Correlation between polyhedral distortions and phase transitions in spinel FeMn2_{2}O4_{4}

Spinel compounds AB$_{2}$X$_{4}$ consist of both tetrahedral (AX$_{4}$) and octahedral (BX$_{6}$) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating properties and their correlations with structural features is critical in understanding these materials. FeMn$_{2}$O$_{4}$ has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn$_{2}$O$_{4}$ by employing the high-resolution neutron powder diffraction. While a large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra along the $c$ axis occurs at $T_{S} \approx$ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn$^{3+}$ cation. A strong magnetoelastic coupling is unveiled at $T_{N1}\approx 400$ K as manifested by the appearance of N\`{e}el-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constant $c$ and a weak anomaly of $a$. Upon cooling below $T_{N2}\approx65$ K, it evolves to a noncollinear ferrimagnetic order with a canting of half B-site $Mn^{3+}$/$Fe^{3+}$ spins in the pyrochlore lattice, which is a unique magnetic order among spinels. Such a noncollinear order induces modifications of the O-B-O bond angles in the octahedra without affecting much the bond lengths of the tetrahedra/octahedra. Our study indicates that FeMn$_{2}$O$_{4}$ is a wonderful platform to unveil interesting magnetic order and to investigate their correlations to polyhedral distortions and lattice.Comment: 28 pages, 10 figures, submitted for publicatio

Characterization of liquid crystals with Miconazole in biphasic systems by polarized microscopy

Los autores desean expresar su agradecimiento al Esp. Farm. Osvaldo Di Sapio y a la Dra. Victoria Rodríguez del Área Farmacobotánica de la Facultad de Ciencias Bioquímicas y Farmacéuticas (UNR) por su colaboración para la obtención de las imágenes microscópicas.Introducción: las emulsiones son un tipo de preparado farmacéutico muy utilizado en aplicación tópica consistentes en sistemas bifásicos de aceite-agua o agua-aceite, donde el principio activo deseado se incorpora en una de las fases dependiendo de su solubilidad. Diversos estudios demuestran que la estabilidad es mayor en emulsiones con estructuras líquido-cristalinas. El principio activo liposoluble Miconazol, que actúa como antimicótico, se solubiliza en la fase oleosa de la emulsión y en la fracción de la cadena hidrocarbonada de los cristales líquidos. Método: se utilizaron técnicas microscópicas para analizar las características de una emulsión convencional y de otra con cristales líquidos, a las que se les incorporó el principio activo Miconazol. Se determinaron las dimensiones de las gotas de la fase interna y mediante microscopía de polarización se caracterizaron los cristales líquidos. Resultados: el análisis de las imágenes microscópicas permitió determinar que en las formulaciones con cristales líquidos con y sin Miconazol, aproximadamente el 80 % de las gotas tienen dimensiones en el intervalo 0,5µm - 1µm. Las observaciones microscópicas con luz polarizada nos permitieron determinar que los cristales líquidos tienen birrefringencia con la formación de cruces de extinción uniáxicas negativas, las cuales son características de las fases liotrópicas laminares con texturas cónicas focales. Conclusiones: los resultados muestran que el agregado de Miconazol, no interfiere con la formación de la estructura de los cristales líquidos, por lo que estas dependen de los componentes de la formulación y de la técnica de preparación.Introduction: emulsions are a type of pharmaceutical preparation widely used in topical applications consisting of two-phase systems of oil-in-water or water-in-oil, where the desired active ingredient is incorporated into one of the phases depending on its solubility. Several studies show that stability is greater in emulsions with liquid-crystalline structures. The liposoluble active substance Miconazole, which acts as an antifungal agent, is solubilized in the oil phase of the emulsion as well as in the fraction of the hydrocarbon chain in liquid crystals. Method: microscopic techniques were used to analyze the characteristics of both a conventional emulsion and another one containing the liquid crystals. Miconazole was incorporated into both emulsions; drop dimensions in the internal phase were determined and the liquid crystals were characterized by polarization microscopy. Results: through the analysis of the microscopic images of the formulation with liquid crystals with Miconazole and without Miconazole, it was possible to determine that approximately 80% of the drops have dimensions ranging from 0.5µm - 1µm. Microscopic observations with polarized light allowed us to determine that liquid crystals have birefringence with the formation of negative uniaxial extinction crosses, which are characteristic of laminar lyotropic phases with focal conical textures. Conclusions: the results show that the addition of Miconazole does not interfere with the formation of the structure of the liquid crystals. Therefore, the formation of liquid crystals depends both on the components of the formulation and the preparation technique