Temperature- and pressure-dependent lattice behaviour of $RbFe(MoO_4)_2$.

Trigonal RbFe(MoO(4))(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 P3m1 to P3. 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 MoO(4) and FeO(6) 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 P3m1 to P3. 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