Tin(II) oxide: structure refinement and thermal expansion

SnO, tetragonal, P4/nmm, a = 3.8029 (5), c = 4.8382 (8) A (neutrons, 293 K), Z = 2. Profile and integrated-intensity refinements at room temperature confirm the regular square-pyramidal coordination of Sn It, with S n O 2 .224(8) /k . Final R for the integrated-intensity refinement was 4 .3% for 28 measured intensities. The thermal expansion is less anisotropic than expected for a layer structure. Introduction. Various forms of tin monoxide, SnO, have been reported (Moore & Pauling, 1941; Donaldson, Moser & Simpson, 1963; Kwestroo & Vromans, 1967). The blue-black modification of SnO is isostructural with the tetragonal form of PbO but it is the only example of a regular square-pyramidal tin(II) coordination. The present neutron study, which was undertaken to check this tin(II) configuration, confirms the previous X-ray powder structure determination (Moore & Pauling, 1941). The sample material was powdered SnO obtained from OSI. High-temperature X-ray powder data were recorded on a C G R diffractometer with a hightemperature device (Barret, Gerard & Watelle-Marion, 1968) using pelleted samples and working under dry nitrogen. The unit-cell parameters were obtained by least-squares refinement from 20 lines (Cu K+~; 5 ° < 0 < °) and were subsequently fitted by least squares by a polynomial of the form a(T) = a o + a~ T + a 2 T 2 where a denotes the cell constant a or c in /k and T the temperature in °C. Neutron diffraction patterns at 293 K were collected on the D1B diffractometer of the Institut Laue-Langevin with 2 = 1.282 A. The sample was inserted in a 10 mm diameter vanadium tube. Data were collected in two steps (2 ° < 20 < 82 ° and 65 < 20 < 145 ° ) of two and a half hours and reduced using conventional ILL programs (Wolfers, 1970). Integrated intensities were determined by fitting the shape of the Bragg peaks to Gaussians. The method of profile-fitting structure refinement (Rietveld, 1969; Hewat, 1973) was applied in the space group P 4 / n m m [tin in 2(c) and oxygen in 2(a)]. The refinement included the scale factor, three half-width parameters, the zero-point correction, the lattice constants, one positional parameter, two isotropic thermal parameters, the preferred-orientation parameter and the peak-asymmetry parameter for peaks below 20 = 30 °. The conventional integrated-intensity refinement used a local program (L'Helgoualch, Fonteneau & Pan0567-7408/80/112763-03501.00 © 1980 International Union of Crystallography 2764 T I N ( I I ) O X I D E netier, 1975) allowing for the refinement of a preferredorientation paramete r G (Rietveld, 1969); the refinement included the scale factor, one positional parameter, two isotropic thermal parameters and the preferredorientation paramete r and was based on 28 measured intensities (42 hkl) . The scattering amplitudes b(Sn) = 6 .2 and b(O) = 5.8 fm were used. The final pa ramete rs are given in Table 1;* there is fair agreement between the results of the two methods, both for the refined parameters and their s tandard deviations. The cell pa ramete rs in the range 273 K to 593 K are as follows (T in °C) : a (/k) = 3 .7984 (6) + 3.31 (36) x 10 -5 T + 7.9 (16) × 10 -8 T 2 c (A) = 4 . 8 3 6 8 (7) + 1 .25 (4) x 10 -4 T + 6.2 (19) x 10 -8 T 2. Discussion. The regular square-pyramidal coordinat ion of Sn II in te t ragonal SnO is confirmed; as expected (Shannon, 1976), the S n O distance (Table 1) is shorter for this regular coordinat ion than the average S n O distances observed in distorted square pyramids : 2 .302 A in NaESn(C204) 3 (Donaldson, Donoghue & Smith, 1976), 2 .312 A in SNC204 (Gleizes & Galy , 1979), 2 .288 A in , S n W O 4 (Jeitschko & Sleight, 1972). However , the most impor tant feature of the SnO structure does not lie in the S n O distance but rather in the O S n O angles: the coordinat ion of Sn n is usually described as being derived from an octahedron distorted by the lone pair (Brown, 1974) which leads, for the fourfold coordinat ion, to two sets of O S n O angles * A list of structure factors has been deposited with the British Library Lending Division as Supplementary Publication No. SUP 35531 (2 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. Table 1. Fina l ref ined p a r a m e t e r s Neutrons a Neutrons b a (A) 3.8036 (12) 3.8029 (5) c (/k) 4.8385 (15) 4.8382 (8) c/a 1.2721 (5) 1.2722 (2) z(Sn) 0.2384 (11) 0.2383 (8) B(Sn) (A z) 1.02 (14) 0.57 (7) B(O) (A 2) 1-06 (13) 0.99 (9) G c 0.063 (16) 0.023 (11) R v, Rw, ~, (%) 4.3, 4.3 4.4 R e , Rwe (%) 10.9, 11.6 d(Sn-O) (/k) 2.224 (8) 2.224 (6) X-ray 3. 804 (6) a 4.826 (8) a 1.269 0-2356 (19) e