THE MAGNETIC-PROPERTIES AND STRUCTURES OF THE TRANSITION-METAL PYROARSENATES M(2)AS(2)O(7) (M=NI, CO, MN)

The bulk magnetic properties and magnetic structures of the pyroarsena tes M(2)As(2)O(7) (M = Ni, Co, Mn), whose crystal structures are relat ed to thortveitite Sc2Si2O7 have been investigated by susceptibility ( chi) measurements and neutron powder diffraction. All three compounds behave as three-dimensional antiferromagnets. The Weiss constants thet a are -92(1), -49(1), and -28(1) K, and the T(N)determined from the ma xima in d(chi)/dT are 29.1(1), 11.63(5), and 8.5(1) K, respectively. I n each case, four magnetic reflections were identified in the neutron diffraction profiles below TN The magnetic structure of Ni2As2O7 was r efined on the basis of an approximate low-temperature crystal structur e. The moments make an angle of similar to 30 degrees to the c-axis in the ac-plane. Each moment has three antiparallel nearest neighbors. T he antiferromagnetic sheets are further stacked in antiferromagnetic s equence along c. Comparison is made with the magnetic structure of Mn2 P2O7. (C) l995 Academic Press, Inc.