Compressibility and Structural Stability of Nanocrystalline TiO2 Anatase Synthesized from Freeze-Dried Precursors

Acoustic Shock Wave-Induced Rutile to Anatase Phase Transition of TiO2 Nanoparticles and Exploration of Their Unconventional Thermodynamic Structural Transition Path of Crystallization Behaviors

Titanium dioxide (TiO2) is one of the most well-known and long-standing polymorphic materials in the transition metal oxide group of materials. The transition from rutile to anatase is one of the long-standing fundamental questions among materials science researchers because seeking the nucleation site at the beginning of the phase transition is highly challenging. Until now, there have been no studies on the unconventional structural phase transition of TiO2 nanoparticles by acoustic shock waves. In the present study, this work provides the first evidence on the solid-state nanostructure of the rutile-to-anatase phase transition of TiO2 by acoustic shock waves whereby these phase transition results are evaluated by Raman spectroscopy, thermal calorimetry, X-ray photoelectron spectroscopy, and microscopic techniques. We propose a novel mechanism for the occurrence of the rutile-to-anatase phase transition based on thermophysical properties and shock wave-induced melting concepts. Under shocked conditions, the R-A phase transition occurs because of the anatase phase's lower interfacial energy (γL/A) and surface energy compared to rutile. We strongly believe that the present work can provide in-depth insight into understanding the crystallization concepts of the TiO2 NPs under extreme conditions, especially with regard to the rutile-to-anatase phase transition.

High-Pressure Behavior of Ca2SnO4, Sr2SnO4, and Zn2SnO4

The pressure-induced structural evolution of Ca2SnO4, Sr2SnO4, and Zn2SnO4 has been characterized by powder X-ray diffraction up to 20 GPa using the ALBA synchrotron radiation source and density functional theory calculations. No phase transition was observed in Ca2SnO4 and Zn2SnO4 in the investigated pressure range. The observation in Zn2SnO4 solves contradictions existing in the literature. In contrast, a phase transition was observed in Sr2SnO4 at a pressure of 9.09 GPa. The transition was characterized as from the ambient-condition tetragonal polymorph (space group I4/mmm) to the low-temperature tetragonal polymorph (space group P42/ncm). The linear compressibility of crystallographic axes and room-temperature pressure-volume equation of state are reported for the three compounds studied. Calculated elastic constants and moduli are also reported as well as a systematic discussion of the high-pressure behavior and bulk modulus of M2SnO4 stannates.

An Investigation of the Pressure-Induced Structural Phase Transition of Nanocrystalline α-CuMoO4

The structural behavior of nanocrystalline α-CuMoO4 was studied at ambient temperature up to 2 GPa using in situ synchrotron X-ray powder diffraction. We found that nanocrystalline α-CuMoO4 undergoes a structural phase transition into γ-CuMoO4 at 0.5 GPa. The structural sequence is analogous to the behavior of its bulk counterpart, but the transition pressure is doubled. A coexistence of both phases was observed till 1.2 GPa. The phase transition gives rise to a change in the copper coordination from square-pyramidal to octahedral coordination. The transition involves a volume reduction of 13% indicating a first-order nature of the phase transition. This transformation was observed to be irreversible in nature. The pressure dependence of the unit-cell parameters was obtained and is discussed, and the compressibility analyzed.

Comparative study on the pressure-induced phase transformation of anatase TiO2 hollow and solid microspheres

Nanostructured anatase TiO₂ undergoes pressure-induced phase transformation, and the transformation sequence is significantly different from the bulk counterpart. The size and the morphology are found both playing a critical role in the phase transformation behavior. In this work, we prepare anatase TiO₂ microspheres using a hydrothermal method. By controlling the reaction time, hollow and solid spheres of similar diameters are prepared. TEM and XRD analysis reveals that these microspheres are aggregates of anatase nanocrystalline of size between 15-16 nm. The phase transformation behaviour under high temperature is examined in situ using both Raman spectroscopy and synchrotron x-ray diffraction. We find that although both solid and hollow spheres are micron-sized, they undergo phase transformation sequence similar to nanomaterials with size of several tens of nanometers. Hollow spheres exhibit a higher compressibility than the solid spheres. A detailed analysis based on the formation mechanism of the spheres is performed to explain the unique phase transformation behavior of these materials.

