Thermal expansion measurements have been performed on single crystals of $R_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ( $R=\mathrm{Pr}$, $\mathrm{Nd}$, and $\mathrm{Dy}$) 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. The anisotropic distortion is enhanced with the establishment of long-range antiferromagnetism, whereas it is quenched by long-range ferromagnetic order.

• Received 9 September 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.995