二十面体Sc13，Sc13¬+1，Sc13¬-1团簇的稳定性与电子结构研究

采用密度泛函理论框架下的广义梯度近似(DFT/GGA),对Sc13团簇进行了几何结构优化,得到13原子钪团簇的基态结构为正二十面体(Ih),在此基础上对二十面体(Ih)Sc13, Sc13¬+1和Sc13-1团簇的稳定性、电子结构磁矩进行研究.结果发现Sc13,Sc13¬+1和Sc13¬-1团簇都在Ih结构时最稳定,该尺寸团簇的稳定性主要由二十面体密堆积构型决定;带电能使团簇的结构稍稍收缩从而使团簇的稳定性有所增强;团簇的键长和对称性对团簇的磁矩有很明显的影响.     

Sc12X（X=C、Si、Ge、Sn、Pb）团簇的几何构形和电子结构研究

基于第一性原理,在密度泛函理论框架下,用广义梯度近似(GGA)研究二十面体Sc12X(X=C、Si、Ge、Sn、Pb)团簇的几何构形和电子结构,系统计算了它们的束缚能(BE)、最高占据轨道(HOMO)与最低未占据轨道(LUMO)之间的能隙(ΔE)、局域磁矩( )及团簇的平均键长( )。研究表明,用C、Si、Ge、Sn、Pb分别替代Sc13团簇中心或表面原子可以使其成为更稳定结构(除Sc12Pb外)。掺杂团簇Sc12X中,当杂质原子X比Sc原子的原子量大很多时,具有C5V对称性的构形比具有Ih对称性的构形更稳定;当杂质原子X比Sc原子的原子量小时,具有对称Ih对称性的构形比具有C5V对称性的构形更稳定(除Sc12C团簇外)。Sc12C团簇的稳定性出现异常,其原因与轨道杂化有关。     

密度泛函理论研究ZrnCo(m=1-13)团簇的结构和磁性

利用密度泛函理论中的广义梯度近似对ZrnCo(n=1-13)团簇进行了结构优化、能量和频率的计算,研究了ZrnCo团簇的平衡几何结构、稳定性、电子性质和磁性.结果表明:Zr4Co,Zr7Co,Zr9Co和Zr12Co团簇的基态稳定性较高,是幻数团簇,尤其是Zr12Co团簇基态为Ih对称性的二十面体结构且稳定性特别高.ZrnCo团簇的磁矩随尺寸的变化可以分三个阶段:n=1-3有稳定的磁矩,从n=4开始磁矩出现振荡性的猝灭,直至n≥8磁矩完全猝灭.体系的磁矩主要来自局域d电子的贡献,ZrnCo团簇磁矩发生猝灭的主要原因是电荷转移和强烈的spd杂化效应.同时发现,过渡金属掺杂在不同特性材料中所形成的团簇体系,其结构、稳定性和磁性有些非常有意思的相似,如TMX12(TM代表过渡金属Fe或Co,X代表Si和Be)团簇、Zr13TM团簇.对此,值得进一步研究.     

Sc12X团簇(X=B,C,N,Al,Si,P)的电子结构和稳定性

基于第一性原理,在密度泛函理论框架下,用广义梯度近似(GGA)研究二十面体Sc12X(X=B、C、N、Al、Si、P)中性和荷电团簇的电子结构和稳定性,系统计算了它们的基态束缚能(BE)、原子间平衡间距、最高占据轨道(HOMO)与最低占据轨道(LUMO)之间的能隙、局域电荷以及HOMO电子构型.研究表明,用C、Si原子或荷电后的B、N、Al、P离子分别替代团簇Sc13中心原子可以使其成为稳定的结构.Sc12X团簇束缚能改变的原因在于掺杂改变了中心原子或离子与表面原子的轨道杂化.     

