NEW DEFECTIVE BRANNERITE-TYPE VANADATES .1. SYNTHESIS AND STUDY OF MN1-X-Y-PHI-XNAYV2-2X-YMO2X-SOLUTIONS(YO6 SOLID)

MnV2O6 of the brannerite-type structure (below 540-degrees-C) doped wi th MoO3 and Na2O forms isomorphous solid solutions MnNaphi = Mn1-x-yph ixNayV2-2x-yMo2x+yO6 (phi, cation vacancy in the original Mn position) , belonging to the pseudoternary MnV2O6-NaVMoO6-MoO3 system. Particula r cases are MnNa = Mn1-yNay V2-yMoyO6 (x = 0), Mnphi = Mn1-xphixV2-2xM o2xO6 (y = 0), and Naphi = Na1-xphixV1-xMo1+xO6 (X + y = 1). MnV2O6 an d NaVMoO6 show miscibility in the entire composition range (MnNa). The opposite boundary of MnNaphi passes through the (100x, 100y) points ( 45, 0), (33, 30), and (30, 70). The phase diagram of the pseudobinary MnV2O6-NaVMoO6 system (determined with DTA) shows (i) a narrow double- lens-type solidus-liquidus pp at high values of y, (ii) two peritectic meltings at lower y (yielding the high temperature beta-MnNa and Mn2V 2O7), and (iii) little area of beta-MnNa. Lattice parameters of MnNa ( determined with X-ray diffraction) reveal small deviations from Vegard 's law. As the ionic radii of both dopants (Na+ and Mo6+) are, respect ively, larger than those of mother ions (Mn2+ and V5+), the unit cell increases in all directions with rising y along the MnNa series of sol id solutions. However, due to the anisotropy of the structure, paramet er c is strongly sensitive to Na/Mn substitution, b is ruled by Mo/V, and a is weakly influenced by Mo/V. Close analogy to the behavior of t he previously studied MnV2O6-LiVMoO6-MoO6 system is discussed. (C) 199 4 Academic Press, Inc.