Impurity Effect on a Structural Phase Transition Intermediate between the Order-Disorder and Displacive Types.

1.为了从理论上阐明杂质对从位移型到有序-无序型结构相变的影响，设计了高精度的热容量热法。在本项目开始之前，已经完成了位移系统和有序-无序系统的实验。由于剩余的样品是商业上不可用的，所以选择适用于非常小的样品的交流量热法作为实验技术。研制了具有高热稳定性的斩波式交流热量计.用淬灭4，4”-二氟对三联苯（DFTP）和3，6-二苯基-1，2，4，5-四嗪（DPTZ）的溶液，制备了薄而均匀的固溶体样品，并用于交流量热法。随着DPTZ含量的增加，DFTP的热反常受到强烈的抑制，这表明对三联苯的中间相变比有序-无序相变对杂质更敏感，与联苯的位移相变相似。从理论上对杂质对结构相变的影响建立统一的认识是非常必要的.在一个简单的模型上进行了分子动力学模拟，显示了位移相变。部分证实了关于置换型体系中强杂质效应机理的讨论。也就是说，这种强效应与晶格振动的局部化有密切联系，即，软模式。

1. In order to clarify the impurity effect on structural phase transition ranging from displacive type to order-disorder type thermodynamically, heat capacity calorimetry of high precision was planned. The experiments on the displacive and order-disorder systems had been completed before this project started. Since the sample remained was commercially unavailable, the ac calorimetry applicable to very small samples was chosen as the experimental technic. The chopped-light ac calorimeter with high thermal stability was developed.2. By quenching liquid solution of 4,4"-difluoro-p-terphenyl (DFTP) and 3,6-diphenyl-1,2,4,5-tetrazine (DPTZ), thin uniform sample of the solid solution was prepared and used for the ac calorimetry. Strong suppression of thermal anomaly of DFTP upon increasing DPTZ contents showed an intermediate phase transition was much more sensitive to the impurity than order-disorder transition in p-terphenyl and similarity to the displacive transition in biphenyl. It is highly desired to establish a unified understanding the impurity effect on structural phase transitions theoretically.3. Molecular dynamics simulations were made on a simple model showing a displacive phase transition. A discussion concerning the mechanism of the strong impurity effect in displacive systems was confirmed in part. That is, the strong effect has a close connection to localization of lattice vibration, I.e., the soft-mode.