Pressure-induced transition from J(eff)=1/2 to S=1/2 states in CuAl2O4

The spin-orbit entangled (SOE) J(eff) state has been a fertile ground to study quantum phenomena. Contrary to the conventional weakly correlated J(eff) = 1/2 state of 4d and 5d transition metal compounds, the ground state of CuAl2O4 hosts a J(eff) = 1/2 state with a strong correlation of Coulomb U. Here, we report that, surprisingly, Cu2+ ions of CuAl2O4 overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the J(eff )= 1/2 state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only S = 1/2 state for pressure P-c > 8 GPa. This phase transition from the Mott insulating J(eff) = 1/2 to the S = 1/2 states is a unique phenomenon. Our study offers an example of the SOE J(eff) state under strong electron correlation and its pressure-induced transition to the S = 1/2 state

High-pressure spectroscopic study of hydrous and anhydrous Cs-exchanged natrolites

Structural phase transitions in hydrous Cs-exchanged natrolite (Cs-NAT-hyd) and anhydrous Cs-exchanged natrolite (Cs-NAT-anh) have been investigated as a function of pressure and temperature using micro-Raman scattering and synchrotron infrared (IR) spectroscopy with pure water as the penetrating pressure medium. The spectroscopic results indicate that Cs-NAT-hyd undergoes a reversible phase transition around 4.72 GPa accompanied by the discontinuous frequency shifts of the breathing vibrational modes of the four-ring and helical eight-ring units of the natrolite framework. On the other hand, we observe that Cs-NAT-anh becomes rehydrated at 0.76 GPa after heating to 100 &deg;C and then transforms into two distinctive phases at 2.24 and 3.41 GPa after temperature treatments at 165 and 180 &deg;C, respectively. Both of these high-pressure phases are characterized by the absence of the helical eight-ring breathing modes, which suggests the collapse of the natrolite channel and formation of dense high-pressure polymorphs. Together with the fact that these high-pressure phases are recoverable to ambient conditions, our results imply a novel means for radionuclide storage utilizing pressure and a porous material.<br /