Synthesis and investigation of boron phases at high pressures and temperatures DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) an der Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT) der Universität Bayreuth vorgelegt von Gleb Parakhonskiy aus Twer Bayreuth, 2012 TABLE OF CONTENTS SUMMARY ............................................................................................................................... 4 ZUSAMMENFASSUNG ........................................................................................................... 6 1 INTRODUCTION ................................................................................................................ 9 1.1 Motivation and background ........................................................................................... 9 1.2 Elemental boron modifications .................................................................................... 11 1.2.1 α-boron .................................................................................................................... 11 1.2.2 β-boron .................................................................................................................... 12 1.2.3 The controversy on the ground state of boron and a relative stability of the two modifications (α-B and β-B) at ambient conditions ......................................................... 14 1.2.4 γ-boron .................................................................................................................... 15 1.2.5 Tetragonal boron, T-50 and T-192 phases .............................................................. 16 1.2.6 The phase PT diagram of boron .............................................................................. 18 1.3 Experimental techniques .............................................................................................. 20 1.3.1 High pressure techniques ........................................................................................ 20 1.3.1.1 Multi-anvil apparatus ....................................................................................... 20 1.3.1.2 Piston cylinder apparatus ................................................................................. 22 1.3.1.3 Diamond anvil cell ........................................................................................... 22 1.3.2 Analytical techniques .............................................................................................. 24 1.3.2.1 Raman spectroscopy ......................................................................................... 24 1.3.2.2 X-ray diffraction ............................................................................................... 25 1.3.3 Electron microscopy techniques .............................................................................. 26 1.3.3.1 Scanning electron microscopy and electron microprobe analysis.................... 26 1.3.3.2 Transmission electron microscopy ................................................................... 27 2 SYNOPSIS (scope of the articles) ................................................................................... 29 2.1 Synthesis of single crystals of α-boron ........................................................................ 29 2.2 Investigation of single crystals of α-boron ................................................................... 30 2.3 Synthesis of single crystals of β- and γ-boron .............................................................. 34 2.4 The phase PT diagram construction and the ground state determination .................... 37 2.5 Synthesis of metastable boron phases: δ- and ε-boron ................................................. 40 2.5.1 Tetragonal metastable boron phase (δ-boron) ......................................................... 40 2.5.2 Newly synthesized rhombohedral metastable boron phase (ε-boron) ..................... 42 3 LIST OF MANUSCRIPTS AND STATEMENT OF AUTHORS CONTRIBUTION ..... 47 4 MANUSCRIPTS ................................................................................................................ 49 4.1 High pressure synthesis of single crystals of -boron ................................................. 49 4.1.1 Abstract ................................................................................................................... 49 4.1.2 Introduction ............................................................................................................. 49 4.1.3 Experimental ........................................................................................................... 50 4.1.3.1 Sample preparation ........................................................................................... 50 4.1.3.2 Synthesis technique .......................................................................................... 51 4.1.3.3 Analytical techniques ....................................................................................... 52 4.1.4 Results and discussion ............................................................................................. 53 4.1.5 Conclusion ............................................................................................................... 58 4.2 Raman spectroscopy investigation of α-boron at elevated pressures and temperatures .............................................................................. 61 4.2.1 Abstract ................................................................................................................... 61 4.2.2 Introduction ............................................................................................................. 61 4.2.3 Experimental ........................................................................................................... 62 4.2.4 Results and discussion ............................................................................................. 64 4.2.5 Conclusions ............................................................................................................. 69 2 4.3 Experimental pressure-temperature phase diagram of boron: resolving the long- standing enigma .................................................................................................................. 72 4.3.1 Abstract ................................................................................................................... 72 4.3.2 Introduction ............................................................................................................. 72 4.3.3 Results ..................................................................................................................... 75 4.3.3.1 Boron phases .................................................................................................... 75 4.3.3.2 Boron phase diagram ........................................................................................ 81 4.3.4 Discussion ............................................................................................................... 83 4.4 High pressure synthesis and investigation of single crystals of metastable boron phases ..................................................................................................... 92 4.4.1 Abstract ................................................................................................................... 92 4.4.2 Introduction ............................................................................................................. 92 4.4.3 Experimental details ................................................................................................ 93 4.4.4 Results and discussion ............................................................................................. 96 4.4.5 Conclusion ............................................................................................................. 104 FULL LIST OF PUBLICATIONS (WITH PAPERS NOT INCLUDED INTO THE THESIS) ........................................................................................................................... 107 5 BIBLIOGRAPHY ............................................................................................................ 109 6 ACKNOWLEDGMENTS ................................................................................................ 116 3 SUMMARY Boron, discovered as an element in 1808 and produced in pure form in 1909, still remains one of the most complicated light elements full of surprises. Even the number of pure boron polymorphs is a subject of intensive discussions. It is proven the existence of α-, β- and γ-boron phases. Structural details of the most common boron phase (β-B) are still not fully revealed. For decades boron remained the last stable element in the periodic table, whose ground state was not determined. It has been a subject of a longstanding controversy, whether α-B or β-B is the thermodynamically stable phase at ambient pressure and temperature. The existence of the α-tetragonal boron phase T-50 has been an open question since its first discovery. It was not clear if T-50 could be synthesized as a pure boron phase or its structure must be stabilized by the presence of carbon or nitrogen. Theorists claimed that T-50 could not exist at all because of its unstable electronic configuration. The tasks of the present work were (a) to develop a methodology of a reproducible synthesis of known boron phases (first of all α-boron),