- The 5-th element. A new high pressure high temperature allotrope (2010)
- Boron is the fifth element in the Periodic Table known for its structural complexity, unusual types of bonding, and for a rich variety of compounds that it forms. Even the number of its existing or hitherto proven elemental modifications is still under discussion. The present study of boron behavior at high pressures and high temperatures (HPHT) comprises various experimental techniques for HP synthesis and material characterization, as well as theoretical modeling. A series of high pressure (up to 20 GPa) and high temperature (up to 1900 °C) synthesis experiments were conducted in multianvil apparatus at Bayerisches Geoinstitut. For testing of the reproducibility of HPHT experiments different types of boron precursors and assemblages for large-volume presses were used. The synthesis products were studied by X-ray diffraction and spectroscopic methods. The diffraction patterns collected from samples synthesized from highly purified boron powders (> 99.99 % purity) resemble those published by R.H. Wentorf in 1965 and described as a new boron form, but later effectively forgotten. To eliminate any possible contaminations that could take place during the synthesis process, the purity of the HP boron samples was confirmed by scanning electron and transmission electron microscopy and electron microprobe analysis. The structure of this HPHT boron polymorph was first unknown, but subsequently determined from our X-ray powder diffraction data and further refined using single-crystal synchrotron diffraction data. The structure was found to be orthorhombic with a Pnnm space group and composed of B12 icosahedra and B2 dumbbells. The unit cell contains 28 atoms (two icosahedra and two dumbbells) and will be called here B28. Atoms in B28 are bonded covalently as revealed experimentally by single-crystal X-ray diffraction studies and calculated Electron Localization Function. The X-ray density of 2.52 g cm-3 of this boron allotrope is the highest among its other known modifications. In order to determine the phase transition boundary between beta-and B28 boron phases, in situ HP laser heating experiments were performed at European Synchrotron Radiation Facilities (ESRF). It was demonstrated that B28 is a stable phase above 9 GPa. Systematic investigations aimed at the development of the technique of the HP single crystal growth of B28 were undertaken. It was found that single crystals of the orthorhombic boron can be grown from metal solutions (Au, Cu, or Pt), i.e. after dissolution in metals at high temperature boron precipitates in form of single crystals with temperature decrease. Experimental products were free from any borides only when Au was used. The maximal length of synthesized B28 crystals was ~100 µm and after their isolation high quality X-ray diffraction data was obtained. Synthesized single crystals of the orthorhombic high-pressure boron phase were studied by means of polarized Raman spectroscopy at ambient conditions. Among all possible 42 Raman-active modes, 32 modes were registered and assigned. Investigation of the high pressure behavior of B28 orthorhombic boron was carried out through combining single crystal X-ray diffraction up to 65 GPa and Raman spectroscopy measurements performed up to 105 GPa. Above 40 GPa discontineous behavior in the mode Grueneisen parameter was detected. At the same pressure an inflection in the dependence of the relative volume of a unit cell versus pressure occurs. However, the structure of the material does not change. It is suggested that B28 undergoes an isostructural phase transformation, probably due to reducing of the polarity of covalent bonds. The orthorhombic B28 boron reveals extraordinary physical properties. Due to very strong covalent bonding it has a very high Vickers hardness HV=58 GPa, making B28 to be the second hardest elemental material after diamond. The electrical resistance and IR-spectroscopic measurements demonstrated the semiconducter nature of B28. Combination of these properties makes the high-pressure boron polymorph a prospective material for industrial applications.