Li 2B 4O 7 crystal structure in anharmonic approximation at 20, 200, 400 and 500 °C

Abstract

Li 2B 4O 7 crystal structure was studied at elevated temperatures and refined in anharmonic approximation: 20 °C, a = 9.475(3), c = 10.283(6) Å, 883 F( h k l), R = 0.015; 200 °C, a = 9.501(3), c = 10.275(6) Å, 826 F( h k l), R = 0.021; 400 °C, a = 9.535(3), c = 10.295(8) Å, 809 F( h k l), R = 0.022; and 500 °C, a = 9.550(3), c = 10.295(6) Å, 779 F( h k l), R = 0.029. B O bond lengths were corrected in a view of O and B atomic vibrations. Refinement of anharmonic components of atomic temperature factors has revealed high asymmetry of Li thermal vibrations. The behaviour of tetraborate groups was studied. A stability of rigid B O groups configuration was indicated, as in the case of β CsB 5O 8, LiB 3O 5 and β-Na 2B 8O 13, studied by us earlier. The boron–oxygen framework expands as a three-dimensional hinge; B O B and O B O angles inside tetraborate groups practically do not change, while B1 O1 B2 angles between the groups increase to 1.6° on heating up to 500 °C due to O1 atoms shifts letting Li atom to fall through oxygen atoms along c-axis causing the contraction in this direction.