Phase transitions in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mi>HD</mml:mi><mml:mtext>−</mml:mtext><mml:mi mathvariant="normal">D</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> mixtures up to 350 GPa

Utilizing the high-pressure low-temperature Raman and optical transmission/absorption spectroscopies, we have mapped out the phase diagram of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi mathvariant="normal">H</a:mi><a:mn>2</a:mn></a:msub></a:math>-HD-<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi mathvariant="normal">D</c:mi><c:mn>2</c:mn></c:msub></c:math> alloy with the initial H:D concentration 50:50 up to 350 GPa between 10 and 300 K. We followed the phase lines between all known solid phases [I, II, III, IV, and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mrow><e:mi>IV</e:mi></e:mrow><e:mo>′</e:mo></e:msup></e:math> (<f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>IV</f:mi><f:mo>+</f:mo><f:mi mathvariant="normal">V</f:mi></f:mrow></f:math>)] of pure <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi mathvariant="normal">H</h:mi><h:mn>2</h:mn></h:msub></h:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi mathvariant="normal">D</j:mi><j:mn>2</j:mn></j:msub></j:math> constraining their locations in the wide space. We trace the phase boundary separating phases III and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mi>IV</l:mi><l:mo>(</l:mo><l:msup><l:mrow><l:mi>IV</l:mi></l:mrow><l:mo>′</l:mo></l:msup></l:mrow></l:math>)/V demonstrating that this phase line has a steep negative slope (<m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mo>−</m:mo><m:mn>0.95</m:mn></m:mrow></m:math> K/GPa), which becomes shallower at above 250 GPa (<n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mo>−</n:mo><n:mn>0.55</n:mn></n:mrow></n:math> K/GPa). The phase line follows the trend observed in the pure species. Additionally, we find that the band gap of the mixture between 300 and 360 GPa is close to those of the pure species suggesting that alloying one isotope by another would not significantly shift the metallization pressure. These observations imply that hydrogen deuteride (HD) has very similar properties to those exhibited by the pure species.