Optical and mechanical stability of Tutton’s salt crystal of Cu²⁺ doped ammonium cobalt(II) sulfate hexahydrate

Cu²⁺ doped ammonium cobalt(II) sulfate hexahydrate (ACSH) crystals were investigated for LED applications. Cu²⁺ doped ACSH crystals obtained via slow evaporation technique were analyzed for their structural, morphological, optical, thermal, and mechanical properties. The compound formed a crystalline structure within the monoclinic system (P2_1/a). FTIR and Raman spectral analyses ascertained peaks pertaining to ν(O–H), and ν(N–H) corresponding to NH₄⁺ and M–H₂O groups. The presence of N, O, S, Co, and Cu was demonstrated through EDAX. The peaks at 450 and 530 nm are ascribed to the [Co(H₂O)₆]²⁺ center due to ⁴T_1g → ⁴T_1g(P) and ⁴T_1g → ⁴A_2g transitions. The photoluminescence spectrum showed a red emission at 778 nm on excitation at 520 nm. The EPR spectrum indicated a strong tetragonal distortion in [Cu(H₂O)₆]²⁺, and the crystal is comprised of a H-bonded structure involving [Co(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺, NH₄⁺, and SO₄²⁻ networks. The TG–DTA–DSC study showed the loss of water molecules and the subsequent decomposition of the compound. The material exhibited a reverse indentation size effect (n = 3.9) according to Kick's law. The mechanical properties, such as microhardness, yield strength, stiffness constant, fracture toughness, and brittleness of the crystal were calculated. The laser-induced damage threshold of the ACSH crystal doped with Cu²⁺ was determined to be 29.89 GW/cm², which is superior to those of urea or KDP crystals. The results support that the developed crystal can be effectively employed in light-emitting diode applications.