Volume 217, Issue 2

Many crystalline materials show chemical short range order and relaxation of neighboring atoms. Local structural information can be obtained by analyzing the atomic pair distribution function (PDF) obtained from powder diffraction data. In this paper, we present the successful extraction of chemical short range order parameters from the x-ray PDF of a quenched Cu 3 Au sample.

The crystal structures of yellow Cu 3 PS 4 and of black Cu 3 SbS 4 were refined from single crystal X-ray diffraction data. Cu 3 PS 4 crystallizes orthorhombic in an ordered wurtzite superstructure type with the space group Pmn 2 1 (no. 31), a = 7.282(1) Å, b = 6.339(1) Å, c = 6.075(1) Å, V = 280.38(8) Å 3 , and Z = 2. The refinement converged to R = 0.0276, and wR 2 = 0.0710 for 737 unique reflections and 44 parameters. Cu 3 SbS 4 crystallizes tetragonal in an ordered sphalerite superstructure type with the space group I 4̅2 m (no. 121), a = 5.391(1) Å, c = 10.764(1) Å, V = 312.83(9) Å 3 , and Z = 2. The refinement converged to R = 0.0213, and wR 2 = 0.0532 for 492 unique reflections and 14 parameters. The crystal structures of the title compounds and related normal tetrahedral structures are discussed with respect to the preference of either hexagonal or cubic packing of the anions.

The crystal structures of two K 1- x Cs x BSi 2 O 6 solid solutions have been refined at room temperature by the Rietveld method: x = 0.12, a = 12.6858(4) Å, R wp = 7.66%, R F = 5.56% and x = 0.50, a = 12.8480(2) Å, R wp = 7.64%, R F = 3.10%. They are isostructural to cubic KBSi 2 O 6 with the space group I 4̅3 d . The structure is built up from (Si,B)O 4 tetrahedra linked in four-, six- and eightfold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi 2 O 6 and K 0.5 Cs 0.5 BSi 2 O 6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi 2 O 6 has been found in situ under heating. The new polymorphic I 4̅3 d (cubic) – Ia 3̅ d (cubic) transition and the new Ia 3̅ d cubic polymorphic phase has been proposed for K 1- x Cs x BSi 2 O 6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.

The structural variations within the aegirine (NaFeSi 2 O 6 ) – Li–aegirine (LiFeSi 2 O 6 ) solid-solution series have been determined from structure refinements of single–crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C 2/ c . Replacing Na + by Li + in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi 2 O 6 , the bond length decreases by 7.1% with increasing replacement of Na + by Li + . The decrease of the M2–oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li–aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe 3+ O 6 – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na 1- x Li x FeSi 2 O 6 with Li + concentrations x > 0.75 crystallize in the space group P 2 1 / c . For the compounds with x < 0.75, no phase transition from C 2/ c → P 2 1 / c was observed down to low temperatures (30 K). A x – T phase diagram for the stability of the low temperature P 2 1 / c structure is provided. The characteristica of the P 2 1 / c low temperature form of the (Na,Li)FeSi 2 O 6 compounds agree with those of pure LiFeSi 2 O 6 , recently described by Redhammer et al. (2001).

The crystal structure of Na 3 HP 2 O 7 ⋅ H 2 O has been determined from X-ray powder diffraction data. The structure is monoclinic, space group P 2 1 / n (No. 14), with unit cell dimensions a = 10.4242(1) Å, b = 6.8707(1) Å, c = 10.0837(1) Å, β = 99.2258(7)°, V = 712.87(2) Å 3 , Z = 4, D m = 2.43 g/cm 3 and D x = 2.41 g/cm 3 . The structure has been solved by direct methods using EXPO program and refined by FULLPROF package. The final R Bragg value was 7.6%. Positions of hydrogen atoms were not defined. There are three sodium cations in the asymmetric unit. Two of them are five-coordinated and one cation has sixfold oxygen coordination. Each HP 2 O 7 3– anion is hydrogen bonded to its two adjacent neighbours forming infinite chains parallel to the b axis. Water molecules are located in infinite channels extended along the b direction. The oxygen atom of the water molecule is connected with two sodium cations and hydrogen bonded with one oxygen atom of diphosphate group.

We have studied the crystal structure of ((phenyl) 4 As) 2 C 60 Cl at room temperature and below the phase transition at T c = 125 K by single-crystal x-ray diffraction. At room temperature tetragonal lattice parameters were found as a = b = 12.588(1)Å and c = 20.345(1)Å. At T = 120 K the lattice parameters were determined as a = b = 12.5060(1)Å and c = 20.4420(1)Å. The room temperature and low-temperature structures were found to be isostructural, with space group I 4/ m . Structure refinements were performed with restrictions on the C 60 molecules according to the noncrystallographic isohedral point symmetry I h . Derivations from this symmetry could not be found, suggesting that any possible Jahn-Teller distortion of the C 60 radical anions will be smaller than 0.01Å. The phase transition observed in ESR at T c = 125 K is proposed to be a glass transition. At room temperature there is dynamic disorder between two orientations of the C 60 molecules as they are related by the 4-fold rotation. Below T c the disorder becomes static. Two types of twinning have been observed in different crystals. The first type is represented by a rotation over 180 about [1,1,1] axis. It results in different orientations of the 4-fold unique axes with angles of 82.4 degrees between them. Secondly, merohedral twinning was observed corresponding to the two orientations of the structure with 4/ m symmetry on the tetragonal lattice. Both twinnings result in extra orientations of the C 60 molecules, and they should be taken into account, when analysing the anisotropy of the physical properties of crystals of this compound.

The crystal structure of the globular organic molecule R-thiocamphor (C 10 H 16 S) has been studied at ambient and low temperatures by high resolution powder X-ray diffraction using synchrotron radiation. At around 260 K thiocamphor transforms on cooling to an ordered orthorhombic structure from the orientationally-disordered body-centered cubic phase with a = 8.1477(1)Å at room temperature. The structure of the low-temperature phase was solved by auto-indexing, then by intensity extraction and direct methods using the EXPO package and refined by the Rietveld Method. The low temperature structure has a = 9.0490(1)Å, b = 14.5887(3)Å, c = 7.2931(1)Å, space group P 2 1 2 1 2 1 , Z = 4, ρ calc = 1.161 g cm -3 .

The synthesis and the X-ray crystal and molecular structure of the N,N'-Dimethylurea picrate [C 3 H 9 N 2 + C 6 H 2 N 3 O 7 - ] are reported. It crystallizes in the triclinic system, space group P 1̅ with a = 6.882(7), b = 9.050(2), c = 11.149(12) , α = 97.54(13), β = 104.02(9), γ = 96.78(14), V = 659.7(2)Å 3 , Z = 2, D calc = 1.60 g/cm 3 .The structure consists of infinite layers of anions alternating with cations linked together through an extensive hydrogen bonding involving most of the potential donors and acceptors. Semiempirical calculations for the picrate complex reproduce well the observed geometry.