Investigation of the structure of harmotome by X-ray (293 K, 100 K) and neutron diffraction (15 K)

The structure of harmotome (Ba (sub 2) [(AlO (sub 2) ) (sub 4) (SiO (sub 2) ) (sub 12) ] . 12 H (sub 2) O) was investigated by single crystal X-ray (T = 293 K and 100 K) and neutron diffraction (T= 15 K) on three different crystals. The cell parameters are: T= 293 K: a = 9.8688 (21) A, b= 14.1295 (7) A, c = 8.7092 (9) A, beta = 124.74 (1); T= 100 K: a = 9.841 (16) k, b= 14.085 (19) A, c = 8.680(15) A , 0= 124.78 (1); T= 15 K: a = 9.884(6) A, 6 = 14.100(5) A, c = 8.655(4)A, beta = 124.59(4). The refinement was carried out in space group P2 (sub 1) /m. The final agreement factors are: T= 293 K: R(F) = 4.0 %, R (sub W) (F)= 6.6 %; T = 100 K: R(F) = 4.1 %, R (sub W) (F) = 7.4 %; T= 15 K: R(F) = 9.2 %, R (sub W) (F) = 7.3 %. The differences in the mean T-O distances indicate partial Al/Si ordering in the tetrahedra. A slight displacive phase transition was observed at T = 15 K without pronounced change of the cell parameters. Hydrogen positions of the water molecules were obtained from neutron data. Only one molecule, coordinated with barium, is relatively stable with two symmetric hydrogen bridges. The remaining molecules of the barium - water complex reveal three sites for the hydrogens, two of them are not fully occupied. As regards the water not bonded to barium, hydrogen atoms could only be located for two out of four molecules. The temperature factors of the framework atoms and especially of the water molecules are too large for a temperature of 15 K and indicate high positional disorder in the harmotome structure.