Structural changes in metastable γ-Na2B4O7 between -150 °C and 720 °C

Abstract

Temperature-dependent structural studies of the metastable phase γ-Na2B4O7 (triclinic, P̄1̄) have been performed by low temperature single crystal X-ray diffraction at -150, -120, -75, -50 and 27 °C and by high temperature X-ray powder diffraction in the range of 27 to 780 °C. γ-Na2B4O7 melts at 720 ± 10 °C. The thermal expansion of the crystal structure is sharply anisotropic. Between -150 °C and 600 °C the main coefficients of the thermal expansion tensor are α11 = 14, α22=-1, α33 = 9, αV = 22×10-6 K-1. γ-Na2B4O7 consists of a three-dimensional framework with two crystallographically independent pentaborate groups as fundamental building blocks that are connected via two tetrahedra. Upon heating from -150 up to 27 °C, the pentaborate groups are stable but the structure expands due to changing B-O-B angles. Owing to nearly parallel orientation of the pentaborate groups in the structure, a minimal expansion in the direction of the axes of the groups and maximum expansion perpendicular to the axes might occurs as expected. The orientation of the displacement ellipsoids of the sodium atoms does not change with temperature and the anisotropy of the displacement parameters increases with temperature. The change of the 〈Na-O〉 distances is close to 0.007 Å for all of the different polyhedra (-150 - 27 °C). The coefficient of the thermal expansion of the 〈Na-O〉 distances is α〈Na-O〉 = 18×10-6 K-1. The averaged value αV is 49×10-6 K-1 for all of the Na-O polyhedra.