Several The Abo <sub> 3 </ Sub> Structure Of Lanthanum Manganese Oxide Combination Of Energy And Materials In The Ion Valence

The manganites R_1-xM_xMnO₃ (R= La, Nd, Pr; M=Ca, Sr, Ba, etc.) have attracted much attention because of their characteristic magnetic and transport properties such as colossal magnetoresistance (CMR). These manganites possess an ABO₃ type perovskite crystal structure. In general, R and M are located at the A-site, and Mn at the B-site. Up to now, a lot of theoretical and experimental results have been obtained. However, in this field, some of basic physical problems can not be explained yet. Furthermore, ABO₃ perovskite material is one of the simplest multi-atomic compounds, a further research on the perovskite material data obtained in the past years may result in a new explanation about relative phenomena in multi-atomic compound. In this paper, two physical phenomena have been explained as follows:Firstly, how to estimate the number ratio between different valence cations in ABO₃ perovskite manganites? La and Ca are trivalent and divalent respectively in the La_1-xCa_xMnO₃ system. The system therefore can be written as La³⁺_1-xCa²⁺_xMn³⁺_1-xMn⁴⁺_xO₃. The valence of the cation is related to its ionization energy, satisfying the condition of valence balance. However, the fourth ionization energies of La and Mn are 49.95eV and 51.20eV, respectively, while the third ionization energy of Ca is 50.91eV. It is therefore puzzling why there are Mn⁴⁺ ions, but no La⁴⁺ or Ca³⁺ ions in this system. Until now, no satisfactory explanation about this phenomenon has been given. We hypothesize that the main reason for the absence of La⁴⁺ and Ca³⁺ ions is that the valence of the cation is related not only to the ionization energy of the cation, but also to the distance between the cation-anion pair.We suppose that there is a potential barrier between cation and anion. The height of the potential barrier is proportional to the ionization energy of the cation, and the width is related to the distance between neighboring cations and anions. The number ratio of the different valence cations is therefore related to the probability of their last ionized electrons transmiting through the potential barrier. Based on this model, some experimental results regarding ABO₃ mixed-valence lanthanum manganites are explained satisfactorily.Secondly, the influence of cohesive energy on unit cell volume of perovskite manganites. In general, ionic size is considered to be an important factor affecting the unit cell volume of manganites with an ABO₃ structure. For La_1-xSr_xMnO₃, however, the unit cell volume decreases with increasing x when x≤0.5 although the average effective ion radius of all cations taken together increases. In the case of La_1-xNa_xMnO₃, the unit cell volume reaches a minimum when x=0.3. Up to now, no satisfactory explanation about these phenomena has been found.In order to explain the above experimental phenomena, we estimated the cohesive energy of perovskite manganites La_1-xNa_xMnO₃ and La_1-xSr_xMnO₃. The total cohesive energy consists of two parts:the ionic cohesive energy and the metallic energy resulting from double exchange electrons. The magnitude of the additional metallic cohesive energy is about 6% of the Coulomb energy. The calculated denpendence of the total cohesive energy on the Mn⁴⁺ ion content in the samples, is consistent with the dependence of the equivalent cubic cell parameter on the Mn⁴⁺ ion content obtained from experiment results. Therefore the dependence of the crystal cell volume on the doping level in both of the perovskite manganites La_1-xNa_xMnO₃ and La_1-xSr_xMnO₃ is explained satisfactorily.