Strength and elasticity of niobium under high pressure

High purity polycrystalline niobium contained in boron-epoxy gasket was compressed in a diamond anvil cell (DAC). The pressure was increased in steps of 3 GPa and the diffraction patterns recorded at each pressure with the incident x-ray beam perpendicular to the load axis of the DAC (radial diffraction). The maximum pressure reached was 37.6 GPa. The compressive strength (differential stress) derived from the radial diffraction data is 0.44(1) GPa at 2.1 GPa and shows a shallow maximum at 5 GPa, and then decreases to 0.35(5) at 12 GPa. At higher pressures, strength increases nearly linearly and the extrapolated value at 40 GPa is 0.94(6) GPa. At any pressure, the single-crystal elastic moduli derived from the diffraction data can be made to match well those obtained from the extrapolation of the elasticity data at ambient pressure by adjusting the weight parameter a that appears in the lattice strain theory. The parameter a is found to decrease from 2.00(8) at 2.1 GPa to 1.35(4) at 37.6 GPa. VC 2011 American Institute of Physics. [doi:10.1063/1.3594748]