The temperature dependence of the crystal structure of berlinite, a quartz-type form of AlPO4

The temperature dependence of atomic positions, mean-square displacements and probability density functions in a synthetic specimen of berlinite was analyzed using X-ray single crystal data measured at fourteen values of temperature. The characteristic features and the temperature dependence of the structure of berlinite were found to be quite similar to those of quartz, in detail. With increasing temperature, the equilibrium po sition of an atom, if expressed in fractional coordinates, appears to move steadily toward its beta-position along a straight line, and finally attain the beta-phase positions after abrupt displacements in a narrow temperature range around 583 degrees C. The temperature dependence of the displacements of atoms from the corresponding high-symmetry beta-positions, in alpha-berlinite, is well fitted with a classical Landau type expression of first-order phase transitions. The highly anisotropic mean square displacements, [U-2], of O atoms increase with increasing temperature, especially in a narrow range just below the alpha-beta transition, in the directions nearly perpendicular to the librating Al-O-P bonds, attaining a local maximum of 0.115-0.117 Angstrom(2) just above the transition point. With varying temperature, the principal axes for the O atoms change their directions smoothly toward those of the high temperature phase, while the axis with the largest [u(2)] of the Al or P thermal ellipsoid remains in [100]. These diffraction results are interpreted in terms of displacive structure transition involving both the ordered alpha- and beta-forms. The Al-O and P-O bond distances ale nearly constant around 1.73 and 1.53 Angstrom, respectively, through the experimental range of temperature.