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 ${\text{FeCl}}_{2}$ $({T}_{N}\ensuremath{\approx}24\text{ }\text{K})$ together with resistivity $(R)$ studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP $^{57}\text{F}\text{e}$ M\"ossbauer 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 $55\ifmmode^\circ\else\textdegree\fi{}$, 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 $p\text{\ensuremath{-}}d$ correlation breakdown, leading to an insulator-metal transition at $\ensuremath{\sim}45\text{ }\text{GPa}$ and is not due to a spin crossover $(S=2\ensuremath{\rightarrow}S=0)$. 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, $Pg32\text{ }\text{GPa}$, 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 $0.13\text{ }\text{\AA{}}$, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus ${K}_{0}=35.3(1.8)\text{ }\text{GPa}$. 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 $\ensuremath{\sim}3.5%$ accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the ${\text{CdI}}_{2}$ 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.