用局域密度泛函线性丸盒轨道原子球近似超元胞法计算(Ba1-xKx)BiO3的Hopfield常数与Tc随组分x的变化

用局域密度泛函线性丸盒轨道原子球近似(LDF-LMTO-ASA)超元胞法计算(Ba_1-xK_x)BiO₃特定组分x的电子结构,其中x=0.0,0.5,1.0三种组分由扩大1倍的元胞计算;x=0.25和0.75两种组分由扩大4倍的元胞计算,所得能带参数总态密度TDOS(E_F)、分波态密度PDOS(E_F)以及自洽晶体势V^t(r),结合由实验测定的Θ_D≈210K,按McMillan强耦合公式以及Gaspari-Gyorffy近似,分别计算各原子的Hopfield常数η_t,电声子耦合常数λ以及超导转变温度T_c随组分x的变化。计算结果λ≈1而T_c最大约10K,且随x变化缓慢。与实验结果对比似乎暗示,在(Ba_1-xK_x)BiO₃中除电声子机制外,随组分变化的复杂结构相变亦将起重要的作用。 关键词：     

Ce3+和Mn2+掺杂的Ba9(Y2-xScx)(SiO4)6光谱特性和温度特性研究

采用高温固相法制备了Ba₉(Y_2-xSc_x)(SiO₄)₆:Ce³⁺,Mn²⁺(x=0,0.5,1.0,1.5,2.0)样品。在该体系中,当Sc³⁺含量从x=0逐渐增加至x=2时,Ce³⁺的蓝光发射强度提高了1.7倍;同时,Mn²⁺的红光发射强度提高了1.9倍,显示了优良的红光特性。样品的发射光谱和漫反射光谱表明,Ce³⁺、Mn²⁺发射强度的增加与Ce³⁺吸收能力和Ce³⁺向Mn²⁺能量传递的提升有直接关系。研究了样品Ba₉Sc₂(SiO₄)₆:Ce³⁺,Mn²⁺的热稳定性。随着温度的升高,Mn²⁺的红光发射呈现先升后降的态势。当温度从室温升至488 K时,Mn²⁺发射强度仅下降至室温时的84%,表现出优良的热稳定性。高亮的红光发射和优良的热稳定性表明该荧光材料可为紫外基白光LED提供良好的红色光源。     

离子注入对超导转变温度的影响

本文中提出了超导转变温度T_c与离子注入剂量φ的经验关系式,T_c=1.13θ_Dexp[-1/(A/(Φ+1)+B)].将这一关系式和若干实验结果进行了对比,结果表明,计算值与实验结果符合得很好。最后我们讨论了这一关系式的物理基础,认为由于费密面附近出现了杂质能带,导致了样品T_c的变化。 关键词：     

非晶锂离子导体B2O3-Li2O-LiCl一Al2O3中VO2+的电子自旋共振

两种非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃-0.1V₂O₅(x=0.05和0.15)的电子自旋共振谱研究表明:(i)ESR线型是高斯型,证实V₂O₅添加量适当;(ii)超精细结构来源于VO²⁺络离子,具有四角对称性,属C_4v群。越精细耦合张量的平行分量平均值A_//=0.0175cm^-1,垂直分量A_⊥=0.0063cm^-1。由g_//(g_⊥)求出其基态²B_2g与第一激发态²E_g的能级间距△₁=2.46×10⁴ cm^-1,基态与第二激发态²B_1g的能级间距△₂=3.03×10⁴ cm^-1;(iii)变温实验证实:Al₂O₃组分较少(x=0.05)的非晶ESR强度比x=0.15的非晶高3倍至2倍,而Al₂O₃组分越多则ESR强度随温升下降越小。     

含外力项的广义KdV方程的类孤子解

本文利用广义KP方程的B?cklund变换,获得了含外力项的广义KdV方程u_t+6uu_x+u_xxx+6f(t)u=g(t)+x(f′+12f²) (1)的类孤子解。 关键词：     

德拜特征温度的各向异性与非均匀性

由于德拜特征温度标识着原子或离子在晶体内的平均振动能量,而不同原子在晶体内的振动模式,或同一原子在非立方晶体内的振动模式显然不同,这就发生了特征温度的非均匀性及各向异性问题。本文提出了从粉末X射线衍射强度测定晶体特征温度的非均匀性及各向异性的方法。测定了CaF₂的特征温度。结果发现:Ca²⁺离子在CaF₂晶体内的特征温度为400K,而F^-离子的特征温度为476K,二者相差76K。 关键词：     

