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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