MN0.15V0.3MO0.7O3, A NEW COMPOUND IN THE MNV2O6-MOO3 SYSTEM

In the past we studied the system MnV2O6 (monoclinic, C2/m, brannerite -type structure)-orthorhombic MoO3, including the Mn Phi = Mn(1-x)Phi( x)V(2-2x)Mo(2x)O(6) solid solutions (Phi = cation vacancy in the origi nal Mn site, X = 100x). Mn Phi's isomorphous with the MnV2O6 matrix ap peared to be stable upto X-sat = 42 at room temperature or atmost X-sa t = 45 at 520 degrees C. Beyond these limits, Mn Phi(sat) and o-MoO3 w ere observed to coexist. Now, a new phase Mn0.15V0.3Mo0.7O3 = Mn0.3V0. 6Mo1.4O6 (or almost, referred to as the Y phase) has been identified i n the MnV2O6-MoO3 system at formal X = 70. It is monoclinic P2/m (P2 o r Pm) with a = 11.829(2) Angstrom, b = 3.657(1) Angstrom, c = 10.330(2 ) Angstrom, beta = 101.54(1)degrees, and V = 437.8(3) Angstrom(3). The Y phase prepared by a citrate precursor method starts to show reasona ble (broadened) XRD reflections at 300 degrees C, becomes predominant at 450 degrees C, and decomposes slowly to Mn Phi(sat) and o-MoO3 at h igher temperatures (above 450 degrees C). Apparently, due to the paral lel course of the solid state reactions, an entirely pure Y phase has never been obtained. Samples with 65 less than or equal to X = 70 alwa ys contain some o-MoO3 traces whereas those with 70 = X greater than o r equal to 80 are contaminated with Mn Phi(sat). (C) 1998 Academic Pre ss.