M. Fayard

Crystallographic description of coincidence‐site lattice interfaces in homogeneous crystals

Coincidence-site lattice interfaces (CSLI) are frequently observed in crystals where a rigid framework remains invariant on both sides of the interface. They also seem to minimize the interface energy, for example, in metals where, empirically, the greater the density of the coincidence-site lattice the more stable the grain boundary becomes. Group-theory considerations allow the determination of all the possible interface operations which leave a given sublattice invariant. A classification of these CSLI with respect to the number of equivalent sublattices they leave invariant is a guide for the prediction of the most stable type of interfaces with respect to the sublattice considered. Examples from different types of crystals illustrate the method, which also applies for translation boundaries, twins and grain boundaries.

Etude par diffraction X et microscopie electronique des composes inedits de formule AnBnO3n+2 dans les systems La2Ti2O7CaTiO3, Nd2Ti2O7CaTiO3 et Ca2Nb2O7CaTiO3

Abstract New compounds corresponding to the formula A n B n O 3n+2 with n = 4,5; 5 and 6 have been found in the two following systems Nd 2 Ti 2 O 7 CaTiO 3 and Ca 2 Nb 2 O 7 CaTiO 3 . These compounds have been investigated by electron microscopy. The results obtained are in good agreement with our structural hypothesis.

Etude par microscopie electronique de quelques termes de la serie (Na, Ca)n Nbn O3n+2

Abstract A regular stacking of perovskite type blocks has been observed by electron microscopy in the (NaCa)n Nbn O3n+2 phases, with n = 4, 5 and 6. Observation of ordered intergrowth with n = 4.5 has shown an alternation of blocks of different thickness n = 4 and n = 5. An other ordered intergrowth with n = 4.33 has been observed.

Mise en evidence par microscopie electronique de marches a la surface de cristaux minces de sesquioxydes de terres rares

Abstract Evaporated rare earth sheets have been oxidized and observed in an electron microscope. Observations indicate the existence of numerous and various steps on the surfaces of oxide cristals. The formation of this steps is related to the transformation cubicmonoclinic. The steps are generally parallel to the faces of the tetraedra O Ln 4 , the linkage of which constitutes the layers of the layered oxide structure.

Modèle de défaut d'empilement dans la pyrite

Abstract A MODEL FOR STACKING FAULTS IN PYRITE A model for stacking faults laying in the (001) plane of pyrite is proposed which is in accord with previously published experimental observations. This stacking fault has an a [0, x, 1/2] vector not parallel to the fault plane. It has a coherent and conservative character and half of the sulphur atoms are located on a coincidence sublattice. Through the fault, the atoms take the positions of the marcassite structure.

Elastic energies of dissociated Frank loops in f.c.c. metals and alloys

Abstract Elastic energies of dissociated non-regular hexagonal Frank loops have been calculated in the approximation of isotropic linear elasticity theory. A model has been used in which attractive Shockley dislocations could eventually recombine. It has been shown that small loops are strongly dissociated provided that the stacking fault energy is not too high. This is a possible explanation for black spot defects. An evolution scheme for loop growth is suggested. In particular, Frank loops and stacking fault tetrahedra could have a similar origin to that considered in early studies of vacancy clustering.

Transformations β → ω dans les alliages metalliques du systeme ternaire TiZrO☆

Abstract The X-ray study of the metallic alloys in the ternary system TiZrO shows the existence of phases α, β, ω (o) and of a new phase ω ′ (o) . This phase ω ′ (o) appears in the alloy Ti 2 ZrO after a long anneal at low temperature. Its structure is described in the space groupe Cmmm ( a = 4,8387 A ; b = 8,1486 A ; c = 6,1042 A ; 8 Ti atoms in 8 n with y = 0,338 and z = 0,254; 4 Zr atoms in 2 a + 2 d; 4 0 atoms in 4 e). The filiation of the phases shows the existence of ordering phenomena in the distribution of Ti and Zr atoms during the transformation β → ω (o) , and in the distribution of 0 atoms during the transformation ω (o) → ω ′ (o) .

General extinction criteria in multi-beam electron microscopy for interfaces in homogeneous crystals

Abstract General criteria for interface extinction in homogeneous crystals are obtained under multi-beam diffraction conditions. They depend on the nature and the location (with respect to the elements of symmetry (hi|ti) of the structure) of the operator (R|τ) characterizing the defect. New information may then be obtained for the location of atoms on both sides of the boundary.

Enantiomorphous ordered domains in stoechiometric lithium aluminate LiAl5O8

Abstract Nucleation and growth of equally probable enantiomorphous P4 1 32 and P4 3 32 ordered structures lead, in the case of LiAl 5 O 8 , to the formation of seven types of boundaries: three ordinary antiphase boundaries and four inversion ones. As in the case of LiFe 5 O 8 a complete analysis of the nature of boundaries has been done by electron transmission microscopy. The contrast conditions allow a determination of the inversion centers.