Elasticity and equation of state of Li2B4O7

A single crystal X-ray diffraction study on lithium tetraborate Li2B4O7 (diomignite, space group I4(1) cd) has been performed under pressure up to 8.3 GPa. No phase transitions were found in the pressure range investigated, and hence the pressure evolution of the unit-cell volume of the I4(1) cd structure has been described using a third-order Birch-Murnaghan equation of state (BM-EoS) with the following parameters: V (0) = 923.21(6) a<<(3), K (0) = 45.6(6) GPa, and K' = 7.3(3). A linearized BM-EoS was fitted to the axial compressibilities resulting in the following parameters a (0) = 9.4747(3) a<<, K (0a) = 73.3(9) GPa, K' (a) = 5.1(3) and c (0) = 10.2838(4) a<<, K (0c) = 24.6(3) GPa, K' (c) = 7.5(2) for the a and c axes, respectively. The elastic anisotropy of Li2B4O7 is very large with the zero-pressure compressibility ratio beta (0c) /beta (0a) = 3.0(1). The large elastic anisotropy is consistent with the crystal structure: A three-dimensional arrangement of relatively rigid tetraborate groups [B4O7](2-) forms channels occupied by lithium along the polar c-axis, and hence compression along the c axis requires the shrinkage of the lithium channels, whereas compression in the a direction depends mainly on the contraction of the most rigid [B4O7](2-) units. Finally, the isothermal bulk modulus obtained in this work is in general agreement with that derived from ultrasonic (Adachi et al. in Proceedings-IEEE Ultrasonic Symposium, 228-232, 1985; Shorrocks et al. in Proceedings-IEEE Ultrasonic Symposium, 337-340, 1981) and Brillouin scattering measurements (Takagi et al. in Ferroelectrics, 137:337-342, 1992).