Sc_(13)团簇的稳定性和磁性研究

使用梯度修正的密度泛函理论,对Sc_(13)团簇的稳定性和磁性进行了广泛的研究.与邻近钪团簇,或者与其它13-原子3d过渡金属团簇相比,Sc_(13)团簇的基态结构二十面体具有最高对称性,结构和磁性均表现出高的稳定性.由于二十面体的高对称性,Sc_(13)团簇的对称性分子轨道为晶体场分裂壳层9A_g(1),10T_(1u)(3),8H_g(5),10A_g(1),6T_(2u)(3),11T_(1u)(3),3G_g(4),3G_u(4),and 9H_g(5).20个价电子以类似于凝胶模型的1S~21P~61D~(10)2S~2壳层填充前四组壳层,其余19个价电子依据洪德法则填充后五组壳层,从而产生了19.0μB的磁矩.5个半填充的晶体场分裂壳层与离域价电子填充的1S~21P~61D~(10)2S~2类壳层导致了稳定的高磁矩Sc_(13)团簇.     

Fe、Co、Ni混合团簇磁性的密度泛函研究

基于13原子二十面体结构,采用密度泛函方法系统计算研究了Fe、Co及Ni单质及二元混合团簇的磁性.发现有限温度下团簇磁性随结构畸变的敏感性随Fe、Co、Ni顺序逐渐减弱,同时发现二十面体结构Fe_(13)及Co_(13)均具有不同磁矩的近简并低能态.对FeNi及CoNi混合团簇、其磁矩随组分的变化不存在反常现象,但对于FeCo混合团簇、其磁矩随组分的演化行为存在个别反常现象.我们认为:这种反常现象能够对FeCo非晶合金中的实验观测结果提供一种可能的理论解释.     

密度泛函理论研究混合团簇Al12X(X=Sc、Ti、V、Gr、Mn、Fe、Co、Ni、Cu、Zn)的结构和磁性

以四种13原子高对称性(Ih、Oh、D5h、D3h)密堆积结构为初始构型,通过不等价位原子替换,利用密度泛函理论系统研究了Al12 X(X=Sc、Ti、V、Cr、Mn、Fe、Co、Ni、Cu、Zn)掺杂团簇的结构及磁性.结果表明:除Al12 Cr和Al12Fe以外,其它Al12X团簇的基态结构均以Ih替换结构为主,其中前3d元素(X=Sc、Ti、V、Mn)倾向于表面位置替换,而后3d元素(X=Co、Ni、Cu、Zn)则倾向于中心位置替换;Al12 Cr和Al12Fe团簇以Oh结构的表面替换为基态结构;对多数3d元素(X=Ti、V、Cr、Fe、Co、Zn)其掺杂团簇均出现明显的近能同分异构现象;相较纯Al13团簇掺杂团簇普遍体现出磁性增强效应.     

铝团簇Al13的结构、电子性质和氧原子吸附的第一原理计算

利用密度泛函理论对Al13团簇的几种典型异构体进行了计算.对几种不同对称性(Ih,D5h,D3h,Oh)的中性和带电团簇Al13、Al13-、Al13 进行了结构优化和总能计算,得到了各种带电状态下的最低能量结构,并计算了Al13团簇的电离势、电子亲和能和结合能.理论结果与实验值比较,比以前的理论计算符合的更好.从中性和负电状态下的正二十面体的最低能量结构出发,研究了氧原子在Al13和Al13-上的吸附行为,并与单个氧原子在铝fcc表面的吸附行为做了比较.计算发现,高稳定性的幻数团簇如Al13-,较中性团簇和铝表面,更不易于氧化.     

笼状Au20内掺M13(M=Fe,Ti)团簇磁性的密度泛函计算研究

采用密度泛函理论中的广义梯度近似方法,对M13(M =Fe,Ti)以及M13内掺Au20团簇的几何结构和磁性进行了计算研究.结果表明:M13和M13内掺Au20团簇的几何结构在0.006-0.05nm误差范围内保持着Ih对称性.Fe13团簇最低能态的总磁矩为44μB,内掺到Au20笼中后形成的Fe13内掺Au20团簇的最低能态总磁矩为38μB,且Au原子与内掺Fe13团簇之间存在着弱铁磁相互作用.Ti13团簇在总磁矩为6μB时能量最低,掺入Au20笼后形成的Ti13内掺Au20团簇最低能态总磁矩是4μB,内表面12个Ti原子与表面Au壳之间是弱铁磁相互作用,而与中心Ti原子之间是弱反铁磁相互作用.由于Au20笼状外壳的影响,Fe13内掺Au20和Ti13内掺Au20团簇中Fe13和Ti13的磁矩比无金壳的Fe13和Ti13团簇的磁矩分别减少了6.81μB和2.88μB.     

