Description des polytypes a phases de Laves de certains oxydes intermetalliques. Ces structures consistent en unites [D,T] de 6 atomes reparties en deux reseaux 3 6 dans (001) comprenant D et un reseau de Kagome et deux reseaux 3 6 dans (001) comprenant T, ces deux reseaux 3 6 etant partages suivant [001] avec les unites voisines [D,T]. Les unites successives [D,T] sont empilees dans des sequences cubiques (c) ou hexagonales (h) ou une combinaison des deux. Ce type d'arrangement de Laves a une grande influence sur les proprietes physiques

Synthesis of Mn-bearing ilvaites, CaFe 2−x 2+ MnxFe3+ [Si2O7/O/(OH)], with 0 ≤ x ≤0.19, have been performed under hydrothermal conditions at 2 and 3 kbars, T = 300 -400°C and at oxygen fugacities defined by the Fe2O3/Fe3O4 - and the Ni/NiO -buffer. As shown by X-ray diffraction, the substitution of Fe2+ by Mn2+ decreases the monoclinic angleβ and causes a phase transition from monoclinic to orthorhombic at x = 0.19. The Fe-distribution has been determined by Mössbauer spectroscopy.

Magnetisation measurements of three synthetic single-crystals A2SiO4 of the olivine family are presented (A=Mn, Fe and Co). All three compounds order antiferromagnetically in the range 45-65 K. For A=Mn, weak ferromagnetism is present. For A=Fe and Co, the magnetisation is strongly anisotropic. A model is proposed, which extends existing published explanations of the magnetic structures. The magnetic cations are represented by spin Hamiltonians, and their interactions by mean fields. Analytical relationships are derived between Curie temperatures, Curie constants and Neel temperatures on the one hand, and spin Hamiltonian parameters on the other. From the latter parameters the maximum possible information on orbital levels is derived. This is enough to calculate magnetic entropies in good agreement with published data, together with specific heat measurements, and to point out the limits of validity of the linearisation processes that are necessary to obtain usable analytical relations between experimental and atomic parameters. Below TN, the susceptibilities are analysed using the conclusions of previous magnetic structure studies, especially those concerning the role of competing exchanges, single-ion anisotropies, and symmetry requirements.

Ya. P. Yarmolyuk and P. I. Kripyakevich (Kristallographiya 19, 539 (1974)) showed that all tetrahedrally close packed (t.c.p.) structures have coordination formulae PpQqRrXx → (PX2)i(Q2R2X3)j (R3X)k, where P, Q, R, and X represent coordination numbers (CN) 16, 15, 14, and 12 polyhedra respectively: p, q, r, and x indicate the numbers of such polyhedra in the unit cells of t.c.p. structures and i, j, and k are positive integers. We propose and demonstrate a limitation to the above formulation: if i ≥ 1 and k ≥ 1, then j ≥ 1 (or if both p > 0 and r > 0, then q > 0). We give reasons for this and discuss the Aufbauprinzip of t.c.p. structures and the results of C. B. Shoemaker and D. P. Shoemaker (Acta Crystallogr. B 42, 3 (1986)).

The mixed valent iron silicate ilvaite CaFe 2 2 +Fe3+ [Si2O7/O/(OH)] has been synthesized under hydrothermal conditions at temperatures between 300 and 500°C, pressures between 1.5 and 4 Kbars and oxygen fugacities controlled by the solid state buffers Fe3O4/Fe2O3, Fe/Fe3O4 and Ni/NiO. All these ilvaites are monoclinic (P21/a with cell parameters a0 = 13.0065 (9) Å, b0 = 8.8073 (7) Å, c0 = 5.8580 (4) Å, and β = 90.332 (6)°. The quality of the samples has been checked by Mössbauer spectroscopy. Scanning electron microscopic pictures show small euhedral crystals with a size up to 30 μ.

