A neutron diffraction determination of the crystal structures of thiourea and deuterated thiourea above and below the ferroelectric transition

Abstract

410 THE CORRECTION OF MEASURED INTEGRATED BRAGG INTENSITIES 0( 4 is the corresponding value calculated for the finite aperture (0-20 scan) and it can be seen that the approximation of an intinite slit in this case gives an error of the order of 300/0 in the TDS contribution to the peak scan. The values obtained for a spherical volume (~5) are in this case also quite good approximations for 0 > 40 °; the effective scan range being + 1-47 °. A sphere of equal volume would give an effective scan range of + 1.43 ° resulting in values of 7 30/0 lower than ~5. These effective scala ranges may be compared with the true scan range of + 1.2 °; a spherical volume using the true scan range would thus give values of c~ of the order of 20% too small in this case. The error in the intensities resulting from ignoring the TDS correction would again be less than about 1% with a corresponding error in the mean temperature factor. The relative agreement between the various ~ values is also illustrated in Fig. 2.We have also estimated the 'mosaic spread' effect for the barium fluoride measurements. The full width at half height of the mcasured peaks varied from 0.2 ° up to 0.8 ° resulting in a reduction of the TDS correction between 1% and 10%. This can therefore be an important factor particularly if the width of the measured peaks changes rapidly with the scattering angle and passes through a focusing position. The structures of thiourea, SC(NH2)~,, and deuterated thiourea, SC(ND2)2, have been determined at room and liquid nitrogen temperatures from three-dimensional neutron diffraction data. No significant structural change on deuteration has been found. N-H.--S hydrogen bonds occur in both materials at both temperatures with N-S distances of 3.35-3-43/~ and N-H-S angles of 169-171 ° and, apart from these hydrogen atoms, the molecules are planar to within 0-010/~. An analysis of the thermal parameters of the atoms in each molecule in terms of rigid vibration parameters shows that at liquid nitrogen temperature the molecules are fairly rigid whereas at room temperature there are serious deviations from rigidity. Excellent agreement has been found between the thermal vibrations of the molecules at room temperature and the observed structure change to the lower ferroelectric state. A qualitative theory of the fcrroelectric nature of thiourea is proposed which explains the observed temperature variation of the spontaneous polarization and coercive field in the lower ferroelectric region, in terms of a variable molecular orientation and a single hydrogen bond which is switched from one sublattice to the other during ferroelectric reversal.

Preparation and X-ray analysis of a 1:1 adduct of hexamethylenetetramine oxide and thiourea

“…(NH2)2CS (Table 2) are in good agreement with corresponding values in orthorhombic thiourea (Truter, 1967;Elcombe & Taylor, 1968) and other molecular complexes of hexamethylenetetramine oxide (Lam & Mak, 1978b;. The thiourea molecule is planar within experimental error.…”

Carbon tetrachloride–thiourea (1/3) adduct at 170 K

“…Tables 2 and 3 contain the derived parameters for the carbon tetrachloride/thiourea adduct. * The bond lengths and angles for the thiourea molecule compare well with those previously observed for pure crystalline thiourea (Mullen, Heger & Treutmann, 1978;Elcombe & Taylor, 1968). The hydrogen bonding in the walls of the thiourea honeycomb structure is shown in the stereopacking plot (Fig.…”

A simple refinement of density distributions of bonding electrons. IX. Bond electron density distribution in thiourea, CS(NH2)2, at 123K

“…A :significantly better R value is obtained for this model (0.017) compared with the usual spherical atom refinement (0.025). Core parameters agree within 2.5o with those from the neutron diffraction study [Elcombe & Taylor (1968). Acta Cryst.…”

“…However, all parameters for nonhydrogen atoms agree to within 2.5a. (Elcombe & Taylor, 1968 Molecule (2) S(2)-C(2) -0-033 (7) 0-5 0.234 (7) 107 (21) 92 (14) 246 ( (4) 74 (8) 63 (8) 62 (6) --13 (6) 18 (6) S(2)(l.p.) 0) 0.041 (7) 0.475 (10) 0.098 (7) 74 (8) 135 (17) 168 (17) 51 (11) 34 ( (2) S ( (10) C (1) 5-45 (7) 5.45 (7) N(1) 5.72 (I1) 6.40 (11) H (1) 1.20 (6) 0.88 (6) H (2) 1.32 (5) 0-96 (5) Molecule (2) (a) (b) S(2) 14.33 (10) 14.33 (10) C (2) 5-35 (7) 5-35 (7) N(2) 5.75 (11) 6-40 (11) H (3) 1.12 (6) 0.84 (6) H (4) 1.34 (5) The absolute agreement with parameters from a high-order X-ray refinement (sin 0/2 > 0.60) is much better, as can be seen from Table 1. However, since the standard deviations for both sets of parameters compared are quite small, the agreement ranges to as much as 7a.…”