An accurate structural study on the β-phase of LiNaSO4 was carried out. This study shows that twin crystals are usually obtained when crystallization takes place from solution, which explains the observed low spontaneous polarization. Raman scattering of Li2SO4, Na2SO4 and LiNaSO4 is explained from the structural data. The phase diagram of the binary system Li2SO4-Na2SO4 was determined by x-ray diffraction and the DTA method. Mixed crystals of the low-temperature phase of Li2-xNaxSO4 with 1⩽x⩽1.22 were observed for the first time. The temperatures of the process in this diagram are high, depending on the cooling rate. A more congruent diagram was obtained working at a lower cooling rate.

Results are presented of a detailed x-ray diffraction, Raman and thermal analysis study of the structural phase transitions in LiNH4SO4 carried out in the temperature range 298-600 K. Either two or just one phase transition was recorded in the temperature range between 335 and 461 K, depending on the warming process rate. Crystal structures at 298, 318, 383, 423 and 483 K during a slow warming process and at 298 and 523 K during a fast warming process were solved. The transition at 335 K is reported for the first time and consists of a rotation by 41° of the sulphate ion around an S-O bond. The new obtained phase above 335 K is close to the enantiomeric phase at room temperature. The high temperature phase crystallized in the non-polar P21nb space group, but the crystal structure obtained removed the spontaneous polarization, according to the calculations made using an ionic model, and agrees with the experimental results.

The effects of Pr substitution in the structure and ferroelectric response for the Sr(1-x)Pr(x)Bi(2)Ta(2)O(9) (SBT-Pr) compound have been studied. Rietveld refinement of the x-ray diffraction patterns indicates that the Pr ion progressively replaces the Sr site in the A 2(1)am space group structure. The solubility of Pr in solid solution is around 15%. The replacement induces a change in the crystal structure and, as a consequence, the dielectric properties are affected. The ferroelectric transition at T(m)∼558 K is shifted to lower temperatures, T(m)∼413 K for x = 0.15 composition. Apparently, the tilt angle (α) associated with the c-axis does not play an important role since it remains essentially constant. However, the rotation in the ab-plane (β) as well as the octahedral distortion observed are strongly related to the coupling between T(m) and x. A relaxor-type transition is observed as Pr is increased, leading to polar microregions above the nominal ferroelectric transition. The local disorder induced by the Pr ion is confirmed by the continuous increase in the diffuseness coefficient according to Isupov's model. These facts hinder the displacement of the TaO(6) octahedra with respect to Bi(2)O(2) along the polarization axis, decreasing the polarization values.

Se realiza un análisis fractal de los eventos de lluvia registrados en Baja California, México, una región semiárida que presenta amplia variabilidad climatológica. Se utilizan series de precipitación de 92 estaciones climatológicas con longitudes de registro mayores a 30 años. Se determinan patrones y características en las series de precipitaciones a partir de valores espaciales y temporales del exponente de Hurst, así como su relación con la temperatura y precipitación media anual, altitud y distribución climatológica. Se emplean con éxito los métodos de rango reescalado, conteo de cajas y análisis multifractal de fluctuación sin tendencia, lo cual permite obtener el valor promedio del exponente de Hurst para diferentes escalas de tiempo. Los datos muestran que la series de precipitación diaria tienden a presentar un patrón persistente; además, los valores del exponente de Hurst se relacionan con el tipo de clima, la altitud, el régimen de lluvias y la temperatura en la zona de estudio. El análisis del exponente de Hurst para diferentes escalas de tiempo evidencia que dicho exponente aumenta a medida que la escala de tiempo en consideración es menor; por lo tanto, la persistencia de la serie se hace más fuerte. Por otra parte, se puede confirmar que la teoría fractal permite analizar el comportamiento de una variable climática, en este caso, la precipitación.

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