High Pressure Synthesis of Rubidium Superhydrides

<p> Through laser-heated diamond anvil cell experiments, we synthesize a series of rubidium superhydrides and explore their properties with synchrotron x-ray powder diffraction and Raman spectroscopy measurements, combined with density functional theory calculations. Upon heating rubidium monohydride embedded in <math display='inline'> <mrow> <msub> <mrow> <mi mathvariant='normal'>H</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </mrow> </math> at a pressure of 18 GPa, we form <math display='inline'> <mrow> <msub> <mrow> <mi>RbH</mi> </mrow> <mrow> <mn>9</mn> </mrow> </msub> <mtext>−</mtext> <mi mathvariant='normal'>I</mi> </mrow> </math> , which is stable upon decompression down to 8.7 GPa, the lowest stability pressure of any known superhydride. At 22 GPa, another polymorph, <math display='inline'> <mrow> <msub> <mrow> <mi>RbH</mi> </mrow> <mrow> <mn>9</mn> </mrow> </msub> <mtext>−</mtext> <mi>II</mi> </mrow> </math> is synthesised at high temperature. Unique to the Rb-H system among binary metal hydrides is that further compression does not promote the formation of polyhydrides with higher hydrogen content. Instead, heating above 87 GPa yields <math display='inline'> <mrow> <msub> <mrow> <mi>RbH</mi> </mrow> <mrow> <mn>5</mn> </mrow> </msub> </mrow> </math> , which exhibits two polymorphs ( <math display='inline'> <mrow> <msub> <mrow> <mi>RbH</mi> </mrow> <mrow> <mn>5</mn> </mrow> </msub> <mtext>−</mtext> <mi mathvariant='normal'>I</mi> </mrow> </math> and <math display='inline'> <mrow> <msub> <mrow> <mi>RbH</mi> </mrow> <mrow> <mn>5</mn> </mrow> </msub> <mtext>−</mtext> <mi>II</mi> </mrow> </math> ). All of the crystal structures comprise a complex network of quasimolecular <math display='inline'> <mrow> <msub> <mrow> <mi mathvariant='normal'>H</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </mrow> </math> units and <math display='inline'> <mrow> <msup> <mrow>. Authors: Kuzovnikov, Mikhail A.; Wang, Busheng (ORCID:0000000277439471); Wang, Xiaoyu; Marqueño, Tomas (ORCID:0000000153711082); Shuttleworth, Hannah A. (ORCID:0000000251131575). DOE Contract: AC02-06CH11357