Crystal structure of HgTlBa 2 CuO x studied by high-resolution electron microscopy
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Crystal structure of HgTlBa2 CuOx studied by high-resolution electron microscopy Takeo Oku Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Satoru Nakajima Department of Chemistry, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan (Received 26 July 1997; accepted 15 August 1997)
The structure model for HgTlBa2 CuOx was proposed from high-resolution electron microscopy using residual indices. Averaged digital high-resolution image of the HgTlBa2 CuOx showed the existence of separated Hg layers and oxygen vacancies in the double (Hg, Tl) layers. Image calculations based on the proposed structure model of HgTlBa2 CuO5 agreed well with the observation, and showed low residual values. The present result indicates the stability of the (Hg, Tl) double layer structure would be due to the formation of oxygen vacancies in the Hg layers.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURES
Since the discovery of HgBa2 CuOx superconductor,1 various types of the Hg–Ba–Ca–Cu–O system have been reported.2–10 HgBa2 CuOx with Hg single layer structure was easily prepared as a single phase under the ordinary pressure.3,4 Hg2 Ba2 (Y, Ca)Cu2 Ox with double Hg layer structure was synthesized only by using a high-pressure, high-temperature technique.5,6 Recently, the double (Tl, Hg) layer structures of (Tl, Hg)2 Ba2 CuOx ,9 (Tl, Hg)2 Ba2 CaCu2 Ox , and (Tl, Hg)2 Ba2 Ca2 Cu3 Ox 7,8 were reported to have been synthesized by the substitution of Tl by Hg under ordinary pressure. In our previous work,9 Hg22z Tlz Ba2 CuOx was shown to be single phase for z > 1.0 although HgBa2 CuOx was formed for z , 1.0. This result indicates that the (Tl, Hg)2 Ba2 CuOx with double (Tl, Hg) layer would have a peculiar structure, which results in the stability of Hg22z Tlz Ba2 CuOx (z > 1.0) and instability of the Hg2 Ba2 CuOx structure with double Hg layer structure. The purpose of the present work is to obtain information on the origin of the structural stability of (Tl, Hg)2 Ba2 CuOx with double (Tl, Hg) layer. HgTlBa2 CuOx was selected because of the boundary phase between Hg22z Tlz Ba2 CuOx and HgBa2 CuOx phases.9 To determine the crystal structure of HgTlBa2 CuOx , highresolution electron microscopy was carried out combined with imagePcalculation. In addition, a residual index P j Iobs 2 Ical j y Iobs ) was used for image (RHREM analysis11,12 because of the simple form and usefulness for determination of the defocus value and crystal thickness. The crystal structure of high-Tc superconductor of Tl2 Ba2 CuO6 was investigated by these R HREM values, and the Tl occupancy was determined from observed images.11 The present results will show the origin of structural stability of Hg22z Tlz Ba2 CuOx .
HgTlBa2 CuOx samples were prepared by a solidstate reaction using a sealed quartz tube technique. The starting material is a mixture of HgO (yellow), Tl2 O3 , BaO2 , and CuO with a metal ratio Hg : Tl : Ba : Cu 1 : 1 : 2 : 1. A mixture of 400 mg
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