Search results for "Monoclinic crystal system"
showing 10 items of 344 documents
Spin crossover behaviour in the iron(II)-2,2-dipyridilamine system: Synthesis, X-ray structure and magnetic studies
2005
Abstract The monomeric compounds [Fe(dpa)2(X)2] · solv [X = NCS−(solv = 0.5H2O) (1), N ( CN ) 2 - (2) and dpa = 2,2-dipyridilamine] have been synthesised and characterised. They crystallise in the P21/n and in the Cc monoclinic systems, respectively. Four of six nitrogen atoms coordinated to the Fe(II) ions belong to two dpa ligands which lie in cis conformation. The remaining positions are occupied by two nitrogen atoms of the pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that compound 1 exhibits an incomplete spin crossover behaviour (T1/2 ≈ 88 K), whereas compound 2 remains in the high-spin configuration. Pressure studies performed on c…
Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4
2012
We have performed an experimental study of the crystal structure, lattice dynamics, and optical properties of PbCrO 4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band gap have been accurately determined. X-ray-diffraction, Raman, and optical absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is from a monoclinic structure to another monoclinic structure. It maintains the symmetry of the crystal but has important consequences in the physical properties; among others, a band-gap collapse is induced. The second one invo…
Structure, phase transitions and molecular dynamics in ferroelastic crystal pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6]
2005
Abstract The crystal structure of the pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6], abbreviated PCA, has been determined by means of X-ray diffraction at 300 and 340 K. The space groups are monoclinic P 2 1 / n (phase III) and orthorhombic Pmnb (phase II), respectively. The crystal undergoes two structural phase transitions: first-order type at 356/329 K (heating/cooling) from phase (I) to (II) and second-order type at 323 K from phase (II) to (III). Dielectric studies suggest the plastic crystals behaviour above 356 K (phase I). Proton spin-lattice relaxation time ( T 1 ) and second moment ( M 2 ) of polycrystalline [C4H8NH2][SbCl6] have been determined at 77–370 K, at 90 and 25…
The structure, phase transition and molecular dynamics of [C(NH2)3]3[Sb2Br9]
2005
The crystal structures of [C(NH2)3]3[Sb2Br9] (Gu3Sb2Br9) at 300 K and of [C(NH2)3]3[Sb2Cl9] (Gu3Sb2Cl9) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb2X93− (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu3Sb2Br9 the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton 1H NMR second-…
Crystal structure, phase transition and ferroelectric properties of the [(CH3)3NH]3[Sb2Cl9(1 x)Br9x] (TMACBA) mixed crystals
2003
The paraelectric–ferroelectric phase transition in the [(CH3)3NH]3[Sb2Cl9(1−x) Br9x] (TMACBA) mixed crystals is investigated by the differential scanning calorimetry, dilatometric and dielectric methods. The phase transition is found at 363.5, 362.7, 360.5 and 350.9 K (on cooling) for TMACBA crystals with x = 0, 0.02, 0.17 and 0.42, respectively. The crystal structures of the pure bromine analogue (x = 1), TMABA, and the mixed crystal TMACBA (x = 0.55) are determined at 297 K. TMABA crystallizes in the trigonal space group Rc: a = 15.098(2) A, c = 21.906(4) A, Z = 3, R1 = 0.0393, wR2 = 0.0746. Its structure is built up of discrete Sb2Br93− bioctahedra and disordered trimethylammonium cation…
Structure, phase transitions and molecular motions in ferroelastic (C4H8NH2)SbCl6·(C4H8NH2)Cl
2002
The crystal structure at 293 K of the new pyrrolidinium chloroantimonate (V) analogue, (C4H8NH2)SbCl6(C4H8NH2)Cl, has been determined by x-ray diffraction as monoclinic, space group P21/c, Z = 8. The crystal is built up of isolated SbCl6- anions, two types of inequivalent pyrrolidinium cation and isolated Cl- ions. It undergoes five solid-solid phase transitions: at 351/374 K of first-order type (cooling/heating, respectively), at 356 and 152 K second order and at 135/141 and 105/134 K first order, detected by differential scanning calorimetry, dilatometric and dielectric measurements. The ferroelastic domain structure appears between 152 and 135 K. The proton nuclear magnetic resonance sec…
The Synthesis and Structure of Palladium 2,4-dimethyl-8-hydroselenoquinolinate
2014
The internal complex palladium 2,4-dimethyl-8- hydroselenoquinolinate Pd[C9H4(CH3)2NSe]2 (I) has been synthesized in the course of study of the complexing activity of 8- hydroselenoquinoline and investigation of the nature of chemical bond metal-selenium in the five-membered metal-containing ring. X-ray diffraction data for I: Monoclinic, space group P21/n, a = 9.0092(4), b = 16.3290(7), c = 14.1073(6) A, = 106.710(2)o, V = 1987.7(2) A3, Z = 4, R1=0.0477, wR2=0.1182 for 4499 reflections (diffractometer Bruker-Nonius KappaCCD, MoK). The crystal structure of the complex I is formed by neutral asymmetric molecules Pd[C9H4(CH3)2NSe]2 in which the central atom palladium is connected bidentic…
Structural diversity of cyanide-bridged bimetallic clusters based on hexacyanometallate building blocks
2002
Abstract Four new cyanide-bridged bimetallic complexes, with different geometries and nuclearities, have been synthesized and characterized by single crystal X-ray crystallography. The trinuclear compound {〚Zn(bpy)2(OH2)〛2〚Fe(CN)6〛}Cl (1)·2 MeOH·12 H2O crystallizes in the triclinic space group P 1 , with a = 12.010(2) A, b = 16.316(3) A, c = 16.325(3) A, α = 97.47(3)°, β = 92.14(3)°, γ = 92.43(3)°, V = 3166(1) A3 and Z = 4. The tetranuclear compound {〚Zn(phen)2〛2〚Fe(CN)6〛2}2– (2) cocrystallizes with 〚Zn(bpy)3〛2+ and 32 H2O molecules in the monoclinic space group C2/c with a = 42.047(8) A, b = 13.541(2) A, c = 28.781(5) A, β = 120.23(3)°, V = 14158(4) A3 and Z = 2. The pentanuclear compound …
X-ray investigation and discussion of the magnetostriction of Gd3T(T= Ni, Rh, Irx) single crystals
2000
The variations of the lattice parameters of Gd3T(T= Ni, Rh, Irx) single crystals with temperature were measured in the range 10–300 K. The compounds with Ni and Rh crystallize in the orthorhombic space groupPnma, but the compound with Ir crystallizes as Gd5Ir2in the monoclinic space groupA2/a. The three compounds exhibit an anomalous anisotropic spontaneous magnetostriction below the magnetic ordering temperature.
Molecular association in low-temperature and high-pressure polymorphs of 1,1,1,2-tetrachloroethane
2010
Interactions and aggregation of 1,1,1,2-tetrachloroethane molecules, Cl3CCH2Cl, have been investigated at low temperature and high pressure. Isobaric and isochoric crystallizations led to two polymorphs, characterized by single-crystal X-ray diffraction. The low-temperature polymorph α is monoclinic, space group C2/c, with molecules orientationally disordered in two sites at the temperature independent 70:30 rate. In isothermal conditions (295 K) 1,1,1,2-tetrachloroethane freezes at 0.73 GPa. The high-pressure polymorph β is monoclinic, space group P21/c, with the molecules fully ordered. The molecular aggregation at varied thermodynamic conditions results from the interplay of halogen inte…