Our recently proposed method for automatic detection, calibration and evaluation of Debye-Scherrer ellipses using pattern recognition techniques and advanced signal filtering¹ was applied to 2D powder diffraction data of the non-ferroelectric, acentric NLO compound α-BiB₃O₆ in dependence on pressure. The measurements were performed at station 9.5HPT at Daresbury. At ambient conditions, BiB₃O₆ crystallizes in space group C2. In the pressure range between P = 6.09 and P = 6.86 GPa, it exhibits a first order phase transition into a structure with space group B1 (phase II at P = 7.3 GPa: a=7.0228 Å, b=6.6671 Å, c=4.2019 Å, α=114.54°, β=95.00°, and γ=90.28°) (Fig. 1). A non-linear compression behavior over the entire pressure range is observed, which can be described by two Vinet relations in the ranges from P = 0. to 6.09 GPa, and from P = 6.86 to 11.6 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to K0 = 38(1) GPa for phase I, and K0 = 114(10) GPa for phase II. The crystal structures of both phases are refined against X-ray powder diffraction data measured at several pressures between 0.0 and 11.6 GPa. The structural phase transition of α- BiB3O6 is mainly characterized by a reorientation of the BO₃ triangles and BO₄ tetrahedra, in order to optimize crystal packing. With increasing pressure, the lone pair which is localized at Bi³⁺ increasingly adopts pure s-character.

Fig. 1: Volume as a function of pressure for BiB₃O₆. The new high pressure crystal structure of BiB₃O₆ at p= 8 GPa is shown in the inset.

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