A-site ordered perovskite CaCu_3Cu_2Ir_2O_(12-δ) with square-planar and octahedral coordinated Cu ions

A novel CaCu_3Cu_2Ir_2O_(12-δ) polycrystalline sample was synthesized at 8 GPa and 1373 K.Rietveld structural analysis shows that this compound crystallizes in an AA'_3B_4O_(12)-type A-site ordered perovskite structure with space group Im-3.Xray absorption spectra reveal a +2-charge state for both the square-planar and octahedral coordinated Cu ions,and the valence state of Ir is found to be about +5.Although the A-site Ca and the A'-site Cu~(2+) are 1:3 ordered at fixed atomic positions,the distribution of B-site Cu~(2+) and Ir~(5+) is disorderly.As a result,no long-range magnetic ordering is observed at temperatures down to 2 K.Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior.The crystal structure is stable with pressure up to 35.7 GPa at room temperature.     

Giant pressure effect in oxygen deficient YBa2Cu3Ox

The pressure effect on T_c of polycrystalline and single crystalline YBa₂Cu₃O_x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dT_c/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dT_c/d P = 0.4 K/GPa which points to pressure effects on T_c aside from charge transfer. In the underdoped region x < 6.93 the dT_c/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT_c/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT_c/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa₂Cu₃O_x structure and can be explained by considering the anisotropic structure of YBa₂Cu₃O_x. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO₂-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dT_c/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T_c to the equilibrium value T_c(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dT_c/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dT_c/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible T_c increase (dT_c/d p)_O due to pressure induced oxygen ordering via oxygen motion between unit cells.     

压力下碱金属铁硒基超导体中的现象与物理

在凝聚态物理研究中, 压力作为对物质状态调控的独立变量得到了广泛的应用. 压力对发现物质的新现象、新规律及对其形成机理的理解和对相关理论的验证起到了重要的作用, 尤其在超导电性的研究中取得了巨大的成功. 文章简要的介绍了通过利用压力手段对具有相分离结构的碱金属铁硒基超导体A_xFe_2-ySe₂ (A=K, Rb, Tl/Rb)开展的系列研究所取得的实验结果, 以及其他一些文献中报道的在此方面的主要实验与理论研究工作, 包括压力导致的超导再进入现象和其产生的量子临界机理、其特有的反铁磁绝缘体相在该类超导体实现超导电性中的作用、化学负压力对超导电性的影响、构成该类超导体的反铁磁序与其寄居的超晶格的关系等.     

Surface atomic imaging of La2CuO4 and superconducting Tl2Ba2Ca2Cu3O10

Surface structures of the high T_c oxide superconductors Tl₂Ba₂Ca₂Cu₃O₁₀ and La₂CuO₄ have been studied by high resolution electron microscopy. In addition to the single atomic layer of La on the (113) surface observed previously, several atomic layers of the C-form of La₂O₃ were found to coat the (001) surfaces of La₂CuO₄. Surface reconstruction with the incorporation of excess Ba on the (110) surface of Tl₂Ba₂Ca₂Cu₃O₁₀ has been observed together with a stepped (11l) surface produced by a combination of the (110) and (001) planes. The (001) surface of Tl₂Ba₂Ca₂Cu₃O₁₀ was also found to terminate with a Ba-O layer. Possible mechanisms for the formation of these surface structures are discussed.     

Performance and reliability improvement of La_2O_3/Al_2O_3 nanolaminates using ultraviolet ozone post treatment

La-based binary or ternary compounds have recently attracted a great deal of attention as a potential candidate to replace the currently used Hf-based dielectrics in future transistor and capacitor devices for sub-22 generation. However, the hygroscopic nature of La₂O₃ hampers its application as dielectrics in electron devices. To cope with this challenge, ultraviolet (UV) ozone post treatment is proposed to suppress the moisture absorption in the H₂O-based atomic layer deposition (ALD) La₂O₃/Al₂O₃ nanolaminates which is related to the residual hydroxyl/hydrogen groups after annealing. The x-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (AFM) results indicate that the moisture absorption of the H₂O-based ALD La₂O₃/Al₂O₃ nanolaminates is efficiently suppressed after 600 ℃ annealing, and the electrical characteristics are greatly improved.     

