Volume and anisotropic spontaneous striction in layered manganites: role of charge localization and magnetic interactions.

Thermal expansion measurements have been performed on single crystals of R2-2xSr1+2xMn2O7 (R=Pr, Nd, and Dy) over a wide temperature range (4-900 K). Our analysis allows us to give a general explanation for the large lattice anomalies observed in the layered manganites. Localization of the carriers gives rise to volume and anisotropic anomalies in the paramagnetic regime.