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本文用中子飞行时间方法对C-15相的超导材料V2Hf,V2Ta和V2Hf0.8Ta0.2以及V2Zr0.5。Hf0.5和V2Zr0.5Hf0.33Ta0.17的热中子非弹性散射谱作了测量,并计算出相对的广义声子态密度。结果与早先发表的Nb对C-15相V2Zr和V2(Hf0.5Zr0.5)系列声子性能的影响一致:声子频率随超导转变温度Tc增加而软化,随Tc减小而硬化。这表明,对于此类材料弹性软化在一定程度上对提高Tc起了作用。结果还进一步表明V2Zr或V2Hf与V2(Zr0.5Hf0.5)之间有着质的差别,V2Hf加Ta后,Tc增加,声子频率软化,而V2(Zr0.5Hf0.5)加Ta后,Tc减小,声子频率则略有硬化。这与V2Zr和V2(Hf0.5Zr0.5)加Nb的结果是一致的。此结果可以用角动量分波表象的能带论方法分析电-声耦合相互作用得出的杂化理论来定性解释。
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本文中研究了C-15结构V2Hf1-xNbx,V2Zr1-xNbx和V2Hf0.5Zr0.5-xNbx系列的超导转变温度Tc与Nb合量x的关系,发现V2Zr,V2Hf加Nb后与V2Hf0.5Zr0.5加Nb后性能显著不同。测定了V2Zr,V2Hf和V2Hf0.5Zr0.5的X射线光电子能谱。结果表明,当Hf原子和Zr原子共存于AB2化合物的A位上时,发生了一种增强原子间相互作用的新的电荷转移。这个事实支持由角动量分波表象能带论方法分析电声耦合超导原理的结果对四元V2(HfZrNb)系列的超导行为提出的一种可能解释:4d-5d原子配位可能有助于提高4d导带的杂化程度,从而有利于提高超导Tc。
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本文研究了C-15结构化合物(Zr_(0.5)Hf_(0.5)-xTa_x)V_2(x≤0.2)和(Hf_(0.5)Zr_(0.5-x)Nb_x)V_2(x≤0.2)的超导转变温度T_c及其压力效应T_c/P。报道了实验方法与结果。与(Hf_(1-x)Ta_x)V_2和(Zr(1-x)Ta_x)V_2的情况不同,在(Zr_(0.5)Hf_(0.5-x)Ta_x)V_2中Ta的引入使常压下的T_c下降,然而T_c/P却大为提高。因此高压下Zr_(0.5)Hf_(0.45)Ta_(0.05)V_2的T_c反而比Hf_(0.5)Zr_(0.5)V_2更高。导出了一个基于角动量分波表象能带论方法的描述压力效应的新关系式。它指出导带电子波函数中的高角动量成分变化对T_c的压力效应影响比较重要。这个公式有助于理解上述复杂的实验现象,并能合理地解释某些元素(如Cs,Ba,La等)的T_c随压力剧增的事实。 相似文献
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本文报道,通过对Y1-xCaxBa2Cu3-xMxO7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa2Cu3-xMxO7-δ(M=Fe,Ni)体系进行比较,发现Y1-xCaxBa2Cu3-xFexO7-δ体系的Tc显著地高于相应x值的YBa2Cu3-xFexO7-δ体系,而Y1-xCaxBa2Cu3-xNixO7-δ体系则相反,Tc低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(nH)变化相互补偿,抑制了仅Fe替代时引起的nH和Tc急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y1-xCaxBa2Cu3-xFexO7-δ体系属于CuO2平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y1-xCaxBa2Cu3-xNixO7-δ体系,由于Ni2+离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni2+离子的拆对效应是引起Tc下降的直接原因。
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基于第一性原理方法,探索M-Al-N(M=Ti,Zr,Hf)结构的稳定性,计算其力学性质.计算M-Al-N化合物的能量,发现除实验已知的结构Ti2AlN和Ti4AlN3、Zr2AlN、Hf2AlN外,还存在两种新的热力学稳定结构Zr4AlN3、Hf4AlN3.弹性常数和声子谱的计算,表明这两个结构是力学稳定和晶格动力学稳定的.计算M2AlN和M4AlN3的力学性质,发现它们具有高的体模量、剪切模量、弹性模量、维氏硬度等;分析其力学性质随组分比例、组成元素的变化规律,为该类材料的选择和应用提供理论依据.最后计算M2AlN和M4AlN3的电子态密度和分态密度、电子密度分布、Mulliken群分析等. 相似文献
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Mahfoud Ziyad Rachid Ahmamouch Mohamed Rouimi Slimane Gharbage Jacques C. Vdrine 《Solid State Ionics》1998,110(3-4):311-318
Phosphates of general formula M0.5Hf2(PO4)3 with M=Cd2+, Ca2+, Sr2+ and Cu2+ were prepared by coprecipitation and characterized by several physical techniques. The compounds containing Cd2+, Ca2+, Sr2+ belong to the Nasicon-type structure, whereas Cu0.5Hf2(PO4)3 exhibited substantially different DRX patterns. Combined temperature programmed reduction (TPR) and temperature-programmed oxidation (TPO) showed that the copper in Cu0.5Hf2(PO4)3 was distributed between two energetically different