Volume 139. number 8 PHYSICS LETTERS A 21 August 1989 MAGNETIC PROPERTIES OF SINTERED BaCuO 2 C.B. AZZONI, A. PALEARI, G.B. PARRAVICINI, G. SAMOGGIA Dipartimento di Fisica “A. Volta”, Unirersitã di Pavia, Via Bassi 6, 27100 Pavia, Italy F. PARMIGIANI and M. SCAGLIOTTI CISE, Materials Department, P.O. Box 12081,20134 Milan, Italy Received 26 September 1989; revised manuscript received 17 May 1989; accepted for publication 22 June 1989 Communicated by D. Bloch EPR susceptibility and XPS measurements were performed on sintered BaCuO,. A structured sharp EPR signal superimposed on a broad band was observed. It is shown that the strucmred line results from the superposition of two signals associated with divalent copper ions lying in two different distorted octahedral sites. These ions are probably placed on the outermost layer of the sintered grains. After the discovery of superconductivity in me- superconducting CuO and cuprates, as it was for- tallic oxides the physical and chemical properties of merly observed in earlier studies [12]. these systems have been intensively investigated. In On the other hand, it is now clear that the mech- order to study the symmetry characteristics of the anism of high temperature superconductors criti- Cu 2~ ion sites, electron paramagnetic resonance cally depends on the local character of the copper— (EPR) measurements were carried out on high tern- oxygen bond. This fact makes of interest the study perature superconductor (HTSC) oxides [1—5].All of slightly different chemical compounds or of HTSC these experiments revealed the presence of a sharp chemical components in order to understand the EPR line which was assigned first to octahedral co- subtle difference existing between copper corn- ordinated Cu2 ± ions contained in a small portion of pounds that are superconducting and those which are non-superconducting material spread in the super- not. conducting matrix [11 or, in turn, to Cu2~ions in In this light the aims of the present Letter are to a metallic phase present in both the superconducting study the dependence of EPR signals generated by and non-superconducting structure [2]. Recently Cu2~ions in BaCuO 2 on the oxygen partial pressure these problems have been clarified [6—10], all the P02 and to contribute to clarify the origin of the sharp EPR spectra were shown to be not intrinsic to the EPR signal in Cu—Ba—O compounds. superconducting phase, and rather due to low con- Among the HTSC chemical components BaCuO2 centrations of spurious phases typically present in is particularly suitable for our purpose because it has the samples. Cu 2~ions with several different coordinations [13]. Besides these problems, however, the supercon- Samples were prepared in the form of pellets by ducting properties of HTSC systems have been shown the solid state reaction of pure grade BaCO 3 and CuO to be largely dependent on thermal cycles and on the in a temperature controlled tube furnace at 950°C oxygen partial pressure during the heat treatment and in 02 flow for 12 h. After this first calcination [111. process the powder was ground and pressed. The pel- Changes in the oxygen content were also observed lets obtained were annealed at 950°C for 12 h under to produce significant modifications in the crystal- 02 flow followed by furnace cooling over a period of lographic, electric and magnetic behavior of the non- 6h down to 25°C. 411