Li2B4O7 crystal structure in anharmonic approximation at 20, 200, 400 and 500 oC

Li2B4O7 crystal structure was studied at elevated temperatures and refined in anharmonic approximation: 20 oC, a = 9.475(3), c = 10.283(6) A, 883 F(h k l), R = 0.015; 200 oC, a = 9.501(3), c = 10.275(6) A, 826 F(h k l), R = 0.021; 400 oC, a = 9.535(3), c = 10.295(8) A, 809 F(h k l), R 0.022; and 500 oC, a = 9.550(3), c = 10.295(6) A, 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-CsB5O8, LiB3O5 and β-Na2B8O13, 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 1.6o on heating up to 500 oC due to O1 atoms shifts letting Li atom to fall through oxygen atoms along c-axis causing the contraction in this direction