Observation of Hendricks-Teller partial order in a tetragonal cuprate superconductor: ${\mathrm{La}}_{1.68}$${\mathrm{Nd}}_{0.14}$${\mathrm{Na}}_{0.10}$${\mathrm{K}}_{0.082}$${\mathrm{CuO}}_4$

The cuprate superconductor, ${\mathrm{La}}_{1.68}$${\mathrm{Nd}}_{0.14}$${\mathrm{Na}}_{0.10}$${\mathrm{K}}_{0.082}$${\mathrm{CuO}}_4$, was prepared by selective precipitation from a NaOH/KOH eutectic melt at 350 °C. Synchrotron x-ray-diffraction data indicate that the structure is related to the tetragonal T-structure type of doped ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, but the presence of superlattice reflections shows the unit cell to be tripled along the c direction [a=3.763(1) Å and c=39.63(2) Å]. The existence of this supercell is confirmed by electron microscopy and electron diffraction, which show that the tripling is due to cation ordering. Careful examination of the positions and widths of the superlattice reflections in the synchrotron data indicates that the dopant cations are in fact only partially ordered, and a simple Hendricks-Teller model using an ordering scheme suggested by the electron microscopy data agrees well with the diffraction data. There is no structural change (orthorhombic distortion) between room temperature and 28 K, just above the onset temperature for superconductivity determined from magnetic-susceptibility measurements. The Meissner fraction, estimated to be 80% at 5 K, indicates that this material is a bulk superconductor.