Co-crystallization of red emitting (NH₄)₃Sc(SO₄)₃:Eu³⁺ microfibers: structure–luminescence relationship for promising application in optical thermometry

The (NH4)3Sc(SO4)3:0.005Eu3+ phosphor has been synthesized as rod-like microparticles by crystallization from an aqueous solution. Its crystal structure belongs to the monoclinic system, space group P21/c, Z = 4. An enantiotropic first-order phase transition from monoclinic to high-temperature rhombohedral structure (space group R3c) occurs at 350–355 K and compound decomposition begins only above 573 K. Under 248 nm excitation, the compound exhibits luminescence corresponding to the characteristic 5D0 → 7F2 transitions in Eu3+ ions. The absence of an inversion center in the sites occupied by Eu3+ causes a high intensity of spectral lines at 602–642 nm associated with hypersensitive induced electric dipole 5D0 → 7F2 transitions. Heating of the (NH4)3Sc(SO4)3:0.005Eu3+ sample to a temperature of 393 K is accompanied by considerable distortion of the Eu3+ coordination by the phase transition, manifesting a gradual increase in the asymmetry ratio (R) between the integral intensities of the lines at 602–642 nm (5D0 → 7F2 transition) and at 581–602 nm (5D0 → 7F1). When the heating is suppressed, the R level returns to the original value, confirming the reversibility of the structural transformation. A good reproducibility of the obtained R values has been revealed during 4-fold thermal cycling.