• Proceedings of the Korea Crystallographic Association Conference
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• 2003.05a
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• pp.8-8
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• 2003

Electron-phonon interaction plays a significant role in forming of colossal magnetoresistance effect (CMR). Polaron formation was observed by neutron diffraction and by extended X-ray absorption fine structure (EXAFS) analysis. Local probe as given by the EXAFS is a useful method to study the polaronic charge and its dependence on temperature and ions size. Here we present the EXAFS study of polaronic charge in La/sub 0.7/Ca/sub 0.3-X/Ba/sub X/MnO₃ compositions. The single phase La/sub 0.7/Ca/sub 0.3-X/Ba/sub X/MnO₃ manganites (x=0; 0.03; 0.06, ..., 0.3) were prepared by ceramic technology [1]. The Curie temperature was determined by extrapolation of the temperature dependence of the magnetization (down to zero magnetization). EXAFS experiments were carried out at the 7C EC beam line of the Pohang Light Source (PLS) in Korea. The atomic pair distribution functions (PDF) were obtained by re-regularization method [2] from filtered spectra. The PDF for the x=0.3 sample showed a single peak function and for x=0.0, 0.03, 0.06, 0.09, 0.12 compositions were asymmetric in agreement with a small Jahn-Teller elongation of two (short and long) bonds of the MnO/sub 6/ octahedron. Dispersion, σ/sub Min-O//sup 2/, and asymmetry, σ/sub Min-O//sup 3/, of the Mn-O bond distances varied significantly with x and showed a maximums at x=0.09. The maximum of σ/sub Min-O//sup 2/ is caused by increase of dynamic rms displacements of the Mn-O distances near the T/sub C/. The observed x dependence of σ/sub Min-O//sup 3/ reflects the reduction of charge carriers mobility at approaching to T/sub C/ from low as well as high temperatures.

• Journal of the Korean Ceramic Society
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• v.38 no.5
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• pp.424-430
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• 2001

Cut-off 전압 변화에 따른 충방전 특성을 알아보기 위하여 Mn을 다른 전이 금속이 Co와 Ni로 소량 치환시킨 Li(M $n_{1-{\delta}}$ $n_{\delta}$)$_2$ $O_4$(M=Ni, Co, $\delta$=0, 0.05, 0.1, 0.2)를 고상 반응법으로 80$0^{\circ}C$에서 48시간 동안 유지하여 합성하였다. 충방전의 cut-off 전압은 2.5~4.4V, 3.0~4.5V, 3.5~4.5V, 3.5V~4.7V의 네 가지 전압범위고 하였다. 충방전 실험결과, Li(M $n_{1-{\delta}}$ $n_{\delta}$)$_2$ $O_4$의 용량은 각각 Co와 Ni의 $\delta$=0.1에서 최대를 보였다. Co 치환 조성 재료와 순물질 모두에서 최대의 용량을 보인 cut-off 전압대는 3.5~4.5V 이었는데 이때의 Li(M $n_{0.9}$ $Co_{0.1}$)$_2$ $O_4$와 LiM $n_2$ $O_4$의 초기 충전용량과 초기 방전용량은 각각 118, 119mAh/g과 114, 104mAh/g 이었다. 또한 모든 cut-off 전압대에서 Li(M $n_{0.9}$ $Co_{0.1}$)$_2$ $O_4$는 순수한 LiM $n_2$ $O_4$보다 더 높은 용량과 우수한 싸이클 성능을 보였으며 그 결과는 밀착형 전지구성에서도 일치하였다.하였다.

• Bulletin of the Korean Chemical Society
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• v.10 no.6
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• pp.504-509
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• 1989

The effect of oxygen-deficiency on the charge distributions and orbital energies for small copper oxide clusters representing the superconducting materials $YBa_2Cu_3O_x (6{\leq}x{\leq}7)$ were investigated by the extended Huckel molecular orbital (EHMO) method with the tight-binding model. Our calculations show +3 oxidation state of Cu(1) in the $CuO_3$ chain and +2 or +1 of Cu(2) in the $CuO_2$ layers for $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 (or +5), while for $YBa_2Cu_3O_6$ +1 oxidation state of Cu(1) and +3 (or +2) of Cu(2) in the $CuO_2$ layers with the nominal charge of $Cu_3$ = +7 (or +5). For $Cu_3O_{12}$ cluster representing $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 the Cu(2) $d_{{x^2}-{y^2}}$ orbitals in the $CuO_2$ layers is a typical Jahn-Teller $d^9$ system with the partial hole and the Cu(1) $d_{{_z2}-{_y2}}$ orbital in the $CuO_3$ chain contains hole occupancy. For $Cu_3O_{10}$ cluster representing $YBa_2Cu_3O_6$ with the nominal charge of Cu = +5 the orbital character of the highest partially occupied MO (HPOMO) and the lowest completely unoccupied MO (LCUMO) of $Cu_3O_{12}$ representing $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 is reversed, and the character of Cu(1) $d{{x^2}-{y^2}}$ orbital of LCUMO of the $Cu_3O_{12} $cluster is vanished. It is suggested that the local crystal field environment of Cu(1) by the oxygens in the Cu(1) chain may play a vital role in conductivity and superconductivity, either alone or through cooperative electronic coupling with the Cu(2) layers in $YBa_2Cu_3O_7.$.