Correlation between structure and photoluminescence of the europium doped glaserite-type phosphate Na_2SrMg(PO_4)_2

摘要

A new phosphate Na_2SrMg(PO_4)_2 has been synthesized and investigated by X-ray diffraction, DTA, 31P NMR spectroscopy and photoluminescence measurements. This compound crystallizes in the space group P2i/a of the monoclinic system with the cell parameters: a = 9.158(1) A, b = 5.267(1) A, c = 13.498 (1) A, ß = 90.01(1)° and four formula units per cell. Its structure is closely related to that of the mineral glaserite K_3Na(SO_4)_2 and thus it can be described by the general formula XY_2M(TO_4)_2. The X, Y, M and T sites are fully occupied by Sr~(2+), Na~+, Mg~(2+) and P~(5+) cations, respectively. The anionic framework is consisted by the stacking along the [001] direction of two kinds of alternating [MgP_2O_8~(4-) ]_∞, mixed layers parallel to the (a, b) plane and resulting from a corner-sharing between MgO_6 octahedra and PO_4 tetrahedra. The Sr~(2+) cations are located within the interlayer space, while those Na~+ are found in large cavities bounded to the layers. The DTA analysis showed a congruent melting of this compound at 1374 K. The ~(31)P NMR spectroscopy confirmed the presence of two distinct phosphors sites in the structure. Optical studies were performed on the Na_2SrMg(PO_4)_2 compound doped with Eu~(3+) and Eu~(2+). The trivalent europium was used as a local probe, replacing strontium and sodium, what gives complementary and consistent results to the crystallographic analyses. The divalent europium was used to analyze preliminary the potentiality for this compound to be integrated as phosphor in Light Emitting Diode (LED).