Knowledge about high-pressure, high-temperature behavior of elemental materials is important for fundamental understanding of bonding evolution, phase transformations, and establishing of PT phase diagrams, which are of high significance for materials’ synthesis and applications. However, theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities.
Boron, the fifth element of the Periodic Table, has five currently established allotropes (α-B, β-B, γ-B, δ-B (T-50), and ε-B), and all of them are based on various arrangements of B12 icosahedra, since three valence electrons of boron are insufficient to form a simple covalent structure. However, theoretical calculations suggest a possibility of the existence of a non-icosahedral boron allotrope with the α-Ga type structure.
In the present work in order to verify the theoretical predictions we have investigated the behavior of β-B under extreme conditions using high-purity single crystals synthesized by the high-pressure high-temperature large volume press technique. Laser heating of the diamond anvil cell compressed to 115 GPa led to the synthesis of predicted non-icosahedral boron allotrope (we denoted it as ζ-B). Synchrotron in situ single-crystal X-ray diffraction revealed the α-Ga-type orthorhombic structure (space group Cmce) with the unit cell parameters a = 2.7039(10) Å, b = 4.8703(32) Å, c = 2.9697(6) Å (Z=8). It may be described as a stacking along the (010) direction of distorted and corrugated hexagonal nets with the 36 topology. Measured precisely interatomic distances and linear compressibilities along the major crystallographic directions do not allow interpreting the structure as layered, as earlier proposed. The newly synthesized ζ-B studied in the pressure range from 115 to 132 GPa was found to be less compressible than any other of previously known boron allotropes.