Magnetic coupling near the metal-insulator transition in RNiO3

In the temperature versus geometric tolerance factor t phase diagram of the RNiO$_{3}$ perovskite family, the N\'{e}el temperature T$_{N}$ increases with t, $i.e.$ the (180\r{ }- $\phi )$ Ni-O-Ni bond angle, until it is intercepted by an insulator-metal transition occurring at T$_{IM}$ that decreases with increasing t. Recent XAS data reveal that as T$_{N}$ approaches T$_{IM}$ in the insulator phase, large and small NiO$_{6/2}$ octahedra emerge locally although neutron and x-ray diffraction are fit well by an orthorhombic rather than monoclinic space group. The bonding has been shown to be vibronic where T$_{N}$ approaches T$_{IM}$. In order to probe how the interatomic exchange interactions evolves where the bonding is vibronic with T$_{N} \quad <$ T$_{IM}$, we have carried out a systematic measurement of the pressure dependence of T$_{N}$. This dependence was determined by tracking under pressure an anomaly of the resistivity $\rho $(T) that occurs at T$_{N}$. The coefficient dlnT$_{N}$/dP of GdNiO$_{3}$ falls into the range of values for magnetic insulators well-described by superexchange theory. However, this coefficient increases dramatically as t increases, reaching a maximum at lower pressure in SmNiO$_{3}$ before falling to zero; in Nd$_{0.5}$Sm$_{0.5}$NiO$_ {3}$ it is zero in the pressure interval where T$_{N} \quad <$ T$_{IM}$. A schematic magnetic phase diagram of T$_{N}$ versus the Ni-O-Ni electron transfer integral is presented.