High pressure structural phase transitions of TiO_2 nanomaterials

Recently, the high pressure study on the TiO_2 nanomaterials has attracted considerable attention due to the typical crystal structure and the fascinating properties of TiO_2 with nanoscale sizes. In this paper, we briefly review the recent progress in the high pressure phase transitions of TiO_2 nanomaterials. We discuss the size effects and morphology effects on the high pressure phase transitions of TiO_2 nanomaterials with different particle sizes, morphologies, and microstructures. Several typical pressure-induced structural phase transitions in TiO_2 nanomaterials are presented, including size-dependent phase transition selectivity in nanoparticles, morphology-tuned phase transition in nanowires, nanosheets,and nanoporous materials, and pressure-induced amorphization(PIA) and polyamorphism in ultrafine nanoparticles and TiO_2-B nanoribbons. Various TiO_2 nanostructural materials with high pressure structures are prepared successfully by high pressure treatment of the corresponding crystal nanomaterials, such as amorphous TiO_2 nanoribbons, α-PbO_2-type TiO_2 nanowires, nanosheets, and nanoporous materials. These studies suggest that the high pressure phase transitions of TiO_2 nanomaterials depend on the nanosize, morphology, interface energy, and microstructure. The diversity of high pressure behaviors of TiO_2 nanomaterials provides a new insight into the properties of nanomaterials, and paves a way for preparing new nanomaterials with novel high pressure structures and properties for various applications.     

Spin--polarized electronic properties of NiHe0.25 under pressures

This paper studies the effects of He atom on the spin-polarized electronic properties of nickel under pressures using ab initio pseudopotential plan-wave method.Under high pressures,the compound of NiHe0.25 can exist and helium-bubble can not create in Ni.A pressure-induced ferromagnetic to paramagnetic phase transition has been predicted in NiHe0.25 at about 218 GPa.It is found that under pressures,the magnetic property of Ni atoms is more strongly affected by He atom than by H atom and that the behaviour of He atom in Ni are completely different from that of H atom,like the bonding characteristics and the electron transfer.     

纳米硫化锌球壳的高压相变研究

 采用同步辐射能量色散X射线衍射（EDEX）技术和金刚石对顶砧高压装置,对纳米硫化锌球壳进行了原位高压X射线衍射实验。最高压力达33.3 GPa。常压下纳米硫化锌球壳为纤锌矿结构和闪锌矿结构共存的混相结构。压力达到11.2 GPa时,纳米硫化锌空心球中的纤锌矿结构全部转变为闪锌矿结构。压力达到16.0 GPa时,发生了由闪锌矿结构向岩盐矿结构的相变,在17.5 GPa和21.0 GPa时分别出现未知峰,33.3 GPa时基本完全转变为岩盐矿结构。两个相变均为可逆相变。     

Pressure-induced superconducting ternary hydride H3SXe: A theoretical investigation

In general, heavy elements contribute only to acoustic phonon modes, which are less important for the superconductivity of hydrides. However, it was revealed that the heavier elements could enhance the phonon-mediated superconductivity in ternary hydrides. In the H₃S–Xe system, a novel H₃SXe compound was discovered by first-principle calculations. The structural phase transitions of H3SXe under high pressures were studied. The R-3m phase of H₃SXe was predicted to appear at pressures above 80 GPa, which transitions to C2/m, P-3m1, and Pm-3m phases at pressures of 90, 160, and 220 GPa, respectively. It has been anticipated that the Pm-3m-H₃SXe phase with a similar structural feature as that of Im-3m-H₃S is a potential high-temperature superconductor with a T_c of 89 K at 240 GPa. The T_c value of H₃SXe is lower than that of H₃S at high pressure. The “H₃S” host lattice of Pm- 3m-H₃SXe is a crucial factor influencing the T_c value. The Xe atoms could accelerate the hydrogen-bond symmetrization. With the increase of the atomic number, the T_c value linearly increases in the H₃S–noble-gas-element system. This indicates that the superconductivity can be modulated by changing the relative atomic mass of the noble-gas element.     

