Structure and Magnetism in CrTa2O6: A Trirutile Oxide Based on Cr2

CrTa2O6, a rare example of an oxide containing Cr, was prepared by solid state reaction from a mixture of Cr2O3, Ta2O5, and Ta in a sealed silica tube at 1050°C. It exhibits a slightly distorted trirutile structure described in P21/n with a 5 4.738(1), b 5 4.7421(6), c 5 9.2972(9), and b 5 90°55@(1) as determined from Guinier–Hagg data and CuKa1 radiation, in excellent agreement with previous reports. The crystal structure was refined from powder neutron diffraction data at 20 K. The Cr environment is distorted from that usually found in trirutile oxides with Cr–O distances ranging from 2.06(1) to 2.270(9) As consistent with a static Jahn–Teller distortion as expected for high-spin Cr. Magnetic susceptibility data show a Curie–Weiss behavior with C 5 2.79(1) emu mol K, which compares well with the expected value of 3.0 emu mol K for the high-spin state. A h # 5 230.0(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 5 10.3(1) K and a complex magnetic structure with ordering wave vector kK 5 (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 9.0K. The properties of CrTa2O6 are compared with other known trirutile MTa2O6 and MSb2O6 materials. ( 1998 Academic Press