中性及带电混合团簇Al12X(X=Li、Na、K、Rb、Cs结构和磁性的密度泛函研究

以四种13原子高对称性(Ih、Oh、D5h、D3h)密堆积结构为初始构型,通过不等价位原子替换,利用密度泛函理论系统研究了中性及带电Al12X(X=Li、Na、K、Rb、Cs)混合团簇的结构及磁性.结果表明:带电影响团簇稳定性序列且带正电影响显著强于负电情形;相较负电,对中性Al12X及正电Al12X+团簇基态均出现明显的近能同分异构现象;相较中性团簇,带电各混合团簇化学活性普遍降低;全部混合团簇基态均体现出磁矩最小化效应.     

Cross sections of 43Sc, 44Sc, 46Sc isotopes formed in the 45Sc + 3He reaction

Reactions ⁴⁵Sc(³He, αn)⁴³Sc, ⁴⁵Sc(³He, α)⁴⁴Sc and ⁴⁵Sc(³He, 2p)⁴⁶Sc in the ³He energy range of 5 to 24 MeV are investigated in experiments performed with a ³He ion beam during the irradiation of U-120M cyclotron scandium targets. Activation is used to determine the yield of nascent Sc isotopes. The γ activity induced in targets is measured using a high-resolution HPGe detector. The character of the excitation function changes during the formation of these ions and differs from the excitation functions for deuterons, despite the low bond energy of ³He and the positive values of the Q reactions leading to the formation of ⁴⁴Sc and ⁴⁶Sc isotopes. The cross sections of ⁴⁴Sc formation reach their maximum value at the Coulomb barrier of the reaction, due to the stable ⁴He nucleus that accompanies the formation of ⁴⁴Sc. The contribution from different reaction mechanisms to the cross sections of ⁴³Sc, ⁴⁴Sc, and ⁴⁶Sc isotope formation are considered.     

Cross sections for production of 43Sc, 44Sc, and 46Sc isotopes in the 45Sc + 3He reaction

Reactions ⁴⁵Sc(³He, αn)⁴³Sc, ⁴⁵Sc(³He, α)⁴⁴Sc, and ⁴⁵Sc(³He, 2p)⁴⁶Sc, resulting from the irradiation of scandium targets with a beam of ³He ions with energy from 5 to 24 MeV, are investigated in experiments on the U120M cyclotron of the Nuclear Physics Institute (Rez, Czech Republic). The activation technique is used to find the yield of the produced Sc isotopes. The induced γ activity in the targets is measured using a high-resolution HPGe detector. Despite the low binding energy of ³He and positive reaction Q values, which leads to formation of the ⁴⁴Sc and ⁴⁶Sc isotopes, the behavior of the excitation functions for the formation of these isotopes differs from the behavior of the excitation function for deuterons. Scandium-44 formation cross sections reach their maximum at the reaction Coulomb barrier. This is because not only ⁴⁴Sc but also a stable ⁴He nucleus is formed in the reaction.     

Excitation functions for 44Sc, 46Sc, and 47Sc radionuclides produced in the interaction of 45Sc with deuterons and 6He

Experimental excitation functions are presented for ⁴⁵Sc(d, p)⁴⁶Sc, ⁴⁵Sc(d, t)⁴⁴Sc, ⁴⁵Sc(⁶He, ⁵He*)⁴⁶Sc and ⁴⁵Sc(6He, α)⁴⁷Sc reactions at projectile energies near the Coulomb barrier. The obtained excitation functions for reactions ⁴⁵Sc(d, p)⁴⁶Sc and ⁴⁵Sc(⁶He, ⁵He*)⁴⁶Sc have similar behavior and have a maxima near the Coulomb barriers of these reactions. The compilation of the available experimental data, obtained at deuteron- and ⁶He-energies near the Coulomb barrier, showed that the values of the cross sections at the maxima of the excitation functions obtained in (d, p) reactions and the reactions for one-neutron pickup from the ⁶He projectiles have a different Z-dependence.     

