Volume 215, Issue 4

Structures and properties of ternary and quaternary sulfides and selenides containing Gallium and /or Indium and transition elements like Sc, V, Ti, Cr, Mn, Fe, Co, Ni, Zn, Cd, and Hg crystallising in layered structures of the ZnIn 2 S 4 type are reviewed. A survey of the different polytypic structures observed in these systems is given along with a compilation of the known compounds, their structures, their range of homogeneity and their lattice parameters. As far as available data on high pressure phase transformations, thermal stability, and band gap energies of these compounds are tabulated, too.

We show that although the crystalline structure of Strontium Dithionate Tetrahydrate can be well described in either P6 4 and P3 1 space groups (or respectively, P6 2 and P3 2 ) and electro-optic measurements have indicated the 32 point group, optical second harmonic measurements performed on an oriented crystal are only compatible with point group 3.

Four new apatitic phases were prepared and their structures determined. The structure of Ca 10 (PO 4 ) 6 S was refined from single crystal X-ray data and the structures of Sr 10 (PO 4 ) 6 S, Ba 10 (PO 4 ) 6 S and Ca 10 (PO 4 ) 6 Se from powder X-ray data using the Rietveld method. The four apatites are isostructural and crystallize in the trigonal space group P-3 with the chalcogenide ion positioned at (0 0 ½). The sulfoapatites show no ability to absorb H 2 S in the way that oxyapatite absorbs H 2 O at elevated temperatures. This can be attributed to the position of the sulfide ion and the way it influences the crystal structure around the vacant chalcogenide position at (0 0 0).

The crystal structure of monoclinic CuNH 4 PO 4  · H 2 O has been determined from X-ray powder diffraction data. The space group is P2 1 /c. The cell dimensions are a=8.6676(1) Å, b=7.5312(4) Å, c=7.4054(4) Å, β=94.508(3)o, Z=4. The structure of CuNH 4 PO 4  · H2O was solved ab initio using EXPO program. The structure refinement has been carried out by FULLPROF package. The final R Bragg value was 3.4%. The compound has a layered structure. Copper atoms link PO 4 tetrahedra and water molecules to layers which are parallel with yz plane of unit cell. Ammonium cations are in the interlayer space and couple these layers by N-H···O hydrogen bonds.

Structural changes accompanying thermal transformation in Na-montmorillonite samples up to a temperature of 500°C have been investigated by X-ray line profile analysis. The method of Fourier initial slope and variance analysis of X-ray line profiles have been used to calculate the different microstructural parameters like crystallite size, r.m.s. strain (<e 2 > 1/2 ), variation of inter layer spacing (g), fraction of planes affected by such defects (γ), etc. at different stages of the dehydration process. The dehydration mechanism also has been explained based on thermogravimetric analysis (TGA).

The crystal structures of two new hydrogen monofluorophosphates, CsHPO 3 F and Cs 3 (NH 4 ) 2 (HPO 3 F) 3 (PO 3 F), have been determined at 180 K. The crystals of CsHPO 3 F are monoclinic, space group C2/m, with the unit cell dimensions of a = 14.478(5) Å, b = 5.929(2) Å, c = 5.413(2) Å, β= 103.30(3)°, V = 452.2(3) Å3, and Z = 4, R 1 = 0.0155 and wR 2 = 0.0378. Cs 3 (NH 4 ) 2 (HPO 3 F) 3 (PO 3 F) crystallizes in the monoclinic space group P2 1 /c, with a = 20.619(4) Å, b = 12.076(2) Å, c = 15.856(3) Å, β= 102.58(2)°, V = 3853(1) Å3, and Z = 8, R1 = 0.0455 and wR 2 = 0.0976. In the structure of CsHPO 3 F, two symmetrically disordered hydrogen bonds (2.527(2) Å) connect two HPO 3 F units to form closed dimers. Cs 3 (NH 4 ) 2 (HPO 3 F) 3 (PO 3 F) contains a complex hydrogen bond network (N-H···O and O-H···O) with two different types of P tetrahedra, PO 3 F and HPO 3 F. The structure contains a complex network of hydrogen bonds. Units of 2NH 4 and PO 3 F (I) and Cs and HPO 3 F (II) are found in and around planes parallel to (x0z). Fluorine is not involved in the hydrogen bonding in either of the structures.

A number of lanthanide compounds LnX 2 (Ln = Sc, Y, La-Lu; X = S, Se, Te), LnY2 (Y = P, As, Sb), and LnZ2 (Z = Si, Ge) are known to crystallize with a structure derivable from only one aristotype, the ZrSSi type. This structure type contains square layers of silicon atoms (in general: square layers of the element of medium electronegativity). The structural deviations observed within the different compounds are usually small and principally affect this square layer. The space groups of the derivative structures under consideration are subgroups of that of the aristotype and the group-subgroup relations are outlined. Distortions of the square layers caused by electronic reasons can mostly be understood by applying simple electron counting rules. A number of compounds with chalcogen deficiency LnX 2 -δ is noted, too, in which the occurrence of vacancies in the square layers often leads to even larger superstructures.

Crystal structures of a monoclinic and a hexagonal monosaccharide, 40 atoms per molecule, were successfully predicted by a potential energy minimisation method. The intermolecular interactions were described by three terms covering electrostatic, attraction and repulsion contributions to the total energy. Two types of atomic charges based on the Hartree-Fock wave function, Mulliken and derived from the electrostatic field generated by a molecule, were combined with two force fields called Williams and Dreiding. With the cell parameters fixed, the correct structure was found in approximately 75 % of cases. With the cell parameters optimised no reasonable structure was found for the hexagonal monosaccharide. The accuracy of the predicted structures was evaluated by means of X-ray Rietveld refinements.

The solid state structures of the complexes Co(Hdmg) 2 (NCS)(NH 3 ) (Hdmg = dimethylglyoximate) and CoCl(Hdmg) 2 (H 2 O) were determined by single crystal X-ray diffraction. The thiocyanato complex crystallizes in the orthorhombic space group Pnma with lattice constants a = 14.626(3) Å, b = 9.222(3) Å, c =11.575(3) Å, V = 1561.2(7) Å 3 and Z = 4 while the chloro complex is monoclinic, space group P2 1 /m with lattice constants a = 7.572(2) Å, b = 8.820(4) Å, c = 10.351(5) Å, β=96.29(3)º, V = 687.2(5) Å 3 and Z = 2. In these coordination compounds the pseudooctahedral molecules are located on crystallographic mirror planes; both bridging hydrogen atoms in each complex are close to one of the chelating ligands whereas chemical intuition would rather suggest two monoprotonated Hdmg moieties. This conflict is resolved when the correlation between intra and intermolecular hydrogen bonds is taken into account.