Evolution of superconductivity and charge order in pressurized RbV3Sb5

The kagome metals AV₃Sb₅(A=K,Rb,Cs)under ambient pressure exhibit an unusual charge order,from which superconductivity emerges.In this work,by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV₃Sb₅,we find that the charge order becomes suppressed under a modest pressure pc(1.4 GPa3Sb₅.Our findings point to qualitatively similar temperature-pressure phase diagrams in KV₃Sb₅ and RbV₃Sb₅,{and suggest a close link}between the second superconducting dome and the high-pressure resistance anomalies.     

Synthesis and properties of La1-xSrxNiO3 and La1-xSrxNiO2

Superconductivity has been realized in films of La_1-xSr_xNiO₂. Here we report synthesis and characterization of polycrystalline samples of La_1-xSr_xNiO₃ and La_1-xSr_xNiO₂ (0 ≤ x ≤ 0.2). Magnetization and resistivity measurements reveal that La_1-xSr_xNiO₃ are paramagnetic metal and La_1-xSr_xNiO₂ exhibit an insulating behavior. Superconductivity is not detected in bulk samples of La_1-xSr_xNiO₂. The absence of superconductivity in bulk La_1-xSr_xNiO₂ may be due to the generation of hydroxide during reduction, a small amount of nickel impurity, or incomplete reduction of apical oxygen. The effect of interface in films of La_1-xSr_xNiO₂ may also play a role for superconductivity.     

An electron spin resonance study of Eu doping effect in La_(4/3)Sr_(5/3)Mn_2O_7 single crystal

The effect of Eu3+ion doping in the La sites of single-crystal La4/3Sr5/3Mn2O7was investigated. Electron spin resonance(ESR) was applied to La4/3Sr5/3Mn2O7and(La0.8Eu0.2)4/3Sr5/3Mn2O7single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance(PMR)and anisotropic ferromagnetic resonance(FMR) lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic(PM) phase and the ferromagnetic(FM) phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal(La0.8Eu0.2)4/3Sr5/3Mn2O7below the temperature of spin freezing Tf(～ 29.5 K).     

Na2TiGeO5: Crystal structure stability at low temperature and high pressure

The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a small instability below T_c≈278 K, indicating ferroelastic properties of Na₂TiGeO₅. The behavior is related to the specific crystal structure built of polyhedral layers with shared TiO₅ pyramids and GeO₄ tetrahedra, alternating with layers of Na⁺ cations. Antiparallel alignment of the short apical titanyl bond in adjacent rows of the polyhedral layer gives rise to spontaneous strain, when a distortion of the TiO₅ groups occurs. Single-crystal structures determined at room temperature and 120 K suggest that {1 1 0} domains, developing below T_c, entail a tetragonal-to-orthorhombic symmetry change. The mechanism is attributed to a shortening of the O–O distance between the polyhedral layers, and to minor shifts of the positions of the Ti atoms and the correlated oxygen atoms along the c-axis. The structure distortion, however, is too small to allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as B₀=89(2) GPa and . A pressure-induced phase transformation takes place at P_c≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by ΔT_c/ΔP≈1.76 K/GPa.     

Structural modulation and physical properties of cobalt-doped layered La_2M_5As_3O_2(M=Cu,Ni) compounds

We investigate the structural variation and physical properties of layered La_2M_5As_3O_2(M=Cu,Ni) compound upon Co doping. It is found that the substitution of Co ion just induces the monotonous change of lattice constants without observing the anomalous kink in superconducting La_2(Cu~(1-x)Nix)_5 As_3O_2 solid-solutions. Meanwhile, this doping barely changes As–As bond length in [M_5As_3]~(2-) subunit(±2%), being significantly smaller than 7% shrinkage of that in La_2(Cu~(1-x)Nix)_5 As_3 O_2. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba~(1-x)K_xFe_2 As_2 without the interference of As1–As 2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La2(Cu1-xCox)~5As_3O_2 turns into itinerant ferromagnetic metal, while La2(Ni1-x Cox)5 As3 O2 shows paramagnetism and suppressed structural phase transition upon Co-doping. The distinct structural variation and absence of superconductivity provide important clues to understand the effect of As–As bond in [M_5 As_3]~(2-) subunit.     

