Cation Exchange as a Route to Introduce Magnetism to Hybrid-Improper Polar Phases

The pseudo Ruddlesden-Popper phase Li₂CaTa₂O₇ is converted to ZnCaTa₂O₇, FeCaTa₂O₇, or CoCaTa₂O₇ by reaction with the corresponding transition-metal dichloride. Diffraction data reveal that ZnCaTa₂O₇ adopts a polar crystal structure (P2cm) with the Zn²⁺cations ordered into stripes within the interlayer coordination sites, and the TaO₆ units adopt an a^-b^-c⁺/-(a^-b^-)c⁺ tilting pattern. In contrast, FeCaTa₂O₇ and CoCaTa₂O₇ adopt polar structures (P2₁nm) with the transition-metal cations ordered in a checkerboard pattern within the interlayer coordination sites, and the TaO₆ units adopt an a^-b^-c⁺/ b^-a^-c⁺ tilting pattern. The different polar structures adopted are rationalized on the basis of the size of the interlayer transition-metal cation. On cooling, FeCaTa₂O₇ (T_N = 40 K) and CoCaTa₂O₇ (T_N = 25 K) adopt antiferromagnetically ordered states with spins aligned parallel to the crystallographic stacking axis and arranged in a G-type manner. Close inspection of the NPD data collected from FeCaTa₂O₇ at low temperature reveals a diffuse component to the magnetic scattering, which, in combination with magnetization data, suggest a glassy component to the low-temperature magnetic state. Neither FeCaTa₂O₇ nor CoCaTa₂O₇ shows significant lattice parameter anomalies around their respective Néel temperatures, in contrast to the previously reported manganese analogue MnCaTa₂O₇.