Pressure-induced structural, electronic, and magnetic phase transitions in FeCl2 studied by x-ray diffraction and resistivity measurements

High-pressure (HP) synchrotron x-ray diffraction (XRD) studies were carried out in FeCl2 (TN 24 K) together with resistivity (R) studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP F 57 e Mössbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by 55°, and (ii) collapse of the magnetism concurring with a sharp decrease in the isomer shift. The R (P,T) studies affirm that the cause of the collapse of the magnetism is a PI p-d correlation breakdown, leading to an insulator-metal transition at ∼45 GPa and is not due to a spin crossover (S=2→S=0). The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30-57 GPa, and culminating in a high-pressure phase, P>32 GPa, can clearly be quantified. The IP-HP phases coexist through the 32-57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the c axis increases and the a axis shrinks by 0.21 and 0.13, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus K0 =35.3 (1.8) GPa. The intralayer Cl-Cl distances increase but no change is observed in Fe-Cl bond length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-Cl bond length and an abrupt drop in V (P) by ∼3.5% accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the CdI2 structure. The bulk modulus of the HP phase is practically the same as that of the LP-IP phases suggesting negligible changes in the phonon density of state.