PrMnO $_3$和CaMnO $_3$中声子模的红外研究
Infrared study of the phonon modes in PrMnO$_3$ and CaMnO$_3$
摘要：
研究了正交锰钙钛矿PrMnO $_3$和CaMnO $_3$在300 ~ 4 K温度范围内光学声子模频率范围内的红外反射谱。用基于Lorentz模型的拟合方法分析了这两种材料的红外声子谱，并与晶格动力学计算结果进行了比较，确定了$Pnma$结构的振动模式。计算已进行的框架内的壳模型，使用短程Born-Mayer-Buckingham和长程库仑势，其参数已被优化，以便计算的拉曼和红外活性声子频率，晶格参数与它们的实验值相匹配。我们发现PrMnO $_3$和CaMnO $_3$的红外声子频率值之间存在密切的对应关系，这表明Ca $^{2+}$取代Pr $^{3+}$离子导致中、高能红外声子频率降低，而低能红外声子频率增加.然而，实验获得的两种材料的红外声子振幅似乎是无关的。这些模式的振动模式的比较研究表明，它们中的大多数对应于复杂的原子振动显着不同的PrMnO $_3$的CaMnO $_3$，不能只分配到一个给定类型的振动（外部，弯曲，或拉伸模式）。特别地，这些结果证实了CaMnO $_3$的结构与理想的（立方）钙钛矿结构相当远。
英文摘要：
The infrared (IR) reflectivity spectra of orthorhombic manganese perovskites PrMnO$_3$ and CaMnO$_3$ are studied in the frequency range of optical phonon modes at temperatures varying from 300 to 4 K. The IR phonon spectra of these two materials are analyzed by a fitting procedure based on a Lorentz model, and assigned to definite vibrational modes of $Pnma$ structures by comparison with the results of lattice dynamical calculations. The calculations have been performed in the framework of a shell model using short range Born-Mayer-Buckingham and long range Coulomb potentials, whose parameters have been optimized in order that the calculated Raman and IR active phonon frequencies, and lattice parameters match with their experimental values. We find a close correspondence between the values of the IR phonon frequencies of PrMnO$_3$ and CaMnO$_3$, which shows that the substitution of the Pr$^{3+}$ ions with Ca$^{2+}$ results in a reduction of the frequency of medium- and high-energy IR phonons, and an increase of the frequency of those of low-energy. Nevertheless, the experimentally obtained IR phonon amplitudes of the two materials appear to be unrelated. A comparative study of the vibrational patterns of these modes reveals that most of them correspond to complex atomic vibrations significantly different from PrMnO$_3$ to CaMnO$_3$ which cannot be assigned only to a given type of vibration (external, bending, or stretching modes). In particular, these results confirm that the structure of CaMnO$_3$ is quite far from the ideal (cubic) perovskite structure.
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