Relieving geometrical frustration through doping in the Dy Ca BaCoO swedenborgites.

The geometrically frustrated antiferromagnet DyBaCoO is investigated through a combination of x-ray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy Ca BaCoO causes a lifting of the geometrical frustration resulting in a structural transition from a trigonal P31c to an orthorhombic Pbn2 symmetry at x  =  0.4. This structural transition can also be accessed as a function of temperature, and all our orthorhombic specimens exhibit this transition at elevated temperatures. The temperature at which this structural transition occurs is observed to scale linearly with the mean ionic radius of the R site ion. However, CaBaCoO which has an equal number of Co and Co ions clearly violates this quasilinear relationship, indicating that charge ordering could also play a critical role in stabilizing the orthorhombic distortion in this system. Using thermoremanent magnetization measurements to circumvent the problem of the large paramagnetic background arising from Dy ions, we chart out the phase diagram of the Dy Ca BaCoO series.