An Electron Microscopy Study of New Chemically Twinned Phases in the MgS‐Yb 2 S 3 System

Four new phases in the MgS‐Yb 2 S 3 system have been identified and characterised by means of transmission electron microscopy and associated techniques, such as microdiffraction and X‐ray energy dispersive spectroscopy analysis. The observed stoichiometries are: MgYb 2 S 4 [ a = 0.3752(2), b = 1.259(1), c = 1.268(2) nm] and Mg 2 Yb 6 S 11 [ a = 0.37297(4), b = 1.2592(2), c = 3.4853(3) nm] with orthorhombic symmetry (space group Cmcm ) and Mg 3 Yb 8 S 15 [ a = 1.2637(1), b = 0.37176(5), c = 2.4132(2) nm; β = 100.40(1)°] and MgYb 4 S 7 [ a = 1.2578(2), b = 0.3735(3), c = 1.1417(7) nm; β = 104.88(6)°] with monoclinic symmetry (space group C 2/ m ). Their structures can be described and derived from the NaCl‐type by the crystallographic operation of chemical twinning at the unit‐cell level along the {113} NaCl planes. The a and b lattice parameters are common in all the structures while the c parameter depends on the total extension of the twin slabs’ sequence, n , such that c ≈ n  ×  d (113) MgS (for orthorhombic phases) or d (001) ≈ n  ×  d (113) MgS (for monoclinic phases). All the unit‐cell parameters were refined from the X‐ray powder diffraction data after the unit‐cell symmetries had been deduced from electron microscopy data. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)