Temperature- and pressure-dependent lattice behaviour ofRbFe(MoO4)2

Abstract

Trigonal RbFe(MoO4)2 is a quasi-two-dimensional antiferromagnet on a triangular lattice belowTN = 3.8 K, The crystal exhibits also a structural phase transition atTc = 190 K related to symmetry change from to . We present the temperature- and pressure-dependent characteristics of this material inthe context of ambiguous opinions on the symmetry and crystal properties belowTc. A single-crystal x-ray diffraction shows that the temperature-dependent evolution of the unitcell in the range 100–300 K is strongly anisotropic with markedly discontinuous changes atTc. The transition is connected with a spontaneous strain developing in effect of the volumedecrease. The structure releases the strain by rotation of corner-sharing rigidMoO4 andFeO6 polyhedrain the (a,b) basal plane. The temperature dependence of the IR vibrational wavenumbers exhibits weak changesnear Tc, which are consistent with the symmetry transformation from to . High-pressure x-ray powder diffraction indicates that the material is extremely softbut with some stiffening at high pressure. The zero-pressure bulk modulus isB0 = 7.9(6) GPa and the pressurederivative is B0′ = 10(1). The compression curve can be described by a single equation of state, corresponding tothe trigonal cell, up to 5 GPa. An amorphization appearing above 5 GPa and increasinggradually on further pressure increase suggests the thermodynamic instability of thehigh-pressure structure.