X-ray study of chromium- and boron-doped synthetic Ga2Al(AlSi)O7 gehlenites

Gehlenite single crystals of the compositions Ca2Al[AlSi]O7 (I), Ca2Al[AlSi]O7:Cr (II), and Ca2Al[AlSi]O7:Cr,B (III) were synthesized by the zone melting technique. The x-ray study (CAD-4F Enraf-Nonius automatic diffractometer, correction for absorption) of the synthesized crystals has shown that the crystals are tetragonal with the unit cell parameters a=7.6895(7), c=5.0662(5) A (I); a=7.6877(3) and c=5.0659(5) A (II); and a=7.690(1) and c=5.0714(3) A (III). The structure refinement was performed up to the reliability factors R=0.0215 (546Fhkl) (I), R=0.0190 (531Fhkl) (II), and R=0.0196 (535Fhkl) (III). The comparative analysis of the crystal structures determined leads to an assumption that Cr4+ ions are located only in tetrahedra of diorthogroups. A specific Al-tetrahedron also contains some amount of silicon. Simultaneous doping of crystals with chromium and boron cations results in the substitution of some Al atoms in the specific tetrahedron by boron, and of some (AlSi) atoms in the tetrahedron of the diorthogroup by chromium. The composition of the crystals synthesized can be written as Ca2Al(Al0.53+Si0.54+)2O7 (I), Ca2(Al1-xSix)(Al0.5+0.5x3+Si0.5-0.5x-y4+ Cry4+)2O7 (II), and Ca2(Al1-xBx)(Al0.53+Si0.5-y4+Cr4+y)2O7(III)