ELSEVIER Journal of Alloys and Compounds 232 (1996) 63-66 Investigation of Cu,Ga, Cr, Se, single crystals A. Winiarski”, I. Okonska-Kozlowskab, J. Heimann”, M. Neumannc “ Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland blnstitute of Chemistry, Silesian University, Szkolna 9, 40-006 Katowice, Poland ‘Department of Physics, University of Osnabriick, 49069 Osnabriick, Germany Received 5 September 1994; in final form 26 June 1995 Abstract From X-ray photoelectron spectroscopy data and X-ray microprobe analysis the formula Cu,Ga,Cr,Se, (x + y + z = 3) for the investigated single crystals was determined. Replacement of Cu by Ga in tetrahedral sites of the spinel-type structure was observed. A negative chemical shift of the Cu 2p levels as well as an absence of Cu shake-up satellite lines was noticed. T, dependency on the Ga content was found from susceptibility measurements. Effective and saturation magnetic moments at low concentrations of gallium (y cO.16) were found to be higher than in CuCr,Se,. For gallium-rich samples the values of the moments decrease rapidly. The problem of metal valency in this system is discussed in relation to charge distribution. Covalency and stability of the spinel-type structure is also discussed. Keywords: Spinels; XPS; Magnetic susceptibility; Covalency 1. Introduction Two main problems appear in doped spinel-type crystals - the valency of metals and the occupation of tetrahedral and octahedral sites. The structure of CuCr,Se, is known to be a normal spinel-type with the space group Fd3m [l-3]. Cu,,,Ga,.,Cr,Se, was the first gallium-doped chal- cogen spine1 single crystal to be investigated [4]. The results of experiments on polycrystalline material were reported in 1970 [5]. The system Cu, _,Ga,Cr,Se, was investigated by Okonska-Kozlowska [6] and some phase relationships were found. The growth conditions of these spinel-type single crystals are reported in Ref. [7]. In this paper one can also find measurements of lattice constants and suggestions concerning the va- lency of copper. Speculations about the valency of Cu and Cr in CuCr,Se, were published in Refs. [Z&9]. The authors assume the valencies of Cu to be +l and +2 with those of Cr being +3 and +4. An important remark was made by von Philipsborn [lo]: covalency increases in the sequence 0, S, Se, Te. Magnetic properties of the polycrystalline (Cu, Zn)Cr,Se, system were investigated by Okonska- Koz#owska and Krok [ll]. The measured saturation moment per chromium atom in CuCr,Se, was found 0925-8388/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDZO925-8388(95)01961-8 to be 2.38, which is much lower than the expected value (3.00). The value of the saturation moment close to the expected value is reported in Ref. [ll] for ZnCr,Se, (p,,,/Cr at.% = 2.87). This indicates that copper influences the magnetic moments in chal- cogenic spinels. 2. Experimental The single crystals were grown by the chemical vapor transport method [3,7] in the form of octahedra with dimensions up to 2 mm. The octahedrals were not usually perfectly shaped and skeletonic crystals were also formed. The surface of “as-grown” crystals were covered by a thin layer of selenium which was visible under an optical microscope. This layer was formed in the latter part of the growth process. Photoelectron spectra were recorded with an ESCA PHI 56OOci instrument from Perkin Elmer using monochromatized Al-KIY radiation. Crystals were in- vestigated “as-grown”, split in air or under UHV conditions. X-Ray microprobe analyses were carried out using a Cameca system. By varying the growth conditions, we obtained crystals which belonged to five