Phase transformations in multiferroics Bi$_{1-x}$Ca$_{x}$Fe$_{1-x}$Mn$_{x}$O$_{3}$

The crystal structure and the magnetic properties of multiferroics Bi$_{1-x}$Ca$_{x}$Fe$_{1-x}$Mn$_{x}$O$_{3}$ ($x\le$ 0.22) have been studied. It has been found that the stoichiometric compositions undergo a crystal-structure transformation from the rhombohedral (space group $R3c$) polar phase ($x\le$ 0.18) to the orthorhombic (space group $Pnma$) nonpolar phase ($x\ge$ 0.20) via a two-phase structural state. The polar phase is antiferromagnetic at $x<$ 0.10 and exhibits a metamagnetic behavior. The polar ($x\ge$ 0.10) and nonpolar phases are weak ferromagnets at room temperature with a spontaneous magnetization close to 0.07 emu/g ($x$ = 0.18 and 0.22). A decrease in temperature leads to the transition to a state close to an antiferromagnetic one.