Nitridoaluminosilicate CaAlSiN3 and its Derivatives - Theory and Experiment
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1040-Q10-09
Nitridoaluminosilicate CaAlSiN3 and its Derivatives - Theory and Experiment Masayoshi Mikami, Hiromu Watanabe, Kyota Uheda, and Naoto Kijima Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshidacho, Aoba-ku, Yokohama, 227-8502, Japan ABSTRACT Nitridoaluminosilicate AAlSiN3 (A=Mg, Ca, Sr) and several solid-solutions, CaAlSiN3Si2N2O and CaAlSiN3-SrAlSiN3, have been investigated theoretically and experimentally. The lattice constants have been satisfactorily predicted by first-principles band calculation based on virtual crystal approximation of heterovalent ions (Al/Si and N/O). Calculated heats of formation give useful hints on the feasibility of the synthesis of the CaAlSiN3 derivatives. INTRODUCTION Nitridoaluminosilicate CaAlSiN3 (CASN) has attracted more and more attention owing to the fact that the material doped with rare-earth element exhibits efficient luminescence under InGaN diode irradiation. In particular, the red phosphor, Eu2+-doped CASN, has small thermal quenching of luminescence and sufficient chemical durability for white LED use.1,2 Still, for the lineup of various kinds of white color – “warm”(reddish) white to “cold”(bluish) white – , it is more desirable to tune the wavelength of the red luminescence with other physical/chemical properties kept as possible. Remembering Dorenbos’ works on Eu/Ce 4f-5d transition energies for inorganic compounds and the empirical formulas as a function of the ligand species (i.e. optical polarizabilities of anions), the species of the second nearest cations, and the bond lengths between the luminescent center and the ligand anions,3 we expect that CASN-type compounds with different chemical composition may have different red colors when Eu2+ is doped. To pursuit the possibilities, we have investigated CASN-type compounds, e.g. MgAlSiN3 (MASN)/SrAlSiN3(SASN) as well as Si2N2O(sinoite). The AAlSiN3 structures (A=Mg, Ca, Sr) can be regarded as distorted AlN-based wurtzite superstructure (Cmc21, No.36) with Al and Si disordered on 8b site and A occupying 4a site. Sinoite can be derived from the CASN structure by missing out Ca atoms and substituting Al atoms with Si atoms and two-coordinated N (N[2]) atoms with O atoms to satisfy the charge neutrality. Hence it is natural to expect solid solutions, CASN-Si2N2O and CASN-SASN. The question is synthetic feasibility of the CASN-derivatives. In this paper, we focus on AAlSiN3 in the first place. The lattice constants have been satisfactorily reproduced or predicted in comparison with experimental results. We will see the synthetic feasibility of MASN and SASN in comparison with CASN. Crucial are the heats of formation of a rate limiting step (ASiN2+AlN AAlSiN3) and a typical side reaction (2ASiN2 + A2Si5N8). Next, we will discuss the solid solutions, CASN-Si2N2O and CASN-SASN. Si3N4 The calculated results appear useful to understand our experimental results. CALCULATION METHOD The present results have been obtained owing to the ABINIT code4 that is based on ab initio pseudopoten
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