Mössbauer studies of57Co-doped layered perovskites

Mössbauer spectra of57Co-doped polycrystalline or single-crystal samples of layered perovskites La2Mo4 (M=Cu, Co, Ni) and R2CuO4 (R=Nd, Eu, Gd) were recorded at room temperature and below. Of the samples studied, only La2CuO4 shows a widely separated doublet at room temperature and a single clearly resolved sextet well belowTN.


Introduction
The layered perovskite LazCuO4 is the parent compound from which the first high-T c superconductors were obtained [1] by doping with divalent elements. La2CuO 4 is orthorhombic at and below room temperature [2]. Oxygen-deficient La 2CUO4_,. is antiferromagnetic with T N as high as 328 K [2,3], while oxygen-rich La2CuO4+.v is superconducting with T c -30 K [4]. La2CoO4 is orthorhombic at room temperature and becomes tetragonal below -135 K; it becomes antiferromagnetic below about 275 K [5]. LazNiO 4 is very sensitive to oxygen content, the stoichiometric compound being orthorhombic and not magnetically ordered between 95 K and 4 K [6] while La2NiO4.05 undergoes a tetragonal to orthorhombic transition in cooling below about 240 K and becomes antiferromagnetic below 70 K [7].

S. Jha et al. /M.S. of 57Co-doped layered perovskites
Recently the layered perovskites R2CuOa_r, where R = Pr, Nd, Sm [8] or Eu but not Gd [9], have been found to be electron-type superconductors below about 20 K when 15% of R is replaced with tetravalent Ce or Th. The parent compounds R2CuO4_y are tetragonal and semiconductorlike, and in muon spin rotation experiments reveal static magnetic order of Cu moments below 300 K [10]. Antiferromagnetic ordering of Cu in R2CuO 4 below room temperature has been suggested by susceptibility measurements for R = Eu [11] and Gd [12]. The compounds R2CuOn_y, where R = light rare earth element Pr through Gd, differ from La 2 MOa_y in that the former have oxygen atoms square-planar coordinated about Cu, while in the latter oxygen atoms are octahedrally coordinated about Cu [5,8,131.

Experimental results
Polycrystalline samples of La2MO4_ r and R2CuO 4 were made by mixing stoichiometric amounts of the constituents by co-precipitation and by repeated grinding and firing [14]. Samples were cooked at 500~ overnight, in flowing helium gas to produce the oxygen-deficient form, or in air to produce the stoichiometric form. X-ray powder diffraction showed the samples to be singlephase. Thin, platelike single-crystal samples were grown from PbO-and CuObased fluxes; the tetragonal c-axis was perpendicular to the thin face [11,12].
For M~Sssbauer source experiments, carrier-free 57Co was deposited on one face of the single-crystal sample or on a disk made by compressing a polycrystalline sample; after repeating the final anneal such sources were used with a 0.3  Table 1 MtSssbauer data for 57Co-doped perovskites. T is the sample temperature. Sample form is single crystal (SC) or polycrystalline (PC); absorber is PFC unless indicated ~. In mm/s, 6 is the isomer shift relative to c~-Fe, F is the FWHM for all lines in the spectrum, and A is the splitting of the doublet or A EQ. Error in the least significant digit is given in ().  (5) Sign not determined where not explicitly given. h Ref. [14]. mg/cm 2 57Fe enriched potassium ferrocyanide (PFC) or stainless steel (SS) absorber at room temperature.

Sample
M/3ssbauer spectra are given in figs. 1 and 2, and the data are presented in table 1. Spectra are fitted with a doublet for 295 K, and for R2CuO 4 (R = Eu, Gd) below 200 K, with a doublet and 1 or 2 sextets. Comparison [14] of 5VCo-and 57Fe-doped La2CuO4_~, showed that the electron capture aftereffect does not significantly broaden the lines in this M/fssbauer source experiment. Broad lines might be expected for magnetic interaction weak relative to quadrupole interaction just below T N (for La2MO4, M = Cu, Co at 295 K). However, it is not clear why the lines are broader at 295 K for the singlecrystal samples than for the polycrystalline samples of the same material; final heat treatments were the same.

Discussion
The MiSssbauer spectra for 57Co(La2CoO4_~.) from 295 K to 78 K do not reflect the magnetic ordering and structural phase change reported [5] in this temperature range.
Zeeman splitting of the Mt3ssbauer spectra for 57Co(R2CuO4) is consistent with the presence of antiferromagnetic ordering of the copper moments for temperatures below about 150 K in EuzCuO 4 and below about 200 K in Gd2CuO 4 [15], as had initially been hinted in susceptibility measurements [11,12]. The fitting of two sextets for Gd2CuO 4 indicates two different types of site for the 57Fe probe.