THE TEMPERATURE-DEPENDENCE OF THE CRYSTAL-STRUCTURE OF BERLINITE, A QUARTZ-TYPE FORM OF ALPO4

The temperature dependence of atomic positions, mean-square displaceme nts and probability density functions in a synthetic specimen of berli nite was analyzed using X-ray single crystal data measured at fourteen values of temperature. The characteristic features and the temperatur e dependence of the structure of berlinite were found to be quite simi lar to those of quartz, in detail. With increasing temperature, the eq uilibrium po sition of an atom, if expressed in fractional coordinates , appears to move steadily toward its beta-position along a straight l ine, and finally attain the beta-phase positions after abrupt displace ments in a narrow temperature range around 583 degrees C. The temperat ure dependence of the displacements of atoms from the corresponding hi gh-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 incre ase with increasing temperature, especially in a narrow range just bel ow the alpha-beta transition, in the directions nearly perpendicular t o 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 la rgest [u(2)] of the Al or P thermal ellipsoid remains in [100]. These diffraction results are interpreted in terms of displacive structure t ransition involving both the ordered alpha- and beta-forms. The Al-O a nd P-O bond distances ale nearly constant around 1.73 and 1.53 Angstro m, respectively, through the experimental range of temperature.