Structure and magnetism in CrTa2O6: A trirutile oxide based on Cr2+

CrTa2O6, a rare example of an oxide containing Cr2+, was prepared by solid state reaction from a mixture of Cr2O3, Ta2O5, and Ta in a sealed silica tube at 1,050 C. It exhibits a slightly distorted trirutile structure described in P21/n with a = 4.738(1), b = 4.7421(6), c = 9.2972(9), and β = 90 degree 55'(1) as determined from Guinier-Hagg data and CuKα1 radiation, in excellent agreement with previous reports. The crystal structure was refined from powder neutron diffraction data at 20 K. The Cr2+ environment is distorted from that usually found in trirutile oxides with Crsingle bondO distances ranging from 2.06(1) to 2.270(9) angstrom consistent with a static Jahn-Teller distortion as expected for high-spin Cr2+. Magnetic susceptibility data show a Curie-Weiss behavior with C = 2.79(1) emu mol-1 K-1, which compares well with the expected value of 3.0 emu mol-1 K-1 for the high-spin state. A θc = -30.0(8) K indicates the dominance of antiferromagnetic exchange. A susceptibility maximum at 11 K is evidence for long-range antiferromagnetic order. Low-temperature neutron diffraction data confirm a TN = 10.3(1) K and a complex magnetic structure with ordering wave vector k = (1/4 1/4 1/4) as seen for some other trirutile oxides. For T > TN, two-dimensional, short-range correlations are seen in the form of a Warren line shape. Interestingly, the two-dimensional correlations persist into the ordered regime, at least to 9.0 K. The properties of CrTa2O6 are compared with other known trirutile MTa2O6 and MSb2O6 materials