Phase composition of the Mg2V2O7-Ni2V2O7 system in the subsolidus region

High-temperature X-ray powder diffraction experiment was carried out to stu dy the phase composition of Mg2V2O7-Ni2V2O7 mixtures from room temperature to 850 degreesC. alpha --> beta phase transition in Mg2V2O7 was shown to in volve a 2.25% reduction in the unit cell volume. No phase transition was ob served in Ni2V2O7. Solid solutions in nickel pyrovanadate and in the two ma gnesium pyrovanadate polymorphs were the equilibrium products of a reaction bt tween the pyrovanadates. With increasing nickel concentration in Mg2(1- x)Ni2xV2O7, where 0 less than or equal to x less than or equal to 0.03, the phase-transition point elevates from 760 to 820 degreesC. The end-member o f beta -Mg2V2O7(ss) at 820 degreesC contains (2.5 +/- 0.5) mol % Ni2V2O7. N i2(1-y)Mg2yV2O7(ss), where 0 less than or equal to y less than or equal to 0.93, is thermally stable over the entire range of temperatures.