Unconventional Crystal Structure of the High-Pressure Superconductor La3Ni2O7</mml…

The discovery of high-temperature superconductivity in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>La</a:mi></a:mrow><a:mrow><a:mn>3</a:mn></a:mrow></a:msub><a:msub><a:mrow><a:mi>Ni</a:mi></a:mrow><a:mrow><a:mn>2</a:mn></a:mrow></a:msub><a:msub><a:mrow><a:mi mathvariant="normal">O</a:mi></a:mrow><a:mrow><a:mn>7</a:mn></a:mrow></a:msub></a:mrow></a:math> at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:msub><d:mrow><d:mi>NiO</d:mi></d:mrow><d:mrow><d:mn>6</d:mn></d:mrow></d:msub></d:mrow></d:math> octahedral bilayers stacked along the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>c</f:mi></f:math>-axis direction was consistently posited in initial studies on <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:msub><h:mi>La</h:mi><h:mn>3</h:mn></h:msub><h:msub><h:mi>Ni</h:mi><h:mn>2</h:mn></h:msub><h:msub><h:mi mathvariant="normal">O</h:mi><h:mn>7</h:mn></h:msub></h:math>. Here, we reassess this structure in optical floating zone-grown <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>La</k:mi><k:mn>3</k:mn></k:msub><k:msub><k:mi>Ni</k:mi><k:mn>2</k:mn></k:msub><k:msub><k:mi mathvariant="normal">O</k:mi><k:mn>7</k:mn></k:msub></k:math> single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mrow><n:msub><n:mrow><n:mi>NiO</n:mi></n:mrow><n:mrow><n:mn>6</n:mn></n:mrow></n:msub></n:mrow></n:math> octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:msub><p:mi>La</p:mi><p:mn>3</p:mn></p:msub><p:msub><p:mi>Ni</p:mi><p:mn>2</p:mn></p:msub><p:msub><p:mi mathvariant="normal">O</p:mi><p:mn>7</p:mn></p:msub></p:math>, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity. Published by the American Physical Society 2024