X-RAY-DIFFRACTION INVESTIGATIONS OF THE CRYSTALLOGRAPHIC PARAMETERS AND THERMAL-EXPANSION OF BETA-BAB2O4 CRYSTALS

The lattice parameters a and c of beta-BaB2O4 crystals have been measu red in the temperature range 80-300 K by the x-ray diffraction method. The thermal expansion coefficients alpha are calculated from the meas ured values of the parameters, A substantial anisotropy of the thermal expansion is found, It is shown that the thermal expansion coefficien t alpha(c) along the c axis is an order of magnitude greater than the thermal expansion coefficient alpha(a) in a plane perpendicular to thi s axis, It is established that alpha(a) becomes negative in the temper ature range 80-190 K. (C) 1997 American Institute of Physics.