Exploration On Crystallization Habit And Spectrum Study Of Fluoride Crystal Re:BaY₂F₈

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.