High-pressure synthesis and neutron scattering study of tantalum hydride <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>TaH</mml:mi><mml:mrow><mml:mn>1.23</mml:mn><mml:mo>(</mml:mo><mml:mn>5</mml:mn><mml:mo>)</mm

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>580</a:mn><a:msup><a:mspace width="0.16em"/><a:mo>∘</a:mo></a:msup><a:mi mathvariant="normal">C</a:mi></a:mrow></a:math> using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>P</d:mi><d:mi>m</d:mi><d:mtext>−</d:mtext><d:mn>3</d:mn><d:mi>n</d:mi></d:mrow></d:math>, Ta atoms at the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mn>2</e:mn><e:mi>a</e:mi></e:mrow></e:math> and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mn>6</f:mn><f:mi>c</f:mi></f:mrow></f:math> Wyckoff positions) and a lattice parameter of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mi>a</g:mi><g:mo>=</g:mo><g:mn>5.510</g:mn><g:mo>(</g:mo><g:mn>5</g:mn><g:mo>)</g:mo><g:mspace width="0.16em"/><g:mi>Å</g:mi></g:mrow></g:math> at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>T</i:mi><i:mo>=</i:mo><i:mn>85</i:mn></i:mrow></i:math> K. The hydrogen content determined by hot extraction was <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:mrow><j:mi mathvariant="normal">H</j:mi><j:mo>/</j:mo><j:mi>Ta</j:mi></j:mrow><j:mo>=</j:mo><j:mn>1.23</j:mn><j:mo>(</j:mo><j:mn>5</j:mn><j:mo>)</j:mo></j:mrow></j:math>. Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mi mathvariant="normal">Δ</l:mi><l:mi mathvariant="normal">H</l:mi><l:mo>/</l:mo><l:mi>Ta</l:mi><l:mo>=</l:mo><l:mn>0.2</l:mn></l:mrow></l:math> was desorbed at around <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mo>−</o:mo><o:mn>70</o:mn><o:msup><o:mspace width="0.16em"/><o:mo>∘</o:mo></o:msup><o:mi mathvariant="normal">C</o:mi></o:mrow></o:math>, and then the rest of the hydrogen was desorbed between <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mn>100</r:mn><r:msup><r:mspace width="0.16em"/><r:mo>∘</r:mo></r:msup><r:mi mathvariant="normal">C</r:mi></r:mrow></r:math> and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:mrow><u:mn>390</u:mn><u:msup><u:mspace width="0.16em"/><u:mo>∘</u:mo></u:msup><u:mi mathvariant="normal">C</u:mi></u:mrow></u:math>. The <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"><x:mrow><x:mi mathvariant="normal">A</x:mi><x:mn>15</x:mn><x:mtext>-type</x:mtext></x:mrow></x:math> metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of <z:math xmlns:z="http://www.w3.org/1998/Math/MathML"><z:mrow><z:mi mathvariant="normal">A</z:mi><z:mn>15</z:mn><z:mtext>−</z:mtext><z:msub><z:mi>TaH</z:mi><z:mrow><z:mn>1.23</z:mn><z:mo>(</z:mo><z:mn>5</z:mn><z:mo>)</z:mo></z:mrow></z:msub></z:mrow></z:math> demonstrated that hydrogen atoms occupy the <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML"><bb:mrow><bb:mn>24</bb:mn><bb:mi>k</bb:mi></bb:mrow></bb:math> and <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML"><cb:mrow><cb:mn>16</cb:mn><cb:mi>i</cb:mi></cb:mrow></cb:math> Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the <db:math xmlns:db="http://www.w3.org/1998/Math/MathML"><db:mrow><db:mn>24</db:mn><db:mi>k</db:mi></db:mrow></db:math> and an increase of the <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML"><eb:mrow><eb:mn>16</eb:mn><eb:mi>i</eb:mi></eb:mrow></eb:math> site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML"><fb:mrow><fb:mi mathvariant="normal">A</fb:mi><fb:mn>15</fb:mn><fb:mtext>−</fb:mtext><fb:msub><fb:mi>TaH</fb:mi><fb:mrow><fb:mn>1.23</fb:mn><fb:mo>(</fb:mo><fb:mn>5</fb:mn><fb:mo>)</fb:mo></fb:mrow></fb:msub></fb:mrow></fb:math> at 72, 135, 145, and <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML"><hb:mrow><hb:mn>166</hb:mn><hb:mspace width="0.16em"/><hb:mi>meV</hb:mi></hb:mrow></hb:math>, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML"><jb:mrow><jb:mn>24</jb:mn><jb:mi>k</jb:mi></jb:mrow></jb:math> and <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML"><kb:mrow><kb:mn>16</kb:mn><kb:mi>i</kb:mi></kb:mrow></kb:math> sites, respectively. No superconductivity was found in <lb:math xmlns:lb="http://www.w3.org/1998/Math/MathML"><lb:mrow><lb:mi mathvariant="normal">A</lb:mi><lb:mn>15</lb:mn><lb:mtext>−</lb:mtext><lb:msub><lb:mi>TaH</lb:mi><lb:mrow><lb:mn>1.1</lb:mn></lb:mrow></lb:msub></lb:mrow></lb:math> and hydrogen-free A15-Ta at temperatures down to 1.5 K. Published by the American Physical Society 2024