combination of rotation and volume shrinkage of the FeO6 octahedra while siderite undergoes the isostructural volume collapse and electronic spin transition.

Experimental and kinetic modeling study of tetralin pyrolysis at low pressure

The pyrolysis of tetralin was studied from 850 to 1500K in an electrically heated laminar flow reactor at 30Torr. Synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry was used for isomeric identification and mole fraction measurements of pyrolysis products, especially free radicals. A kinetic model with 149 species and 554 reactions was developed in this work and validated by measured mole fraction profiles of pyrolysis species. Rate of production (ROP) analysis and sensitivity analysis were performed for mechanistic analysis of tetralin decomposition and aromatic growth processes. Contributions of four overall decomposition pathways of tetralin proposed in previous high-pressure pyrolysis studies were evaluated at the low-pressure condition based on both experimental observations and ROP analysis. It is concluded that tetralin mainly decomposes to dihydronaphthalenes, naphthalene, indene, indenyl radical and styrene via unimolecular decomposition reactions and H-abstraction reactions in low-pressure pyrolysis. Special modeling efforts on the formation pathways of indene were made to explain its high concentration in tetralin pyrolysis. Because of the high concentration levels in the pyrolysis of tetralin, indenyl radical and naphthalene play significant roles in the formation of large polycyclic aromatic hydrocarbons (PAHs), which explains the high sooting tendency of tetralin compared with alkylbenzenes.

Powder X-Ray Diffraction Study of the Hydration and Leaching Behavior of Nontronite

AbstractNontronite is a significant component of commercially important nickel laterite ores. Its behavior during high-pressure acid leaching of such ores may have an impact upon the efficiency of the process. The present study was conducted in order to further investigate the response of this material during high-pressure acid leaching. In situ synchrotron powder X-ray diffraction data were collected from a number of nontronite samples during hydration and leaching reactions at ambient and elevated temperatures. The present study followed previous high-pressure acid-leaching studies of nontronite where unexpected contraction and expansion behavior of the clay was observed by means of in situ X-ray diffraction. In the earlier studies the data sets only extended to ~20 Å so that when the nontronite expanded to greater than 19.5 Å (hydrated) the main 001 peak was only partially visible in the observed d-spacing range. The aim of the current work was to collect similar in situ diffraction data over a greater d-spacing range to observe more fully the movement of the main 001 reflection in order to better understand the changes taking place. This work was undertaken at the powder diffraction beamline of the Australian Synchrotron which was configured such that an upper d-spacing limit of ~34.5 Å could be achieved. Suggestions arising out of the previous work were confirmed along with additional information from testing of samples from the Source Clays Repository of The ClayMinerals Society. These results also show contradictory behavior of clays with the layer charge distributed over tetrahedral and octahedral sheets.

High‐pressure Raman study on CeO2 nanospheres self‐assembled by 5 nm CeO2 nanoparticles

AbstractCeO2 undergoes a first‐order phase transition from fluorite to α‐PbCl2‐type structure under high pressure. To evaluate the changes in physical properties of CeO2 nanomaterials as the particle size decreasing, high‐pressure Raman study under quasi‐hydrostatic condition has been performed on CeO2 nanospheres self‐assembled by 5 nm CeO2 nanoparticles at room temperature. Surprisingly, as the pressure elevate to 34 GPa, the CeO2 nanospheres still retain the cubic fluorite‐type structure, indicating the sample is more stable than the bulk counterpart. Whereas, previous high‐pressure studies show the phase transition at 22.3/26.5 GPa for 12 nm CeO2 nanoparticles, which is less stable than the bulk materials. The enhancement of phase stability might be attributed to the increase of surface energy of CeO2 nanospheres as the size of the building units decrease.