Structural, optical and magnetic properties of MCuSi4O10 (M = Ba and Sr) blue pigments

Inspired by the ancient art work and recent scientific advances in inorganic blue pigments, we have synthesized polycrystalline ceramic samples of barium copper tetrasilicate (BaCuSi4O10) and strontium copper tetrasilicate (SrCuSi4O10) using solid state reaction route to understand their crystallography, magnetism and optical behaviour. Powder X-ray diffraction patterns confirmed single phase tetragonal crystal structure with space group P4/ncc for both the pigment samples. The unit cell lattice parameters and other structural parameters of these samples were determined using the Rietveld profile refinements of powder diffractograms. Both of these blue pigments show swift optical absorption below 300 nm and considerable absorption in the visible and infrared regions. The obtained values of band gap for BaCuSi4O10 and SrCuSi4O10 were found to be 3.61 and 3.52 eV respectively. Temperature and field dependent magnetization measurements are suggestive of paramagnetic state at room temperature and superparamagnetic state at low temperatures in both these compounds. The estimated values of effective magnetic moments were found to be 1.833 and 1.91 mu(B) for BaCuSi4O10 and SrCuSi4O10 respectively. Interestingly these obtained values of magnetic moments are very close to the theoretical value of 1.73 mu(B) for Cu2+ ion.