The controlled-valence properties of La& "Sr"Fe03 and La& "Sr"Mn03 are studied by means of softx-ray absorption spectroscopy. A comparison between the transition-metal 2p spectra and atomicmultiplet calculations is used to determine the 3d count. The 0 1s spectrum is used to characterize changes in unoccupied states that contain oxygen p character. The results indicate that the holes induced by substitution for both series are located in states of mixed metal 3d-oxygen 2p character. The ground state of LaFe03 is mainly 3d' and becomes 3d'L (where L denotes a ligand hole) in the Lal "Sr"Fe03 series for low Sr concentration. The main component of the ground state of LaMn03 is 3d and becomes a mixture of 3d' and 3d L in the Lal "Sr"Mn03 series. The trends in controlledvalence properties of similar oxides across the transition-metal series can be rationalized within the framework of the Zaanen-Sawatzky-Allen model.
This paper serves the twofold purpose of demonstrating a numerical technique that appears to have potential in studies of nonlinear reactive transport problems and of presenting a case study where the implications of reactive transport processes are profound. The case study relates to the Fuhrberger Feld aquifer in northern Germany, which provides the bulk of the water supply for the half‐million city of Hannover. Part of the aquifer receives a strong influx of nitrate from agricultural activities. The nitrate in the aquifer is microbially transformed into sulfate, the transformation depending on the availability of reduced sulfur compounds present in the sediment, and the sulfate is also transformed further. The individual transformations occur within distinct reaction zones in the aquifer. The relevant dynamic processes can be formulated as a nonlinear transport problem involving multiple interacting species in dissolved or solid form. The system is simulated using a finite element‐based technique that generates a symmetric coefficient matrix for the transport equation, while providing second‐order accuracy in time. The form of the matrix allows the use of a highly efficient and robust symmetric conjugate gradient solver. The technique provides an ample spatial resolution capacity at reasonable cost and handles grids with irregular geometry in two or three dimensions. The convenience and versatility of conventional finite elements is retained. A vertical‐section simulation identifies different reaction zones in the aquifer and gives insight into the effect of the controlling parameters. The simulation study is a first step in the development of a prognosis for the fate of the groundwater resource.
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