Elasticity and rheology of platinum under high pressure and nonhydrostatic stress

Using radial x-ray diffraction under nonhydrostatic compression in a diamond-anvil cell, we determine a lower bound of the yield strength of polycrystalline platinum to be 2.2 to 3.3 GPa in the pressure range of 4.2 to 22.4 GPa at room temperature. The elastic anisotropy S=2(S₁₁-S₁₂)/S₄₄ of platinum is also evaluated, and is equal to 1.47(16) throughout this pressure range. In addition, platinum shows a time-dependent relaxation under nonhydrostatic stress at both ambient temperatures and, in a separate set of experiments, during laser heating. Average strain rates measured for platinum are 10^-7 s^-1 at 300 K and 10^-5 s^-1 at 1200 K, resulting in effective creep viscosities of 10¹⁵ Pa s at 300 K and 10¹³ Pa s at 1200(300) K.