Polytypism of incommensurately modulated structures of crystalline bromine upon molecular dissociation under high pressure

Polytypism of incommensurately modulated structures was hitherto unobserved. Here, it was found in solid bromine and iodine upon molecular dissociation under pressure up to 112 GPa. Single-crystal synchrotron x-ray diffraction in laser-heated diamond anvil cells revealed a number of allotropes of bromine and iodine, including polytypes of Br-IIIγ <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mo>(</a:mo><a:mrow><a:mi>F</a:mi><a:mi>m</a:mi><a:mi>m</a:mi><a:mi>m</a:mi><a:mo>(</a:mo><a:mrow><a:mn>00</a:mn><a:mi>γ</a:mi></a:mrow><a:mo>)</a:mo><a:mi>s</a:mi><a:mn>00</a:mn><a:mo>)</a:mo></a:mrow></a:math> with γ varying within 0.18 to 0.3, coexisting at a given pressure. This finding opens a research area in the field of solid-solid phase transitions, disordered states of solid matter, and high-pressure structural studies.