Relieving Geometrical frustration through doping in the Dy$_{1-x}$Ca$_{x}$BaCo$_{4}$O$_{7}$ Swedenborgites

The geometrically frustrated antiferromagnetic Swedenborgite DyBaCo$_{4}$O$_{7}$ is investigated by a combination of xray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy$_{1-x}$Ca$_{x}$BaCo$_{4}$O$_{7}$ 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. In line with previous reports, the temperature at which this structural transition occurs scales linearly with the mean ionic radii of the $A$ site ion. However, CaBaCo$_{4}$O$_{7}$ which has an equal number of Co${^{2+}}$ and Co${^{3+}}$ 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${^{3+}}$ ions, we chart out the phase diagram of the Dy$_{1-x}$Ca$_{x}$BaCo$_{4}$O$_{7}$ series.

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