HIGH-TEMPERATURE SUPERCONDUCTORS AND RELATED COMPOUNDS STUDIED BY MOSSBAUER SPECTROSCOPY

57Fe, 119Sn, and 151Eu Mossbauer spectroscopy was used to study the chemical structure, phonon mode 
changes, and low-temperature phase transformation around the Tc as well as suppression of superconductivity 
in high-temperature superconductors and related compounds. Anomalous temperature-dependent 
changes in the total 57Fe spectral area fraction and in the Mossbauer line shift were simultaneously 
found around the Tc in a EuBa2(Cu0.9957Fe0.01)3O7-d superconductor. These anomalous changes 
were attributed to phonon softening and low-temperature phase transformation occurring around 
the superconducting transition. Significant differences were observed between the 57Fe Mossbauer 
spectra of superconducting EuBa2(Cu0.9957Fe0.01)3O7-d and the isostructural non-superconducting 
PrBa2(Cu0.9957Fe0.01)3O7-d. The differences were interpreted in connection with the suppression of superconductivity 
(by hole filling or hybridization of Pr, Cu and O states) in the Pr-containing compound. 
The unusually high isomer shift value observed in the Pr-containing material can give evidence for the 
charge transfer mechanism between the Cu(1) chains and the Cu(2) planes and for its role in the 
suppression of superconductivity.


INTRODUCTION
M6ssbauer spectroscopy can be advantageously applied to study the following topics of high temperature superconductors: This wide range of applications becomes possible because M6ssbauer spectros- copy can detect the energy changes between the levels of M6ssbauer nuclei due to different hyperfine interactions between the nucleus and the electrons, with high sensitivity.
For the M6ssbauer investigations, the high temperature superconductor samples must contain M6ssbauer nuclides.They can either be constitutents (e.g., Eu, Sn) or impurities (e.g., Fe, Co).The latter ones must be introduced as probes for the M6ssbauer measurement in a small amount (usually enriched 57Fe, 9Sn or 57Co) to minimize the disturbance of superconducting properties of the material.
The high interest in this field is demonstrated by a large number of publications referenced in [1].The authors have been investigating a large number of super- conducting oxides  since 1986, and they have been studying all of the topics listed above.
Instead of giving a summary of our previous works, we would like to call attention to one of them that is closely related to the mechanism of high Tc superconductivity.Temperature-dependent anomalous changes were found around the Tc in 57Fe M6ss- bauer spectra of a Tl-containing superconductor [13].These changes were inter- preted by us as a relaxation process due to Cooper pairing with energy predicted by the BCS theory [26].Namely, an evidence for the possibility of BCS type superconductivity in high temperature superconductors was obtained by M6ssbauer spectroscopy.
In the present paper we show a few results obtained recently by M6ssbauer investigation of high-Tc superconductors and related compounds.
We succeeded in preparing a single-phase material of a PrBaz(Cuo.99YFeo.o)307_ocompound that was synthetized from Ba(NO3)2, CuO, Pr60l, and Fe203 materials at 930C for 12 hours in oxygen atmosphere.
The synthetized material was given subsequent heat treatments (400C/120 min, 600C/40 min, 600C/180 min, 450C/480 min, 450C/120 min) and finally was heat treated at 600C for 180 min and 450C for 600 min either in a nitrogen (sample A) or in an oxygen (sample B) atmosphere.
