Simulation of Mg-K and S-K energy-loss near-edge structures in MgS and MgYb2S4 compounds

E. URONES-GARROTE; M.S. MORENO; L.C. OTERO-DÍAZ

Manchester

Congreso; 15th European Microscopy Congress; 2012

The formation of MS-R2S3 solid solutions (M: alkaline-earth metal; R: rare-earth metal) is well documented [1], adopting NaCl-type and related superstructures for M = Ca or Mg and for the heavy rare-earth metals. The solid solution for M = Mg and R = Yb was characterized in detail recently [2,3] due to its potential properties as environmentally-friendly inorganic pigments. The MgS-Yb2S3 system can be formulated as Mg1-xYb(2/3)x ?(1/3)xS (0 ? x ? 0.75), where ? = cation vacancies. The system presents exclusively the NaCl-type structure for x ? 0.30 and from that nominal composition the existence of crystals with spinel-type structure is also detected employing TEM and associated techniques. The coexistence of both structure types can be attributed to an order-disorder transition from NaCl-type into spinel-type, since they share the same basic framework, which is the structure of atacamite [3].     MgS compound (x = 0) presents NaCl-type structure, where both S and Mg are located in octahedral coordination: {MgS6} and {SMg6}. In the case of MgYb2S4, where x = 0.75, Mg and S are located in tetrahedral coordination: {MgS4} and {S4(Mg,Yb)}. Since the energy-loss near-edge structure (ELNES) of S-K and Mg-K edges is sensitive to coordination variations the main objective of this work is the study of those edges in MgS and spinel-type MgYb2S4 compounds together with their calculation using the FEFF code.     The samples (MgS and MgYb2S4) were prepared treating the corresponding nitrates as precursors - Mg(NO3)2·6H2O and Yb(NO3)3·5H2O - under a flow of H2S (10 % in Ar) and a flow of Ar bubbling in CS2 at a temperature of 1000ºC for 10 hours. TEM observations were performed in a Philips CM200FEG electron microscope fitted with a GIF 200. EEL spectra were acquired in diffraction mode, with a collection semi-angle of   4.6 mrad, a dispersion of 0.1 ev/pixel and an acquisition time of 8 seconds. The energy resolution was   0.9 eV, measured from the FWHM of the zero-loss peak. More than 15 spectra from S-K and Mg-K edges were collected from each metal sulphide.     The spectra for both compounds are shown in Figs. 1 and 2. It can be observed clear differences between both materials which allows us to identify and distinguish between these phases. In consequence these spectra can be used as end members for an empirical fingerprinting of composition changes in the solid solution.    In order to correlate ELNES spectral features to cation and oxygen contributions we have modeled the S-K and Mg-K edges by means of real-space multiple scattering calcultions using the FEFF9.1 program as described in [4]. This version allows to consider a number of experimental conditions, for example collection angle and beam energy. The calculations include core-hole effects by using the RPA approximation.    The calculated density of states reveals a covalent nature of these phases. The structures appearing within the first 10 eV above the threshold arise from a covalent mixing of mainly S 2p and Mg s-p (plus Yb in MgYb2S4) states.     It can be realized that in all cases the calculations reproduce all details present in the fine structure. A very good agreement with experiment is achieved for these covalent compounds.