【摘要】 对于提拉法合成的BiVO₄单晶进行了X射线衍射分析。室温下该晶体属于单斜晶系,空间群为B2/b,z=4,晶胞参数为:a=7.247A,b=5.096A,c=11.702A,γ=134.18°。进行了二维、三维帕特逊分析和三维电子密度计算。全矩阵最小二乘法修正给出335个独立反射的R=0.067,R_w=0.058。经吸收和次级消光校正后,R=0.058。Bi和V原子各占据一套4e特殊点系,O原子占据两套一般点系。V³⁺是四配位,形成规则的VO₄四面体。Bi³⁺是八配位,形成畸变Bi-O十二面体。BiVO₄在T_c温度上下空间群各为I4₁/a和I2/b（或B2/b）。由四方到单斜相变没有正交中间变体。相变显然是由Bi-O多面体孤对畸变所驱动。Bi³⁺和V⁵⁺沿c轴上下交替位移,这就显示单斜BiVO₄具有层状结构的特征。这一特征由光学观察得到证实。该晶体在垂直干c轴的方向极易开裂而产生平坦的解理面。 选取具有胞状结构圆盘形晶体、自然解理晶片和粉末晶粒三种试样进行了光学观察,并研究了晶体透过率与波长关系。更多还原

【Abstract】 A single-crystal X-ray diffraction analysis has been performed on fergusonite BiVO4 synthesized by pulling method. At room temperature the crystal belongs to monoclinic system, with space group B2/b, z = 4, and cell parameters a = 7.247 A, b=5.096 A, c = 11.702 A and r =134.18OOO. The structure has been analysed by two and three-dimensional Patterson procedure, and calculated through three-dimensional electron density. A full-matrix, least square refinement gave R=0.067, Rw =0.058 for 335 independent reflections. An absorption and secondary extinction correction gave final R = 0.058. The Bi and V atoms occupy a respective set of 4e special positions and the O atoms two sets of general positions. The oxygen atoms take part in four-fold coordinations around vanadium in the sense that they form regular VO4 tetrahedra. The structure also shows eight-fold oxygen coordinations around bismuth and form distorted Bi-O dodecahedron. The space groups for temperatures above and below Tc are found to be I41/a and 12/b (or B2/b) respectivelv. The phase transition from tetragonal to monoclinic does skip over a possible intermediate orthorhombic modification. The phase transition is apparently driven by a lone-pair distortion of the Bi-O polyhedron. Bi3+ and Vs+ displace alternatively up and down along the c axis, resulting in a layer-like Character for monoclinic BiVO4 The character has been proved by optical observations. The BiVO4 crystal cleaves easily to give smooth (cleavage) faces parallel to plane perpendicular to the c axis.Three kinds of samples, i.e crystals with cell-like structure and disc shape, crystal ribbon with natural cleavage and powder crystal grain, were used for optical observation, and the relationship between the transmittance of BiVO4 and optical wave-length was studied.更多还原