Twinning and epitaxial growth of taaffeite-type modulated structures in BeO-doped MgAl2O4

A recent study of (111) twins in MgAl2O4 spinel crystals from Mogok (Burma) implied that twinning could be triggered by the presence of Be during crystal growth. Here we demonstrate that twinning in spinel is in fact chemically induced and is not a consequence of an accidental attachment of crystals. The growth conditions were reproduced in a tube furnace at 1200 °C by liquid-phase assisted reaction of Al2O3 and MgO with the addition of BeO, while PbF2 was used as melt-forming agent. BeO does not only cause abundant {111} twinning of spinel, but also complex topotaxial overgrowths of BeMg3Al8O16 taaffeite when the concentration of BeO exceeds 12.5 mol%. A HRTEM study of the spinel–taaffeite interface confirmed the following epitaxial relationship: [10]·{111}sp∥[110]·{0001}taf. The basic structural unit, observed on (111) twin boundary in spinel (i.e. Be2+ replacing for Mg2+ on the interfacial tetrahedral interstices), is identical to hexagonal close-packed (hcp) sequences observed in ternary taaffeite compounds, suggesting that twinning is probably a preparatory stage of polytype formation. The formation {111} twins in spinel is explained in the context of twin-induced exaggerated grain growth mechanism. This phenomenon can be exploited for the production of spinel crystals with complex morphologies.