Insights into the structural symmetry of YCrO₃ from synchrotron X-ray diffraction

A high-resolution synchrotron X-ray diffraction study of a single-crystal YCrO₃ compound was employed to obtain its crystallographic information, such as lattice parameters, atomic positions, bond lengths and angles, and local crystalline distortion size and mode. The measurements were taken at 120 K (below the antiferromagnetic transition temperature T_N ≃ 141.5 K), 300 K (between T_N and the ferroelectric transition temperature T_C ≃ 473 K) and 500 K (above T_C). Using the high intensity of synchrotron X-rays, it was possible to refine collected patterns with the previously proposed noncentrosymmetric monoclinic structural model (P12₁1, No. 4) and determine detailed structural parameters. Meanwhile, for a controlled study, the data were refined with the centrosymmetric orthorhombic space group (Pmnb, No. 62). The lattice constants a, b and c and the unit-cell volume increased nearly linearly upon heating. With the P12₁1 space group, the distributions of bond lengths and angles, as well as local distortion strengths, were observed to be more dispersed. This implies that (i) the local distortion mode of Cr2O₆ at 120 K correlates with the formation of canted antiferromagnetic order by Cr1–Cr2 spin interactions, primarily via intermediate O3 and O4 ions; and (ii) the previously reported dielectric anomaly may have a microscopic origin in the strain-balanced Cr1—O3(O4) and Cr2—O5(O6) bonds as well as the local distortion modes of Cr1O₆ and Cr2O₆ octahedra at 300 K. Local crystalline distortion is shown to be an important factor in the formation of ferroelectric order. The comprehensive set of crystallographic information reported here allows for a complete understanding of the unique magnetic and ferroelectric properties of YCrO₃.