Polytypic modifications in heavily Tb and Eu doped gadolinium aluminum borate crystals

The evidence of a new polytypic phase was demonstrated in double doped gadolinium aluminum borate (GAB) crystals grown by the high temperature top seeded solution growth (HTTSSG) method. The structures of the rhombohedral and the new monoclinic phases in the Eu0.02Tb0.12Gd0.86Al3(BO3)4 and Eu0.18Tb0.19Gd0.63Al3(BO3)4 compositions were determined by single crystal and powder X-ray diffraction methods. The monoclinic crystals with different compositions have identical structures with space group C2/c. It was concluded that the starting crystallization temperature was the dominant factor in formation of the rhombohedral and monoclinic phases. The crystals with non-centrosymmetric rhombohedral structure (R32) may possess similar compositions as the previous ones but their starting crystallization temperature was lower. ► Polytypic modifications of Eu and Tb codoped GAB crystals have been found. ► Symmetry has been reduced with increasing crystallization temperature (Tcr). ► Number of molecules per unit cell has been increased with growing Tcr.