A. New functional metal oxynitrides by ammonia nitridation through citrate route

1. Introduction

    Metal oxynitrides are attracting attention as photo catalysis in visible light and phosphors for white LED. These applications use the narrower band gap in nitrides than that in oxides due to their covalent bond nature. We are developing various kinds of new effects induced by the copresence of nitride and oxide ions especially in double metal oxynitrides. They can only be prepared by nitridation of oxide precursor obtained from the homogeneously mixed solution of more than two kinds of metal cations. Traditional high temperature preparation is not effective for the preparation at all.

2. Research results

(1) Wurtzite type gallium oxynitride and its cation doping
New wurtzite type gallium oxynitride (Ga_0.89□_0.11)(N_0.66O_0.34) having about 10 at% of gallium vacancy was obtained by ammonia nitridation of the oxide precursor prepared through citrate route. Both Mn³⁺ and Fe³⁺ could be doped in 5 at% as their oxide cluster because their nitrides were thermally metastable. The doped products showed antiferromagnetic behavior. Only 1 at% of Cr³⁺ was doped because it prefers to be in octahedral site. Zn²⁺ could be doped in a wide solid solution limit of 33 at% to improve the crystallinity of the gallium oxynitride. Its absorption edge at around 3eV became sharper with the amount of doped Zn²⁺ as shown in Fig.1.

Fig. 1 Reflection spectrum for Ga_1-yZn_y(N,O) oxynitrides.

(2) New phosphors in SiAlONs
Spinel type AlON crystallized after the post annealing in nitrogen atmosphere of the nitride products through citrate route. Magnetoplumbite type aluminum oxynitride coexisted in the products doped with a small amount of divalent europium. Its c-lattice parameter and nitrogen content changed with the amount of Eu²⁺. Luminescence in blue to white color was observed in its excitation at 254nm. The relative emission intensity changed with the amount of Eu²⁺ among those at 400nm, 475nm and 520nm as shown in Fig. 2. These three emissions may be assigned for Eu²⁺ coordinated with different way and amounts of nitride and oxide ions. New phosphors were also found out in SiAlON polytypoids.

Fig. 2 Photoluminescece spectrum for Eu₂₊ doped aluminum oxynitrides.

3. Future view

  Various kinds of new metal oxynitrides are now coming out as a new group of inorganic solids. Nitride and oxide ions may be ordered in the crystal structure because of their different degree of covalency. The ordering is expected to induce useful properties such as the above emission in multiple wave lengths. New oxynitride (Nb_0.56Al_0.44)(O_0.38N_0.37□_0.25) with an average crystal structure of rock salt type showed a superconductivity with Tc at around 15K.

4. Publications

1. S. Kikkawa, K. Nagasaka, T. Takeda, M. Bailey, T. Sakurai and Y. Miyamoto, "Preparation and lithium doping of gallium oxynitride by ammonia nitridation via a citrate precursor route", J. Solid State Chem., 180, 1984-1989 (2007).
2. S. Kikkawa, N. Hatta and T. Takeda, "Preparation of aluminum oxynitride by nitridation of a precursor derived from aluminum-glycine gel and the effects of the presence of europium", J. Amer. Ceramic Soc., 91(3), 924-928 (2008).
3. S. Yamamoto, S. Kikkawa, Y. Masubuchi, T. Takeda, H. Wolff, R. Dronskowski and A. Yoshiasa, "Chemical synthesis, structure elucidation, and quantum-chemical modeling of a Cr3+ doped gallium oxynitride made by precursor nitridation", Solid State Commun., 147, 41-45 (2008).