sites in proportions respectively equal to 40 and 60%. Electron Paramagnetic Resonance (EPR) investigations confirmed the TPR/TPO results and revealed that the two sites hosting the Cu2+ ions are of orthorhombic symmetry. Moreover, the Cu2+ ions might be reduced by hydrogen to Cu+. These results were also supported by the UV–visible studies that showed the disappearance, under reducing conditions, of the band corresponding to crystal field transitions of Cu2+ ions and the emergence of a new peak attributed to the transitions between (3d)10 and (3d)9(4s)1 Cu+ levels. At the same time, IR spectroscopy confirmed that protons entered the open lattice framework of the material and gave rise to a new protonated phase containing monovalent copper Cu0.5IH0.5Hf2(PO4)3. This redox process was proven to be reversible without any subsequent change in the network of the phosphate. 相似文献
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Based on the Landau-Dovenshire theory, the thermodynamic potential of (111) oriented films is constructed to investigate the pyroelectric properties of Pb(Zr1-xTix)O3 thin films. Due to the presence of nonlinear coupling terms resulted from the (111) epitaxy with substrates, the effects of misfit strain and electric field on the phase transitions at room temperature are more complex than that of (001) and (110) oriented films. Pb(Zr0.5Ti0.5)O3 thin films with the Ti composition around the morphotropic phase boundary (MPB) have the giant dielectric and pyroelectric properties. Moreover, Pb(Zr1-xTix)O3 thin films grown on the tensile substrates which induce the triclinic γ phase have the larger dielectric and pyroelectric properties than that on the compressive substrates. Therefore, the physical properties of (111) oriented Pb(Zr1-xTix)O3 thin films can be adjusted by choosing the suitable substrates. 相似文献
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The crystal structure and magnetic properties of R2Fe17-xCrx(R=Dy,Er,0≤x≤3) compounds have been investigated by me ans of X-ray diffraction and magnetization measurements. These compounds have hexagonal Th2Ni17-type structure. The unit-cell volumes decrease with the increase of Cr concentration x. The Curie temperature Tc of the Er2Fe17-xCrx compounds increases from 320 K for x=0 to 403 K for x=1.0 and then decreases with further increase of x. The Cur ie temperat ure Tc of Dy2Fe17-xCrx compounds increases from 364 K for x=0 to 435 K for x=1.0 and then decreases with further increase of x. The saturation magnetization of these compounds shows an approximately linear decrease with the increase of x. Spin reorientation transitions occur s in Er2Fe17-xCrx(x=2.0 and 3.0). 相似文献
