The compression behavior and stress state of nanocrystalline tungsten boride (WB) were investigated using radial x-ray diffraction (RXRD) in a diamond-anvil cell under non-hydrostatic compression up to 60.4 GPa. The compression properties and stress state are analyzed using lattice strain theory. Experiments were conducted at beamline X17C of the National Synchrotron Light Source. The radial x-ray diffraction data yield a bulk modulus that is qualitatively consistent with density functional theory calculations and demonstrate that WB is a highly incompressible material. A maximum differential stress, t, of about 14 GPa can be supported by nanocrystalline WB at the highest pressure. This corresponds to about 5% of the shear modulus, G, which is smaller than the values of t/G ($8%-10%) observed for BC 2 N, B 6 O, TiB 2 , and c-Si 3 N 4 at high pressures. Thus, while WB is highly incompressible, its strength is relatively low at high pressures compared to other hard ceramics. V
Generally
speaking, for a semiconductor, the temperature dependence
of excitonic emission corresponds to that of its band gap. However,
an anomalous behavior is exhibited by the excitonic luminescence of
diamond where as the temperature increases (from 10 to 300 K), its
indirect exciton luminescence peak displays a spectral-distinguishable
blue shift, whereas the indirect band-gap absorption shows a weak
red shift. According to experimental high-resolution deep-ultraviolet
spectra and theoretical analysis, the weak red shift of its indirect
band gap is ascribed to its large Debye temperature (ΘD ≈ 2220 K), which makes the lattice constant change comparatively
little in a large temperature range, so the change of its band gap
is relatively small; in this case, as the temperature rises, the thermal
population of valence-band holes that moves to a high-energy state
far away from the Fermi surface contributes to the macroscopic blue
shift of its excitonic emission.
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