Volume 228, Issue 6 -

The temperature-dependent evolution of the glass into a crystalline phase is studied for a rubidium borosilicate glass of composition 16.7 Rb 2 O · 16.7 B 2 O 3 · 66.6 SiO 2 employing X-ray diffraction (XRD) data. A glass sample was prepared by melt quenching from 1500°С within 0.5 hour. The glass sample was step-wise annealed at 13 distinct temperatures from 300 °C up to 900 °C for 1 h at every annealing step. To investigate changes in the glass structure, angle-dispersive XRD was applied by using an energy-resol v ing semiconductor detector. The radial distribution functions (RDFs) were calculated at every stage. For polycrystalline states the crystal structure of the samples with different thermal history was refined using the Rietveld method. Comparing correlation distances estimated from RDFs of glass and polycrystalline samples and mean interatomic distances calculated for polycrystalline samples by using atomic coordinates after Rietveld refinement, it is concluded that the borosilicate glass under study is converted into the crystalline state in the temperature range of 625–750 °C (i.e. in the temperature range close to the glass transition range 620–695 °C as determined by differential scanning calorimetry by using of heating rate of 20 K/min) at an average heating rate of about 0.35 K/min. When the heating rate is increased up to 10 or 20 K/min, the crystallisation temperature shifts sharply up to 831–900 °C and 878–951 °C, respectively. XRD data give evidence that distinctive traces of cubic RbBSi 2 O 6 appear from glass at about 625 °C and a two-phase range exists up to 750 °C. After annealing at higher temperatures (800–900 °C) the crystal structure practically does not change any more.

In this third part of a series about 30 different structures of AB 2 O 6 compounds are reviewed in an extensive study. We compare the near range co-ordination of all ions and try to derive general rules which hold in such complex arrangements. We examine the validity of known rules of crystal chemistry in a large field of structures and derive new ones which should help in developing criteria for a prediction of structures.

The 1 : 1 co-crystallisation experiments conducted between 1,2,4,5-benzenetetracarboxylic acid, LH 4 , and each of 2,2′-dipyridyldisulfide (2-PA) and 4,4′-dipyridyldisulfide (4-PA) resulted in the formation of two salts of distinct formulation, i.e . [LH 4 ] 0.5 [2-PAH][LH 3 ] ( 1 ) and [LH 2 ][4-PAH 2 ] ( 2 ), respectively, indicating that proton transfer had occurred during crystallisation. In 1 , a framework structure constructed by hydrogen bonded (O—H … O) LH 4 and LH 3 -0 species defines channels in which reside 2-PAH cations that feature syn geometries and intramolecular N—H … N hydrogen bonds. By contrast, in 2 , direct N—H … N links between cations and anions lead to zigzag supramolecular chains.

The co-crystallisation of two equivalents of 2,6-dinitrobenzoic acid ( 1 ) with one equivalent of a member of the isomeric N,N′-bis(pyridin-n-ylmethyl)ethanediamides, n = 2, 3 and 4, resulted in the anticipated 2 : 1 salts, 2 – 4 . The central core, i.e. CH 2 N(H)C(=O)C(=O)N(H)CH 2 , of the isomeric di-cations are superimposable and chemically equivalent pairs of carbonyl, amide and pyridinium groups have an anti conformation. By contrast, the relative orientations of the terminal pyridinium rings from the central core vary significantly across the series as do the relative disposition of the acidic-H atoms. This feature of the structure influences the nature of N—H…O hydrogen bonding and, therefore, the observed supramolecular aggregation patterns. Thus, in 2 , the syn-disposition of the nitrogen-bound hydrogen atoms enables the benzoate residue to interact via the two oxygen atoms with both pyridinium- and amine-H atoms to yield a zero-dimensional aggregate. By contrast, in each of 3 and 4 , having further distance between the acidic-H atoms, the benzoate residues still interact (either through one both or both oxygen atoms) with pyridinium- and amine-H atoms but those derived from symmetry related molecules to generate one-dimensional aggregates. The three-dimensional architectures of 1 – 3 are consolidated, in the main, by C—H…O interactions. The structure of 1 revealed the benzoic acid residue to be perpendicular in contrast to the co-planar conforämations of the nitro groups. In the crystal packing, O–H…O hydrogen bonding lead to a helical supramolecular chain.

Three new ionic [Fe( phen ) 3 ](N(CN) 2 ) 2 · 4.5 H 2 O ( 1 ), [Fe( dmeophen ) 3 ](N(CN) 2 ) 2 · 1.5 H 2 O ( 2 ) and [Fe( dmeophen ) 3 ](C(CN) 3 ) 2 ( 3 ) complexes have been synthesized ( phen is 1,10-phenanthroline, dmeophen is 4,7-dimethoxy-1,10-phenanthroline). X-ray structural analysis (( 1 ): C 2/ c , a = 18.1041(3), b = 16.8244(3), c = 24.6691(5) Å, β = 92.138(2)°; ( 2 ): C 2/ c , a = 22.0291(10), b = 18.8654(5), c = 11.6914(5) Å, β = 114.872(5)°; ( 3 ): Pbca , a = 12.0841(5), b = 22.0624(13), c = 34.2729 (16) Å) revealed that the structures of all discussed complexes consist of one [Fe( phen ) 3 ] 2+ or [Fe( dmeophen ) 3 ] 2+ complex cation and two uncoordinated tcm or dca counteranions (tcm is tricyanomethanide, dca is dicyanamide); water molecules were found in ( 1 ) and ( 2 ), too. In the cations, the Fe(II) atom is coordinated by six nitrogen atoms of chelate coordinated neutral ligands forming a distorted octahedron coordination geometry. One of the anions in each structure is disordered. Weak π–π interactions between planar dca anions and pyridine rings of phen in ( 1 ) and O—H … O and O—H … N hydrogen bonds in ( 1 ) and ( 2 ) stabilize their structures.