Hydrogen and the tetragonality of FeNi (tetrataenite)

Calculations have been conducted to show that it may be feasible to enhance the tetragonality ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>c</mml:mi> <mml:mo>/</mml:mo> <mml:mi>a</mml:mi> </mml:math> ratio) of tetrataenite ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>L</mml:mi> <mml:msub> <mml:mn>1</mml:mn> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>;</mml:mo> <mml:mi>P</mml:mi> <mml:mn>4</mml:mn> <mml:mo>/</mml:mo> <mml:mi>m</mml:mi> <mml:mi>m</mml:mi> <mml:mi>m</mml:mi> </mml:math> ) by charging it with hydrogen, a process that can be achieved at ambient temperature, well below the ordering temperature of the FeNi compound. Although the saturation magnetisation is slightly compromised, the magnetocrystalline anisotropy should increase significantly. It is now worth doing experiments to establish or disprove the results, because the energy of inserting a single hydrogen atom into the location that enhances tetragonality is slightly greater than that in a sub-optimal site in the context of increasing the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>c</mml:mi> <mml:mo>/</mml:mo> <mml:mi>a</mml:mi> </mml:math> ratio. Changes in tetragonality as a function of the location of the hydrogen atom can be explained in terms of the relative strengths of the Fe-H and Ni-H bonds.