Doped lanthanum manganite compounds exhibit a range of conducting, electronic and magnetic phases, and, in the case of the Ca-doped series, a phase transition from a ferromagnetic metal to an antiferromagnetic insulator occurs. Here, the authors use ultrahigh-resolution synchrotron X-ray diffraction to track the complex structural behavior of La1-xCaxMnO3 below the charge-ordering temperatures. In the model manganese perovskites La1-xCaxMnO3, several important phenomena have been observed, including ferromagnetic metallic/insulating states, colossal magnetoresistance effects, and charge- and orbital-ordered states. In the past, only compounds with x = 1/2, 2/3 and 3/4 and an insulating ground/antiferromagnetic state have been studied. To fully understand the crystal and electronic structures of these materials, it is important to study compounds with doping levels in the range of 0.5 < x < 2/3. Here we study the crystal structure in a series of compounds with 0.5 < x & LE; 0.6 using ultrahigh-resolution synchrotron X-ray diffraction. The experimental results reveal that all compounds undergo a structural transition at T < T-CO(x) & AP; 200 - 220 K with the concomitant emergence of superlattice Bragg peaks, which can be indexed assuming a superstructure with a modulation propagation vector, & tau;. At the base temperature of 5 K, the modulation vector of the superstructure & tau; = [& tau;(a), 0, 0] is parallel to the a-axis, with & tau;(a) varying linearly with x, as & tau;(a) & AP; 1 - x. Our results may aid attempts to understand more deeply phenomena related to spin, charge, and orbital ordering, as well as colossal magnetoresistance and symmetry breaking and emergent order in quantum states.

During the last decades, the utilization of imaging modalities such as single photon emission computed tomography (SPECT), positron emission tomography (PET), and magnetic resonance imaging (MRI) in every day clinical practice has enabled clinicians to diagnose diseases accurately at early stages. Radiolabeled iron oxide nanoparticles (RIONs) combine their intrinsic magnetic behavior with the extrinsic character of the radionuclide additive, so that they constitute a platform of multifaceted physical properties. Thus, at a practical level, RIONs serve as the physical parent of the so-called dual-modality contrast agents (DMCAs) utilized in SPECT/MRI and PET/MRI applications due to their ability to combine, at real time, the high sensitivity of SPECT or PET together with the high spatial resolution of MRI. This review focuses on the synthesis and in vivo investigation of both biodistribution and imaging efficacy of RIONs as potential SPECT/MRI or PET/MRI DMCAs.

The La1-xCaxMnO3 series of compounds with antiferromagnetic ground states (x 1/2) have been extensively studied due to the novel spin, orbital, and charge-ordering states observed when the calcium concentration is a simple fraction (x = 1/2, 2/3, and 3/4). The ground states of these compositions have been explained by the Goodenough charge, orbital, and spin ordering model. An important issue remaining is the elucidation of how the ground state changes when x is not a simple number. Here we study the magnetic structure of La1-xCaxMnO3 for 0.51 x 0.69 using powder neutron diffraction measurements supported by magnetization data. For compositions with 0.51 x 0.56, the magnetic structure, which we term as an incommensurate charge exchange (CE) structure can be described by two propagation vectors kC = [1/2, 0, 1/2] and kE = [6E, 0, 1/2]. In the second one, the component parallel to the a* axis of the reciprocal lattice changes with the Mn4+ concentration x as 6E approximate to x - 1/2 providing, thus, an unambiguous signature of the adoption of an incommensurate magnetic structure. As x gradually increases, the diffraction data reveal that two magnetic phases-one adopting the incommensurate CE, and one adopting the commensurate "2/3" magnetic structure-co-exist in the concentration regime of 0.57 x 0.61. Around the simple fraction x = 2/3, the magnetic structure can be also described by three propagation vectors, the commensurate kE = [0, 0, 1/2], kC = [1/2, 0, 1/2], and an incommensurate k2/3 = [1/3 + 62/3, 0, 1/2] propagation vector with 62/3 taking negative/zero/positive values for x smaller than/equal to/larger than 2/3, respectively. Our experimental results for 0.51 x 0.56 are neither in favor of a stripe structure consisting of a fine mixture of x = 1/2 and x = 2/3 phases (phase separation) nor of a defect structure in which an appropriate amount of Mn3+-O sheets have been replaced by Mn4+-O sheets (defect structure). A sinusoidal modulated structure has been used as a possible candidate in explaining the experimental neutron diffraction magnetic Bragg peaks. This result may be linked to the presence of a mixed orbital state of the manganese ions.