No abstract

No abstract

Struvite, MgNH4POn.6H20, Mr=245.4, is orthorhombic, Pmn2~. To establish the correct polarity, two refinements were completed from diffractometer Friedel-pair intensities measured with graphitemonochromatized Cu Ka radiation (A = 1.54178/~) using two crystals grown under different, controlled, conditions: a = 6.955 (1), b = 6.142 (1), c = 11.218 (2)A (average cell parameters obtained and refined from 20 reflexions, 55°<-20<-110°), V= 479.2 (1) A3, Z = 2, Dx = 1.70 Mg m -3, F(000) --260, /z(Cu Ka) = 3"64 mm -~, room temperature. Final R values for the correct and incorrect polarity were 0.0322, 0.0402 for the first crystal (764 reflexions, 723 Friedel pairs) and 0.0331, 0.0419 for the second crystal (863 reflexions, 805 Friedel pairs). These values, taken with the relative Fo, Fc values of the Friedel pairs and with the results from a test proposed by Rogers [Acta Cryst. (1981), A37, 734-741 ], establish the correct polarity, identical for both crystals, at a very high confidence level. In order to complete a more accurate, room-temperature, determination, a third crystal was employed for graphite-monochromatized Mo Ka diffractometry (3. = 0.71069/~): a = 6.966 (1), b = 6.142 (1), c = 11.217 (2) A (parameters from 24 reflexions, 22 °< -20<-43°), V= 479-9 (1) A 3, Dx = 1.70 Mg m -3, /x(Mo Ka) = 0.40 mm-l. Final R--0.0286 for 989 independent reflexions. Within the chosen experimental conditions, the ammonium ion does not appear to be disordered contrary to the results from former investigations of struvite and the isomorphous arsenstruvite. The correct polarity has been correlated with crystal morphology and with surface features of some important complementary forms.

No abstract

The structures of the title modifications of synthetic SrGa2Ge208 and BaGa2Ge208 have been determined from counter intensities and refined by Fourier and full-matrix least-squares methods. Both compounds crystallize in space group P21/a with Z=4. The mean tetrahedral distances for SrGa2Ge208 are:(Ge-O)= 1.748 (2), (Ga-O)= 1.829 (2) and for BaGa2Ge208: (Ge-O)= 1.753 (4), (Ga-O)= 1.820 (4) /~. The two structures may be considered very highly, or completely, ordered. A comparison of the present results with those obtained for the paracelsian form of SrGa2Si208 is given. The relations between the structural features and the stability conditions are briefly discussed.