• Title/Summary/Keyword: Jahn-Teller

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Bonding and Electronic Considerations in the Apex Bridged $M_2(OR)_6L_x(\mu-X)$ Complexes (M = Mo ; R = -t-Bu, -i-Pr ; L = Py ; x = 0 or 2 ; X = CO, SO$_2$, HCCH, H, and Cl)

  • Ko, Jae-Jung
    • Bulletin of the Korean Chemical Society
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    • v.9 no.3
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    • pp.121-126
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    • 1988
  • The molecular interaction of $Mo_2(OR)_4({\mu}-OR)_2$$ unit has been studied for carbonyl and acetylene ligands by means of extended Huckel calculations. We have extended the extended Hckel calculations to unknown apex bridged $Mo_2(OR)_6({\mu}-X)$ complexes (X = $SO_2$H, and Cl) in order to compare the stability of the complexes. In the $C_2H_2$and H apex bridged complexes, one finds a relatively small HOMO-LUMO gap. However, both complexes are stabilized due to Jahn-Teller distortion. The stability of both complexes is comparable to that of $Mo_2(OR)_6(CO)$ complex. The comparisons and predictions in the stability of known and unknown complexes are the subject of this study.

Structure and Bonding of Perovskites A($Cu_{1/3}Nb_{2/3}$)$O_3$ (A=Sr, Ba and Pb) and their Series of Mixed Perovskites

  • Park Hyu-Bum;Huh Hwang;Kim Si-Joong
    • Bulletin of the Korean Chemical Society
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    • v.13 no.2
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    • pp.122-127
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    • 1992
  • Some perovskites $A($Cu_{1}3}Nb_{2}3}$)O_3(A=$Sr^{2+}$$, $Ba^{2+}$ and $Pb^{2+}$) and their series of mixed perovskites have been prepared by solid state reaction. Single perovskite phase was obtained in Sr or Ba rich samples, but pyrochlore phase was found in Pb rich samples. The stability of perovskite phase is dependent on the ionicity of bonding as well as the tolerance factor. All the obtained perovskites have tetragonal symmetry distorted by Jahn-Teller effect of $Cu^{2+}$. In the case of $Sr(Cu_{1}3}Nb_{2}3})O_3$, some superlattice lines caused by threefold enlarging of fundamental unit cell were observed. And, the symmetry of B site octahedron and the bonding character of B-O bond have been studied by IR, ESR and diffuse reflection spectroscopy. It appeared that the symmetry and the bonding character are influenced by such factors as the size and the basicity of A cation.

Lattice Deformation and Electronic Structure of the $C_{60}{^+}$ Cation

  • 이기학;이한명;전희자;박성수;이왕로;Park, T. Y.;Xin Sun
    • Bulletin of the Korean Chemical Society
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    • v.17 no.5
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    • pp.452-457
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    • 1996
  • The effects caused by the ionization on the electronic structure and geometry on C60 are studied by the modified Su-Schriffer-Heeger (SSH) model Hamiltonian. After the ionization of C60, the bond structure of the singly charged C60 cation is deformed from Ih symmetry of the neutral C60 to D5d, C1, and C2, which is dependent upon the change of the electron-phonon coupling strength. The electronic structure of the C60+ cation ground state undergoes Jahn-Teller distortion in the weak electron-phonon coupling region, while self-localized states occur in the intermediate electron-phonon region, but delocalized electronic states appear again in the strong electron-phonon region. In the realistic strength of the electron-phonon coupling in C60, the bond structure of C60+ shows the layer structure of the bond distortion and a polaron-like state is formed.

A Molecular Orbital Study of the Electronic Structure and the Ring Inversion Process in$Cp_2TiS_3$ Complex

  • Sung Kwon Kang;Byeong Gak Ahn
    • Bulletin of the Korean Chemical Society
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    • v.15 no.8
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    • pp.658-662
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    • 1994
  • Ab initio and extended Huckel calculations have been applied to discuss the electronic structure, ring inversion barrier, and geometry of the $Cp_2TiS_3$ compound. The deformation of four membered ring in the planar geometry is originated from a second-order Jahn-Teller distortion due to the small energy gap between HOMO and LUMO on the basis of extended Huckel calculations. The puckered $C_s$ geometry is stabilized by the interaction of the $x^2-y^2$ metal orbital with the hybrid orbital in sulfur. Ab initio calculations have been carried out to explore the ring inversion process for the model $Cl_2TiS_3$ compound. We have optimized $C_s$ and $C_{2v}$ structures of the model compound at the RHF level. The energy barriers for the ring inversion are sensitive to the used basis set. With 4-31$G^*$ for the Cl and S ligands, the barriers are computed to be 8.41 kcal/mol at MP2 and 8.02 kcal/mol at MP4 level.

