The study of rutile-type structures has been undertaken in order to investigate the main features of the electron density in these compounds. The results of X-ray diffraction experiments carded out on VF2 have been published elsewhere [de Almeida & Costa (1985). Port. Phys. 16,[181][182][183][184][185][186][187][188][189][190]. Significant contour levels observed in difference Fourier maps were attributed to the asphericity of the 3d-electron distribution in the crystalline field of the rutile-type structure. Attempts were made to interpret these effects quantitatively by carrying out: (i) a fit of the parameters which represent the occupancy of each of the 3d orbitals, based on comparison of the observed structure factors with those calculated from 3d wave functions; (ii) a multipole refinement of the parameters which determine the occupation of the 3d 0108-7681/87/040346-07501.50 orbitals in the crystal field. Crystal data: vF2, Mr = 88.94, tetragonal, P42/mnm, a=4.806(10)
IntroductionThe study of rutile-type structures, MF2, where M is a first-series transition metal, has been undertaken in our laboratory. The aim of this work is a careful determination of the electron-density distribution with a degree of accuracy which would enable a quantitative analysis of the charge-density asphericity © 1987 International Union of Crystallography M. M. R. COSTA AND M. J. M. DE ALMEIDA 347 to be carried out. This paper describes the results obtained for vanadium difluoride. A detailed discussion of the experimental conditions used in data collection and of data analysis, as reported in a previous paper (de Almeida & Costa, 1985), will be summarized in the following paragraphs. An attempt was made to interpret in a quantitative way the effects observed on difference Fourier maps by carrying out a refinement of the parameters which define the occupancy of the 3d orbitals in the crystal field.
Data collectionSpecimens I and II with approximate dimensions 0.04 × 0.05 × 0.10 mm and 0.06 × 0.07 × 0.12 mm respectively were cut from the same large single crystal, grown by B. J. Garrard, Clarendon Laboratory, Oxford (England) (Wanklyn, Garrard & Wondre, 1976). The lattice parameters were determined from 23 reflections with 37 < 20 < 52 ° for crystal I and 25 reflections with 18 < 20 < 63 ° for crystal II using the standard technique developed for X-ray diffractometry: a = b = 4.806 (10), c = 3.237 (7) A,. X-ray intensities diffracted by both crystals were measured on a CAD-4 four-circle diffractometer.Two independent experiments, A and B, were performed: a set of 2447 integrated intensities from crystal I I were observed and measured using Ag Ka radiation in experiment A. A similar set of data, consisting of 2882 reflection intensities from crystal I, were obtained in experiment B, using Mo Ka radiation.Reflections within a sphere of reciprocal space (-10 <_ h _<10, -10