Temperature- and pressure-dependent lattice behaviour of RbFe(MoO4)(2)

Trigonal RbFe(MoO4)(2) is a quasi-two-dimensional antiferromagnet on a triangular lattice below T-N = 3.8 K, The crystal exhibits also a structural phase transition at T-c = 190 K related to symmetry change from Pm1 to P. We present the temperature-and pressure-dependent characteristics of this material in the context of ambiguous opinions on the symmetry and crystal properties below T-c. A single-crystal x-ray diffraction shows that the temperature-dependent evolution of the unit cell in the range 100-300 K is strongly anisotropic with markedly discontinuous changes at T-c. The transition is connected with a spontaneous strain developing in effect of the volume decrease. The structure releases the strain by rotation of corner-sharing rigid MoO4 and FeO6 polyhedra in the (a, b) basal plane. The temperature dependence of the IR vibrational wavenumbers exhibits weak changes near T-c, which are consistent with the symmetry transformation from P (3) over bar m1 to P (3) over bar. High-pressure x-ray powder diffraction indicates that the material is extremely soft but with some stiffening at high pressure. The zero-pressure bulk modulus is B-0 = 7.9(6) GPa and the pressure derivative is B-0' = 10(1). The compression curve can be described by a single equation of state, corresponding to the trigonal cell, up to 5 GPa. An amorphization appearing above 5 GPa and increasing gradually on further pressure increase suggests the thermodynamic instability of the high-pressure structure.