Crystal and magnetic structures of the brownmillerite Ca₂Cr₂O₅

Abstract

Powder neutron diffraction and magnetic susceptibility measurements at 10-300 K have been used to determine the crystal and magnetic structures of brownmillerite type Ca(2)Cr(2)O(5), which was obtained by reduction of the high pressure phase CaCrO(3) through hard-soft chemistry. The ambient temperature crystal structure of Ca(2)Cr(2)O(5) is refined in space group I2mb and the unusual tetrahedral coordination of Cr(3+) results in local structural distortions. Cr(3+) spins order antiferromagnetically below 220 K and a substantial observed canting of moments shows that Heisenberg exchange is weak or frustrated and competes with antisymmetric Dzialoshinskii-Moriya interactions.

High-pressure BaCrO3 polytypes and the 5H–BaCrO2.8 phase

“…[9] The most reduced phase, Ca 2 Cr 2 O 5 , is an unusual Cr 3+ brownmillerite structure with antiferromagnetic order at 220 K and a substantial canting of ordered moments. [10] The above ACrO 3 (A = Ca, Sr) precursor perovskites are only formed in a cubic-type (3C) polytype at moderate pressures, but a variety of hexagonal perovskite polytypes are known for the A = Ba analogue.…”

Fluorination and reduction of CaCrO3 by topochemical methods

“…For example, it is well known the tetravalent chromium cation strongly favors tetrahedral coordination over octahedral coordination and the ionic radius is too small to be incorporated into perovskite structures, although these observations are not without exceptions. 10 Previously, alkaline chromium oxide perovskites such as ACrO3 (A = Ca, Sr) have been stabilized under high pressures, 11,12 and Arevalo-Lopez and Attfield et al have discovered new oxygen-vacancy ordered phases CaCrO3-x (x = 0.33, 0.4, 0.5) 13,14 and SrCrO3-y (y = 0.2, 0.25), 15 which were synthesized by reduction of ACrO3 (A = Ca, Sr) with hydrogen gas. CaCrO2.5 was found to adopt the brownmillerite structure.…”

Hard–soft chemistry of Sr1−xCaxCrO3–δsolid solutions

“…6 Contrary to the SrCrO3-δ phases, CaCrO3-δ show vacancy ordering along the (100) direction of the cubic perovskite cell, and are related to the brownmillerite structure, see Figure 1c. The brownmillerite CaCrO2.5 has Cr 3+ in an unusual tetrahedral environment and shows antiferromagnetic ordering at 220 K. 7 In view of the very different vacancy superstructures of SrCrO3δ and CaCrO3-δ phases, it is of interest to discover how these structures evolve across the Sr1-xCaxCrO3-δ series, and whether any new intermediate structures might be formed. Here we report the 'hard-soft' synthesis of Sr1-xCaxCrO3-δ phases for x = 0.25, 0.5 and 0.75, and the vacancy superstructures adopted.…”