New Routes to Nitrides: Synthesis of a Highly Colored Lithium Tantalum Oxynitride
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JOEL D. HOUMES AND HANS-CONRAD ZUR LOYE Department of Chemistry, Massachusetts Institute of Technology Cambridge, MA 02139 ABSTRACT The synthesis of a new lithium tantalum oxynitride is described. The compound, LiTaO3_3xN2x was synthesized by reacting LiTaO 3 with anhydrous ammonia at temperatures ranging from 773 K to 1173 K. Depending on the reaction temperature the sample color changes from white to tan (773 K) to bright orange (973 K) to red (1123 K). The color appears to be the result of a p-d transition consistent with a charge transfer from localized nitrogen states to a band of predominately metal character. Electrical and magnetic measurements indicate that the material is a diamagnetic insulator. INTRODUCTION Recently there has been a push among several groups to explore new methods to synthesize nitride materials[1-7]. One method, the ammonolysis of oxide precursors, has led to the synthesis of a large number of new nitride and oxynitride materials, the latter the result of the incomplete ammonolysis of the oxide starting material[8-13]. For some elements, such as tantalum, there exists a rich oxynitride chemistry, which can be accessed by incomplete nitridation of metal oxides[141. While many nitride structures are unique and different from those of oxides, some oxynitrides take on structures commonly found in oxides. For example, when the pyrochlore La 2 W 2 0 9 is reacted with ammonia, the resulting product
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Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
(LaW(O,N) 3 ) has the perovskite structure[8, 91. In fact, the substitution of only small amounts of nitrogen (< 5 wt. %) for oxygen could occur with retention of the oxide structure. While the structural effects of such small doping levels can be almost insignificant, the effect of nitrogen substitution on the electronic properties can be quite large. For example, the magnetism of the nitrogen-doped rare-earth iron magnets is greatly enhanced over that of the metal alloys[15]. We have been exploring the use of tantalum oxides as precursors to ternary tantalum nitrides. LiTaO 3 is an attractive starting material for nitride synthesis through oxide ammonolysis, because it is readily available and relatively inexpensive. Furthermore, Li2MoO 6 and Li 2 WO 6 have been reacted successfully with flowing ammonia to form the ternary nitrides LiMoN 2[16] and LiWN2117]. In this paper we describe the synthesis of a new highly colored lithium tantalum oxynitride material, formed by a slight substitution of nitrogen for oxygen, with retention of the oxide precursor structure. EXPERIMENTAL LiTaO 3 _3 ,N 2 x was synthesized by reacting LiTaO 3 (Johnson-Matthey Specialty Products, 99.998%) with anhydrous ammonia (160 cc/min.) in a gas flow-through system between 773 K and 1173 K for 12 hours and was then quenched by turning off and opening the furnace. The ammonia was used as received and not dried further. X-ray diffraction analysis of all the lithium tantalum oxynitride products was performed on a Siemens D5000 diffractometer with Cu Ka
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