Pressure-Induced Non-crystallization in Eu2+ Activated Sr-Borates

High pressure-induced crystallization in disordered or non-crystalline matter is well known. Pressure-induced disordering or non-crystallizing in crystals, however, has been seldom found so far. In the present paper, under the pressure of 3.0~7.0 GPa, the pressure-induced non-crystallization phenomena in Eu2+ activated orthorhombic Sr-borates crystals were investigated by using X-ray diffraction, emission and excitation spectra of Eu2+, Raman spectra and SEM topography. After treating at 3.0 GPa, the X-ray diffraction spectra of the sample exhibit a new character in which a set of clearly widened peaks of the crystal were superposed on the diffuse peaks of non-crystalline state; the intensity of the emission spectrum peaking at 370 nm, which is given by 5d4f transition of Eu2+ located in orthorhombic crystal, decreased and widened obviously, and a broad band of emission spectrum peaking at 440 nm, which is given by Eu2+ located in non-crystalline state, was also observed; the excitation spectra for the two emission bands differed with each other considerably; the intensity of Raman peaks decreased and their half width increased; the particle size observed by SEM remained in the order of micrometer of orthorhombic crystal grain synthesized under normal pressure. As the pressure increasing, the width of X-ray diffraction peaks for the crystalline state increased, but the total diffraction intensity further decreased, shile the total intensity of diffuse peaks for the non-crystalline state further increased; the emission band peaking at 370 nm was not observed, only a very low emission peak was recorded at 440 nm; Raman peaks were further decreased and widened, too; the particle size observed by SEM did not vary apparently. Analyses from the experimental results of X-ray diffraction show the pressure effects in the fining of crystal grains, the non-crystallization of crystalline state, and the grain size reduction from the order of micrometer to nanometer. They are 2 m (0.1 MPa), 49.4 nm (3.0 GPa), 29.7 nm (5.0 GPa), 25.1 nm (7.0 GPa), respectively. The volume ratio fo crystalline to non-crystalline state with increasing pressure was found, they are 70/30 (3.0 GPa), 63/37 (5.0 GPa), 57/43 (7.0 GPa), respectively. The symmetry of crystal field around Eu2+ in crystalline and non-crystalline state formed under the high pressure is different and lowering. The lowering and widening of Raman peaks result from pressure-induced fining of grain to nanometer in size. Combining the results about the grain size of SEM and X-ray diffraction, it seems that the pressure-induced nano-grains exhibits in the form that there exist sub-grains in the larger grains of micrometer. The interface zone between the sub-grain is probably in non-crystalline state.