Studies of Tie Electronic Structure By XPS f HTSC and Related Compounds
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STUDIES OF TEE ELECTIONIC STRUCTURE BY IPS OF ITSC AND RELATED COMPOUNDS
1. 2. 3. 4. 5.
S.Myhrat, A.E. Bocquett, J.F. Dobson', P. Goodman 2 , P.C. Healyl, A.M. Stewart 3 , J.G. Thompson 4 and T.J. Vhites Div. Sci.and Tech., Griffith University, Q. 4111, Australia. Sch. of Physics, Melbourne University, V. 3052, Australia. Res. Sch. of Physical Sciences, ANU, ACT, 2601, Australia. Res. Sch. of Chemistry, ANU, ACT, 2601, Australia. Electron licroscope Centre, Queensland University, Q.4067, Australia.
ABSTUCT Measurements by photoemission spectroscopy have shown that the chemical environments of alkaline earth cations in HTSC compounds are different from the environments of these ions in the prototype perovskite structures. A recently discovered HTSC compound (Pb2 Sr 2 Ca0.5Yo.sCu 3Os) exhibits the exceptional feature of having a typical HTSC signature for Sr, while Ca shows a perovskite-like signature. The crystal-chemical features of the relevant compounds have been correlated with these observations. These correlations may be indicative of inter-layer electronic interactions. INTRODUCTION It is well-established for a range of HTSC compounds that photoemission analyses have detected high (HBE) and low (LBE) binding energy contributions to the characteristic line shapes of alkaline earth (Ca,Sr,Ba) cations [e.g., 1,2,3]. There is an ongoing controversy about the relative magnitudes of these contributions, but widespread agreement that the LBE components are associated with bulk electronic phenomena. It is significant, we believe, that the observed LBEs are not only lower than those for insulating alkaline earth divalent model compounds, but in some cases also lower than those for the alkaline earth metals [4]. The UBE components, on the other hand, are consistent with divalency and are, at least in part, associated with surface artifacts [e.g., 5,6,7]. The anomalous BEs of the alkaline earth cations may be relevant in the context of HTSC compounds and the hypothetical inter-layer interactions required by several theories, some of which have invoked the effects of polarization [e.g., 8,9,10]. This paper discusses recent extensions of photoemission measurements to the HTSC compound Pb2 Sr 2Yo. 5 Ca0 . 5 Cu3 08 (PSYCC), and the correlations with the results for some of the other HTSC compounds, and their structural prototype n(Ca,Sr)Ti0 3 . (Ca,Sr)O layer perovskites. EXPERIMENTAL DETAILS The layer perovskite, n(Cal-xSr1 )TiO 3.(Cal-.Sr.)O (n = 2) specimens with x = 0.2 and 0.8, were prepared by sintering in air at 14500C. At this temperature (near-liquidus) the resultant disks (12mm diameter and 2mm thickness) were oxygen deficient; some interpretations of the low BEs for the alkaline earth species in the HTSC compounds have invoked the effects of oxygen vacancies [2]. Details of the fabrication procedures and the specimen characteristics have been described elsewhere [11]. The PSYCC specimen (a pellet of 10mm diameter and 2mm thickness) was synthesized in accord with procedures in the literature [12]; the microstru
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