CONFIGURATION-SYMMETRY ANALYSIS OF INTERSTITIAL POINT DEFFCTS IN H.C.P. CRYSTALS AND DISTRIBUTION OF DIFFUSE RADIATION-SCATTERING INTENSITY IN RECIPROCAL SPACE
Valentin A. Tatarenko
Dept. of Solid State Theory, Inst. for Meta1 Pyrysics, 36
Academician vernadsky Blvd, UA-252680 Kyiv-142, Ukraine
tatar@imp.kiev.ua
Keywords: interstitial solid solution, static displacements of ions, strain-induced interaction of impurity atoms, electrochemical' interaction of atoms, short-range order, diffuse scattering of radiation, long-range order.
| The semiphenomenological model [1-3] of interatomic interactions within interstitial solid solutions based on hexagonal close-packed (h.c.p.) crystals (a-Ti, Tc, a-Y, a-Sc, etc.) is extended. The calculated energy parameters [4, 5] are applied in the statistical-thermodynamic analysis of the ordering (decomposing) interstitial solid solutions as well as in the study of peculiarities of the radiation scattering by such distorted solutions. It is shown that, for instance: i) the thermodynamics of the orientational ordering and spatial correlations of the tetrahedral (or octahedral) interstitial H/D(O-atoms' arrangement in h.c.p. rare-earth metals (or a-Ti) is governed by the 'electrochemical' 'repulsion' between interstitial atoms, and peculiarities of the diffuse neutron scattering by such solutions are controlled by both this 'repulsion' and H/D(O)-induced distortion effects; ii) the module of the total energy of strain-induced 'attraction' may exceed substantially the corresponding quantity of 'electrochemical' 'repulsion' between one chosen H atom and a rest of H atoms on the octahedral interstices of h.c.p. Tc, and iii) phase transformations of the solid solution of H atoms on the octahedral interstices of h.c.p. Tc and the diffuse neutron scattering by this solution are governed by the strain-induced H-H `attraction' as well as by H-induced strain effects in h.c.p. Tc. The comparison of substantially distinct isodiffuse neutron-scattering patterns, calculated within the scope of the kinematical theory, with corresponding experimental patterns, given implanted impurity atoms having the octahedral or tetrahedral coordination (see Figs. 1, 2, and Fig. 3 respectively) in h.c.p. host crystals, furnished and identification of the symmetry type of interstitial positions occupied by implanted atoms in actual disordered solid solutions trending towards the atomic ordering or decomposition reactions (see Figs.1, 3, and Fig. 2 respectively). | |