Magnetic properties and helical spin structure of another compound Ca3SbFe3Si2O14 from the family of iron-containing langasites

A strong interest is attracted to the langasite-type compounds containing magnetic 3d ions. Coexistence of electric and magnetic order parameters in such materials can provide a new variety of multiferroics. In our work, another Fe-based langasite with a nearly stoichiometric composition Ca3SbFe3Si2O14 was synthesized. X-ray diffraction and X-ray fluorescence analyses, studies of magnetic susceptibility, specific heat and M & ouml;ssbauer spectroscopy were performed. The crystal structure of Ca3SbFe3Si2O14 (CSFS) was found to be hexagonal, space group P321. From full-profile analysis by the Le Bail method, the lattice parameters a = 8.115(1) & Aring;, c = 5.058(1) & Aring;, V = 288.4(1) & Aring;(3) were determined. The temperature dependence of magnetic susceptibility chi(& Tcy;) indicates the formation of long-range magnetic order at temperatures below T-N = 34 K. The negative value of Weiss temperature theta(CW) indicates the domination of antiferromagnetic exchange interaction. From the value of the Curie constant, the effective magnetic moment per Fe3+ ion mu (eff) = 6.12 mu (B) was estimated, which is slightly higher than the expected value 5.9 mu (B) for the 3d(5) - S = 5/2 state of iron. Measurements of the specific heat allowed a Debye temperature value Theta(D) = 390(10) K to be obtained. Based on the assessment of the magnetic contribution to the specific heat, the value of magnetic entropy Delta S-mag similar to 47.8 +/- 5.6 J/mol K was determined. Hyperfine interaction parameters obtained from M & ouml;ssbauer spectra indicate a helical magnetic ordering of iron moments in langasite Ca3SbFe3Si2O14. The period of a magnetic helicoid is about five periods of the crystal structure along the c axis. The calculated value of the angle (alpha) between the main axis of the electric field gradient Z ' at iron nuclei and the crystallographic c axis (67.2 degrees) is significantly greater than in other langasites Ba3SbFe3Si2O14 (28.8 degrees), Ba3NbFe3Si2O14 (36 degrees), Ba3TaFe3Si2O14 (34.8 degrees). This may be due to the more contracted FeO4 tetrahedra in CSFS compared to those in the other compositions.