A new family of manganese(II,III) - Rare earth pyrochlores

Polycrystalline samples of a new family of blackish brown pyrochlores of co mposition Mn2R2-[MnSb3]O-14 (R = Y, In, La, Pr, Nd, Sm-Lu), with two Mn2+ a nd one Mn3+ per formula, have been prepared by heating mixtures of reagent grade MnO2, R2O3, and Sb2O3 to 1280 or 1300 degrees C in stoichiometric mol ar ratios. The composition was established by chemical analysis and confirm ed by X-ray powder diffraction methods, neutron powder diffraction data for the Y and Ho compounds, and magnetization measurements. These pyrochlores crystallize in the Fd (3) over bar m space group (no. 227) with Z = 4. All of the atoms are in special positions and show only one variable position p arameter, u. The unit-cell dimensions, a (Angstrom), range from 10.1227(8) (R = In), to 10.347(2), (R = La). The oxygen positional u parameters were d etermined from X-ray powder diffraction data for 2Mn+2R at (c) sites, Mn+3S b at (d) positions, and the origin at the inversion center; and vary from u = 0.420 (R = Y) to u 0.429 (R = Pr). Neutron diffraction data for R = Y an d Ho gave u = 0.42167(7) and u 0.42162(9), respectively. Interatomic distan ces are given. Magnetization measurements from 4 to 350 K are consistent wi th high spin Mn2+ and high spin Mn3+ at (c) and (d) positions, respectively . The possibility of predicting u parameters in pyrochlores from a values a nd ionic radii is discussed.