The SnSrCa0.Y0.sCu3Os'+ material was prepared from a 1:1 mixture of SnO and Sr2Ca0.sY0.sCu307_ by subsequent heat treatments (300C/2h, 400C/2h, 820C/3h, 400C/5h) in a nitrogen atmosphere.After grinding and homogenization, a pellet was formed and heat treated at 800C for 3 hours and then at 200C for 4 hours.Precursors SrCa0.sY0.sCu307_d were prepared from a well-homogenized stoi- chiometric mixture of SrCO3, CaCO3, Y203, and CuO.The mixture was calcined between 950-1000C for 4 hours and then kept at 750C for 4 hours.After cooling, it was powdered and heat treated at 1000C for 8 hours.The SnO precursor was prepared from SnCI2.H20 dissolved in HC1, neutralized with a Na2CO3 solution, washed with H20, and dried at l l0C.57Fe, SEu and ll9Sn M6ssbauer spectra of the powdered samples were recorded at transmission geometry.A temperature-controlled cryostat was used for the low- temperature measurements.Isomer shift values are given relative to alpha-iron, EuF3, and CaSnO3.Providing the gamma rays 3.109Bq 57C0, 10 Bq 15tEuF3 and 3.10 Bq CallgSnO3 sources were used.The evaluation of the spectra was performed by conventional least-square fitting of lines using MOSFUN and SIRIUS programs.RESULTS
The M6ssbauer spectra (Figs.2-4) of superconductor samples are considered as superpositions of subspectra belonging to M6ssbauer nuclides occupying different sites with various surroundings in the lattice.The 151Eu spectrum of the EuBa2(Cu0.9957Fe0.01)3OT_dsuperconductor (Fig. 2) can be evaluated as a singlet corresponding to Eu situated at the normal rare earth site.
The l9Sn spectrum of the EuBa2(Cu0.9919Sn0.01)3OT_dsuperconductor (Fig. 3) is decomposed into a singlet and a doublet, which is associated with Sn atoms at the Cu(2) and Cu(1) sites.
The 57Fe (and the 57Co) spectra could be fitted with three (or four) doublets (D1, D2 and D3 in Fig. 4) representing Fe atoms at the Cu(1) and the Cu(2) sites.
Comparing the relative area fractions of M6ssbauer lines corresponding to dif- ferent Cu sites, it was suggested [6,10] that iron and cobalt prefer the Cu(1) site while tin prefers the Cu(2) site in these superconductors.

Valence State of Cations
The valence state of 57Fe, 57Co, 119Sn, or 15Eu substituted into the high-T super- conductors can be established by the determination of isomer shift from the M6ss- bauer spectra.
The isomer shift depends on the electron density at the nucleus and can be expressed under certain conditions [27] by the formula" a (2 R/R {ldo)l [xI)'a(0)l2} where [qrs(0) [2 and [a(0)l are the s electron densities at the nucleus and AR/R is the relative change in the radius of nucleus.From the known interrelation [27] between the isomer shift and the valence state the latter can be obtained.
In each superconductor, Eu and Sn was found in the valence state Eu(III) and Sn(IV).However, the valence state of Fe and Co could be either three or four [2][3][4][5]. 1.3.Checking the Solid State Reaction Used for Preparation M6ssbauer spectroscopy can be utilized as a fingerprint method [33] to check the product of the solid-state reaction used for the preparation of high-T supercon- v (ram/s) tlgSn M6ssbauer spectrum of Sn02 precursor of SnSr2Cao..Yo..sCu308+,.
In the case of Sn, which has several possibilities to form perovskite compounds other than the desired superconductor, one of the first questions is whether or not one has succeded in preparing the right compound.Since the valence state of the regular Sn environment (an Sn-O layer) in the 1-2-4 compound SnSr2Ca0.sY0.sCu3Os+d is supposed to be similar to that in SnO (i.e., (II)), we used M6ssbauer spectroscopy to determine whether or not this is really the case.
Fig. 6 shows the M6ssbauer spectrum of an SnO precursor.As expected, the main part of the spectrum is a doublet characteristic of Sn(II).The parameters of this component correspond to those observed with SnO.Fig. 7 shows a typical 19Sn spectrum of SnSr2Cao.sYo.Cu3Os+d.It is obvious from the spectrum that there is no Sn(II) present here.All Sn atoms have valence state IV.A closer look reveals at least two components belonging to Sn atoms with different surroundings.
1.4.Variations with Oxygen Content Fig. 8 shows 57Fe M6ssbauer spectra of EuBa2(Cu0.9957Fe0.01)306.8 and EuBa2(Cu0.99-57Fe0.01)306.95superconductors.It can be well seen that the M6ssbauer spectra reflect sensitively the changes in the oxygen content of the superconductor by the change in the relative intensity of doublet D2 associated with Fe atoms at five-coordinated Cu(1) site.Similar changes have already been found in other cases [31].Another interesting variation of M6ssbauer parameters with the oxygen content was also found in a previous work [21].The magnitude of the isomer shift of the M6ssbauer probe at the Cu(1) site in YBa2Cu306/x was found to increase contin- uously with decreasing oxygen content (Fig. 9) and to undergo a sharp leveling off just below 06.4, in which the oxygen content corresponds to a superconductor (metal)-to-insulator transition [28].This can be interpreted in terms of electronic charge transfer between the chain and plane sites.
