Ruddlesden-Popper (RP) phases with the formula (CaO)(CaMnO 3 ) n , where n ) 1, 2, 3, and ∞, have been prepared using the Pechini citrate gel process at temperatures as low as 900 °C under flowing oxygen. The compounds were characterized by X-ray powder diffraction, Rietveld profile analysis, thermal gravimetric analysis, iodometric titrations, Mn K-edge X-ray absorption spectroscopy, temperature-dependent magnetic susceptibility, and resistivity. Rietveld analysis shows that there is an elongation in the apical Mn-O distances of the MnO 6 octahedra, which increases with decreasing dimensionality. Mn K-edge X-ray absorption spectroscopy and iodometric titrations show no detectable amount of Mn 3+ (a Jahn-Teller distorted d 4 ion) in the samples indicating that the MnO 6 deformation may be associated with a splitting in the half-filled t 2g levels of Mn 4+ . Magnetic susceptibility measurements show spontaneous magnetic ordering to antiferromagnetic states at ∼125 K for CaMnO 3 and Ca 4 Mn 3 O 10 and at ∼110 K for Ca 2 MnO 4 and Ca 3 Mn 2 O 7 . The effective magnetic moment is greatly suppressed in all of the RP manganates, even in the threedimensional CaMnO 3 , n ) ∞ phase, and decreases dramatically with decreasing dimensionality. Large deviations from Curie-Weiss behavior are observed above the ordering temperature, which increase with decreasing n. These observations are discussed in terms of two-dimensional magnetic fluctuations and alternative mechanisms. The compounds are all poor electronic conductors with room-temperature resistivities in the range of 10 2 -10 4 Ω cm.

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.

The La 2Ϫ2x Ca 1ϩ2x Mn 2 O 7 (0.6рxр1.0) series has been successfully synthesized using a citrate gel technique in order to study their structure and properties as a function of x in the Mn 4ϩ -rich region of the phase diagram. Rietveld refinement of powder x-ray diffraction data, combined with electron microscopy, shows that the phases are of high purity and adopt the nϭ2 Ruddlesden-Popper structure comprised of perovskite bilayers separated by rocksalt layers. The lattice parameter c reaches a minimum at xϳ0.7 and the increase with x is attributed to elongation of the apical Mn-O bonds due to preferential A-site occupancy and/or splitting of the t 2g levels of Mn 4ϩ . X-ray absorption spectroscopy measurements confirm that the Mn-valence variation in these series tracks formal valence expectations. In the region 0.6рxр0.8 the magnetic susceptibility manifests a peak or shoulder at ϳ280 K due to charge and orbital ordering, and antiferromagnetic order develops at lower temperatures (T N ϳ150-200 K) with quasi-two-dimensional antiferromagnetic fluctuation effects being evidenced above T N . The magnetic properties change significantly at 0.825рxр1.0: at higher temperatures two-dimensional magnetic coupling is observed and at ϳ115 K the system spontaneously orders antiferromagnetically, but with a ferromagnetic moment. The transport results indicate insulating behavior at all compositions, but with an enhanced localization upon charge/orbital ordering in the xϭ0.7 material.

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Crystallography, using conventional Bragg diffraction, and the study of atomic correlation functions, using total diffraction, have historically been carried out separately. There exist two different scientific communities, which in the case of neutron diffraction normally use different instruments. However, modern time-of-flight neutron diffractometers allow data to be collected to high maximum momentum transfer, Qmax, and with good reciprocal-space resolution, d/d. The high Qmax yields correlation functions with good real-space resolution, whilst the good reciprocal-space resolution yields data well suited to conventional crystallographic analysis. We show how the Liquids and Amorphous Diffractometer, LAD, at the ISIS spallation neutron source at the Rutherford Appleton Laboratory has been used to obtain new information on a number of disordered crystalline molybdates, Li2MoO3, LiMoO2 and D2MoO3. The average crystal structures are determined using Rietveld refinement of the Bragg diffraction data, whilst the local structures are determined by modelling the correlation functions, T(r), obtained from total neutron diffraction data. Reconciling the information from the two techniques provides a deeper understanding of structures than is possible using either technique in isolation. Finally, we discuss how the next generation of instruments will allow the development of this technique with specific reference to the new General Materials Diffractometer, GEM, at ISIS.

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