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

The manganates La0.80Ca0.20MnO3, La0.70Ca0.30MnO3 and La0.70Sr0.30MnO3 exhi bit the colossal magnetoresistive (CMR) effect. Total neutron diffraction w as employed to yield information on both the avenge and local atomic struct ure 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 tr ansformation of the total diffraction pattern to yield the total correlatio n 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. Jah n-Teller distortions of the MnO6 octahedra are absent at the lowest tempera tures in the metallic phase. As the temperature is raised towards the param agnetic-semiconducting to ferromagnetic-metallic transition at T-c, T(r) ex hibits clear increases in the variance of Mn-O bond distances, which greatl y exceed those expected from the increase in disorder due to atomic thermal motion. This is confirmed by comparing the behaviour of the three material s, which have different values for T-c. The advantage of studying the local structure directly by determining T(r) from total neutron scattering, rath er than extrapolating from the average to local structure from Bragg scatte ring studies, is demonstrated. Comparisons are made with the results obtain ed for the ordered compound LaMnO3,which does not exhibit the CMR effect. T he three CMR manganates studied here donor show a separation of the Mn-O di stances into two well resolved sets above T-c as reported by other workers.