574 BIS-THIOACETATE DE NICKEL F PICOLINE (1:2) Tableau 5. Distances intermoldculaires < 4 fk S-C'(3) 3,87 (1) C(3)--C'(3) 3,52 (2) S-C'(4) 3,65 (1) C(3)--C'(4) 3,53 (2) 0-C'(4) 3,94 (1) C(4)-C'(4) 3,77 (2) 0-C'(5) 3,81 (1) C(4)-C'(7) 3,79 (2) O-C'(8) 3,78 (I) C(5)-C'(5) 3,82 (2) O-C"(8) 3,62 (1) C(5)-C'(6) 3,60 (2) C(2)-C'(2) 3,92 (2) C(5)-C'(7) 3,69 (2) C(2)-C'(3) 3,72 (2) C(6)-C'(6) 3,74 (2) C(2)-C'(4) 3,89 (2) C(6)-C"(6) 3,75 (2) C(2)--C'(8) 3,85 (2) C(6)-C'(7) 3,84 (2) N-C'(4) 3,56 (1) C(6)-C'(8) 3,84 (2) N-C'(5) 3,96 (1) C(7)-C'(8) 3,74 (2) ridine sont obtenues avec les cycles azot6s les plus symdtriques. L'examen des distances interrnol6culaires inf6rieures 4 ~ (Tableau 5) montre que la coh6sion de la struc- ture cristalline est assur6e par des contacts de van der Waals. R6f6renees BOREL, M. M., GEFFROUAIS, A. & LEDI~SERT, M. (1976). Acta Cryst. B32, 2385-2387. BOREL, M. M., GEEEROUAIS, A. & LEDI~SERT, M. (1977). Acta Cryst. B33,568-571. BOREL, M. M. & LEDI~SERT, M. (1976). Acta Cryst. B32, 2388-2391. CROMER, D. T. (1965).Acta Cryst. 18, 17-23. CROMER, D. Z. & WABER, J. T. (1965). Acta Cryst. 18, 104-109. Acta Cryst. (1977). B33, 574-577 The Orthorhombic Phase of WO 3 By EKHARD SAIAE Mineralog&ches Institut der Techn&chen Universitd't Hannover, 3 Hannover, Welfengarten 1, Germany (BRD) (Received 11 August 1976; accepted 30 September 1976) Abstract. From ca 467 to 680°C WO 3 exhibits orthorhombic symmetry. The structure is perovskite- like with space group Pmnb and a = 7.341 (4), b = 7.570(4), c = 7-754 (4) .A. The deviation from the ideal perovskite structure is characterized by a zigzag motion of the W position in the b and e directions as well as a tilt system with tilt angles around a. The rela- tion with other WO 3 phases is discussed. Introduction. WO a has been studied in detail because of its ferroelectric, electrooptic, and semiconducting properties. It also shows the ability to incorporate metals, ammonium ions, and hydrogen to form tungsten bronzes, and displays colour centres on irradiation with UV light, application of an electrical field or doping with protons of high mobility. Unfor- tunately it is very difficult to obtain good single crystals as WO 3 shows a tendency to form substoichiometric shear phases (Sundberg & Tilley, 1974). Hence detailed structural data are lacking. The crystals show five phase transitions in the range -180 to 900°C changing from tetragonal-ortho- rhombic-monoclinic-triclinic-monoclinic during cool- ing (Tanisaki, 1960a; Salje & Viswanathan, 1975). At room temperature a monoclinic (WO3 I) and a triclinic (WO 3 II) modification can be obtained. Only the struc- tures of the monoclinic phase at room temperature (Tanisaki, 1960b; Loopstra & Rietveld, 1969) and at -70°C (WO 3 II) (Salje, 1976b) have been determined. In this paper the structure of the orthorhombic phase is described and compared with the structures of the monoclinic and tetragonal varieties. The structure of the triclinic phase will be reported later. The preparation of single crystals of WO 3 has been described (Salje & Viswanathan, 1975). For the X-ray investigations only thin plates of WO 3 (II) with pseudo- orthorhombic morphology were used. The same crystals were used in all other experiments (Salje, 1974). They show no domain structures. From the results ofBerak & Sienko (1970) it can be assumed that these crystals are stoichiometric although the best single crystals show the triclinic and not the mono- clinic phase at room temperature. They differ from those we obtained by gas transport. These varieties are similar to those described by Schr6der & Felser (1972) and always show domain structures. The crystals were mounted on a Ni wire which was screwed on the top of a goniometer head. The heating element was bent to form a U with the crystal at the bottom. The probe was spliced with 'Leitsilber' (Fa.