Relieving geometrical frustration through doping in the Dy_1−xCa_xBaCo₄O₇swedenborgites

Abstract

The geometrically frustrated antiferromagnetic Swedenborgite DyBaCo4O7 is investigated by a combination of xray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy1−xCaxBaCo4O7 causes a lifting of the geometrical frustration resulting in a structural transition from a Trigonal P31c to an orthorhombic Pbn21 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, CaBaCo4O7 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 Dy1−xCaxBaCo4O7 series.

Intermediate magnetic phase of the magnetoelectric compound (Ca,Sr)BaCo4O7 described with the superspace formalism

“…It has been shown that this is an efficient way to adjust the bond valence requirements of Ba 2+ which is strongly underbonded in this family of compounds [4,38,41] and is the driving force for the successive phase transitions. In the case of the "Ca-114" compound discussed in this work, the highest-symmetry hexagonal P6 3 mc phase is not realized, however the trigonal subgroup P31c has been only observed above 1073 K [42]. By replacing the Ln 3+ by Ca 2+ the orthorhombic Pna2 1 symmetry is favored.…”

Synthesis, Optical, Dielectric, SHG, Magnetic and Visible Light Driven Catalytic Studies on Compounds Belonging to the Swedenborgite Structure

“…The ϵ’ ‐T studies indicate a maximum at ~360 °C, which may be indicative of a structural transition or rearrangement of the various tetrahedral moieties in the structure. It may be noted that there have been reports of a phase transition in swedenborgite compound, CaBaCo 4 O 7 , and other substituted compounds, where a change in symmetry has been observed [73] …”

“…The measurements were carried out over the 2θ range from 5 to 80 degrees, with step size of 0.026 degrees. Initially, the X′pert High score Plus software was employed for preliminary analysis, followed by refinement of the PXRD patterns using the GSAS program [10–105] . During the refinement, various parameters including lattice parameters, scale factors, background (Fourier polynomial background function), pseudo‐Voigt (U, V, W, and X), and isotropic thermal displacement factors (U iso ) were refined.…”

“…It may be noted that there have been reports of a phase transition in swedenborgite compound, CaBaCo 4 O 7 , and other substituted compounds, where a change in symmetry has been observed. [73] The ɛ'-T studies as a function of temperature indicates that the transition at ~360 °C is a diffuse phase transition with a slight shift towards higher temperatures. In addition, the ɛ'-T behavior exhibits a decrease in permittivity with increasing frequency.…”