The Effect of Reaction Temperature for Synthesis of LiMn2O4 by Calcination Process and the Electrochemical Characteristics (소성법에 의한 LiMn2O4의 제조시 반응 온도의 영향과 전기화학적 특성)

  • Lee, Chul-Tae;Lee, Jin-Sik;Kim, Hyun-Joong
    • Applied Chemistry for Engineering
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    • v.9 no.2
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    • pp.220-225
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    • 1998
  • The spinel structured $LiMn_2O_4$ was prepared from $Li_2CO_3$ and $MnO_2$ by calcination at various temperatures in the range of $750{\sim}900^{\circ}C$. It was found that the most suitable cubic structure of $LiMn_2O_4$ was obtained by heating at $850^{\circ}C$ for 12 hrs. However, in the calcination at $900^{\circ}C$, $Mn^{4+}$ of 0.06M was changed to $Mn^{+3}$ by the oxygen loss, so that it has been shown that the formula has changed to $LiMn_2O_{3.97}$. This phenomena were in agreement with the Jahn-Teller distortion by the increment of $Mn^{+3}$ ion on the octahedral sites of the spinel structured $LiMn_2O_4$. The results showed that after 15 charge/discharge cycles in the voltage range from 3.5V to 4.3V versus Li/$Li^+$ with a current density of $0.25mA/cm^2$, the spinel structured $LiMn_2O_4$ that was prepared at $900^{\circ}C$ showed a lower discharge capacity, 82~50 mAh/g, while the $LiMn_2O_4$, prepared at $850^{\circ}C$, showed the discharge capacity of 102~64 mAh/g.

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The Synthesis of LiMn$_2$O$_4$by sol-gel method and properties as electrode materials for lithium secondary battery (Sol-Gel 법에 의한 LiMn$_2$O$_4$의 합성 및 리튬이차전지용 전극물질로의 특성)

  • 이진식;박용성;우제완
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.10 no.3
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    • pp.219-225
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    • 2000
  • The spinel structured $LiMn_2O_4$was obtained by two consecutive heat treatment on xerogel; the first heat treatment was at $150^{\circ}C$ and the second at $350^{\circ}C$ was obtained by sol-gel process using an aqueous solution of lithium hydroxide and manganese acetate. The synthesized $LiMn_2O_4$ by the sol-gel process showed a discharge capacity of 88~56 mAh/g after 15 cycles in Li/lM $LiClO_4$(in PC)/$LiMn_2O_4$at a current density of 0.25 mA/$\textrm{cm}^2$ and the voltage ranged 3.5 V to 4.3 V. For the second heat treatment above $350^{\circ}C$, $Mn_2O_3$was formed as a by-product during the synthesis of $LiMn_2O_4$. The heat treatment at $500^{\circ}C$, for example, showed a lower discharge capacity 81~47 mAh/g, after the 15 charge/discharge cycles. The lower capacity was due to the increment of $Mn^{3+}$ ion and this phenomenon was in agreement with the Jahn-Teller distortion.

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Analysis of Electrochemical Characteristics of the Rechargeable $LiMn_2O_4$ Thin Film Battery (재충전이 가능한 박막전자용 $LiMn_2O_4$ 박막 전지의 전기화학 특성 분석)

  • Kim Joo-Seok;Jung Hunjoon;Kim Chan-Soo;Joo Seung-Ki
    • Journal of the Korean Electrochemical Society
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    • v.3 no.3
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    • pp.131-135
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    • 2000
  • In order to investigate the origin of capacity fading with charge/discharge cycling in $LiMn_2O_4$ thin film battery, impedance studies have been performed with increasing cycling in $LiMn_2O_4/1M\;LiClO_4-PC/Li$ cells. The fitted values obtained from impedance data show good agreements with the experimental results. Especially, the element of charge transfer resistance of $LiMn_2O_4/liquid$ electrolyte interface initially increased, and then saturated with increasing the charge/discharge cycles, which could explain the cause of initial abrupt capacity fading of $LiMn_2O_4$ thin film with cycling due to interfacial reaction. The steady capacity fading is caused by the increasing of Warburg resistance. The chemical diffusion coefficient of Li ions decreased from $5.15\times10^{-11}cm^2/sec$ at 1st cycles to $6.3\times10^{-12}cm^2/sec$ at 800th cycles, which attributed to the Jahn-Teller distortion/Mn dissolution which diminishes tetra hedral sites necessary for Li diffusion in $LiMn_2O_4$.