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采用固相烧结法制备了Zri_(1-x)Al_(2-x)V_(2-x)Mo_xO_7(0≤x≤0.9),并通过调整Al~(3+)/Mo~(6+)对ZrV_2O_7中的Zr~(4+)/V~(5+)离子替代量来实现近零膨胀,对于较小的x值(x≤0.3),材料保持了与ZrV_2O_7相同的立方相结构.随着Al~(3+)/Mo~(6+)替代量的增加,(Al/Zr)~-和(Mo/V)~+之间的库仑相互作用逐渐加强,这种库仑相互作用导致材料中未发生畸变的立方相晶体结构逐渐减少.当x≥0.7时,材料中立方相晶体结构完全消失.在425-750 K温度区间内,Zr_(0.5)Al_(0.5)M_(0.6)O_7展示出近零膨胀性质(-0.39×l0~(-6)K~(-1)).Zr_(0.5)Al_(1.5)V_(1.5)Mo_(0.5)O_7的低热膨胀性能可能与Al~(3+)/Mo~(6+)对ZrV_2O_7中Zr~(4+)/V~(5+)部分替代引起部分晶体结构发生的畸变及其对未替代部分的晶格结构的影响有关. 相似文献
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YTi(Fe1-xCox)11在012型四方结构,居里温度随Co含量的增加而提高,本文为了揭示YTi(Fe1-xCox)11磁性与微观结构的联系,对这一系列化合物进行穆斯堡尔谱研究,在室温下,测量YTi(Fe1-xCox)11(其中x=0.0,0.2,0.4)的穆斯堡尔谱,最佳拟合结果证实在这种化合物中,Co原子优先占据j和f晶位,在所测样品中,超精细场在x=0.2处出现极值,这与YTi(Fe1-xCox)11饱和磁化强度测量结果相符合。并对YTi(Fe1-xCox)11,Y2(Fe1-xCox)14和YTi(Fe1-xCox)11的穆斯堡尔谱的实验结果进行比较,分析在YTiFe11中与Y2Fe14B中的3d电子能带结构的差异。
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Vanadium pentoxide xerogels have shown high electrochemical performance in terms of energy content. The high specific energy and high intercalation capability make the materials promising for thin film lithium battery and electrochromic device applications. In order to enhance the rate capabilities of the host we increased the electronic conductivity by doping the V2O5 xerogels with silver. Samples were prepared by mixing various amounts of silver powder with V2O5 hydrogel. We were able to prepare silver-doped vanadium pentoxide dip-coated thin films with a molar ratio (Ag/V) ranging from 0.005 to 0.5 (AgyV2O3 with y = 0.01, 0.1 and 1). With the successful doping, the electronic conductivity of V2O5 was increased by 2 to 3 orders of magnitude. The insertion capacity of the material was maintained and up to 4 moles of lithium per mole of silver-doped V2O5 (XRG) were found to be reversibly intercalated. 相似文献
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通过对FeSe进行化学插层可以将其超导转变温度(Tc)从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料(Li0.84Fe0.16)OHFe0.98Se和Li0.36(NH3)yFe2Se2开展了高压调控研究,发现压力会首先抑制高温超导相(称为SC-I相),然后在临界压力Pc以上诱导出第二个高温超导相(称为SC-Ⅱ相),呈现出双拱形T-P超导相图.这两个体系的Pc分别约为5和2 GPa,两个体系SC-Ⅱ相的最高Tc分别可以达到约52和55 K,比相应SC-I相的初始Tc提高了10 K.对(Li0.84Fe0.16)OHFe0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的Tc与霍尔系数倒数1/RH(∝载流子浓度ne)具有很好的线性依赖关系.对(Li0.84Fe0.16)OHFe0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此Pc以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构. 相似文献
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过渡金属氧化物二氧化钒(VO2)在温度340 K附近会发生金属绝缘体的转变(metal-insulator transition, MIT). 基于金属绝缘体的转变性质, VO2薄膜材料具有很好的应用前景. 本文首先采用脉冲激光沉积制备了高质量的V2O5薄膜, 再通过高温氢退火还原V2O5薄膜制备出VO2多晶薄膜. 研究了不同的退火温度、退火时间、退火气氛对VO2薄膜制备的影响, 采用X射线衍射、X射线光电子能谱、变温电阻特性测量等手段对样品进行分析, 发现在H2(5%)/Ar退火气氛下, 在一定的退火温度范围内(500–525 ℃), 退火 3 h, 得到了B相和M相共存的VO2薄膜, 具有M相的VO2的MIT特性, 而相同退火温度下退火时间达到4.5 h, 薄膜完全变成B相的VO2. 通过纯Ar气氛下对B相VO2再退火, 得到了转变温度为350 K, 电阻突变接近4个数量级的M相的VO2薄膜. 实现了VO2的B相和M相的相互转变.
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2薄膜')" href="#">VO2薄膜
金属绝缘体转变
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