Application Of Mössbauer Spectroscopy To Determine The Parameters Of The EFG Tensor At Barium Nodes For УBA2CU3O7-X

The method of the Messbaur emission spectroscopy on the Ba-133 isotope, the parameters of the electric field gradient (EFG) tensor are determined. It is established that the discrepancy between the experimental and calculated values of eqкр is explained by the imperfection of the models used to calculate eqкр.


INTRODUCTION
When studying the structural properties of HTSCs, there are many problems, the solution of which is mandatory. In this aspect, one of the main problems in HTSC physics is the problem of determining the location of hole localization, which determine the phenomenon of superconductivity. The most reliable method for identifying holes in HTSC

Application Of Mössbauer Spectroscopy To Determine The Parameters Of The EFG Tensor At Barium Nodes For УBA2CU3O7-X
The American Journal of Applied sciences (ISSN -2689-0992) lattices is considered to be a comparison of the experimentally determined parameters of the electric field gradient tensor (EFG) with the results of their theoretical calculation. This method is most effective when measuring and calculation of the parameters of the EFG tensor. All HTSCs, as a rule, contain copper, and the parameters of the EFG tensor at copper sites are most sensitive to the charges of atomic centers. This work is devoted to the consideration of the experimental and theoretical results of determining the parameters of the EFG tensor in the copper nodes of the UBa2Cu3O7-x ceramics.

MATERIALS AND METHODS
The combination is one of the most typical high-temperature superconductors and is distinguished by a relative prostate synthesis, high values of the temperature of transition to the superconducting state Tc, and the ability to regulate Tc by changing x. It is these features that explain the fact that the overwhelming number of studies on comparing the calculated and experimental parameters of the EFG tensor refers to УBa2Cu3O7-x. Ceramics УBa2Cu3O7-x is characterized by a variable composition (0≤x≤1). The maximum value of Тс (~ 91K) was found for УBa2Cu3O7 (х = 0). As x increases, Tc decreases and compositions with x> 0.65 do not exhibit superconductivity. The experimental determination of the parameters of the electric field gradient (EFG) tensor at the lattice sites of УBa2Cu3O7-x is an urgent problem. This is primarily due to the possibility for the specified ceramics to compare the experimental and calculated parameters of the EGF and, as a result, the ability to assess the validity of those initial assumptions within which the calculation of the lattice sums is performed. The most complete information on the parameters of the EFG at the sites of the crystal lattice can be obtained by the methods of nuclear quadrupole resonance (NQR) and Mössbauer spectroscopy. However, as applied to УBa2Cu3O7-x, both methods refer either to copper centers (NQR on 63Cu and 65Cu isotopes [1]) or to impurity centers in the position of copper atoms (Mössbauer effect on 57Fe (57Co) 119Sn and 67Zn (67Cu) isotopes) , or to the centers of rare earth metals in the position of yttrium atoms (the Mössbauer effect on the isotopes 151Eu, 155Gd, 161Dy, 166Er, 169Tm, and 170Yb This is due to both the difficulties of measuring the NQR spectra on the isotopes 135Ba, 137Ba, and the absence of stable Mössbauer isotopes of barium. In this work, it is proposed to use emission Mössbauer spectroscopy on the 133Ba (133Cs) isotope to determine the parameters of the EFG tensor at barium nodes. Electron capture in the 133Ba mother nucleus leads to the formation of an excited 81-keV level of 133Cs, and the recoil energy during neutrino emission in this case does not exceed 1 eV, so that the daughter cesium atom is in the position of barium atoms. The isomeric transition in 133Cs occurs between states with spins 5/2 and 7/2, so that when a cesium atom is in a crystal field

RESULTS AND DISCUSSIONS
As a preliminary stage, we measured the emission Mössbauer spectrum of 133BaO (Fig.  A) -it is a single line with the width (HEXP. = 0.90 ± 0.02 mm / s) slightly exceeds the doubled natural width 81 -keV of the 133Cs level (2Gest. = 0.54 mm / s). Since BaO has a NaCl-type lattice, we considered that the difference between HEXP. and 2Gest. caused only by instrumental effects and later on, 0.90 mm / s was taken as the instrumental width of the spectral line (i.e., when the experimental Mössbauer spectra of U133BaCu3O7-x were decomposed into quadrupole multiplets, it was assumed that the width of the components could not be less than 0.90 mm / s ). The Mössbauer spectrum of superconducting ceramics УBa2Cu3O7 is a single broadened line (see Fig. B and table), which corresponds to 133Cs + centers at barium sites. In the structure of УBa2Cu3O7-x, barium atoms occupy a single position, and therefore the broadening of the experimental spectrum is associated with unresolved quadrupole splitting. The table shows the values of e2qQ determined from the spectrum in Fig. b. For semiconductor ceramics УBa2Cu3O6, the Mössbauer spectrum is also a single line, but its width will noticeably increase (see Fig. C and table). This spectrum also corresponds to 133Cs + centers at barium sites; the e2qQ values for them are given in the table.
From the experimental values of eq according to the relation eq = (1-γ) eqcr. (here γ is the Sternheimer coefficient for Cs + γ = -121.3), were obtained eqcr values. [3]. As can be seen from the table, the transition from the orthorhombic to the tetragonal modification УBa2Cu3O7-x is accompanied by an increase in eqcr., And there is a qualitative agreement between the experimentally determined and calculated values of eqcr., Although there is no quantitative agreement. As an illustration, Fig. the dashed line shows the theoretical Mössbauer spectrum for the UBa2Cu3O6 sample, and the value of eqcr. given in [4] was used for the calculation.

CONCLUSION
It can be seen that, in contrast to the experimental spectrum, a distinct structure