New ecological pigments in the Ca-Yb-S system

The microstructure (via SEM, TEM and X-ray powder data), magnetic and optic al colour parameters of the solid solution Ca(1-x)Yb(2/3x)square S-1/3x (wi th 0 less than or equal tox less than or equal to1) were investigated. Seve ral samples have been prepared by heating mixtures of CaS and Yb2S3 (white and yellow colour, respectively) in sealed silica tubes and by sulphurating the amorphous precursors of both metals in a H2S +Ar steam in CS2. Scannin g electron micrographs show small particle size, up to 5 mum, in the second preparation samples. All the samples are bluish-green coloured and suitabl e for pigment applications. For x approximate to0.11, the cation vacancies (produced during the process 3Ca(2+)--> 2Yb(3+)?) are randomly distributed and a decrease in the NaCl-type unit cell volume of 1% was observed accompa nied by cluster formation in the image. For x approximate to0.33, we observ ed a cubic superstructure of NaCl-type, with a =2a(0) = 11.280(3) Angstrom; a(0) =5.50 Angstrom for pure CaS. Higher x values give mixtures of two pha ses: (a) rhombohedral symmetry, a(r)=7.987(6) Angstrom and alpha =58.6 (deg rees), and (b), orthorhombic Yb3S4-type, with a=12.641(8) Angstrom, b=3.844 (4) Angstrom and c=13.01(1) Angstrom. (C) 2001 Elsevier Science B.V. All ri ghts reserved.