Local distortions in the colossal magnetoresistive manganates La0.70Ca0.30MnO3, La0.80Ca0.20MnO3 and La0.70Sr0.30MnO3 revealed by total neutron diffraction

Abstract

The manganates La0.80Ca0.20MnO3, La0.70Ca0.30MnO3 and La0.70Sr0.30MnO3 exhibit the colossal magnetoresistive (CMR) effect. Total neutron diffraction was employed to yield information on both the average and local atomic structure of these disordered crystalline materials as a function of temperature. The average structures were determined from Rietveld analysis of the Bragg scattering. Information on the local structures was obtained by Fourier transformation of the total diffraction pattern to yield the total correlation function, T(r). Particular attention is paid to changes in the Mn-O bond distances, which are widely believed to be important in the CMR effect. Jahn-Teller distortions of the MnO6 octahedra are absent at the lowest temperatures in the metallic phase. As the temperature is raised towards the paramagnetic-semiconducting to ferromagnetic-metallic transition at Tc, T(r) exhibits clear increases in the variance of Mn-O bond distances, which greatly exceed those expected from the increase in disorder due to atomic thermal motion. This is confirmed by comparing the behaviour of the three materials, which have different values for Tc. The advantage of studying the local structure directly by determining T(r) from total neutron scattering, rather than extrapolating from the average to local structure from Bragg scattering studies, is demonstrated. Comparisons are made with the results obtained for the ordered compound LaMnO3, which does not exhibit the CMR effect. The three CMR manganates studied here do not show a separation of the Mn-O distances into two well resolved sets above Tc as reported by other workers.