Putting the Squeeze on Lead Chromate Nanorods

We have studied by means of X-ray diffraction and Raman spectroscopy the high-pressure behavior of PbCrO₄ nanorods. We have found that these nanorods follow a distinctive structural sequence that differs from that of bulk PbCrO₄. In particular, a phase transition from a monoclinic monazite-type PbCrO₄ to a novel monoclinic AgMnO₄-type polymorph has been discovered at 8.5 GPa. The crystal structure, Raman-active phonons, and compressibility of this novel high-pressure phase are reported for the first time. The experimental findings are supported by ab initio calculations that provide information not only on structural and vibrational properties of AgMnO₄-type PbCrO₄ but also on the electronic properties. The discovered phase transition triggers a band gap collapse and a subsequent metallization at 44.2 GPa, which has not been observed in bulk PbCrO₄. This suggests that nanoengineering can be a useful strategy to drive metallization under compression.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

High-Pressure Phase Transition of Micro- and Nanoscale HoVO4 and High-Pressure Phase Diagram of REVO4 with RE Ionic Radius

In situ Raman spectra of HoVO₄ micro- and nanocrystals were obtained at high pressures up to 25.4 and 18.0 GPa at room temperature, respectively. The appearance of new peaks in the Raman spectra and the discontinuities of the Raman-mode shift provided powerful evidence for an irreversible zircon-to-scheelite structure transformation for HoVO₄ microcrystals at 7.2 GPa and for HoVO₄ nanocrystals at 8.7 GPa. The lattice contraction caused by the size effect was thought to be responsible for the different phase-transition pressures. Also, the higher stability of HoVO₄ nanocrystals compared with the microcrystals was also confirmed using the Raman frequencies and pressure coefficients. The results of the phase transition of HoVO₄ were compared with previously reported rare-earth orthovanadates, and the phase diagram of REVO₄ with RE ionic radius at different pressures was presented.

Reciprocating Compression of ZnO Probed by X-ray Diffraction: The Size Effect on Structural Properties under High Pressure

Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds.

Structural and vibrational properties of corundum-type In2O3 nanocrystals under compression

This work reports the structural and vibrational properties of nanocrystals of corundum-type In₂O₃ (rh-In₂O₃) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In₂O₃. Nanocrystalline rh-In₂O₃ showed stability under compression at least up to 20 GPa, unlike bulk rh-In₂O₃ which gradually transforms to the orthorhombic Pbca (Rh₂O₃-III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In₂O₃ is discussed.

Pressure-induced amorphization of YVO₄:Eu³⁺ nanoboxes

A structural transformation from the zircon-type structure to an amorphous phase has been found in YVO4:Eu(3+) nanoboxes at high pressures above 12.7 GPa by means of x-ray diffraction measurements. However, the pair distribution function of the high-pressure phase shows that the local structure of the amorphous phase is similar to the scheelite-type YVO4. These results are confirmed both by Raman spectroscopy and Eu(3+) photoluminescence which detect the phase transition to a scheelite-type structure at 10.1 and 9.1 GPa, respectively. The irreversibility of the phase transition is observed with the three techniques after a maximum pressure in the upstroke of around 20 GPa. The existence of two (5)D0-->(7)F0 photoluminescence peaks confirms the existence of two local environments for Eu(3+), at least for the low-pressure phase. One environment is the expected for substituting Y(3+) and the other is likely a disordered environment possibly found at the surface of the nanoboxes.

Ten-fold coordinated polymorph and metallization of TiO2under high pressure

A stable and metallic CaC₂-type structure of TiO₂is identified with the highest coordination number among the known phases.