金属Nb中H的量子行为研究

采用基于密度泛函原理的赝势平面波方法,计算了H在Nb晶体中的势场.通过求解H在该势场中的Schrdinger方程,得到了H的振动状态.计算结果表明,H的基态和第一激发态是局域的,第二激发态是非局域的,可以在T点之间迁移以及做4T/6T环运动. 关键词： 第一性原理 金属氢化物 量子振动     

Phase Transition and Optical Properties of Solid Oxygen under High Pressure: A Density Functional Theory Study

Crystal structures and optical properties of the δ-O2 phase and the ε-O8 phase have been investigated by using the ab initio pseudopotential plane-wave method. It is found that the phase transition is of the first order with a discontinuous volumetric change from the antiferromagnetic δ-O2 phase to the nonmagnetic ε-O8 phase, consistent with the experimental findings. The energy band calculations show that the direct band gap changes into an indirect band gap after the phase transition. The apparent change in the optical properties can be used for identifying the phase transition from δ-O2 to ε-O8.     

Electronic structure and optical properties of LiXH3 and XLiH3 （X= Be,B or C）

The equilibrium lattice constant, the cohesive energy and the electronic properties of light metal hydrides LiXH3 and XLiH3 （X = Be, B or C） with perovskite lattice structures have been investigated by using the pseudopotential plane-wave method. Large energy gap of LiBeH3 indicates that it is insulating, but other investigated hydrides are metallic. The pressure-induced metallization of LiBeH3 is found at about 120 GPa, which is attributed to the increase of Be-p electrons with pressure. The electronegativity of the p electrons of X atom is responsible for the metallicity of the investigated LiXH3 hydrides, but the electronegativity of the s electrons of X atom plays an important role in the metallicity of the investigated XLiH3 hydrides. In order to deeply understand the investigated hydrides, their optical properties have also been investigated. The optical absorption of either LiBeH3 or BeLiH3 has a strong peak at about 5 eV, showing that their optical responses are qualitatively similar. It is also found that the optical responses of other investigated hydrides are stronger than those of LiBeH3 and BeLiH3 in lower energy ranges, especially in the case of CLiH3.     

紫外激光和压力共同作用下C60-peapod的聚合相变研究

采用气相扩散方法将C₆₀分子填充到单壁碳纳米管(SWNTs)中，制备出高填充比率的豆荚形纳米材料C₆₀@SWNT，又称为peapod.用金刚石对顶砧(DAC)装置获得高压，在高压下同时利用紫外激光处理样品，通过激光和压力的共同作用研究了C₆₀分子在碳管内的聚合相变.在21.5GPa高压下，同时紫外激光(325nm)照射30min后，拉曼光谱表明C₆₀分子在碳管内发生了聚合，形成一维链状O相聚合结构，且该相变是不可逆的. 关键词： 60 peapod')" href="#">C₆₀ peapod 紫外激光 高压 拉曼光谱     

Nb2H的电子结构和相互作用

应用基于密度泛函理论的赝势-平面波方法研究了Nb2H的电子结构和H原子占据点之间的关系.计算结果表明:由4个Nb近邻构成的四面体中心点(T点)为H的稳定俘获点,而由6个Nb近邻构成的八面体中心点(O点)则为相互作用势的极大值点,是不稳定点.相邻T点间存在低能量通道,具有鞍点(S点)结构.在T点及近邻低能通道上,H的1s能级展宽较弱,Nb的4d,5s带部分向下延展,与H带杂化后形成孤立带.当H由T向O移动时,孤立的杂化带向金属价带靠近,带隙逐渐消失.H处于完整宿主晶格中的T点时,布里渊区Г点处的光学声子结构色散较强,而在远离Г点处的色散很弱.H在T点邻域及T-T通道上的电子结构稳定性和声子的色散关系表明,H倾向于以稳定负离子形式存在.     

Nb2H 的电子结构和相互作用

应用基于密度泛函理论的赝势-平面波方法研究了Nb₂H的电子结构和H原子占据点之间的关系.计算结果表明：由4个Nb近邻构成的四面体中心点(T点)为H的稳定俘获点，而由6个Nb近邻构成的八面体中心点(O点)则为相互作用势的极大值点，是不稳定点.相邻T点间存在低能量通道，具有鞍点(S点)结构.在T点及近邻低能通道上,H的1s能级展宽较弱，Nb的4d, 5s 带部分向下延展，与H带杂化后形成孤立带.当H由T向