Migración de catión inducida por la presión y expansión de volumen en los pirocloros defectuosos ANbWO6 (A = NH4+, Rb+, H+, K+)
Paris W Barnes1, Patrick M Woodward, Yongjae Lee
1Department of Chemistry, The Ohio State University, Columbus, OH 43210, USA.
Journal of the American Chemical Society
|April 10, 2003
Resumen
Bajo presión, los pirocloros de amonio y rubidio se expanden inesperadamente debido a la inserción de agua en su estructura. Este cambio estructural, impulsado por el desplazamiento catiónico, explica los aumentos de volumen contraintuitivos en estos materiales.
Área de la Ciencia:
- Ciencia de los materiales Ciencia de los materiales.
- Química del estado sólido.
- La cristalografía es una técnica de cristalografía.
Sus antecedentes:
- Los pirocloros defectuosos con la fórmula ANbWO(6) (A = NH4+, Rb+, H+, K+) exhiben comportamientos estructurales complejos.
- Comprender su respuesta a estímulos externos como la presión es crucial para el diseño de materiales.
Objetivo del estudio:
- Investigar la evolución estructural y de composición de los pirocloros bajo alta presión hidrostática.
- Para dilucidar el mecanismo detrás de los cambios de volumen inducidos por la presión en estos materiales.
Principales métodos:
- Estudios de alta presión utilizando una célula de yunque de diamante de hasta 7 GPa.
- Difracción monocromática de polvo de rayos X sincrotrón monocromático para el análisis estructural.
- refinamientos de Rietveld para determinar los parámetros estructurales.
Principales resultados:
- NH4NbWO ((6) y RbNbWO ((6) muestran una expansión de volumen contraintuitiva por encima de presiones de umbral específicas (aprox. 3.4 GPa y 3.0 GPa, respectivamente).
- Esta expansión se atribuye a la inserción de agua en el marco de pirocloro, desplazando a los iones NH4+ o Rb+.
- KNbWO ((6).H2O y HNbWO ((6).H2O exhiben una contracción continua, sin el fenómeno de expansión.
Conclusiones:
- La expansión inducida por presión en NH4NbWO ((6) y RbNbWO ((6) es impulsada por un mecanismo de desplazamiento catiónico impulsado por la hidratación.
- Este mecanismo es único para los pirocloros con cationes monovalentes más grandes que no están ya en su estado hidratado más estable en condiciones ambientales.
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