Sten Samson (1967) gave a detailed description of the Cu4Cd3 structure type, which belongs to the complex intermetallic compounds in giant cells. Laves-Friauf polyhedra and icosahedra arranged around the centres of two interpenetrating diamond complexes have been modeled to understand the structure. A 3-dimensional basic framework can be found consisting of a polyhedron around W* of the 8th order that belongs to the family of W* polyhedra with members of NaCd2 , a-Mn, y-brass, sodalite etc. Sten Andersson (1980) believed to represent an alternative description of the structure, but he had to accept the Laves-Friauf polyhedra around one D complex; for the icosahedral framework he prepares a replacement by octahedra and tetrahedra as blocks or units of the pyrochlore structure, but this — if at all — was successfully only in part. Introduction The motivation of this note is the remark of a colleague to the effect that as we are interested in promoting framework descriptions of intermetallic and inorganic structures, it would be interesting to see if a relatively simple 176 Ε. Hellner and W. Β. Pearson framework description could be given for a really complex intermetallic structure such as Cu4Cd3 (Samson, 1967) F43m, that has 1124 atoms in its unit cell. Here we give three possible descriptions (one of which expresses Samson's verbal and pictorial description of the structure) that we believe are simple enough to be read by those not well versed in the descriptive nomenclature of "frameworks". Indeed the three descriptions together contain no more jargon that than defined in Table 1 — most of which will be selfevident to those familiar with descriptions of crystal structures. The second feature of this note is to address a problem, not so far explicitly discussed in regard to framework descriptions of relatively complex structures: that of redundancy in the description. Because many atoms are common to more than one polyhedron described in the framework expression, their existence may be described two, three or even four times. The effect of this is that e.g. the framework that expresses Samson's Table 1. Jargon used in framework descriptions of Cu4Cd3.

The magnetization density distribution in the ionic ferromagnet Rb 2 CrCl 4 has been determined by polarized neutron diffraction from a single crystal at 4.5 K. Magnetic structure factors F M were derived from the flipping ratios R of 660 main structure reflections (189 independent) and 57 super-structure reflections with sin θ /λ ≼ 0.8 Å -1 (λ = 0.90 Å). The main structure reflections were corrected for extinction using a secondary extinction parameter. The F M were modelled by representing the mag­netization density by a superposition of densities from orbitals centred at each atom : Cr(3d, 4s) and Cl(3s, 3p). The results verify the presence of orbital ordering arising from the cooperative Jahn-Teller distortion in the basal plane, previously postulated as responsible for the ferromagnetic exchange interaction. The proportion of the Cr 2+ moment occupying each 3d orbital is 0.26(2)( yz ), 0.25(2)( xz ), 0.25 0.25(2)( z 2 ) and — 0.01 (2)( x 2 — y 2 ), where z is the principal axis of Jahn-Teller elongation at each Cr site. The optimum refined distribution of the magnetic moment (μ B ) was Cr(3d) 3.16(8), Cr(4s) 0.6(1), equatorial Cl(3s) 0.05(2), Cl(3p) 0.06(3) and axial Cl(3s) 0.03(3), Cl(3p) 0.08(5).

Low-temperature neutron diffraction measurements were performed on single crystals of Rb2Crl-,Mn,ClJ(O c x c 1). From these experiments a concentration versus temperature magnetic phase diagram is obtained. Three concentration ranges with different magnetic behaviour are found. In the same range as the Jahn-Teller-induced superstructure occurs. 0 6 x 6 0.41, a somewhat canted ferromagnetic structure exists. An antiferro- magnetic order is found in the range 0.59 x 1. The Tversus x dependence of the phase boundaries and the magnitudes of the magnetic moments are discussed with respect to percolation behaviour. Within the percolation limits XF = 0.41 for ferromagnetic and xA~ = 0.5Y for antiferromagnetic ordering no magnetic long-range order is observed. This finding is consistent with a proposed spin glass phase for intermediate concentrations.