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.