High pressure study on the superconducting transition temperature and Hall coefficient of Ba1−xKxBiO3 up to 8 GPa

The superconducting transition temperature (T_c) of Ba_0.62K_0.38Bio₃ (T_c=30 K) has been measured under high pressure up to 8 GPa. It is observed that T_c increases initially with pressure, as reported by Uwe et al., Shirber et al. and Huang et al., but decreases above 4 GPa. The Hall coefficient of Ba_0.62K_0.38BiO₃ has been measured up to 1.2 GPa. The absolute value of the Hall coefficient decreases with pressure by 10% GPa, the value of which is almost the same as that obtained in most CuO-based high-temperature superconductors.     

空位缺陷和相变对冲击压缩下蓝宝石光学性质的影响

为了探究冲击压缩下蓝宝石光学性质的变化行为,本文采用第一性原理方法,在180 GPa的压力范围内计算了蓝宝石理想晶体和含空位点缺陷晶体的光学性质.吸收光谱数据表明,仅考虑压力和温度因素不能解释冲击消光实验的结果,而冲击诱导的氧离子空位点缺陷应该是导致该结果的一个重要原因.波长在532 nm处的折射率数据表明:1)蓝宝石的两个高压结构相变将导致其折射率明显上升;在Corundum和Rh_2O_3相区,其折射率将随冲击压力增大而降低;在CalrO_3相区,压力小于172 GPa时,其折射率随冲击压力增大而缓慢地降低,但172 GPa以上时折射率却随冲击压力增大而逐渐增大;2)空位点缺陷对折射率随冲击压力的变化规律有明显的影响.本文结果不仅有助于增强用空位点缺陷的物理机理来解释蓝宝石冲击透明性损伤现象的可靠性,而且对未来进一步的实验研究以及发展新型窗口材料有重要的参考作用.     

Hole distribution and madelung energy in the high-Tc oxide superconductors

Evidence is presented that in high-T_c superconductors the electrostatic interaction as expressed by the Madelung energy is the key factor in determining the charge distribution and in particular the distribution of the holes between the CuO₂-planes and other structural subunits. Specifically, this explains the phase diagram (T_c as a function of oxygen content and oxidizing power) of cation-substituted YBa₂Cu₃O_6+y and the oxygen ordering in YBa₂Cu₃O_6.5. Moreover, we argue that the dependence of T_c on strontium content in La_2−xSr_xCuO₄ is similarly due to the fact that holes go into the (La,Sr)O-sheets as well as into the CuO₂-planes.     

Origin of cathodic degradation and new phase formation at the La0.9Sr0.1MnO3/YSZ interface

New phase formation at the La_0.9Sr_0.1MnO₃/YSZ interface and its effects on the cathodic performances were studied at 900 °C in air. The resistance caused by the interfacial product layer kept increasing with time to reach up to 40% of the total resistance after 500 h. The interfacial product was identified as La₂Zr₂O₇ by XRD measurement. The electrical conductivity of La₂Zr₂O₇ (2.4 × 10⁻⁵ S cm⁻¹ at 1000 °C), measured by AC impedance and current interruption methods, was 4 to 7 orders of magnitude smaller than those of La_0.9Sr_0.1MnO₃ electrode or YSZ electrolyte. Either the electronic conductivity or the electrochemical O₂ reduction activity of La₂Zr₂O₇ was negligible. Combining these results, a conclusion was made that the cathodic degradation comes mainly from the growth of interfacial product layer and its contribution to the cell resistance increment is ohmic in nature.     

La3+存在形式对白云鄂博稀选尾矿微晶玻璃性能的影响

选取白云鄂博稀选尾矿和粉煤灰为主要原料, 采用熔铸法制备了0–4 wt%La₂O₃掺杂的CaO-MgO-Al₂O₃-SiO₂系微晶玻璃. 利用DTA, XRD, SEM, TEM+EDS和性能测试手段研究La³⁺在白云鄂博稀选尾矿微晶玻璃中的存在形式及其对该微晶玻璃显微结构、抗折强度和耐腐蚀性的影响. 结果表明, La³⁺以置换固溶方式进入辉石主晶相, 1 wt%的La₂O₃可促进辉石主晶相形成. 超过1 wt%的La₂O₃与基础玻璃组分反应生成Ca₃La₆(SiO₄)₆第二相, 并与辉石相争夺Ca²⁺和Si⁴⁺离子来阻碍辉石相形成. 添加1 wt% La₂O₃的微晶玻璃综合性能最优, 其抗折强度和密度分别为198 MPa和3.18 g/cm³.     

