Electronic structure of MgS and MgYb 2 S 4 compounds by EELS and realspace multiple scattering calculations
E. Urones-Garrote1, MS Moreno2 and LC Otero-Díaz3
1. Centro Nacional de Microscopía Electrónica, Universidad Complutense, Madrid, Spain
2. Centro Atómico Bariloche, SC de Bariloche, Argentina
3. Departamento de Química Inorgánica, Facultad CC Químicas, Universidad Complutense, Madrid, Spain.
[email protected]
Keywords: EELS, metal sulphides, real-space multiple scattering calculations
The formation of MS-R 2 S 3 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, displaying a color
variation from yellow to green . The MgS-Yb 2 S 3 system can be formulated as Mg 1-x Yb (2/3)x  (1/3)x S (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: {MgS 6 } and {SMg 6 }. In the case of MgYb 2 S 4 , where x = 0.75, Mg and S are
located in tetrahedral coordination: {MgS 4 } and {S(Mg,Yb) 4 }. Since the energy-loss near-edge
structure (ELNES) of S-K and Mg-K edges is sensitive to coordination variations, which also have a
strong influence on color properties, the main objective of this work is the study of those edges in
MgS and spinel-type MgYb 2 S 4 compounds together with their calculation using the FEFF code.
The samples (MgS and MgYb 2 S 4 ) were prepared treating the corresponding nitrates as
precursors – Mg(NO 3 ) 2 ·6H 2 O and Yb(NO 3 ) 3 ·5H 2 O – under a flow of H 2 S (10 % in Ar) and a flow of
Ar bubbling in CS 2 at a temperature of 1000 ºC 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. Clear differences can be observed
between both materials which allow identifying and distinguishing 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 sulphur contributions we have
modelled the S-K and Mg-K edges by means of real-space multiple scattering calculations using the
FEFF9.1 program as described in [4]. This version allows considering 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 the 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 MgYb 2 S 4 ) states.
It can be observed that the calculations reproduce all
details present in the fine structure in all cases. Therefore, a very good agreement with experiment
is achieved for these covalent compounds.
References
[1] J Flahaut in “Handbook on the chemistry and physics of rare earths”, ed. KL Gschneider and L
Eyring, (North-Holland, Amsterdam), vol 4.
[2] E Urones-Garrote, A Gómez-Herrero, AR Landa-Cánovas, F Fernández-Martinez and LC OteroDíaz, Journal of Alloys and Compounds 374 (2004) p. 197.
[3] E Urones-Garrote, A Gómez-Herrero, AR Landa-Cánovas, RL Withers and LC Otero-Díaz,
Chemistry of Materials 17 (2005) p. 3524.
[4]. MS Moreno, K Jorissen and JJ Rehr, Micron 38 (2007) p. 1
[5] The authors gratefully acknowledge funding from projects with reference S-2009/PPQ-1326 and
MAT2010-19460 and from CONICET (Argentina).
Figure 1. Comparison of Mg-K (left) and S-K (right) experimental edges of MgYb 2 S 4 (open circles) and
calculations (blue line) using FEFF9.1.
Figure 2. Comparison of Mg-K (left) and S-K (right) experimental edges of MgS (open circles) and
calculations (blue line) using FEFF9.1.