Resolving the Octahedral Tilt Order in La_2-xSr_xCuO₄

The pattern of tilts of CuO₆ octahedra in 214 cuprates can have a large impact on the electronic order. For example, a phase in which the octahedra rotate about the Cu-O bonds is known to pin stripe order in La_2-xBa_xCuO₄. In La_2-xSr_xCuO₄, it has been believed that the octahedral rotations are only about an axis at 45° to the Cu-O bonds. In a recent neutron scattering study of an x = 0.07 crystal, we found evidence that there is also a tilt component about the Cu-O bonds [1]. To further explore the situation, we have performed a crystallographic study of the parent compound, La₂CuO₄, on the 4-circle diffractometer HB-3A at HFIR (ORNL). While confirming that the crystal has monoclinic symmetry, as previously reported, we have verified a new set of superlattice peaks (hkl) with h and k odd, l even. These reflections are consistent with a tilt component about the Cu-O bonds and require a revision of the space group.
[1] H. Jacobsen et al., Phys. Rev. B 92, 174525 (2015).