Exploring the High Pressure Behavior of Superhard Tungsten Tetraboride

Miao Xie, Reza Mohammadi, Zhu Mao, Matt M. Armentrout, Abby Kavner,

Richard B. Kaner, and Sarah H. Tolbert

Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA

(Work performed at the ALS, HP-CAT, and also using the COMPRES gas-loading system at GSECARS)

Dense transition metal borides are a new class of the growing family of superhard materials. These compounds may find use as cutting tools and protective coatings. In the study, we examined the high pressure behavior of a new superhard material candidate tungsten tetraboride (WB4) in a diamond anvil cell up to 58.4 GPa. The zero-pressure bulk modulus, K0, obtained from fitting the pressure-volume data using the second-order Birch-Murnaghan equation of state is 326 ± 3 GPa. In contrast to the high pressure behavior of superhard material ReB2, we found an anomalous lattice softening of the c-axis in WB4 during compression, which was partially reversible during decompression. The anomaly was assigned to a second-order phase transition. We believe that the three-dimensional, almost isotropic, rigid covalently boron network in WB4 is responsible for both the observed structural change in WB4 and its high intrinsic hardness. In addition, based on our measured bulk modulus and an estimated Poisson’s ratio, a high shear modulus of 249 GPa was estimated for WB4 using an isotropic model.

Fig. 1. WB4 undergoes a pressure-induced second-order phase transition at ~42 GPa. This transition is reversible with some hysteresis, suggesting a mechanical origin. In contrast, ReB2 shows no evidence of a phase transition. The different pressure behavior can be related to difference in crystal structures between these two materials.

Fig. 2. (From left to right)

Crystal structures of ReB2, suggested structure of WB4, and a second suggested structure for WB4(W1.83B9). The presence of the boron-boron covalent bonds in WB4 may account for its distinct high pressure behavior relative to ReB2.

References

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R. Mohammadi, A. T. Lech, M. Xie, B. E. Weaver, M. T. Yeung, S. H. Tolbert, and R. B. Kaner, Proc. Nat. Acad. Sci. 108, 10958 (2011).

M. Xie, R. Mohammadi, Z. Mao, M. M. Armentrout, A. Kavner, R. B. Kaner, and S. H. Tolbert, Phys. Rev. B 85, 064118 (2012).