Volume 218, Issue 12

The crystal chemistry of binary and ternary intermetallic compounds with tetragonal U 3 Si 2 or Zr 3 Al 2 type structure is reviewed. Besides the preparation techniques and crystal chemistry, we especially focus on the structure–property relations and chemical bonding peculiarities.

Single crystals of Ba 2 VSi 2 O 8 have successfully been grown from a high-temperature solution and single crystal X-ray diffraction used to refine its fresnoite type average crystal structure. In contrast to other recently investigated representatives of the fresnoite family of compounds, Ba 2 VSi 2 O 8 did not at first appear to be incommensurately modulated. Neither single crystal X-ray diffraction nor powder neutron diffraction gave any indication for the existence of satellite reflections in addition to the strong average structure Bragg reflections. A careful electron diffraction study, however, has revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave-vectors q 1 = 0.297 [110]* ½ c * and q 2 = 0.297 [11̅0]* + ½ c *. The reciprocal space positioning of these incommensurate primary modulation wave-vectors, the overall (3 + 2)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from the single crystal XRD refinement are all consistent with their arising from condensed Rigid Unit Mode (RUM) modes of distortion of the Ba 2 VSi 2 O 8 parent structure. The weakness of the satellite reflections in Ba 2 VSi 2 O 8 relative to, for example, Ba 2 TiGe 2 O 8 or Sr 2 TiSi 2 O 8 arises from the significantly smaller amplitude of the condensed RUM mode as is clear from the magnitudes of the refined average structure anistropic displacement ellipsoids.

The type B structure of the single rare earth element disilicates has been revised using room temperature and pressure single-crystal X-ray diffraction measurements on Gd 2 Si 2 O 7 , Tb 2 Si 2 O 7 and Ho 2 Si 2 O 7 synthesized at 2–2.5 GPa, 1400–1500 °C in a piston-cylinder apparatus and Dy 2 Si 2 O 7 synthesized hydrothermally at 0.1 GPa. Crystal data are: triclinic, space group P 1̅, Z = 4; Gd 2 Si 2 O 7 – a = 6.6609(2), b = 6.7081(2), c = 12.1390(4) Å, α = 94.277(1), β = 90.577(1), γ = 91.441(1)°, R = 0.033, and D x = 5.929 g/cm 3 ; Tb 2 Si 2 O 7 – a = 6.6331(3), b = 6.6799(2), c = 12.0967(2) Å, α = 94.128(1), β = 90.609(2), γ = 91.541(1)°, R = 0.038, and D x = 6.041 g/cm 3 ; Dy 2 Si 2 O 7 – a = 6.6158(2), b = 6.6604(2), c = 12.0551(4) Å, α = 94.373(2), β = 90.836(2), γ = 91.512(2)°, R = 0.031, and D x = 6.156 g/cm 3 ; and Ho 2 Si 2 O 7 – a = 6.5960(2), b = 6.6328(3), c = 12.0214(6) Å, α = 94.479(1), β = 90.856(1), γ = 91.749(2)°, R = 0.034, and D x = 6.313 g/cm 3 . Substitution of Gd 3+ , Tb 3+ , Dy 3+ , and Ho 3+ cations into the type B structure results in proportional linear decrease in size of the REEO n polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.

X-ray diffraction studies on eleven single crystals having compositional ratios close to Yb/(Fe,Al) = 2:17 revealed four distinctly different structures, two having hexagonal symmetry (LuFe 9.5 and TbCu 7 type) at low and intermediate Al and Yb contents (0–7.5 at Al; 9.6–10.6 at Yb and 11.2–12.5 at Al; 10.5–10.9 at Yb), respectively, and two having rhombohedral symmetry (PrFe 7 (A) and PrFe 7 (A + B) types) at high Al and Yb contents (9.8–19.8 at Al; 10.7–11.6 at Yb). All have a common hexagonal TbCu 7 type subcell and contain chains made up of Yb and Fe(Al) 2 dumbbells running along the c -axis that substitute for each other with various ordering schemes and concentrations. The subcell ratio c / a depends on both the concentration of Fe(Al) 2 dumbbells, for which it generally increases, and the structure type (dumbbell order). We define an order parameter capable of describing the dependency of the subcell ratio c / a on the Fe(Al) 2 dumbbell ordering, irrespective of structure type, as the probability of finding two Fe(Al) 2 dumbbells within a chain that are separated by one Yb atom only. Aluminium substitutes for iron mainly on non-dumbbell sites at low Al contents (LuFe 9.5 and TbCu 7 type) and enters Fe 2 dumbbell sites only at high Al contents (TbCu 7 type and PrFe 7 type derivatives). The bond distances in the mixed Fe(Al) 2 dumbbells increase with increasing Al content. Those in the Fe 2 dumbbells depend on the dumbbell order (within one or between different structure types), and less on the Al content of the crystal.

The crystal structure of CaSiO 3 -walstromite has been solved and refined by a data set collected in situ from an inclusion in a diamond single crystal (1135 observed reflections, Mo K α , Nonius CAD4, R = 3.46). The crystal structure is almost identical to that of walstromite (Ca 2 BaSi 3 O 9 ). The replacement of Ba 2+ by Ca 2+ causes no significant structural changes (ring silicate; according to F. Liebau: dreier single rings). The structural differences to wollastonite-II (Ca 3 Si 3 O 9 ) are also relatively small: The Ca positions are almost the same, but the axial Si 3 O 9 -ring is isomorphously replaced by the same ring with the inverse axiality. This constitutes a new principle of isomorphous replacement. Based on density functional theory the equation of state has been predicted. This allows us to estimate the remaining pressure of the investigated CaSiO 3 inclusion in the diamond to be ≈3 GPa.

In this study 4-(3-coumarinyl)-3-benzyl-4-thi azolin-2-one 4-methylbenzylidenehydrazone 3 was synthesised. An independent proof of the thiazolylhydrazone structure of 3 was achieved by single crystal X-ray diffraction analysis. Elemental analyses and spectral data (IR, 1 H-NMR, 13 C-NMR and EIMS) of 3 was also determined. The coumarin ring system and the thiazole ring are almost planar, the dihedral angle between them is 62.93(5)°. The phenyl groups make dihedral angles of 86.09(7) and 31.67(7)° with the thiazole ring plane. The crystal structure is stabilised by intra-, intermolecular hydrogen contacts and C–H... π type interactions.