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Neutron diffraction data were collected on urea at 60, 123 and 293 Κ, and the nuclear parameters were determined [/? (F) = 0.034,0.033 and 0.043 respectively]. The parameters at 293 Κ are compared with the neutron parameters of Pryor and Sanger by means of half normal plots and χ 2 tests, and the low-temperature parameters with the ls 2 -core parameters which were determined by Mullen and Hellner with X-ray data. Significant differences of0.0030 Â, on average, were found between the t/ 33 components from our low-temperature determination and from the Is 2 -core refinement. X-Ν and X-X(1 s 2 ) maps are calculated for about 100 Κ with two different scale factors and, after an assessment of all parameters, a "best" experimental deformation density is calculated. The effect of large librations of the Ν atom about the C-O axis (4.2 rms degrees) on the density distribution, and the effect of using not fully correct phases are discussed. The errors in the density distribution which are caused by errors in the X-ray data, in the scale factor and in the neutron parameters, are assessed to be 0.07 e -3 at the bond peaks, and 0.45 eA" 3 at the nuclear positions. The "best" experimental deformation density has higher bond peaks (except for the C-O bond) than a (published) theoretical difference density (4-31G basis), but lower peaks in the lone-pair regions of the oxygen atom and no polarization minimum at the oxygen atom.

While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca(2)Co(1 - x)Zn(x)Si(2)O(7) system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P42(1)m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P2(1)2(1)2. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T(1) tetrahedra in the 3 x 3 superstructure. In this structure, the clustering of the low-coordinated Ca(2+) ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.

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.