Ramans study of Y1−xPrxBa2Cu3O7−δ and YB2Cu3−xZnxO7−δ single crystals

Single crystals with known T_c values of Y_1−xPr_xBa₂Cu₃O_7−δ (Y---Pr1:2:3) and YBa₂Cu_3−xZn_3−xZn_xO_7−δ (Y---Zn1:2:3) systems are studied by Raman measurements. The Raman spectra for (Y---Pr1:2:3) single crystals show that the frequencies of Ba and O_z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y---Pr1:2:3 systems. On the other hand, in the Y---Zn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T_c, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO₂ planes. The induced disorder in the CuO₂ planes may be related with suppression of T_c in the Y---Zn1:2:3 system. Thus, the suppression mechanism in the Y---Zn1:2:3 systems seems to be different from that in the Y---Pr1:2:3 systems.     

HoVO4相变的高压拉曼光谱和理论计算研究

利用高温固相反应法制备了纯相的HoVO₄,并在0–21.25 GPa压强范围内测定了HoVO₄的拉曼光谱. 通过分析其拉曼峰的频移和劈裂变化情况,发现HoVO₄在9.3 GPa发生相变. 根据第一性原理选取并优化相似体系的高压晶体结构,将其与HoVO₄的常压锆石矿型I4₁/amd结构进行了能量比较,确认HoVO₄ 相变结构为白钨矿型结构（I4₁/a）. 研究结果表明,HoVO₄具有ScVO₄和YVO₄体系的从锆石矿型结构（I4₁/amd）至白钨矿型结构的相变过程. 分析对应结构相的体积随压强的变化,发现体积坍塌对该相变起重要作用. 上述研究结果有助于了解HoVO₄的高压结构以及该材料在高压特殊条件下的应用. 关键词： 锆石矿型结构 白钨矿型结构 拉曼光谱 第一性原理     

Microwave absorption and paracoherence excess conductivity studies in high-Tc superconductors

Single phase polycrystalline samples of La_1.8Sr_0.2CuO₄, YBa₂Cu₃O_7−δ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y high-T_c superconductors have been studied using XRD, SEM, EDAX, (T), χ_AC and EPR techniques. Detailed investigations on low-field-dependent microwave absorption have been carried out in order to investigate the effects of field exposure, field cooling, zero field cooling and amplitude of field variation on the samples. The results have been discussed on the basis of granular model via Josephson junctions of high-T_c superconductors. Over and above, paracoherence excess conductivity analysis has been carried out in all the three samples. The critical exponent (γ) estimated for all the three samples in close vicinity to their respective transition temperatures has an excellent agreement with the theoretically predicted value for 3-dimensional XY-ferromagnet (γ = 1.33 for D = 3, N = 2).     

Near-zero thermal expansion in β-CuZnV2O7 in a large temperature range

We report a new type of near-zero thermal expansion material β-CuZnV₂O₇ in a large temperature range from 173 K to 673 K. It belongs to a monoclinic structure (C2/c space group) in the whole temperature range. No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment. The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa, respectively. The mechanism of negative thermal expansion in β-CuZnV₂O₇ is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.     

红色荧光粉La2Mo2O9:Eu3+,W6+的制备及发光性能

采用高温固相法合成出La₂Mo₂O₉:Eu³⁺,W⁶⁺系列红色荧光粉,其结构为立方晶系的β- La₂Mo₂O₉。在395 nm光激发下,样品La_1.40Mo₂O₉:0.60Eu³⁺发射出很强的红光,最强发射峰位于616 nm处。适量地掺杂W⁶⁺离子可以提高样品的激发和发射强度,在395 nm光激发下,La_1.40Mo_1.84O₉:0.60Eu³⁺,0.16W⁶⁺荧光粉的Eu³⁺的⁵D₀→⁷F₂跃迁发射强度最大,是样品La_1.40Mo₂O₉:0.60Eu³⁺的1.23倍。最后,将La_1.40Eu_0.60Mo_1.84O₉:0.16W⁶⁺荧光粉与~395 nm发射的InGaN芯片一起制作成红光发光二极管(LED),该LED发射出很强的红光。