Co2+-Ti4+和Cu2+-Nb5+离子取代BaFe12O19单晶体中Fe3+离子的研究

本文对用Co²⁺-Ti⁴⁺和Cu²⁺-Nb⁵⁺离子取代BaFe₁₂O₁₉单晶体中Fe³⁺离子进行了研究,以Bi₂O₃作为助熔剂生长出了BaFe_12-2xCo_x²⁺Ti_x⁴⁺O₁₉(x=0;0.04;0.09;0.13;0.27和0.68)以及BaFe_12-x[Nb_1/3⁵⁺Cu_2/3²⁺]_xO₁₉(x=0;0.28;0.44和0.60)这两系列的单晶体,测定了100—300K温度范围内样品的磁化强度σ与单轴各向异性常数K₁,我们发现,对Co²⁺-Ti⁴⁺取代的样品,当x≤0.09时,其σ与K₁随x的增加而缓慢增加;当x>0.09时,其σ与K₁随x的增加而迅速降低,至x=1.1时,K₁变为零,对Nb⁵⁺-Cu²⁺取代的样品,其σ值在整个成份范围内基本保持不变,且有缓慢增加趋势;而K₁值则随x增加而单调下降,提出了取代离子在M型六角铁氧体中可能的分布模型来解释我们的结果。 关键词：     

CeO2热力学性质的第一原理研究

使用第一性原理方法结合准谐德拜模型研究压力0~30 GPa,温度0~2 000 K,二氧化铈立方结构的热力学性质,包括常压下平衡体积V、体弹模量B₀、热容c_p和熵S随温度的变化以及不同压强下热容c_p、熵S、德拜温度Θ,体膨胀系数α与温度的关系.常压下计算的热容c_p和熵S随温度的变化与实验数据符合很好.     

非晶态Fe87-xSixB13合金的磁化强度、电阻与温度的关系

本文研究了非晶态Fe_87-xSi_xB₁₃(x=0,9.6,14.5)合金的饱和磁化强度、电阻率与温度的关系。得到样品的居里温度T_C和晶化温度T_cr随Si含量的增加而明显提高。低温下的热磁关系符合布洛赫的T^3/2定律,计算出自旋波劲度系数D从x=0时的62meV·A²增加到x=14.5时的111meV·A²。从D值和Handrich理 关键词：     

非中心对称超导体Mg10±δIr19B16?δ的超导电性研究

Mg_10±δIr₁₉B_16δ是具有非中心对称性的超导材料. 通过对两种组分的样品Mg_9.3Ir₁₉B_16.7和Mg₁₁Ir₁₉B₁₅的比热测量, 得到了这类超导体的超导态和正常态的特征参数：包括转变温度T_C, 正常态态密度N(E_F), 德拜温度Θ_D, 上临界场H_C2等. 并由此求出Ginzburg-London相干长度ξ_GL, 穿透深度λ_GL, 下临界场H_C1和热力学临界场H_C. 这些参数因化学组分不同而变化. 高的N(E_F)和Θ_D对应高的T_C, 因此也具有较高的H_C2. 另外比热跃变ΔC/γ_nT_C=1.66 和电声子耦合常数λ=0.58不随化学组分变化, 表明此超导体是中等强度耦合的第二类超导体. 关键词： 非中心对称超导体 比热     

The low temperature heat capacity and electrical resistivity of ReO3

Low temperature heat capacity and electrical resistivity measurements are reported for ReO₃. The heat capacity data give an acoustical mode Debye temperature θ = 327 K, and an electrronic density of states parameter γ = 2.83 mJ/mole-K². The observed temperature dependence of the resistivity is consistent with the existence of electron scattering both from acoustic mode phonons and from optical mode phonons of characteristic temperature θ_E = 1080 K. The above measurements are used to evaluate the electron-phonon interaction parameter λ = 0.24.     

Phenomenological trends for heavy fermi liquids and narrow-band metals III: Characteristic temperatures

A correlation between the ratio of the Fermi temperature T_F¹ and the Debye temperature θ_D and the occurrence of superconductivity or magnetic order is described. Values of T_F¹/θ_D⪢ 1 are found to favor “conventional” superconductivity, whereas T_F¹/θ_D≲ 1 favors magnetic order. “Heavy fermion” superconductivity is found to be restricted to the case T_F¹/θ_D < 1.     

