TEMPERATURE-DEPENDENT LIGHT-SCATTERING STUDIES OF VERWEY TRANSITION AND ELECTRONIC DISORDER IN MAGNETITE
TEMPERATURE-DEPENDENT LIGHT-SCATTERING STUDIES OF VERWEY TRANSITION AND ELECTRONIC DISORDER IN MAGNETITE
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DOI：
10.1103/physrevb.9.5236
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发表时间：
1974-01-01
影响因子：
3.7
通讯作者：
VERBLE, JL
中科院分区：
文献类型：
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作者：
VERBLE, JL
本文研究了磁铁矿（Fe 3 O 4）的光散射谱随温度的变化，试图确定光学声子在Verwey跃迁中所起的作用。长波声子的群论分析被用来分类在立方相和正交相的正常模式。此外，磁铁矿的分子模型进行了描述，并作为基础的推导的立方结构中的所有42个正常模式的对称坐标。实验上，在立方相（T> 119 K）中观察到五个拉曼模，这与群论的预测一致;同样，在正交相（T< 119 K）中观察到五个模，尽管在该相中预期有15个拉曼激活模。在通过转变温度时，在光散射光谱的一般特征或声子本身中没有观察到突然或剧烈的变化。我们解释这种缺乏一个突然的变化的证据表明，拉曼活性声子在负责过渡的物理机制中发挥次要作用。此外，这些结果似乎与Verwey模型和最近提出的各种带模型的预测不一致。拉曼光谱的一个特别不寻常的特征是，即使在低温下，拉曼谱线也非常宽（约30 cm-1）。对于最高频率模式（约680 cm− 1），线宽随着温度升高并超过转变温度而连续增加。我们解释在低温下的加宽是由于电子无序与随机排列的Fe 2+和Fe 3+离子的B网站。这与Verwey模型的基本假设形成鲜明对比。另一方面，它是建议的线宽的温度依赖的部分有关的磁铁矿中的导电机制，我们假设这是由于小极化子跳跃。论点是支持这一假设。
The light-scattering spectra of magnetite (Fe 3 O 4) have been studied as a function of temperature in an attempt to determine the role played by optical phonons in the Verwey transition. A group-theoretical analysis of the long-wavelength phonons is used to classify the normal modes in both the cubic and orthorhombic phases. In addition, a molecular model of magnetite is described and is used as a basis for the derivation of the symmetry coordinates of all 42 normal modes in the cubic structure. Experimentally, five Raman modes are observed in the cubic phase (T> 119 K) in agreement with the predictions of group theory; likewise, five modes are observed in the orthorhombic phase (T< 119 K) even though 15 Raman-active modes are expected in this phase. On passing through the transition temperature, no abrupt or dramatic changes are observed either in the general features of the light-scattering spectrum or in the phonons themselves. We interpret this lack of a sudden change as evidence that the Raman-active phonons play a minor role in the physical mechanism responsible for the transition. Furthermore, these results appear to be inconsistent with the predictions of both the Verwey model and various band models that have been recently proposed. A particularly unusual feature of the Raman spectrum is that the Raman lines are anomalously broad (≈ 30 cm− 1) even at low temperatures. For the highest-frequency mode (≈ 680 cm− 1), the linewidth increases continuously as the temperature is raised through and above the transition temperature. We interpret the broadening at low temperatures as being due to electronic disorder associated with the random arrangement of Fe 2+ and Fe 3+ ions on the B sites. This is in distinct contrast to the basic assumptions of the Verwey model. On the other hand, it is suggested that the temperature-dependent part of the linewidth is related to the conduction mechanism in magnetite, which we assume to be due to small-polaron hopping. Arguments are presented in support of this assumption.