The phase transitions in CsFe(MoO4)2 triangular lattice antiferromagnet, neutron diffraction and high pressure studies

• CsFe(MoO4)2 undergoes two phase transitions (PT) at low temperatures.
• The rotations of (MoO4)2− tetrahedra brake P-3m1 symmetry at 220 K.
• The ‘120°’ spin structure develops at 4.5 K with modulation vector q = (1/3 1/3 0.5).
• This triangular lattice antiferromagnet is possibly multiferroic type II material.
• CsFe(MoO4)2 undergoes reconstructive PT between ambient and 0.7 GPa pressure.

We report on the phase transitions in the triangular lattice antiferromagnet CsFe(MoO4)2 at low temperatures and high pressure using powder neutron and X-ray diffraction, specific heat, magnetic susceptibility and in situ high pressure Raman measurements. CsFe(MoO4)2 undergoes a structural phase transition induced by the rotation of (MoO4)2− tetrahedra at Tc = 220 K. The transformation is associated with a symmetry decrease from P-3m to P-3. Below 4.5 K the antiferromagnetic long range order appears that is related to the nuclear unit cell by the modulation vector q = (1/3 1/3 0.5). The system adopts ‘120°’ spiral spin structure that has been found in several triangular lattice multiferroics. Under pressure CsFe(MoO4)2 undergoes a reconstructive phase transition to a much larger superstructure which preserves the super-exchange interactions at low temperatures and allows an additional magnetic long-range order at T ∼ 20 K.