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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.
Reproduced by kind permission of AVL List GmbH. © AVL List GmbHSpecifications
subject to change without
notice. All
rights reserved
Roditi International 2007