Nuclear Magnetic Resonances and Mn-O-P-O-Mn Superexchange Linkages in Paramagnetic and Antiferromagnetic LiMnPO4

Nuclear magnetic resonances of ${\mathrm{P}}^{31}$ and ${\mathrm{Li}}^7$ have been observed in single crystals of LiMnP${\mathrm{O}}_4$ in the antiferromagnetic and paramagnetic states as a function of orientation from 4.2-300°K. The resonances were found to be shifted from the predicted $\gamma H_0$ in a way that indicates the presence of unpaired electrons in $3s$ (and possibly $3p$) orbitals of the phosphorus atoms separated from the magnetic ${\mathrm{Mn}}^{++}$ ions by an intervening oxygen atom. The Néel temperature is established as 34.85°±0.1°K by observation of the ${\mathrm{Li}}^7$ resonance as a function of temperature. The arrangement of spins in the antiferromagnetic state is established by a combination of NMR and susceptibility data. The structure so determined requires the presence of superexchange interactions along Mn-O-P-O-Mn paths as well as conventional Mn-O-Mn paths. Discussion is given of the observed shifts in terms of chemical bonding and of the connection between the shifts and both the conventional and longer range superexchange linkages. Linewidths and shapes and values of $T_1$ are given and discussed briefly.