SnO2: A comprehensive review on structures and gas sensors

Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

Metal oxides possess exceptional potential as base materials in emerging technologies. In recent times, significant amount of research works is carried out on these materials to assess new areas of applications, including optical, electronic, optoelectronic and biological domains. In such applications, the response and performance of the devices depend crucially, among other factors, on the size, shape and surface of the active oxide materials. For instance, the electronic and optical properties of oxides depend strongly on the spatial dimensions and composition [1] . The large number of atoms on the surface, and the effective van der Waals, Coulombic and interatomic coupling significantly modify the physical and chemical properties of the low dimensional oxide materials vis-a-vis its bulk counterparts. As a result, low dimensional oxide materials, such as nanoparticles, nanospheres, nanorods, nanowires, nanoribbon/nanobelts, nanotubes, nanodisks, nanosheets evoke vast and diverse interests. Thermal and physical deposition, hydro/solvothermal process, spray-pyrolysis, assisted self-assembly, oil-in-water microemulsion and template-assisted synthesis are regularly employed to synthesis one-, two- and three-dimensional nanostructures, which have become the focus of intensive research in mesoscopic physics and nanoscale devices. It not only provides good scopes to study the optical, electrical and thermal properties in quantum-confinement, but also offers important insights for understanding the functional units in fabricating electronic, optoelectronic, and magnetic devices of nanoscale dimension. Tin oxide (SnO 2 ) is one such very important n-type oxide and wide band gap (3.6 eV) semiconductor. Its good quality electrical, optical, and electrochemical properties are exploited in solar cells, as catalytic support materials, as solid-state chemical sensors and as high-capacity lithium-storage. Previously, Chopra et al. [2] reviewed different aspects of transparent conducting SnO 2 thin films. Wang et al. [3] discussed device applications of nanowires and nanobelts of semiconductor oxides, including SnO 2 . Batzill et al. [4] discussed about the surface of single crystalline bulk SnO 2 . However, it is understood that neither there is any comprehensive review on various crystallographic phases, polymorphs, bulk modulus, lattice parameters and electronic states of SnO 2 , nor there is any updated compilation on the recent progress and scope on SnO 2 nanostructures. Therefore, the proposed review covers the past and recent progress on the said topics and is summarized in the following manner. The available theoretical and experimental works on crystal structures, bulk modulus, lattice parameters are reviewed in details. The electronic states and the band structures of these phases are discussed next. Active crystal surfaces of SnO 2 play vital roles in its many interesting properties, including sensing and catalytic applications. So, a short review is written on its different surfaces, its electronic structures and density of states. The discussion on the importance of morphological variations on the properties of SnO 2 is followed by a review on different methods for obtaining such structures. A detail survey on the existing literature on techniques and mechanisms for the growth of nanostructures are included. SnO 2 is efficiently employed in gas sensing applications. A review on such applications is compiled based on the role of morphology and performance. The future course of SnO 2 as an important material in the contemporary research is also discussed.

SnO2: A Comprehensive Review on Structures and Gas Sensors

Electron gas interionic potentials (EGIP) have been developed to determine the equation of state (EOS) of the cubic (C1) and orthorhombic (C23) polymorphs of ${\mathrm{SrF}}_{2},$ including the thermal effects by means of a quasiharmonic Debye model. The zero pressure cell parameter ${(a}_{0}),$ lattice energy ${(E}_{\mathrm{latt}}),$ and bulk modulus ${(B}_{0})$ of the C1 phase are computed with errors smaller than 1.2%, 1.2%, and 7.1%, respectively. The predicted EOS is in good agreement with the observed data and satisfies the universal Vinet EOS. For the C23 phase, the optimized zero-$p$ cell parameters a, b, and c and the six fractional coordinates are reported and the pressure dependence of ${a/a}_{0},$ ${b/b}_{0},$ and ${c/c}_{0}$ explored by fitting independent modified Vinet EOS's to the computed data. The analysis reveals a greater compressibility of the C23 phase along the b and c axes than along the a direction. Our calculation predicts the $\mathrm{C}1\ensuremath{\rightleftharpoons}\mathrm{C}23$ equilibrium to occur at ${p}_{\mathrm{tr}}=3.92$ GPa, which is between the observed values for the $\mathrm{C}\stackrel{\ensuremath{\rightarrow}}{1}\mathrm{C}23$ ${(p}_{\mathrm{tr}}=5.0$ GPa) and $\mathrm{C}23\ensuremath{\rightarrow}\mathrm{C}1$ ${(p}_{\mathrm{tr}}=1.7$ GPa) phase transitions.

Atomistic simulation of Sr F 2 polymorphs

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

A stable compound of helium and sodium at high pressure

The grain-size effect on phase transition induced by pressure in ZnO nanocrystals has been investigated by in situ high-pressure synchrotron radiation X-ray powder diffraction, optical and electrical resistance measurements. The transition pressure of the B4-to-B1 phase transformation for 12 nm ZnO is found to be 15.1 GPa while it is 9.9 GPa for bulk ZnO. Three components: the ratio of the volume collapses, the surface energy difference, and the internal energy difference, governing the change of transition pressure in nanocrystals, are uncovered. The enhancement of transition pressure in ZnO nanocrystals as compared with the corresponding bulk material is mainly caused by the surface energy difference between the phases involved. The high-pressure B1 ZnO phase is not metallic in the pressure range up to 18 GPa at room temperature.

https://iopscience.iop.org/article/10.1209/epl/i2000-00233-9/pdf

Structural stability in nanocrystalline ZnO

The grain-size effect on the semiconductor-to-metal transition in ZnS has been investigated by in situ high-pressure electrical resistance and optical measurements. It is found that the grain-size effect can elevate the transition pressure of ZnS in a larger pressure range. On the basis of the results obtained and results reported in the literature, we demonstrate that the dangers of using the transition pressures of the II–VI compounds as pressure calibrators without a detailed knowledge of their grain-size effects on the transition pressures cannot be overstressed.

/pdf/grain-size-effect-on-pressure-induced-semiconductor-to-metal-4e7vi2skz7.pdf

Grain-size effect on pressure-induced semiconductor-to-metal transition in ZnS

High-pressure experiments on metallic thallium have been carried out using a diamond-anvil cell for pressures up to 32 GPa. A hexagonal close packed → face-centred cubic (f.c.c.) phase change takes place around 4 GPa. Fitting of the experimental data to the Murnaghan and Birch equations of state has been used to obtain values for the bulk moduli and their pressure derivatives for the two phases. Extrapolation of the high-pressure data for the f.c.c. phase to low pressure has been used to obtain an estimate of the pressure to about 0.23 GPa in nanosized metastable f.c.c. thallium inclusions formed in aluminium by ion implantation.

A High-Pressure Study of Thallium

High-pressure X-ray diffraction studies have been performed on ThP using synchrotron radiation and a diamond-anvil cell. The bulk modulus B0 and its pressure derivative B′0 have been determined (B0 = 137 GPa; B′0 = 5.1). A phase transition from the NaCl structure to the CsCl structure was observed at about 30 GPa.

Crystal structure and the equation of state of thorium monophosphide for pressures up to 50 GPa

The phase transformation from NaCl structure (B 1) to CsCl structure (B2) in actinide compounds has been studied using X-ray powder diffraction in the pressure range up to about 60 GPa. It is shown that the transition is sluggish, has a strong hysteresis and is accompanied by a volume change in the range 8–12%. These features are similar to those of the corresponding transition in the alkali halides and other B1 compounds, indicating a common mechanism for the transformation as concerns the lattice geometry.

The pressure‐induced transformation B1 to B2 in actinide compounds

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