Substitution of Mg2+and Fe2+in the Trigonal-Bipyramidal-Coordinated Site inBETA-(Mg, Fe)Al(PO4)O

Solid solution compositions in the system FeAl(PO4)O–MgAl(PO4)O were synthesized hydrothermally at 735°C and 0.2 GPa and controlled oxygen fugacities. X-ray powder diffraction studies reveal that under these conditions a complete solid solution series is formed. It is characterized by the substitution of Mg2+and Fe2+in a fivefold-coordinated trigonal-bipyramidal site. The solid solution compositions are isotypic with the end members, having theBETAʹʹ-MgAl(PO4)O structure, and crystallize in the monoclinic space groupP21/c.57Fe-M?ssbauer spectroscopic measurements show that these phases are paramagnetic at room temperature. They are characterized by an isomer shift between 1.18 and 1.20 mm/s relative to ALPHA-iron and a compositionally dependent quadrupole splitting between 2.52 and 2.61mm/s for high-spin57Fe2+in the trigonal-bipyramidalcoordinatedMe2+site.