Size and morphology effects on the high pressure behaviors of Mn3O4 nanorods

Electron Microscope Study of the Pressure-Induced Phase Transformation and Microstructure Change of TiO2 Nanocrystals

Microstructure transformation of materials under compression is crucial to understanding their high-pressure phase transformation. However, direct observation of the microstructure of compressive materials is a considerable challenge, which impedes the understanding of the relations among phase transformation, microstructure, and material properties. In this study, we used transmission Kikuchi diffraction and transmission electron microscopy to intuitively characterize pressure-induced phase transformation and microstructure of TiO2. We observed the changes of twin boundaries with increasing pressure and intermediate phase TiO2-I of anatase transformed into TiO2-II (α-PbO2 phase) for the first time. The following changes occur during this transformation: anatase (diameter of ∼100 nm) → anatase twins 60° along the [110] zone axis → intermediate TiO2-I twins 60° along the [010] zone axis → TiO2-II twins 90° along the [010] zone axis. These results directly reveal the crystallographic relation among these structures, enhancing our understanding of the phase transformation in TiO2 nanocrystals.

Pressure-Induced Modulation of Tin Selenide Properties: A Review

Tin selenide (SnSe) holds great potential for abundant future applications, due to its exceptional properties and distinctive layered structure, which can be modified using a variety of techniques. One of the many tuning techniques is pressure manipulating using the diamond anvil cell (DAC), which is a very efficient in situ and reversible approach for modulating the structure and physical properties of SnSe. We briefly summarize the advantages and challenges of experimental study using DAC in this review, then introduce the recent progress and achievements of the pressure-induced structure and performance of SnSe, especially including the influence of pressure on its crystal structure and optical, electronic, and thermoelectric properties. The overall goal of the review is to better understand the mechanics underlying pressure-induced phase transitions and to offer suggestions for properly designing a structural pattern to achieve or enhanced novel properties.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Size and Shape's Effects on the High-Pressure Behavior of WS2 Nanomaterials

Exploring the behavior of nanocrystals with varying shapes and sizes under high pressure is crucial to understanding the relationship between the morphology and properties of nanomaterials. In this study, we investigated the compression behaviors of WS2 nanotubes (NT-WS2) and fullerene-like nanoparticles (IF-WS2) by in situ high-pressure X-ray diffraction (XRD) and Raman spectroscopy. It was found that the bulk modulus of NT-WS2 is 81.7 GPa, which is approximately twice as large as that of IF-WS2 (46.3 GPa). This might be attributed to the fact that IF-WS2 with larger d-spacing along the c-axis and higher defect density are more compressible under isotropic pressure than NT-WS2. Thus, the slender NT-WS2 possess a more stable crystal structure than the IF-WS2. Our findings reveal that the effects of morphology and size play crucial roles in determining the high-pressure properties of WS2 nanoparticles, and provide significant insight into the relationship between structure and properties.

Pressure Tuning the Jahn-Teller Transition Temperature in NaNiO2

NaNiO2 is a layered material consisting of alternating layers of NaO6 and Jahn-Teller-active NiO6 edge-sharing octahedra. At ambient pressure, it undergoes a broad phase transition from a monoclinic to rhombohedral structure between 465 and 495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 and 490 K. The 290 and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixed-phase throughout. The unit-cell volume was fitted to a second-order Birch-Murnaghan equation of state, where B = 119.6(5) GPa at 290 K. We observe at 490 K that the fraction of the Jahn-Teller-distorted phase increases with pressure, from 67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in conjunction with neutron diffraction measurements at 490 K on removing pressure from 5.46(9)  to 0.342(13) GPa, we show that the Jahn-Teller transition temperature increases with pressure. Our results are used to present a structural pressure-temperature phase diagram for NaNiO2. To the best of our knowledge, this is the first diffraction study of the effect of pressure on the Jahn-Teller transition temperature in materials with edge-sharing Jahn-Teller-distorted octahedra and the first variable-pressure study focusing on the Jahn-Teller distortion in a nickelate.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

We synthesized single crystals of marokite (CaMn₂O₄)-type orthorhombic manganese (II,III) oxide, γ-Mn₃O₄, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn₃O₄ is a semiconductor with an indirect band gap E_g of 0.96 eV and two antiferromagnetic transitions (T_N) at ∼200 and ∼55 K. The phase stability of the γ-Mn₃O₄ crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn₃O₄ was determined to be B₀ = 235.3(2) GPa with B' = 2.6(6). The γ-Mn₃O₄ phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO₆ polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn³⁺ ions in the octahedra and the Mn²⁺ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.