Cross sections and thermonuclear reaction rates for 42Ca(p, γ)43Sc, 44Ca(p, γ) 45Sc, 44Ca(p,n) 44Sc and 45Sc(p,n)45Ti

The yield of γ-rays from the reaction ⁴²Ca(p, γ)⁴³Sc has been measured as a function of bombarding energy over the range 0.63–3.01 MeV, from ⁴⁴Ca(p, γ)⁴⁵Sc over the range 0.775–4.00 MeV, from ⁴²Ca(p, p'γ)⁴²Ca over the range 2.24–3.01 MeV, and from ⁴⁴Ca(p, p'γ)⁴⁴Ca over the range 1.90–5.03 MeV. The cross section of the reaction ⁴⁴Ca(p, n)⁴⁴Sc has been measured from threshold to a bombarding energy of 5.05 MeV by observation of the 1157 keV γ-ray associated with the residual ⁴⁴Sc activity, and the cross section of the reaction ⁴⁵Sc(p, n)⁴⁵Ti has been measured from threshold to a bombarding energy of 4.00 MeV both by observation of the annihilation radiation associated with the residual ⁴⁵Ti activity and by measurement of the total neutron yield with a wide-angle BF₃ tube and paraffin detector. All these data are compared with statisticalmodel calculations and satisfactory agreement is achieved. Thermonuclear reaction rates for the (p, γ) and (p, n) reactions are calculated for the temperature range 5 × 10⁸-10¹⁰K and the significance of these results for explosive nucleosynthesis in stars is discussed.     

Optical and piezoelectric anomalies of ordered (Sc,Ga) N and (Sc,In) N ternaries

Theoretical results are presented regarding the incorporation of Scandium into wurtzite GaN and InN binaries. The electric, optical and piezoelectric properties of the resulting ScGaN and ScInN systems are reported by using first-principles Local-density approximation (LDA) within density functional theory (DFT), Berry phase approach within modern theory of polarization and phonon calculations within the density functional perturbation theory. Our results predict the existence of breaking-symmetry structural phase transition in ordered Sc_0.5Ga_0.5N and Sc_0.5In_0.5N alloys when subjected to a compressive or tensile strain. Moreover, our results demonstrate the existence of symmetry preserving pressure-induced isostructural phase transitions in ordered ScGaN and ScInN systems for different Sc concentrations. It has been shown that the existence of isostructural phase transitions leads to dramatic changes in optical, acoustic, and piezoelectric properties of ordered ScGaN and ScInN systems under high pressure. In particular, this study demonstrates that the existence of first-order isostructural phase transitions in Sc₁Ga₁N₂ at a critical hydrostatic pressure of 12.3 GPa leads to a huge enhancement of piezoelectricity (i.e., the e ₃₃ piezoelectric coefficient adopts a huge value as large as 13 C/m²). In addition, It has been shown that ordered Sc_0.5Ga_0.5N and Sc_0.5In_0.5N alloys exhibit tremendous piezoelectric response, associated with a breaking-symmetry phase transition from nonpolar P6₃/mcc(D_6h) space group to a polar P6₃ mc(C_6v) structure, at fixed Ga, In and Sc compositions, as a function of the in-plane compressive and tensile strains. We also reveal the reason behind, and consequences of, these unusual properties associated with the strain-induced and pressure-induced structural phase transitions in the novel ScGaN and ScInN ordered structures.     

Energies,Wavelengths, and Transition Probabilities in Sc XIII

Physics of Atomic Nuclei - The fine-structure energy levels and lifetimes are calculated for the lowest 218 levels of the 2s22p5, 2s2p6, 2s22p43s, 2s22p43p, 2s22p43d, 2s2p53s, 2s2p53p, 2s2p53d,...