The crystal structures at room (296 K) and low (173 K) temperature of several alpha-alums have been refined by single-crystal X-ray structure analysis. Many alpha-alums of known structure are disordered, the sulfate anions occupying one of two possible sites. All those studied here exhibited such disorder and the relative occupancies of the two sites are in excellent agreement with those obtained by Raman spectroscopy, where the nu1(SO4) mode is seen as a doublet owing to the presence of two different types of sulfate ion. No phase transitions were noted on cooling but there is less disorder.

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The unit cell edge length, a, of a set of complex cubic perovskites having the general formula A 2+ 2 BB ′ O 6 is predicted using two methodologies: multiple linear regression and artificial neural networks. The unit cell edge length is expressed as a function of six independent variables: the effective ionic radii of the constituents (A, B and B ′ ), the electronegativities of B and B ′ , and the oxidation state of B. In this analysis, 147 perovskites of the A 2+ 2 BB ′ O 6 type, having the cubic structure and belonging to the F m3m space group, are included. They are divided in two sets; 98 compounds are used in the calibration set and 49 are used in the test set. Both models give consistent results and could be successfully used to predict the lattice cell parameter of new members of this series.

In order to demonstrate the possibility of predicting the structural parameters of members in a sequence of isostructural compounds, the kieserite group isotypes (with the general formula M II XO 4 .H 2 O) were chosen since a number of them have accurately re®ned crystal structures. The unit-cell parameters and the fractional atomic coordinates were shown to vary linearly with both cation and anion size. This makes it possible to calculate the structural parameters of a particular member, taking into account only the effective ionic radii of the constituent atoms. Agreement between the calculated and experimentally re®ned (by X-ray diffraction) structural parameters is good. The cell constants and atomic coordinates of FeSeO 4 .H 2 O, iron selenate monohydrate, are predicted in this way.

Transmission infrared, Attenuated Total Reflectance (ATR) and Raman spectra of crystalline methylammonium iodide (MAI) and formamidinium iodide (FAI) in the temperature interval starting from –170 ºC to 200 ºC were studied. The spectra recorded in the region from 4000 to 500 cm–1 enabled resolving the ambiguities associated with the origin of some bands. For the first time a complete and detailed vibrational investigation and assignment of the IR spectra of these compounds based on the differences in the temperature dependent IR spectra for all phases, including the metastable ones, have been made. The findings support the already established crystal structure of the phases for both compounds. The correlation between the overtones and fundamental modes has been confirmed based on the temperature induced isosbestic point.

Perovskite series GdCr 1 -x Co x O 3 (x = 0, 0.33, 0.5, 0.67 and 1) was obtained using a solution combustion method. The powder XRD was used for identification and structural characterization of the obtained perovskites. All compounds crystallize within the space group Pnma. The morphology of samples was studied using SEM. The impedance and AC conductivity of GdCr 1 -x Co x O 3 were studied using impedance spectroscopy in a frequency range from 10 Hz to 10 MHz and in temperature interval 297-337 K. Changes in electric modulus and DC conductivity, with increasing of the value of x in the structures, were observed. The AC conductivity obeyed the universal power law, r(x) = r(0) + Ax n and revealed semiconductor behavior. The calculated activation energies of existing processes varied with the cobalt content and applied frequency. The impedance spectra showed non-Debye behavior with a distribution of relaxation times for relaxation and conductive processes. The conduction mechanism for pure orthochromite and orthocobaltite was defined and two types of conduction were observed in the investigated temperature range for the complex perovskites. In order to explain the results, an equivalent circuit with fitted values of circuit components was proposed.

In this work, the unit cell parameter (a) of the series of cubic ABX 3 perovskites was modeled using counter-propagation artificial neural networks, and the influence of different input variables was examined by using algorithm for automatic adjustment of the relative importance of the variables. The input variables used in this model were the ionic radii of A, B, and X as well as the oxidation state (z) and the electronegativity (x) of the anion.The developed models have good generalization performances-good agreement between experimental and predicted values for lattice parameter. One of the important outcomes from this work is obtained from the results of the automatic adjustment of the relative importance of input variables. That is to say, this analysis gave us an insight that the most pronounced influence on the successful prediction of the unit cell parameter of the analyzed data set of cubic ABX 3 perovskites has the effective ionic radii of B-cation. In addition to this, it may be concluded that the separation of the compounds in different regions of counter-propagation artificial neural networks was predominantly influenced by the input variables with regard to the physical parameters of the anion. Copyright

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