Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides

Tantalum nitride(TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature(HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure–volume(P–V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B'_0= 369(2) GPa with pressure derivatives of B 0= 4 for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result(B_0= 351 GPa), which is close to the recent theoretical calculation result(B_0= 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta–N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.     

LiAl5O8高压相变研究

 运用金刚石压砧同步辐射X射线衍射,对尖晶石结构的LiAl₅O₈进行了高压原位研究。实验发现：在高压下,一组LiAl₅O₈的新相衍射峰出现,随着压力的增加,其新相衍射峰逐渐增强,当压力增加到45.0 GPa时,LiAl₅O₈的低压相衍射峰全部消失,而形成了一组高压新相衍射峰。采用指标化程序对衍射数据进行处理和分析,确定这一高压新相为正交晶系结构,其晶胞参数为a=0.995 9 nm,b=0.644 7 nm,c=0.333 4 nm ,空间群为Pmm2。     

关于完备λ超曲面的第二拼挤定理

欧氏空间R~(n+1)中满足方程H=-X~N+λ的浸入超曲面称为λ超曲面.本文主要研究欧氏空间中完备λ超曲面的第二拼挤问题.设M为R~(n+1)中具有多项式体积增长的n维完备λ超曲面.设M的第二基本形式为A.本文证明存在正的绝对常数γ,如果|λ|≤γ,β_λ≤|A|~2≤β_λ+~1/21,其中β_λ=1/2(2+λ~2+|λ|(λ~2+4)~1/2),那么|A|~2≡β_λ,λ≥0,且M必为n维球面S~n(n~1/2)、n维圆柱面S~k(k~1/2)×R~(n-k)(1≤ k≤ n-1)或S(((λ2+4)~1/2-|λ|)/2)×R~(n-1)之一.     

基于铰链式六面顶压机的二级6-8模超高压大腔体内置加热元件的设计与温度标定

 首次报道了一种新型的基于铰链式六面顶压机的二级6-8模大腔体静高压装置的内置加热元件的设计与温度标定。此加热组装结构简单,升温快,保温效果好,并有效地解决了国外基于两面顶压机构架下的二级6-8模内加热组装中热电偶在施加压力时易断的问题。以低成本的碳管为加热元件,采用直接和间接两种加热方式,用双铂铑（Pt6%Rh-Pt30%Rt）B型热电偶进行温度测量,并根据实验过程中加热功率与腔内实际温度的关系,对不同压力下腔体内的温度进行了标定。实验结果表明：此加热系统的油压达到40 MPa（腔体压力约10 GPa）时,温度可以达到1 700 ℃以上;在油压为30 MPa、样品室温度为1 000 ℃时,保温时间可达2 h,甚至更长;实验中获得样品的直径可达3 mm,高度可达7 mm,实现了在高温超高压条件下大样品的制备,满足了实验对产生高温超高压条件的需要。     

Raman scattering from highly-stressed anvil diamond

The high-frequency edge of the first-order Raman mode of diamond reflects the stress state at the culet of anvil, and is often used for the pressure calibration in diamond anvil cell(DAC) experiments. Here we point out that the high-frequency edge of the diamond Raman phonon corresponds to the Brillouin zone(BZ) center Γ point as a function of pressure. The diamond Raman pressure gauge relies on the stability of crystal lattice of diamond under high stress. Upon the diamond anvil occurs failure under the uniaxial stress(197 GPa), the loss of intensity of the first-order Raman phonon and a stressdependent broad Raman band centered at 600 cm^-1 are observed, which is associated with a strain-induced local mode corresponding to the BZ edge phonon of the L1 transverse acoustic phonon branch.     

Evidence for a High-Pressure Isostructural Transition in Nitrogen

We observed an isostructural phase transition in the solid nitrogen λ-N₂ at approximately 50 GPa accompanied by anomalies in lattice parameters, atomic volume and Raman vibron modes. The anomalies are ascribed to a slight reorientation of the nitrogen molecules, which does not seem to affect the monoclinic symmetry(space group p2_1/c). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the λ-N₂ phase. In addition, a ne...     

Direct observation of melted Mott state evidenced from Raman scattering in 1T-TaS_2 single crystal

The evolution of electron correlation and charge density wave(CDW)in 1T-TaS_2 single crystal has been investigated by temperature-dependent Raman scattering,which undergoes two obvious peaks of A_(1g) modes about 70.8 cm~(-1) and 78.7 cm~(-1) at 80 K,respectively.The former peak at 70.8 cm~(-1) is accordant with the lower Hubbard band,resulting in the electron-correlation-driven Mott transition.Strikingly,the latter peak at 78.7 cm~(-1) shifts toward low energy with increasing the temperature,demonstrating the occurrence of nearly commensurate CDW phase(melted Mott phase).In this case,phonon transmission could be strongly coupled to commensurate CDW lattice via Coulomb interaction,which likely induces appearance of hexagonal domains suspended in an interdomain phase,composing the melted Mott phase characterized by a shallow electron pocket.Combining electronic structure,atomic structure,transport properties with Raman scattering,these findings provide a novel dimension in understanding the relationship between electronic correlation,charge order,and phonon dynamics.     

沥青混合料劈裂试验的细观有限元模拟与测试

为分析沥青混合料的细观尺度破坏机理,采用数字图像技术将AC-25沥青混合料离散为集料、联结料、孔隙三相复合材料,基于ANSYS软件建立马歇尔试件的二维细观有限元模型,输入各相介质的力学参数,虚拟细观劈裂破坏试验。基于非接触式光学方法,测试AC-25马歇尔试件劈裂试验中各相介质应变,并与虚拟细观有限元分析结果进行对比。结果表明,虚拟细观分析的各相介质参数影响较大,细观光学应变测试结果大于虚拟细观分析结果,但虚拟细观应变响应与光学测试结果规律相似,两种分析中沥青混合料破坏的薄弱环节均为联结料与界面。     

High-pressure Raman study of osmium and rhenium up to 200 GPa and pressure dependent elastic shear modulus C44

High-pressure Raman scattering from hexagonal close-packed (HCP) metals Os and Re have been extended up to 200 GPa, and the pressure-dependent shear modulus C₄₄ has been deduced from the Raman-active mode E_2g, which is generated from the adjacent vibration of atoms in hexagonal planes, providing the valuable information about the elastic properties for HCP metals under high pressure. Combined with the available data of HCP metals from previous works, a further study indicates that the $C_{44}^\prime/C_{44}$ ratio would be close to a constant value, 0.01, with increasing atomic number of metals. The results obtained from high-pressure Raman scattering will allow us to probe the elastic anisotropy of the HCP metals at very high pressure.     

三元铁基金属氮化物的高压复分解反应合成北大核心CSCD

以二元金属氧化物(氧化铁、氧化钴、氧化镍)和六方氮化硼为反应前驱体,在大腔体压机提供的高温高压条件下(5GPa、1 673K),通过发生复合高压固相复分解(HPSSM)反应合成组分可调控的三元铁基金属氮化物圆球状块体材料ε-Fe_(3-x)M_xN_(1+δ)(M=Co,Ni)。并利用X射线粉末衍射(XPD)、场发射扫描电子显微镜(FE-SEM)、高分辨率透射电子显微镜(HRTEM)等多种材料表征手段对高压合成的三元铁基金属氮化物进行结构表征,同时基于密度泛函理论(DFT)的第一性原理计算探究压力对HPSSM反应的影响。研究结果表明,高压密闭环境有利于制备高质量金属氮化物,HPSSM反应合成法是制备铁基金属氮化物块体材料的一种有效方法。