NEW DEFECTIVE BRANNERITE-TYPE VANADATES - II - SYNTHESIS AND STUDY OFMN1-X-YAGY-PHI-XV2-2X-YMO2X-SOLUTIONS - AG-MN-STABILIZED HEXAGONAL MOO3(YO6 SOLID)

MnV2O6 (below 540 degrees C) and AgVMoO6 are monoclinic C2/m brannerit e-type structures, They show complete miscibility, forming MnAg = Mn1- yAgyV2-yMo1+yO6 solid solutions. MnAg is able to incorporate an excess of MoO3 and form MnAg Phi = Mn(1-x-y)Phi(x)Ag(y)V(2-2x-y)Mo(2x+y)O(6) phases (Phi = cation vacancy in the original Mn/Ag site) belonging to the pseudoternary MnV2O6-AgVMoO6-MoO3 system. The system can be visua lized as an equilateral triangle with X = 100x and Y = 100y coordinate s along the MnV2O6-MoO3 and MnV2O6-AgVMoO6 arms, respectively, The lim it of the existence of MnAg Phi is almost linear and extends between t he X-Y points S-1 = (45,00) and S-2 = (12,88), As the ionic radii of d opants (Ag+/Mn2+), Mo6+/V5+) are larger than those of the mother catio ns, the lattice constants in both series-MnAg and almost saturated MnA g Phi (located along the S1S2 curve-rise systematically with an increa se in Y. Behind the S1S2 curve, saturated MnAg Phi has been observed a ccompanied by ''hexagonal MoO3'' (h-MoO3), Y phase = Mn0.15V0.3Mo0.7O6 , by (MoxV1-x)(2)O-5, and sometimes by orthorhombic MoO3 and MnMoO4. A ccording to the literature, so-called h-MoO3 is a defective tunnel str ucture stabilized by the inserted free or hydrated L = K+, Rb+, Cs+, N H4+ ions, The formula of dry h-MoO3 is L0.13V0.13Mo0.87O3(I). Ag was b elieved to be too small to stabilize the peculiar h-MnO3. Despite that , we observed Ag-containing h-MoO3 in the field of the MnV2O6-AgVMoO6- MoO3 triangle. Attempts to obtain a pure Ag-containing h-MoO3 have fai led so far; apparently the presence of Mn (beside Ag) stabilizes the h -MoO3 structure and the formula is more complex as compared with I. Th e phase diagram of the MnV2O6-AgVMoO6 system reveals great similarity to those containing Li and Na instead of Ag. All three diagrams involv e a narrow lens-type solidus-liquidus gap at high Y and two peritectic meltings at lower Y, yielding, besides liquid, the high-temperature b eta-MnAg polymorph and Mn2V2O7. (C) 1997 Academic Press.