New High Pressure Crystal Structure and Equation of State of Cesium Hydride to 253 GPa

A new orthorhombic high pressure phase was observed in CsH at approximately 17.5 GPa and ${V/V}_0=0.53\mathrm{}$. This high pressure phase was assigned to the CrB structure with the $\mathrm{Cmcm}$ space group based upon 19 x-ray diffraction peaks. It was studied to ${V/V}_0=0.260\mathrm{}$ at 253 GPa. This is the highest compression measured in the ionic alkali hydrides to date. At 253 GPa, the bulk modulus of CsH (881 GPa) is nearly double that of diamond at atmospheric pressure (442 GPa).