INFLUENCE OF RARE-EARTH ELEMENT ION SIZE ON THE PROCESS OF SPINODAL DECOMPOSITION IN 1:2:3 COMPOUND
D.Kravchuk, S.Sudareva, E.Kuznetsova, T.Krinitsina, I.Bobylev, E.Romanov
Institute
of Metal Physics, Ural Div. of Russian Academy of Sciences, S.
Kovalevskaya Str. 18, Ekaterinburg, GSP-170, 620219, Russia
Nonstoichiometric 1:2:3 compound is thermodynamically unstable at low temperatures and undergoes spinodal decomposition. In the context of Khachaturyan theory [1] the YBa2Cu3O7-d compound (d30,2) decomposes initiallytwo orthorhombic phases (oxygen-enriched and oxygen-depleted) with tweed-structure formation, and, finally, into two stable phases (tetragonal phase with O=6 and orthorhombic phase with O=7). The existence of YBa2Cu3O7-d compound decomposition at 200°C was approved for poly- and single-crystals with various methods of into structure analysis (TEM, X-ray, optical microscopy) and measurement of physical properties [2].
The main goal of the present paper was to study the influence of rare-earth element ion size on the structure of 1:2:3 compound, its decomposition kinetics and the type of tweed contrast. Neodymium with larger ion size than that of yttrium was chosen for the comparison.
TEM investigation have shown that in the YBa2Cu3O7-d compound the lattice unstability in the form of poorly visible tweed contrast in coarse twins is initially observed after annealing at 200°C for 35 h. At further annealing up to 100 h the clearly defined tweed contrast appears resulting from the existence of two systems of transversely-polarized atomic displacement waves of [110] and [11_0] types. The tweed contrast is consistent with diffusion scattering on the electron diffraction patterns in a form of intensity rods along [110] and [11_0] directions.
Decomposition of NdBa2Cu3O7-d compound at low temperature (200°C) anneal starts somewhat earlier. After 16 hours annealing there are no twins in the structure of this compound, and the tweed contrast is observed. The type of diffusion scattering differs from that observed in YBa2Cu3O7-d. Instead of thin and short rods of intensity the diffusion scattering along the reciprocal lattice planes appears.
Obviously, the ion size of a rare-earth element affects fine structure of YBa2Cu3O7-d and NdBa2Cu3O7-d compounds and its behavior at low temperature anneal. The increase of ion size results in higher unstability of 1:2:3 compound.