Mössbauer Study on the Variation in Magnetic Properties of CuO Induced by 57Fe Addition (57Fe 이온이 CuO에 미치는 효과에 관한 Mössbauer 분광 연구)

  • Park, Jae-Yun;Kim, Kwang-Joo
    • Journal of the Korean Magnetics Society
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    • v.19 no.3
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    • pp.113-119
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    • 2009
  • $^{57}Fe_xCu_{1-x}O$(x = 0.0, 0.02) powders were prepared by sol-gel method and their crystallographic and magnetic hyperfine properties have been studied using X-ray diffraction and $M{\ddot{o}}ssbauer$ spectroscopy (MS). The crystal structure of the samples is found to be monoclinic without any secondary phases and their lattice parameters increase with increasing annealing temperature ($T_A$), which is attributed to an increase in oxygen-vacancy content. MS measurements at room temperature indicate that $Fe^{3+}$ ions substitute $Cu^{2+}$ sites and ferromagnetic phase grow with increasing $T_A$. Magnetic hyperfine and quadrupole interactions of $^{57}Fe_{0.02}Cu_{0.98}O$ ($T_A=500^{\circ}C$) in the antiferromagnetic state at 17 K have been studied, yielding the following results: $H_{hf}=426.94\;kOe$, ${\Delta}E_Q=-3.67\;mm/s$, I.S.=0.32 mm/s, ${\theta}=65^{\circ}$, ${\phi}=0^{\circ}$, and ${\eta}=0.6$.

Electrical Characteristics of Cathode Li($Mn_{1-\delta}$$M_{\delta}$)$_2$$O_4$ Substituted by Transition Metals in Li-Ion Secondary Batteries (전이금속 치환 리튬이온 이차전지 정극 Li($Mn_{1-\delta}$$M_{\delta}$)$_2$$O_4$의 전기적 특성)

  • 박재홍;김정식;유광수
    • Journal of the Korean Ceramic Society
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    • v.37 no.5
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    • pp.466-472
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    • 2000
  • As cathode materials of LiMn2O4-based lithium-ion secondary batteries, Li(Mn1-$\delta$M$\delta$)2O4 (M=Ni and Co, $\delta$=0, 0.05, 0.1 and 0.2) materials which Co and Ni are substituted for Mn, were syntehsized by the solid state reaction at 80$0^{\circ}C$ for 48 hours. No second phases were formed in Li(Mn1-$\delta$M$\delta$)2O4 system with substitution of Co. However, substitution of Ni caued to form a second phase of NiO when its composition exceeded over 0.2 of $\delta$ in Li(Mn1-$\delta$M$\delta$)2O4. As the results of charging-discharging test, the maximum capacity of Li(Mn1-$\delta$M$\delta$)2O4 appeared in $\delta$=0.1 for both Co and Ni. Also, Li(Mn1-$\delta$M$\delta$)2O4 electrode showed higher capacity and better cycle performance than LiMn2O4.

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Enhanced Electrochemical Properties of Surface Modified LiMn2O4 by Li-Fe Composites for Rechargeable Lithium Ion Batteries

  • Shi, Jin-Yi;Yi, Cheol-Woo;Liang, Lianhua;Kim, Keon
    • Bulletin of the Korean Chemical Society
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    • v.31 no.2
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    • pp.309-314
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    • 2010
  • The surface modified $LiMn_2O_4$ materials with Li-Fe composites were prepared by a sol-gel method to improve the electrochemical performance of $LiMn_2O_4$ and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and transmission electron microscopy (TEM)-EDS. XRD results indicate that all the samples (modified and pristine samples) have cubic spinel structures, and XRD, XPS, and TEM-EDS data reveal the formation of $Li(Li_xFe_xMn_{2-2x})O_4$ solid solution on the surface of particles. For the electrochemical properties, the modified material demonstrated dramatically enhanced reversibility and stability even at elevated temperature. These improvements are attributed to the formation of the solid solution, and thus-formed solid solution phase on the surface of $LiMn_2O_4$ particle reduces the dissolution of Mn ion and suppresses the Jahn-Teller effect.