静电场作用下KDP和TGS单晶中子衍射增强现象的物理机理

本文提出了KDP和TGS单晶在静电场作用下中子衍射增强现象的物理机理。KDP和TGS都是质子导电晶体。外加直流电压后,晶体中电场有一空间分布。由于压电效应,相关的衍射晶面间距是空间坐标的函数,造成中子衍射增强。对衍射晶面间距为空间坐标函数的情况,导出了透射方式和反射方式的中子衍射强度公式,估计了非均匀的压电效应引起的中子衍射增强的量级,和实验相符。并对进一步验证上述机理提出了新的实验建议。顺便指出,对于α-LiIO₃,在静电场作用下其中子衍射增强,非均匀压电效应不是主要因素。并提出其可能的微观机制是:在非均匀电场作用下,缺陷(包括杂质)在晶体中有一空间分布,引起中子衍射增强。关键词：

MECHANISM OF THE ENHANCEMENT OF NEUTRON DIFFRACTION FROM KDP AND TGS CRYSTALS UNDER THE ACTION OF AN ELECTROSTATIC FIELD

In this article, the mechanism of the enhancement of neutron diffraction from KDP and TGS crystals under the action of an electrostatic field is proposed. KDP and TGS are both proton conducting crystals. In the presence of an applied static voltage, an electric field is set up within the crystal with a certain spatial distribution. Because of the piezoelectric effect, the spacing of the reflecting crystallographic planes depends upon the spatial coordinates, thus the restriction for the angular range of neutron diffraction should be relaxed, and enhancement of neutron diffraction results. Treating the spacings of the crystallographie planes as a function of spatial coordinates, we have derived expressions for the neutron diffraction intensity in both the laue case and the Bragg case, and estimated the order of magnitude of enhancement of the neutron diffraction resulted from inhomogeneous piezoelectric effects. Good agreement with experiment is found. New experiments for further verification of this mechanism are suggested. By the way, we remark that the above mentioned mechanism is not the dominant one responsible for neutron diffraction enhancement observed in a-LiIO₃ under the action of an electrostatic field. A possible micro-mechanism for a-LiIO₃ may be that under the action of an inhomogeneous field, the defects assume a spatial distribution which results in inhomogeneity of spacing between the crystallographic planes and hence enhancement of neutron diffraction.