Pressure-enhanced Insulating State and Trigonal Distortion Relaxation in Geometrically Frustrated Pyrochlore Eu2Sn2O7

The geometrically frustrated pyrochlore Eu 2 Sn 2 O 7 is an insulator with slight trigonal lattice distortion at ambient condition. High pressure is applied to this system to investigate the responses of structural evolution, optical emission and electrical transport properties. In situ high pressure synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence studies are performed in Eu 2 Sn 2 O 7 up to 31.2 and 34.1 GPa, respectively. The abrupt change of the oxygen atomic position without breaking the crystal symmetry is accompanied by disappearing of Raman mode involving SnO 6 octahedron distortion around 17.8 GPa. It indicates a pressure-induced second-order iso-structural transition, which suppresses the trigonal distortion in the SnO 6 octahedron but enhances the local symmetry distortion of EuO 8 hexahedron. Anomalous luminescence of the Eu 3+ 4f–4f transition is observed, which confirms the enhancement of EuO 8 hexahedral distortion at high pressure region. In situ high-pressure electrical transport property is measured by alternating current (AC) impedance spectroscopy up to 32.5 GPa. A rapid increase in resistance with gain of 4 orders of magnitude by applied pressure is observed until 16.6 GPa, and it is followed by a slight decreasing to the highest pressure measured here. All these observations indicate a pressure-enhanced trigonal lattice distortion before the transition pressure, and thus it will enlarge an opening gap at the Fermi energy, followed by releasing distortion at higher pressures.