The hBN to wBN phase transition modified by heavy ion irradiation R. Klein 1 , P. Schouwink 2 , R. Miletich 2 , B. Schuster 1 , C. Trautmann 1 , and R. Neumann 1 1 GSI, Darmstadt, Germany; 2 University of Heidelberg, Germany Boron nitride (BN) shows similarities to carbon, having a hexagonal (graphite-like) structure (hBN) and a cubic dia- mond type (cBN). Under ambient temperature conditions, hBN transforms irreversibly into wurtzitic BN (wBN) at pressures above 10.8 GPa [1]. We investigated the influ- ence of ion irradiation on this displacive phase transition using in situ Raman spectroscopy and synchrotron X-ray diffractometry (XRD). To test the phase transition, powder samples (Sigma-Aldrich) as well as single crystals (fabri- cated in a high temperature / high pressure procedure [2]) were pressurized in a diamond anvil cell (DAC), using rhe- nium as gasket and a methanol-ethanol mixture as pressure transmitting medium. A ruby crystal served as pressure gauge to be read out via fluorescence spectroscopy. After each step of pressure increase, the sample was allowed to relax for 1 h before recording XRD data. The exposure to heavy ions was performed at the UNILAC under vacuum and ambient temperature conditions using Au and Xe ions of 11.1 MeV/u specific energy and flu- ences between 5×10 11 and 1×10 13 ions/cm 2 . Three ir- radiated single crystal samples and a pristine crystal were pressurized simultaneously in order to perform Raman spectroscopy employing a HR800 spectrometer (HORIBA) with a 17 mW He/Ne laser (λ = 632.8 nm). The Raman signal of the intra-layer E 2g vibration between the B and N atoms in hBN single crystals provides sufficiently high in- tensities to allow spectroscopic measurements of the sam- ple in the DAC. The disappearance of the hexagonal phase is evidenced by the simultaneously vanishing E 2g band, with a band maximum at a wavenumber of ν = 1365 cm -1 (P = 0 GPa). We monitored the band intensity as a function of pressure (Fig. 1). For all samples, the vibrational E 2g mode diminishes by a few percent with increasing pres- sure up to 10.8 GPa, the slope becomes steeper for larger fluences. Above a critical value of about 11-12 GPa, the band intensity drops rapidly indicating a significant loss of the hexagonal phase. For the highest irradiation fluence (1×10 13 Xe ions/cm 2 ), the band disappears entirely at 10.8 GPa, whereas for lower fluences this occurs gradually and at higher pressures. At 12 GPa, none of the samples shows traces of hBN. X-ray diffraction studies were conducted on commercially available hBN powder samples. Pristine samples as well as specimens irradiated with Au ions (5×10 11 and 1×10 12 ions/cm 2 ) were investigated at the beamlines ID 27 of the ESRF (Grenoble, France), and ID 16 of the APS Argonne National Laboratory (Chicago, USA), applying pressures up to 25.0 GPa. A MAR345 IP detector was used for data collection. The X-ray beam was focused to 6×15 μm 2 at a fixed wavelength of λ = 0.37 ˚ A (ESRF), and 0.41 ˚ A (APS). Figure 1: Intensity of E 2g Raman band of hBN (normalized to band intensity at 6.17 GPa) as a function of pressure. De- pending on the fluence applied, the band disappears above a critical pressure. Figure 2: Normalized XRD intensities of (100) reflection of hBN compared to wBN as a function of pressure. Unity corresponds to pure hBN, zero indicates pure wBN. As depicted in Fig. 2, only the hBN sample exposed to 1×10 12 ions/cm 2 transformed entirely into wBN at 18.2 GPa. Our Raman and XRD data clearly demonstrate that the irra- diation of hBN with swift heavy ions prior to pressurization enhances the hBN to wBN phase transition. The transfor- mation is more or less complete depending on the applied fluence. The effect is ascribed to damage creation inducing a buckling of the basal planes in hBN [3]. Acknowledgment. Provision of BN single crystals by T. Taniguchi is gratefully acknowledged. References [1] G. Kern et al., Physical Review B 59 (1999) 8551. [2] T. Taniguchi et al., J. of Crystal Growth 303 (2007) 525. [3] H. Wang et al., Solid State Communications 149 (2009) 843. MATERIALS-16 GSI SCIENTIFIC REPORT 2009 438 View publication stats View publication stats