Synthesis and structure of two new quaternary nitrides: Li3Sr2MN4 (M=Nb, Ta)

Two new quaternary nitrides, Li3Sr2MN4 (M = Nb, Ta) have been synthesized from Li, Sr, and Nb or Ta metals under a flowing Ar-NH3 atmosphere at 800 degrees C under ambient pressure, The structures, determined by single crystal X-ray diffraction, are orthorhombic, space group Pnnm (No. 58) with Z = 4, and have lattice parameters a = 8.713(6) Angstrom, b = 9.007(4) Angstrom, c = 7.006(5) Angstrom and a = 8.700(6) Angstrom, b = 9.004(4) Angstrom, c = 7.000(3) Angstrom for M = Nb and Ta, respectively. Refinement based upon F yielded R = 0.033 and R-w = 0.036 for M = Nb and R = 0.035 and R-w = 0.045 for M = Ta. The structures are isotypic with Na2Li3MO4, M = Fe, Ga. The M and two independent Li atoms are tetrahedrally coordinated by N atoms. Each MN4 tetrahedron connects three Li(2)N-4 tetrahedra, one by edge-sharing and two by corner-sharing, to form a two-dimensional layer in the be plane. These layers are linked together by Li(1) atoms to form the three-dimensional structure. The two Sr atoms occupy channels formed by N atoms along the c direction. Crystalline Li3Sr2NbN4 was synthesized by heating a Li3N-Sr2NbN3 mixture at 750 degrees C under an Ar atmosphere. Magnetic susceptibility data for Li3Sr2NbN4 between 4 and 300 K exhibit temperature independent paramagnetism. (C) 1997 Academic Press.