Thermal Expansion of beta-BaB_2O_4 and BaB_4O_7 Borates

摘要

The thermal behavior of beta-BaB_2O_4 and BaB_4O_7 borates is investigated using high-temperature X-ray powder diffraction analysis.The components of the thermal expansion tensor and the tensor orientation with respect to the crystallographic axes are calculated.Thermal deformations are analyzed in relation to the crystal structure.The thermal expansion of the beta-BaB_2O_4 borate is maximum along the c axis and close to zero in the ab plane.The thermal expansion anisotropy correlates with the layered structure,the orientation of the optical indicatrix,and the thermal ellipsoids of atoms.The thermal expansion of the BaB_4O_7 borate is studied in three high-temperature X-ray diffraction experiments(two experiments with heating and one experiment with cooling).The thermal expansion of this compound is strongly anisotropic:the expansion is maximum in the ac mon-oclinic plane,whereas the contraction is observed along the b axis.