  Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magnetically
ordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience further
transformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced in
magnetic susceptibility chi and specific heat Cp measurements. While no anomaly
has been detected in dielectric property of NaCuFe2(VO4)3, the step-like
feature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3.
These data suggest the spin-order-induced ferroelectricity in Li compound and
no such thing in Na compound. On the contrary, the Moessbauer spectroscopy
study suggests similarly wide distribution of hyperfine field in between T* and
TN for both the compounds. The first principles calculations also provide
similar values for magnetic exchange interaction parameters in both compounds.
These observations lead us to conclude on the crucial role of alkali metals
mobility within the channels of the crystal structure needed to be considered
in explaining the improper multiferroicity in one compound and its absence in
other.
