Anion substitution effects on the structure and magnetism of the chromium chalcogenide Cr5Te8 - Part III: Structures and magnetism of the high-temperature modification Cr((1+x))Q(2) and the low-temperature modification Cr((5+x))Q(8) (Q = Te, Se; Te : Se = 5 : 3)

The title compounds were synthesized via the high-temperature (HT) route. The materials are characterized by Rietveld analysis, magnetic measurements, and electronic band-structure calculations. Two different structural modifications depending on the synthesis conditions are observed: a HT modification with trigonal basic cells (space group: P-3m1) for Cr((1-x))Q(2) ((1+x) = 1.25, 1.28, 1.34, 1.37, 1.41, 1.43) and a low-temperature (LT) modification with trigonal super-cell (space group: P-3m1) for Cr((5+x))Q(8) ((5+x) = 5.00, 5.12, 5.36, 5.48, 5.64, 5.72). The crystal structures are closely related to the NiAs-type structure with metal vacancies in every second metal layer. The substitution of Te by Se and the change of the Cr concentration induce significant alterations of the magnetic properties. With increasing Cr content the Weiss constant B changes drastically from negative to strong positive values, i.e., with increasing Cr concentration a shift from predominant anti ferromagnetic exchange to ferromagnetic exchange occurs. At LTs a complex magnetic behavior is observed. For some members a spin-glass (SG) behavior is found with the freezing temperature T-f following the Vogel-Fulcher law. At the highest Cr concentrations ferromagnetic characteristics dominate with spontaneous magnetizations below the Curie temperatures. The differences of the magnetic properties of the LT and HT phases can be explained on the basis of interatomic distances and angles. For a deeper understanding of the experimental results, they have been compared with the results of spin polarized relativistic Korringa-Kohn-Rostoker electronic band-structure calculations and both results are consistent. To explain the features of temperature-dependent magnetization of the compounds, Monte Carlo (MC) simulations based on the Heisenberg model have been performed with the exchange coupling parameters obtained within the ab-initio calculations of the electronic structure. The possible reasons for a modification of the exchange coupling parameters responsible for the temperature-dependent features are also analyzed. (c) 2008 Elsevier Inc. All rights reserved.