High-pressure studies of NaH to 54 GPa

High-pressure x-ray diffraction data on NaH to 54 GPa at room temperature show a B1(NaCl)-to B2(CsCl) phase transition at $P_T$=29.3±0.9 GPa and volume fraction V/$V_0$=0.61±0.01. In the B1 phase the isothermal bulk modulus $B_0$ is 19.4±2.0 GPa, with a pressure derivative $B_0^{\mathcal{’}}$ of 4.4±0.5. These results, together with earlier measurements on KH, RbH, and CsH, indicate that the alkali-metal hydrides show the same strong dependence of $P_T$ on cation as do the alkali-metal halides. The systematically lower bulk moduli of the alkali-metal hydrides as compared to the alkali-metal halides can be attributed to incomplete charge transfer in this series of partly ionic compounds.