A comparative study of the modulated structures of some A2BX4 compounds

The structures of the modulated phases of K₂SeO₄, Rb₂ZnCl₄, Rb₂ZnCl₄ and Cs₂CdBr₄ are compared. Both commensurate and incommensurate modulations are considered. Clear common features can be derived between the polarization vectors of their primary harmonic modulations. Even distortions with very different modulation wavevectors present a clear correlation.     

Supershell structure in alkali metal nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.     

Structural analysis, spectroscopic, and magnetic properties of the 1D triple-bridged compounds [M(dca)2(bpa)] (M = Mn, Fe, Co, Zn; dca = dicyanamide; bpa = 1,2-bis(4-pyridyl)ethane) and the 3D [Ni(dca)(bpa)2]dca·6H2O

The family of compounds [Mn(dca)(2)(bpa)] (1), [Fe(dca)(2)(bpa)] (2), [Co(dca)(2)(bpa)] (3), [Zn(dca)(2)(bpa)] (4), and [Ni(dca)(bpa)(2)]dca·6H(2)O (5), with dca = dicyanamide and bpa = 1,2-bis(4-pyridyl)ethane, has been synthesized. These compounds have been characterized by single crystal (1, 2, 4, and 5) and powder X-ray diffraction (3), by Fourier transform infrared (FTIR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, and by magnetic measurements. Compound 1 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.757(6), b = 9.692(3), and c = 13.073(4) ?, and β = 123.02(2)°; Compound 2 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.588(5), b = 9.661(3), c = 12.970(5) ?, and β = 123.16(3)°; Compound 4 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.519(2), b = 9.643(2), c = 12.943(2) ?, and β = 123.15(1)°; Compound 5 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 18.504(4), b = 19.802(3), and c = 8.6570(18) ?, and β = 99.74(2)°. The compounds 1-4 are isostructural and show a one dimensional (1D) disposition, with the metal(II) ions bridged by double μ(1,5) dca ligands and unusually by a third bridge consisting of the bpa ligand, which adopts a very low torsion angle to accommodate in the structure. This kind of structure is unusual, even considering the voluminous bpa bridge. The compound 5 shows a 3D structure with layers of Ni-bpa joined by single dca bridges. Magnetic susceptibility measurements show antiferromagnetic couplings, increasing for 1-3. Compound 5 shows very slight antiferromagnetic interactions.     

SPECA: a program for the calculation of thermodynamic equilibrium constants from spectrophotometric data

A computer program called SPECA and written in compiled BASIC (Microsoft QuickBasic Ver. 4.5) has been written for the calculation of thermodynamic constants in solution equilibria studies from spectrophotometric measurements. In order to calculate the thermodynamic constants it is necessary to introduce different theories to estimate the activity coefficients of the system involved under investigation. In this version of the program the equations proposed in the models of Debye-Hückel. Specific Interaction Theory (SIT) or the Modified's Bromley Theory can be chosen. To achieve its purpose, the program uses absorbance and activity or free concentration data of one of the components, normally H(+), and it is not limited with respect to the number of components that can be studied. Minimization of the error square sum in the absorbance, which can be absolute or relative, is carried out using the Levenberg-Marquardt-Nash algorithm at three different levels: equilibrium constants in the first level, molar absorptivities in the second and interaction coefficients in the third for those systems using the SIT or the Modified Bromley's Theories.