压致硼酸盐晶态物质的非晶化现象研究

PRESSURE-INDUCED NONCRYSTALLIZATION IN Eu^(2+) ACTIVATED Sr-BORATES

本文用X射线衍射、Eu^(2+)的发射谱与激发谱、喇曼谱以及扫描电镜(SEM)颗粒形貌,研究了常压下合成的具有正交结构的SrB_2O_4(?)Eu^(2+)晶体在3.0-7.0GPa压力下的晶态非晶化现象。分析结果表明,压力导致晶粒细化和晶态非晶化。晶粒尺寸由常压下的微米量级细化为几十个纳米量级,随压力的变化为:2μm(0.1MPa),49.4nm(3.0GPa),29.7nm(5.0GPa),25.1nm(7.0GPa)。晶态与非晶态体积比随压力增加而减小,分别为:70/30(3.0GPa),63/37(5.0GPa),57/43(7.0GPa)。在压力下Eu^(2+)是处在晶态与非晶态两种不同的低对称的环境中。纳米级晶粒是以亚晶粒形式存在于微米级大晶粒中的,压致非晶态可能组成了纳米亚晶粒的界面区。

High pressure-induced crystallization in disordered or noncrystalline matter is well known. Pressure-induced disordering or noncrystallizing in crystals, however, has been seldom found so far. In the present paper,under the pressure of 3. 0-7. 0GPa,the pressure-induced non-crystallization phenomena in Euz+ 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. 0GPa,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 noncrystalline state ; the intensity of the emission spectrum peaking at 370nm, which is given by 5d→4f transition of Eu2+ located in orthorhombic crystal,decreased and widened obviously,and a broad band of emission spectrum peaking at 440nm,which is given by Eu2+ located in noncrystalline 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, while the total intensity of diffuse peaks for the noncrystalline state further increased ; the emission band peaking at 370nm was not observed,only a very low emission peak was recorded at 440nm; Raman peabs 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 noncrystallization of crystalline state,and the grain size reduction from the order of micrometer to nanometer. They are 2μm (0. 1MPa), 49. 4nm (3. 0GPa),29. 7nm(5. 0GPa),25. 1nm(7. 0GPa),respectively. The volume ratio of crystalline to noncrystalline state with increasing pressure was found,they were 70/30(3. 0GPa),63/37 (5. 0GPa),57/43(7. 0GPa),respectively. The symmetry of crystal field around Eu2+ in crystalline and noncrystalline 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 nanograins exhibits in the form that there exist sub-grains in the larger grains of micrometer. The interface zone between the sub-grains is probably in noncrystalline state.