SYNTHESIS, STRUCTURAL AND VIBRATIONAL ANALYSIS OF LIMBO(3) ORTHOBORATES (M=MG,CO,ZN)

The structure of LiZnBO3 obtained by low temperature hydrothermal reac tion has been determined previously in single crystals. We calculated the crystallographic cell of the phases obtained by solid-state reacti on without melting by indexing their X-ray powder diffraction patterns . We found evidence that the structure of LiZnBO3 depends on the prepa ration method. The two structures found are monoclinic probably with t he same Li-Zn cationic disorder, as evidenced by vibrational behavior. The cell parameters for the phase prepared at high temperature are a = 7.9286(6), b = 5.0934(3), c = 6.1464(5) Angstrom, beta = 104.3(5)deg rees, Z = 4. The magnesium and cobalt compounds are isomorphous and fo rm a total solid solution. Their monoclinic cells is not related to th e cells of the zinc derivative. The parameters of the unit cell are a = 9.9155(8), b = 8.8847(11), c = 5.1577(4) Angstrom, beta = 91.231(8)d egrees, Z = 8 for the magnesium compound and a = 10.1100(7), b = 8.851 8(8), c = 5.1311(6) Angstrom, beta = 91.358(7)degrees, Z = 8 for the c obalt compound. The lithium cobalt berate is not very thermally stable and it is best synthesized by a two-step procedure. The effect of the lithium isotopic substitution on the vibrational frequencies shows th at this cation is in tetrahedral coordination in all the phases.