No abstract

No abstract

AbstracL An x-ray singleclystal determination of ihe CuO stmture has been made at 196 K, i.e. below the N6el temperature 230 K, and, as a check, the crystal smclure a1 m m temperature was also determined. The correct space p u p for lhe structure at bolh temperatures was found lo be C c . Earlier mulls of magnnk and neutmn diiimction measuremenls can be explained as antifemmagnetic coupling betwgn copper atoms via oxygen (superexchange) in chains running in the [I 0-11 direction. The s " a l results show changes with temperature in Cu-0 distances in these chains in each -Cu-(M1group lhe longer distance is increased and the shorter decreased when passing from 196 K to m m temperature. This implies a weaW antiferromagnetic mupiing at room temperature. The re6nement of CuO-stmctun at r w m temperature reponed earlier by Asbrink and Norrby (1970) showed the symmetry to be CZ/e. An attempt to refine CUO in the space group C e with the old data was not successful. The diffeerent m u l k obtained Hith different aystals are tentatively explained from published observations regarding valence fluctuations in CuO and non-stoichiometry caused by cation vacancies.

Synchrotron-radiation X-ray diffraction studies of CaF2 at high pressures have been performed on a powder sample up to 45 GPa and on a single-crystal sample up to 9.4 GPa. The bulk modulus of the low-pressure phase was determined to be B o = 87 (5) GPa. A phase transition was observed at about 9.5 GPa. The transition is accompanied by a volume contraction of 11%. The high-pressure phase is orthorhombic PbC12 type (space group Pbnm). The sample only partially reverts to the low-pressure phase upon release of pressure.

Trititanium pentoxide (Tia05) is dimorphic--a rapid, reversible phase transformation occurring at about 120 °C. The high-temperature modification is of the anosovite type (slightly deformed pseudobrookite structure). It can be stabilized at room temperature by small contents of iron.The crystal structure of the low-temperature form has been determined. It is monoclinic, space group C2/m, with a cell content of 4 Ti305 and the unit-cell dimensions a = 9.752, b = 3.802, c----9"442A; fl-----91.55 ° .It may be described in terms of Tie 6 octahedra which are joined by sharing edges and corners to form an infinite three-dimensional framework. The structure is discussed and compared with the anosovite structure.