Abstract
The pseudo Ruddlesden-Popper phase Li(2)CaTa(2)O(7) is converted to ZnCaTa(2)O(7), FeCaTa(2)O(7), or CoCaTa(2)O(7) by reaction with the corresponding transition-metal dichloride. Diffraction data reveal that ZnCaTa(2)O(7) adopts a polar crystal structure (P2cm) with the Zn(2+)cations ordered into stripes within the interlayer coordination sites, and the TaO(6) units adopt an a(-)b(-)c(+)/-(a(-)b(-))c(+) tilting pattern. In contrast, FeCaTa(2)O(7) and CoCaTa(2)O(7) adopt polar structures (P2(1)nm) with the transition-metal cations ordered in a checkerboard pattern within the interlayer coordination sites, and the TaO(6) 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, FeCaTa(2)O(7) (T(N) = 40 K) and CoCaTa(2)O(7) (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 FeCaTa(2)O(7) 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 FeCaTa(2)O(7) nor CoCaTa(2)O(7) shows significant lattice parameter anomalies around their respective Néel temperatures, in contrast to the previously reported manganese analogue MnCaTa(2)O(7).