Anomalous behavior and phase transformation of α-GaOOH nanocrystals under static compression**Project supported by the National Natural Science Foundation of China (Grant Nos. 50772043, 51172087, and 11074089) and the National Basic Research Program of China (Grant No. 2011CB808200)

Abstract

The structural compression mechanism and compressibility of gallium oxyhydroxide, α-GaOOH, are investigated by in situ synchrotron radiation x-ray diffraction at pressures up to 31.0 GPa by using the diamond anvil cell technique. The α-GaOOH sustains its orthorhombic structure when the pressure is lower than 23.8 GPa. The compression is anisotropic under hydrostatic conditions, with the a-axis being most compressible. The compression proceeds mainly by shrinkage of the void channels formed by the coordination GaO3(OH)3 octahedra of the crystal structure. Anomaly is found in the compression behavior to occur at 14.6 GPa, which is concomitant with the equatorial distortion of the GaO3(OH)3 octahedra. A kink occurs at 14.6 GPa in the plot of finite strain f versus normalized stress F, indicating the change in the bulk compression behavior. The fittings of a second order Birch–Murnaghan equation of state to the P–V data in different pressure ranges result in the bulk moduli B0 = 199(1) GPa for P < 14.6 GPa and B0 = 167(2) GPa for P > 14.6 GPa. As the pressure is increased to about 25.8 GPa, a first-order phase transformation takes place, which is evidenced by the abrupt decrease in the unit cell volume and b and c lattice parameters.