On the microstructure of the charge density wave observed in La_1−xCa_xMnO₃

Abstract

We have used low temperature (90 K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La_{1− x} Ca _x MnO₃. It has been stated that Mn³⁺ and Mn⁴⁺ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5 µm in La_0.48Ca_0.52MnO₃ had a wavevector which varied on a scale of tens of nanometres with an average of ⟨q⟩ = 0.450a * and a standard deviation Δq = 0.004 a* in its magnitude and Δθ = 0.56° in its direction at 90 K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90 K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30 K above the ‘charge ordering temperature’.

Acknowledgements

We thank A. J. Williams for synthesising the samples used in this investigation and P. B. Littlewood and J. P. Attfield for helpful discussions. This work was funded by the EPSRC and the Royal Society.