The specific heat of two cubic laves compounds: TiBe2 and ThMg2

The low temperature specific heats of two isomorphous Laves compounds, TiBe₂ and ThMg₂, have been measured. The shape of the transition for the itinerant antiferromagnet TiBe₂ is characteristic of magnetic ordering; the entropy of this transition is quite small, approximately 107 mJ/mole-K, a further indication of itinerancy. The γ, proportional to the density of states, is 42 mJ/mole-K² for TiBe₂, and the Debye temperature, θ_D, is 650 K. These numbers are compared to those for the itinerant ferromagnet, ZrZn₂. The results for ThMg₂ are γ=9.2 mJ/ mole-K² and θ_D = 217 K. Results for both compounds are discussed in regards to the theory of Enz and Matthias.     

Mössbauer studies of 151Eu in europium oxalate, europium bissalen ammonium and europium benzoate

Although a number of europium water insoluble chelates have been prepared for several decades, the covalent nature of these compounds has never been established in any quantitative fashion. Shifts in the I.R. bands and conductivity measurements of these salts were hitherto used to qualitatively compare their molecular nature. In this communique we have used temperature coefficients of ¹⁵¹Eu Mössbauer spectra to determine the Debye temperatures (θ _D) of three europium chelates: namely europium oxalate, europium bissalen ammonium (recently reported) and europium benzoate and compared their θ _D with the measured θ _D of the known ionic EuF₃. Additionally, the mean square amplitude (<x ²) of these four compounds was computed and plotted as a function of temperature giving in each case a reasonable linear plot. It was interesting to note that the θ _D of the most ionic EuF₃ was greatest (283 ± 10 K) followed by the oxalate (166 ± 15 K), then the europium bissalen ammonium (133 ± 5 K) and lastly the europium benzoate with a θ _D of (105 ± 5 K).     

Magnetic and magnetocaloric properties of martensitic Ni2Mn1.4Sn0.6 Heusler alloy

The evolutions of magnetic properties at low temperatures and the influence of magnetic field on the temperature dependence of specific heat in martensitic Ni₂Mn_1.4Sn_0.6 Heusler alloy are studied. The frequency-dependent blocking temperature and considerable exchange bias below it are measured in the martensitic phase. From the analysis of the specific heat curves under magnetic field, a large inverse magnetocaloric effect manifested as the magnetic field induced rise of isothermal magnetic entropy and/or magnetic field induced adiabatic temperature decrease in the vicinity of the reverse magnetostructural transformation and a significant value of the conventional magnetocaloric effect at the Curie temperature are obtained. The Debye temperature and electronic coefficient equal to Θ_D=310±2 K and γ= 16.6±0.3 mJ/K²mol, respectively, do not depend on the magnetic field.     

Non-adiabatic small-polaron hopping conduction in VN–PbO–TeO2 glasses

The dc conductivity of VN–PbO–TeO₂ glasses with different mole percentages of VN, PbO and TeO₂ has been measured in the temperature range 125–450?K. The conductivity of the glasses increases with increasing VN content for a fixed mole percentage of PbO. Neither Mott's variable-range hopping (VRH) model at low temperatures (T<Θ_D/4, where Θ_D is the Debye temperature) nor Greaves’ VRH model at intermediate temperatures (Θ_D/?4<T<Θ_D/2) describe the dc conductivity data for these glasses. Multiphonon tunnelling transport of strongly coupled electrons is also unable to account for the carrier transport. However, at high temperatures (T?>?Θ_D/2), conduction is shown to be due to small-polaron hopping in the non-adiabatic regime. Alteration of the VN content causes a change in the model parameters achieved from best-fitting curves for the glasses. Modulated differential scanning calorimetry analysis shows that the glass transition temperatures T _g in this system vary from 269 to 302°C.     

A model for high-spin/low-spin transitions in solids including the effect of lattice vibrations

A model for high-spin/low-spin transitions in solids is discussed including the effect of low symmetry ligand fields and spin-orbit coupling. These interactions are required in the calculation of quadrupole splitting and magnetic susceptibility. In addition, the contribution from lattice vibrations is taken into account within the approximation of the Debye model. The recently observed entropy change at the transition temperature may be easily explained on this basis. The model is applied in a detailed numerical fit of the experimental data of [Fe(4,7-(CH₃)₂-phen)₂(NCS)₂] where phen = 1,10-phenanthroline. The compound may be characterized by the parameter values Δ₁, = Δ₂ = 400 cm^?, λ = ?80 cm^?1, J = 196 cm^?1 (or J_tot = 205 cm^?1) and N = 10 experimentally based Debye temperatures Θ₁ = 140 K and Θ_h = 130 K have been employed.