Structural aspects of pressure-dependent hole ordering in La1.67M0.33NiO4 (M=Ca,Sr,orBa)

A structural aspect of the hole ordering phenomenon in the La2-xMxNiO4 (x=13 and M=Ca,Sr,orBa) system has been investigated by means of x-ray diffraction experiments and measurements of the hydrostatic pressure dependence of the hole ordering temperature Tc. No significant change of the average lattice parameters was found near Tc, which is consistent with previous arguments that the origin of the ordering is electronic or magnetic in nature. The electrical resistivity conforms to variable range hopping behavior at low temperatures far below Tc and the associated thermal activation energy shows a linear pressure dependence. The highest Tc at ambient pressure was achieved with Sr doping. The pressure dependence of Tc is positive for M=Ba, negative for M=Ca, and very small and negative for M=Sr. The doping and pressure dependences of Tc are examined in the context of the coupling between the holes and the crystal lattice. We suggest that there is an optimum distance of 5.42 between the nearest-neighbor ordered holes which gives the maximum Tc.