Rare-earth tricyanomelaminates [NH4]Ln[HC6N9](2)[H2O](7)center dot-H2O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy): Structural investigation, solid-state NMR spectroscopy, and photoluminescence

摘要

The rare-earth tricyanomelaminates, [NH4]Ln[HC6N9](2)[H2O](7).H2O (LnTCM; Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy), have been synthesized through ion-exchange reactions. They have been characterized by powder as well as single-crystal X-ray diffraction analysis, vibrational spectroscopy, and solid-state H-1, C-13, and N-15 MAS NMR spectroscopy. The X-ray powder pattern common to all nine rare-earth tricyanomelaminates LnTCM (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) indicates that they are isostructural. The single-crystal X-ray diffraction pattern of LnTCM is indicative of non-merohedral twinning. The crystals are triclinic and separation of the twin domains as well as refinement of the structure were successfully carried out in the space group P (1) over bar for LaTCM (LaTCM; P (1) over bar, Z=2, a=7.1014(14), b=13.194(3), c = 13.803(3) angstrom, alpha=90.11(3), beta=77.85(3), gamma=87.23(3)degrees, V=1262.8(4) angstrom(3)). In the crystal structure, each Ln(3+) is surrounded by two nitrogen atoms from two crystallographically independent tricyanomelaminate moieties and seven oxygen atoms from crystal water molecules. The positions of all of the hydrogen atoms of the ammonium ions and water molecules could not be located from difference Fourier syntheses. The presence of [NH4](+) ions as well as two NH groups belonging to two crystallographically independent monoprotonated tricyanomelaminate moieties has only been confirmed by subjecting LaTCM to solid-state H-1, C-13, and N-15{H-1} cross-polarization (CP) MAS NMR and advanced CP experiments such as cross-polarization combined with polarization inversion (CPPI). The H-1 2D double-quantum single-quantum homonuclear correlation (DQ SQ) spectrum and the N-15{H-1} 2D CP heteronuclear-correlation (HETCOR) spectrum have revealed the hydrogen-bonded (N-H...N) dimer of monoprotonated tricyanomelaminate moieties as well as H-bonding through [NH4](+) ions and H2O Molecules. The structures of the other eight rare-earth tricyanomelaminates (LnTCM; Ln-Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy) have been refined from X-ray powder diffraction data by the Rietveld method. Photoluminescence studies of [NH4]Eu[HC6N9](2)[H2O](7).H2O have revealed orange-red (lambda(max)=615nm) emission due to the D-5(0)-F-7(2) transition, whereas [NH4]Tb[HC6N9](2)[H2O](7).H2O has been found to show green emission with a maximum at 545nm arising from the D-5(4)-F-7(5) transition. DTA/TG studies of [NH4]Ln[HC6N9](2)[H2O](7).H2O have indicated several phase transitions associated with dehydration of the compounds above 150 degrees C and decomposition above 200 degrees C.