Relieving geometrical frustration through doping in the Dy_1-x Ca _x BaCo₄O₇ swedenborgites

The geometrically frustrated antiferromagnet DyBaCo₄O₇ is investigated through a combination of x-ray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy_1-x Ca _x BaCo₄O₇ causes a lifting of the geometrical frustration resulting in a structural transition from a trigonal P31c to an orthorhombic Pbn2 ₁ 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, CaBaCo₄O₇ which has an equal number of Co²⁺ and Co³⁺ 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 Dy³⁺ ions, we chart out the phase diagram of the Dy_1-x Ca _x BaCo₄O₇ series.