Crystal structure, thermal behaviour, vibrational spectroscopy and optical properties of new compounds K $$₂$$ 2 Ca(HAsO $$₄$$ 4 ) $$₂\cdot $$ 2 · 2H $$₂$$ 2 O with kröhnkite-type chain

The new kröhnkite compound called potassium calcium-bis-hydrogen arsenate dihydrate K(₂)Ca(HAsO(₄)₂cdot )2H(₂)O was obtained by hydrothermal method and characterized by X-ray diffraction, infrared spectroscopy, Raman scattering, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis and optical (photoluminescence and absorption) properties. It crystallizes in the triclinic space group P(ar{1}) and unit cell parameters (a = 5.971(3)) Å, (b =6.634(3)) Å, (c = 7.856(4)) Å, (alpha =104.532(9)) (^circ ), (eta = 105.464(9)) (^circ ) and (gamma = 109.698(9)) (^circ ). The structure of K(₂)Ca(HAsO(₄)₂cdot )2H(₂)O built up from this infinite, (Ca(HAsO(₄)₂)(H(₂)O)(₂)²⁺), was oriented along an axis resulting from the association of CaO(₆) octahedra alternating with each two HAsO(₄) tetrahedra by sharing corners. Each potassium atom links two adjacent chains by three oxygen atoms of HAsO(₄) tetrahedra. TGA and DSC have shown the absence of phase transition. The existence of vibrational modes corresponding to the kröhnkite is identified by the IR and Raman spectroscopies in the frequency ranges of 400–4000 and 20–4000 cm(^-1), respectively. The photoluminescence measurement show one peak at 507 nm, which is attributed to band–band (free electron–hole transitions) and (bound electron–hole transitions) emissions within the AsO(₄) inorganic part.