【作者】 张守超；

【导师】 阮永丰；

【作者基本信息】 天津大学 ， 材料物理与化学， 2009， 博士

【摘要】 上转换发光现象发现已经有40多年的历史,出现了很多上转换激光晶体材料。BaY₂F₈晶体与其它晶体相比,具有较低的折射率、低声子能量和宽透射波长等特点。本文主要针对BaY₂F₈结晶特点、晶体生长方法、掺杂稀土离子的光谱分析以及晶体生长的缺陷进行了研究。首先,通过对BaY₂F₈晶体结构的分析,结合自发结晶粉晶的XRD数据,从晶体的生长基元为负离子配位多面体理论出发,对自然冷却条件下BaY₂F₈晶体自发结晶习性进行了研究,提出了以Ba2+为中心的近八配位十二面体和以Y3+为中心的近八配位十二面体是该晶体生长的基元。其次,结合BaY₂F₈粉晶X射线衍射（XRD）数据,分析了该晶体的结晶习性。由于BaY₂F₈晶体的熔点较低（约960℃）和在熔点附近熔体十分粘稠,因此用提拉法制备此种晶体存在较大困难。根据以上对其自发结晶习性的分析,首次采用温度梯度法生（TGT）长出质量较好的BaY₂F₈单晶。实验表明,温度梯度为6℃/mm,降温速率小于6℃/h,可以获得BaY₂F₈单晶体,相对较小的坩埚圆锥角度有利于单晶体的获得。X-射线定向分析表明,温度梯度法生长的BaY₂F₈晶体的生长轴平行于[001]方向。结合晶体结构图分析可知,晶体结构中的强键为Y-F2-Y键,该键沿[001]方向伸展成链状,是晶体的优势生长方向。所得到的Er: BaY₂F₈晶体的直径为13mm,长度达40mm,是目前国内最大的此种晶体。根据J-O理论的计算,得到了唯象的晶场参数?i （ i =2,4,6）,并重点分析了Er: BaY₂F₈晶体的各能级的荧光寿命,结果表明,Er: BaY₂F₈晶体的各能级的荧光寿命较之氧化物有很大的提高,其中4I11/2能级的寿命约为5ms,这样大的荧光寿命对于上转换激光的形成极为有利。测试了Er: BaY₂F₈晶体的上转换荧光光谱,对其发光机理进行了讨论最后,对温度梯度法生长Er:BaY₂F₈晶体的开裂现象进行了研究。从理论上讨论了温场分布、生长速率、热应力和稀土离子掺杂对晶体开裂的影响,实验上分析出垂直于解理面方向上膨胀系数的急剧变化是造成晶体开裂的主要因素,通过缓慢地降温,可以成功避免晶体的开裂。实验表明,径向温度梯度越小越好,晶体生长的最佳工艺参数为:轴向温度梯度为6℃/mm,生长速率为0.2-0.3mm/h。更多还原

【Abstract】 The BaY₂F₈ crystal belongs to the monoclinic crystal system, the space group of this biaxial crystal is C2/m. Based on a theoretical model of anionic ligand polyhedrons for crystal growth, the crystallization habit of BaY₂F₈ crystal are studied in this paper. Two kinds of ligand polyhedrons, they are eight-ligand dodecahedral polyhedron with Ba2+ center and eight-ligand dodecahedral polyhedron with Y3+ center, are the growth unites for BaY₂F₈ crystal. Base on the structure characteristics and XRD dates, the crystallization habit of BaY₂F₈ was analyzed. A strong bond of Y-F2-Y stretches along the direction of [001] to form a chain, and this is an advantaged direction for the crystal growth. Because the low melting point （about 960℃） and the large stickiness in melt nearby the melting point, it is difficult to grow BaY₂F₈ crystal by the C-Z method. In this paper the BaY₂F₈ and Er:BaY₂F₈ crystals have been grown by self-crystallization by the temperature gradient technique （TGT） method at the first time. The experiments indicated that the temperature gradient of 6℃/mm and the speed smaller than 6℃/h for temperature decrease are optimized technology parameters to obtain the BaY₂F₈ crystals. The XRD indicated that the crystal growth axis is parallel to [001] direction when grown the crystal by the TGT method. The cracking phenomenon in the crystal growth by the TGT method is analyzed.The influences of the the temperature field distribution, the growth speed, the heat stress and the dopants are theoretically discussed. The quick change of the expansion coefficient in the direction vertical to [001] is the main reason for the crystal cracking.Using the J-O theory, the three phenomenological intensity parameters ?i （i =2,4,6） are fitted, which are 1.43×10-20cm2, 0.49×10-20cm2 and 1.36×10-20cm2 respectively. The fluorescence decay times of Er:BaY₂F₈ crystal are analysed also. The decay time of 4I11/2 state is about 5 ms in Er:BaY₂F₈ crystal, which is quite longer than that in oxide. Under the excitation of 980nm light, the green （520nm, 540nm and 552nm） and red （654nm） up-conversion fluorescence spectra are observed clearly, which are corresponding to the transitions from ground state to the 4F7/2、2H11/2、4S3/2 and 4F9/2 states respectively.更多还原