ZrO_2-Y_2O_3∶Co或Ce晶体热处理前后的杂质电荷态和禁带迁移

Impurity Charge States and Band-Gap Shift in ZrO_2-Y_2O_3:Co or Ce Crystals before and after Heat Treatment

空气中生长(YSZ)ZrO_2-Y_2O_3(88∶12mol%)∶0.3wt%CoO 晶体的透射光谱呈现二个部分:<500nm 短波部分强的非结构吸收归为 Co^(3+)+O^(2-)→Co^(2+)+O^-电荷转移;以及六配位 Co^(2+)离子特征吸收,~4T_1(F)→~4T_1(P)560nm 和~4T_1(F)→~4A_2(F)605nm.该晶体经木炭包裹下加热到900℃后,除了禁带从3.97ev 变为4.43eV 外,短波部分和500～800nm(特别是500～605nm)范围吸收降低,然而,整个谱形尚无变化.未处理晶体包含 Co^(3+)和 Co^(2+),特殊热处理使 Co^(3+)转化为 Co^(2+),晶体由紫色变为蓝色.原来吸收边(3.97eV)是氧的 p 价带的电子到 Co^(2+)+F^+激发态造成的,4.43eV 新的吸收边认为是直接光学还原 Zr^(4+)→Zr^(3+)以及部分未还原 Co^(2+)离子共同作用.YSZ∶0.3wt%CeO_2晶体经真空、1300℃、6h 还原后,连结 Ce^(3+)离子基态~4F_(5/2)(4f)到5d 态跃迁的380～600nm(峰值在460nm)宽带吸收增加,晶体颜色由浅黄色变为桔黄.它的禁带由4.00eV 变为4.43eV.为了证实 YSZ∶Ce 晶体缺陷结构的存在和杂质离子价态,测量了这个晶体热处理前后的电子顺磁共振(ESR).由单电荷(Y_(Zr)V_0)′自由自旋和 Fe^(3+)离子的 ESR 信号变化,反推铈杂质在 YSZ 晶体中的价态变化.

The transmission spectrum of(YSZ)ZrO_2-Y_O_3(88:12mol%):0.3wt%CoO crystal as grownin air exhibited two components:a relatively strong structure-free absorption in the short-wavelength part less than 500nm,which could be attributed to the charge-transfer processCo^(3+)+O^(2-)→Co^(2+)+O^-,as well as the characteristic absorption bands of Co^(2+) ions at siteswith the coordination numbers 6,~4T_1(F)→~4T_1(P) 560nm and ~4T_1(F)→~4 A_2(F)605nm.When the crystals are heated up to 900℃ by wrapping them in charcoal,beside the change ofband-gap from 3.97eV to 4.43eV,the decrease of the absorption in short-wavelength part at500～800nm,particularly in 500～605nm region can be observed,although no change in wholespectral shape.The untreated crystals contain both Co^(3+) and Co^(2+),since special heat treat-ment converts Co^(3+) into Co^(2+) and the crystal colour is from violet to blue.The originalabsorption edge of 3.97eV can be accounted for by an electron from the oxygen P valence bandto excitation state of Co^(2+)+F^+.The new absorption edge of 4.43eV is now determined by theaction of direct photoreduction of Zr^(4+) to Zr^(3+) and the partial unreduced Co^(3+) ions,both.Wide absorption bands from 380 to 600nm peaked at 460nm were increased for YSZ:0.3wt%CeO_2 crystal reduced at 1300℃ in vacuum for 6h,which associated with transition from theground state ~4F_(5/2)(4f) to 5d state for Ce^(3+) ions,the crystal colour changed from yellowishto yellow-orange,and its band-gap is from 4.00 to 4.43eV.Electron spin resonance has beenmeasured in this crystal before and after heat treatment for confirming its defect structure andimpurity valence states.