ИСТИНА

It was demonstrated, that broadband NIR luminescent bismuth species can be prepared by synproportionation reaction of Bi metal and bismuth ions in usual oxidation state +3. This reaction was observed in highly Lux-Flood oxoacidic borate or phosphate glass melts [1], and Lewis acidic chloroaluminate ionic liquid [2]. The various subvalent Bi species, like univalent cation Bi+ and polycation clusters (Bi53+, Bi82+ etc.) were considered as possible products of synproportionation 2Bi + Bi3+ ↔ 3Bi+ or 4Bi + Bi3+ ↔ Bi53+ and candidate NIR luminescent species. The equilibrium shifted toward subvalent species by increasing acidity or/and temperature. To elucidate the role of different subvalent forms of bismuth in observed NIR luminescence we have prepared the phosphate glasses with compositions: 0.8 KMe(PO3)3, 0.2 KBi(PO3)4, where Me = Mg, Ca, Zn. Glasses were fabricated from NH4H2PO4, Bi2O3, ZnO, MgO and CaCO3 starting materials by melting at 1000 ºC in air. During initial batch decomposition, volatilized ammonia had reduced part of Bi3+ to metal. After the complete batch melting this metal synproportionate with remaining Bi3+, forming specific yellow-colored subvalent Bi doped NIR luminescent glasses. Luminescence spectra demonstrate typical picture [1] with 2 maximums near 1200 and 1300 nm. (Fig. 1) Prolonged heating of melts at 1000 ºC in air resulted in oxidation of the subvalent Bi, producing after solidification colorless glasses without NIR luminescence. On the other hand, treatment of glasses at 550-600 ºC in Ar atmosphere induced partial crystallization of glasses with Me = Mg, Ca compositions. Powder diffractograms reveal the presence of KMg(PO3)3 and KCa(PO3)3 benitoite crystal phases in these glass-ceramics. The NIR luminescence spectra of glass-ceramics demonstrate (besides the remaining weakened lines from glasses) the new blue-shifted maximums, resulted from subvalent Bi luminescence in newly formed crystalline phases Taking into account the coincidence of estimated Bi+ crystal radius (1.5 Å) [1] and radius for K+ (1.52 Å in 6-coordinated benitoite environment) we can deduce, that univalent bismuth can isomorphically substitute K+ in phosphate benitoites, whereas it is impossible for large cluster Bi cations. The presence of luminescence in crystalline phase signifies, that at the moment of crystallization, at least part of subvalent Bi exists as NIR luminescent univalent Bi+ cation, which can be trapped in crystals. So, Bi+ is definitely participating in NIR luminescence from subvalent Bi doped materials, whereas the possible role of Bi cluster polycations must be further explored. References 1. A. N. Romanov, Z. T. Fattakhova, D. M. Zhigunov, V. N. Korchak, and V. B. Sulimov, "On the Origin of Near-IR Luminescence in Bi-doped Materials (I). Generation of Low-Valence Bismuth Species by Bi3+ and Bi0 Synproportionation." Opt. Mater., Article in Press.