Gallium Orthophosphate - GaPO4 Crystal Structure
The crystal structure of GaPO4 can be derived from the alpha-quartz structure by alternating replacement of the Si-atoms by Ga and P. The figure shows the lattice in the frame of its unit cell. The number of formula units per unit cell is 3, the same as in quartz, but due to the alternate sequence of Ga and P there is a doubling of the unit cell dimension in the c-direction, compared with quartz. There exist two enantiomorphous structures of GaPO4 with the alpha-quartz space group P3121 nd P3221, yielding the (structural) right-handed and left-handed modifications. The lattice parameters are a = 4,901 Å and c = 11,048 Å at room temperature.
The alpha-beta phase transition to the point group 622, with the occurrence of incommensurable phases common to quartz and other quartz homeotypes like AlPO4 (berlinite) or FePO4 is frustrated in GaPO4. This means that no such alpha-beta phase transition really occurs; however, some advantageous properties, like the stiffening of c66, are still conserved. These properties are believed to be connected with the alpha-beta transition due to the rotation mechanism of the tilted tetrahedrons, which is essentially the same in all crystals of this group. At 970 °C an irreversible reconstructive transition to the so-called beta-cristobalite phase occurs.