Search results for "Monoclinic crystal system"
showing 10 items of 344 documents
Relationships between fatty acid monolayer structure on the subphase and on solid substrates
1991
Abstract Docosanoic acid monolayers with known molecular packings on the water surface have been deposited on thin polymer films and then investigated using transmission electron diffraction at normal and tilted incidence. The diffraction patterns from monolayers deposited under all conditions investigated could be indexed as arising from the same conformationally disordered centred rectangular packing with molecules standing perpendicular to the substrate, although with a spread of unit cell parameters within and between each deposition condition significantly greater than the experimental error. This packing has been seen in monolayers on the water surface, but only under conditions compl…
Nb 4 Te 17 I 4 , a New Pseudo One‐Dimensional Solid‐State Polytelluride
1994
The new ternary compound Nb4Te17I4 has been prepared and structurally characterized. It crystallizes in the monoclinic system, space group C2/c with unit-cell parameters a = 16.199(4), b = 8.128(2), c = 27.355(6) A, β = 110.84(2)°, Z = 4. The structure consists of infinite one-dimensional niobium/tellurium chains running parallel to the crystallographic c direction. The chains are separated by iodine atoms. Short and long metal–metal distances alternate in the sequence of three consecutive short bonds ([d ≈ 3.1 – 3.2 A) and one long (d = 4.268 A) metal–metal separation. Each Nb atom is eight-coordinate. The composition of the chain is ∞11[(Nb5+)2(Nb4+)2(Te22−)4(Te32−)3(I−)4].
Metal ion complexation by tetraester derivatives of bridged calix [4] arenes
1992
A series of bridged calix [4] arenes (5–10 methylene groups in the bridge) were synthesized and transformed into their tetraester derivatives by reaction with ethyl bromoacetate. The stability constants for complexes of the tetraester derivatives with sodium, potassium and silver cations, determined in methanol by spectroscopic or potentiometric techniques, show a drastic decrease (more than 105 for Na +) for the shorter methylene chains. 1H NMR studies demonstrate a conformational rearrangement of the calixarene part to fourfold symmetry to be necessary for the complexation of a cation, which is prevented by the shorter chains. This is further confirmed by the X-ray structure of a tetraest…
Structure and properties of some nitro derivatives of N -methyl- N -phenylnitramine
2000
Abstract Ten mono-, di- and tri-nitro derivatives of N-methyl-N-phenylnitramine were prepared and investigated using spectral and electrooptical methods. Three of them, viz. N-(2, 5-dinitrophenyl)-N-methylnitramine (monoclinic, P21/c, a=8.248(2), b=11.655(2), c=10.404(2) A , β=102.57(2)°), N-(2,3-dinitrophenyl)-N-methylnitramine (monoclinic, P21/c, a=9.224(2), b=7.222(2), c=15.458(4) A , β=101.08(2)°)) and N-(3,5-dinitrophenyl)-N-methylnitramine (monoclinic, P21/n, a=9.814(2), b=12.000(2), c=8.865(2) A , β=114.94(2)°) were examined by the X-ray diffraction method. The nitramino group is nearly planar with the short N(7)–N(8) bond and strongly electron deficient N(8) atom. The nitramino grou…
Synthesis and Crystal Structure of Ln3(OH)(CrO4)4 · 3.5H2O (Ln = Pr, Nd) and Ln2(OH)2(CrO4)2 (Ln = Tb, Gd): Thermal Behavior and Infrared Spectra of …
1993
Abstract Crystal structures of Ln 2 (OH) 2 (CrO 4 ) 2 ( Ln = Gd, Tb) and Ln 3 (OH)(CrO 4 ) 4 · 3.5H 2 O ( Ln = Pr, Nd) were determined by a conventional single-crystal X-ray diffraction technique. Crystals of Ln 2 (OH) 2 (CrO 4 ) 2 ( Ln = Gd, Tb) are monoclinic, space group P 2 1 / c (no. 14) with lattice parameters a = 8.2454(4) A, b = 11.3834(5) A, C = 12.1420(8) A, and β = 136,590(5)°, Z = 4, D x = 4.92 mg · m -3 for Gd 2 (OH) 2 (CrO 4 ) 2 and a = 8.214(1) A, b = 11.348(1) A, c = 12.094(2) A, and β = 136.59(1)°, Z = 4, D x = 5.01 Mg · m -3 for Tb 2 (OH) 2 (CrO 4 ) 2 . Crystals of Ln 3 (OH)(CrO 4 )) 4 · 3.5H 2 O ( Ln = Pr, Nd) are monoclinic, space group C 2/ c (No. 15) with lattice param…
Synthesis, crystal structure and magnetic properties of the first structurally characterized 1,2-dithiocroconato-containing Cu(II) complex, [Cu(bpca)…
1996
Abstract The first crystal and molecular structure of a transition metal complex containing 1,2-dithiocroconate (1,2-dtcr, dianion of 1,2-dimercaptocylopent-1-ene-3,4,5-trione), [Cu(bpca)(H2O)]2[Cu(1,2-dtcr)2]·2H2O (where bpca is the bis(2-pyrdidylcarbonyl)amide anion), has been determined by single crystal X-ray diffraction methods. The compound crystallizesin the monoclinic syste, space group P21/c, with a = 11.661(3), b = 20.255(6), c = 8.265(3) A , s = 107.26(2)° and Z = 2. The structure is formally built of [Cu(1,2-dtcr)2]2− and [Cu(bpca)(H2O)]+ ions and water of hydration. The copper atom of the anion is situated at a crystallographic inversion centre, bonded to four sulfur atoms in a…
Superionicity and polymorphism in calcium fluoride at high pressure.
2014
We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF2 at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists in a decrease of the normal-> superionic critical temperature with compression. This unexpected effect can be explained in terms of a P-induced softening of a zone-boundary $X$ phonon which involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P2_1/c). We discuss the possi…
High-pressure x-ray diffraction study of SrMoO4 and pressure-induced structural changes
2007
SrMoO4 was studied under compression up to 25 GPa by angle-dispersive x-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase to a monoclinic fergusonite phase at 12.2(9) GPa with cell parameters a = 5.265(9) A, b = 11.191(9) A, c = 5.195 (5) A, and beta = 90.9, Z = 4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of t…
High-pressure structural phase transitions in CuWO4
2010
We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experim…
Variation of the optical absorption edge in AgGaS2 single crystals at high pressure
2003
In this paper the optical absorption edge of AgGaS 2 is measured as a function of pressure up to 26 GPa in order to verify the effect of the three phases transitions occurring in that pressure domain. The direct energy gap increases linearly with pressure at the rate of about 4.0 x 10 -2 eV GPa -1 up to 10.2 GPa. The absence of any discontinuity in the energy gap in the pressure range of 4.2-10.2 GPa confirms that the volume change, in the chalcopirite to monoclinic second-order transition, if it exists, is very small. When the pressure is raised above 10.2 GPa, the energy gap drops suddenly by about 1.1 eV and the spectral form of the absorption coefficient is typical of semiconductors wit…