Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La${}_2$SmNi${}_2$O${}_7$

Abstract

We investigated the structural properties of bilayer nickelate La${}_2$SmNi${}_2$O${}_7$ as a function of pressure and temperature. At ambient conditions, we show that the material crystallizes as a monoclinic superstructure distinct from the one previously reported and close to the pseudo-orthorhombic structure of pristine La${}_3$Ni${}_2$O${}_7$. No signatures of satellite reflections associated with charge density wave (CDW) ordering are detected at low temperature. Upon compression, a sequence of pressure-induced structural transitions from monoclinic to orthorhombic 15 GPa and then tetragonal 21 GPa symmetry is observed. Within the superconducting dome, the quality of the X-ray diffraction data enables structural refinements enabling theoretical models to understand the emergence of superconductivity.

8 pages, 5 figures