Exchange interactions in the perovskites Ca1-xSrxMnO3 and RMnO3 (R=La,Pr,Sm) -: art. no. 054403

Measurement of the variation of the Neel temperature T-N with pressure P and the compressibility kappa of the orbitally ordered Mn(III)O-3 array of the RMnO3 family (R=La,Pr,Sm) and of the Mn(IV)O-3 array of the Ca1-xSrxMnO3 system have enabled the determination for each of a systematic increase in kappa, the magnitude of the Bloch parameter alpha(B)equivalent tod ln T-N/d ln V, and T-N with decreasing bending of the (180degrees-phi) Mn-O-Mn bond angles. The data demonstrate that the anomalously high value of -alpha(B)=5.5 reported previously for LaMnO3 is not due to orbital order and its attendant anisotropic magnetic order. A higher \alpha(B)\ than 3.3 and an increasing compressibility characterize a system on the approach from the localized-electron side to a first-order transition from localized to itinerant electronic behavior. Moreover, the puzzling empirical relationship T(N)similar to for the pi-bonding Mn(IV)O-3 array is rationalized by dominance of a semicovalent exchange term over the superexchange term in the perturbation theory for the spin-spin interatomic interactions. The data also provide indirect evidence for a transition from in-phase to out-of-phase stacking along the c axis of the (001)-plane orbital ordering that has been predicted by Mizokawa, Khomskii, and Sawatzky.