2. Phonon Softening and Low Temperature Phase Transition We have found an anomalous temperature dependence of the total area fraction of 57Fe M6ssbauer spectra of a EuBa2(Cuo.9957Feo.ol)307_dsuperconductor at the Tc.This is illustrated in Fig. 9.The change in the area fraction reflects a corresponding change in the M6ssbauer-Lamb factor f (probability of the M6ssbauer effect).
Theoretically in the case of T < Oo the f factor can be given [29] as: where OD is the Debye temperature defined by hwo kOD, where rOD is the characteristic Debye frequency, M is the mass of the nucleus, A is the wavelength of the M6ssbauer gamma radiation, and (x2) is the mean square amplitude of the lattice vibration.In the normal case, the temperature dependence of the f factor is characterized by a single Debye temperature.However, the parts of the plot demonstrated in Fig. 9 belong to different values of OD.The lower value of OD found at temperatures lower than Tc can be attributed to a lower o.This can be interpreted as phonon softening.Our result suggests that phonons must have a role in the mechanism of high-temperature superconductivity.
At the same time, we have also found anomalous temperature dependence of line shifts with the same EuBa2(Cuo.9957Feo.o)3Ov_dsuperconductor between Tonset and Tc (Fig. 10).The temporary decrease in the line shift with decreasing tem- perature around the Tc is in contradiction with the expectation for normal state based on the temperature dependence of second-order Doppler shift [27].This anomaly can be explained by a change in the local environment of Fe atoms.We suppose that the dimension of the environment concerned is shorter than the interatomic distances in the lattice.This means that the temperature dependent line shift anomalies reflect low temperature phase transformation.Assuming correlation between the low temperature phase transformation and phonon softening, the phonon mode changes could be partly attributed to the low temperature phase transformation. 0.18 I-+ +

Suppression of Superconductivity
Superconductivity is lost in EuBa2Cu307_d when Eu is replaced by Pr, although the PrBa2Cu3OT_d is isostructural with the Eu-containing compound.Fig. 11 shows 57Fe M6ssbauer spectra of a PrBa2(Cuo.9957Feo.o1)307_dcompound with different oxygen contents.The spectra of non-superconducting Pr-containing materials are significantly different from those of superconducting perovskites (compare Fig. 6 and Fig. 11).The characteristic isomer shifts of the doublets D1, D2, and D3 are 0.11 mm/s, 0.13 mm/s, and 0.07 mm/s, respectively.These data are unusual for 1-2-3 type superconductors that have isomer shifts 0.03 mm/s, 0.01 mm/s, and 0.15 mm/s for doublets D1, D2, and D3.The differences among the isomer shifts of Prand Eu-containing materials reflect changes in the electronic densities at both Z O -3 0 3 v(mm/s) the Cu(1) and the Cu(2) sites.This can be considered as the consequence of the suppression of superconductivity in the Pr-containing compound.The effect of Pr can be explained by hole filling, which means that the Pr ion having a valence state greater than three gives an extra electron to neutralize a hole in the CuO network, rendering the system non-superconducting.On the other hand, suppression of superconductivity can also be achieved by the hybridization of Pr 4f states with O 2p-Cu 3d states.Both mechanisms result in a change in the electronic density at the Cu(2) site, which is indicated by the change of the isomer shift of doublet D3.A very similar change as we observed for the Cu(2) site was predicted [32] for the suppression of superconductivity by oxygen loss involving the charge transfer mech- anism between the chains and planes [28].The chains can be pictured as charge reservoirs, and the charge transfer can control the T of the superconducting state.
The changes in the isomer shift of doublet D1 and D2 belonging to Fe at Cu (1) site can be considered as evidence for the existence of a charge transfer mechanism between the Cu(1) chains and the Cu(2) planes, because otherwise it would be hard to explain these as an effect of the Pr.On the other hand, the observed differences in the isomer shift of doublet D1 are close to those found by 57Co experiments [21] with decreasing the oxygen content (Fig. 9).