材料物理与化学研究室知识库

本论文在Cs2O-H3BO3、Rb2O-CdO-H3BO3及BaO-ZnO-H3BO3体系中探索获得三种化合物：δ-CsB5O8、BaZn2B2O6和RbCdB3O6。其中，δ-CsB5O8、BaZn2B2O6具有同质多晶现象而RbCdB3O6存在与之类质同晶的KCdB3O6化合物。本论文对这三种化合物的固相合成、晶体结构、物化性质以及理论计算等方面进行了研究。
1. 高温熔液法合成出δ-CsB5O8化合物，其属于正交晶系，空间群为Pccn(No. 56)，晶胞参数：a=21.028(6) Å，b=10.415(3) Å，c=13.294(4) Å，Z =16，V =2911.5(4) Å3。δ-CsB5O8晶体结构是由BO3与BO4基团通过共享顶点氧原子而构成了基本构筑单元的B10O19，它是一种全新的硼-氧的基本结构构筑基元，结构基元相互连接形成了三维网络结构，在[010]方向存在孔道，而三种结晶学占位的Cs+离子就填充在其中。与同质多晶CsB5O8类化合物进行了结构比较，结合差热-热重的热学性质分析，判定CsB5O8同质多晶间相变为位移式相变，且为不可逆的第二类相变。讨论了各相结构间的差异。结构张力计算结果表明δ-CsB5O8由于存在开放的孔道结构而使得键张力减小，导致δ-CsB5O8化合物能够稳定存在。
2. 高温熔液自发结晶的方法合成出单斜晶系的同质多晶化合物BaZn2B2O6，空间群为P21/n(No.14)，晶胞参数：a= 10.154(16) Å，b= 4.928(8) Å，c= 12.085(19) Å，β= 111.227(19) °。BaZn2B2O6晶体结构具有一个二维的∞[Zn2B2O6]2-阴离子层，该层是由BO3、ZnO4基团组成，层与层之间通过Ba2+连接形成三维框架。其中，两种结晶学占位的Zn中有50%的ZnO4四面体之间通过共边的方式连接形成二聚体，这在硼酸盐中并不多见且与泡利第三规则相违背，扩展了化学研究领域。与同质多晶BaZn2B2O6（正交晶系）的结构比较，发现其为重建式相变，整个热分析中并无相变吸热峰而判定为第二类相变，虽然BaZn2B2O6(单斜晶系)粉末样品暂时无法获得，暂时无法界定其相变可逆性，但其GII值却明显小于正交相的同质多晶，通过结构及键价分析，由于共边相连的多面体结构而使键长键角发生了改变，调整整体结构使能量降低，从而导致了BaZn2B2O6化合物结晶于单斜的P21/n空间群。
3. 高温熔液法合成出单斜晶系的存在类质同晶的RbCdB3O6化合物，空间群为C2/c，晶胞参数：a=7.357(2) Å，b= 13.315(4) Å，c= 12.702(4) Å，β = 106.711(3) °，Z=8。晶体的结构是十配位的Rb离子与周围配位的氧原子组成的多面体相互之间共边或共顶点连接形成了一个层状结构，在与RbO10组成的层状垂直方向上，离散的B3O6环状结构与RbO10共用氧原子交叉形成了一个三维的网络结构，Cd离子占据三维网络间的孔道起平衡电荷的作用。对其进行物相检测和各项光学性质表征，RbCdB3O6为非同成分熔融化合物，紫外-可见-近红外漫反射光谱显示化合物紫外截止边为290nm。通过第一性原理计算，RbCdB3O6为直接带隙化合物，能隙为4.21 eV，主要影响来自于O-2p和Rb-6p轨道。由于其结构中存在平行排列的B3O6基团，与双折率晶体α-BaB2O4相似，通过结构比较发现平行排列的B3O6利于大的双折射率产生，对其双折率进行了理论计算在可见光区，计算双折射率值在0.06 -0.07之间。RbCdB3O6与KCdB3O6具有相同的晶体结构，属类质同晶。两者晶体结构中都存在孤立的B3O6层和CdO5多面体，不同处为层与层之间的间距和氧原子配位数不同，这主要是阳离子半径差异引起的，这些差异导致KCdB3O6的全局不稳定因子GII值明显小于RbCdB3O6，而与键长键角波动幅度相关的键张力系数BSI的值基本不变。

hree compounds δ-CsB5O8, BaZn2B2O6 and RbCdB3O6 have been obtained in Cs2O-H3BO3, Rb2O-CdO-H3BO3 and BaO-ZnO-H3BO3 systems, respectively. Though, all the compounds crystalize in the centrosymmetric groups. The δ-CsB5O8 and BaZn2B2O6 compounds have their own polymorphisms. The RbCdB3O6 compound is isomorphic to KCdB3O6. In this thesis, the synthesis, crystal structures, crystal growth and properties of the above compounds are investigated. 1. Single crystal of δ-CsB5O8 is grown by high temperature solution method. δ-CsB5O8 crystallizes in orthorombic space group Pccn (No. 56) with lattice constants a = 21.028(6) ?, b = 10.415(3) ?, c = 13.294(4) ?, Z = 16, V = 2911.5(4) ?3. The 3D network consists of 21 helical chains analogous to DNA which are connected by BO4 units and three types of Cs+ ions filling in the space to balance the charge. And to the best of our knowledge, the FBB B10O19 has not been reported previously in any anhydrous and hydrated borates. 2. The relation and difference of ring structures in all the CsB5O8 polymorphs are discussed to show the exciting diversity of CsB5O8. The work provides an idea to the design and synthesis of new borate compounds. 3. Single crystal of BaZn2B2O6 is grown by high temperature solution method. BaZn2B2O6 crystallizes in monoclinic space group P21/n (No. 14) with lattice constants: a = 10.154(16) ?, b = 4.928(8) ?, c = 12.085(19) ?, β = 111.227(19) °. A layer of ∞[Zn2B2O6]2- which is composed of BO3 and ZnO4 exists in the structure. The layers are connected by the Ba2+ ions. 50 % of the Zn atoms in the structure are connected to each other through oxygen atoms by edge which contradicts the third Pauling’s Rules. 4. By comparison with the polymorphisms, the phase-transition is reconstruction phase change and the thermal analysis endothermic peaks are not obviously observed. The BSI and GII of both BaZn2B2O6 phases are calculated to compare their stability. The results show that BaZn2B2O6 belonging to the monoclinic system may be more stable. This result may be caused by the edge-sharing ZnO4 structure which is similar with KZnB3O6. 5. A novel mixed-borate RbCdB3O6 has been synthesized using Rb2CO3 and H3BO3 as the flux system due to its incongruent melting behavior. The crystal structure has been determined by single-crystal X-ray diffraction method. RbCdB3O6 crystallizes in monoclinic space group C2/c with lattice constants a = 7.357(2) ?, b = 13.315(4) ?, c = 12.702(4) ?, β = 106.711(3) °, Z=8. It is a layer-type structure composed of planar B3O6 rings, which are parallel to each other in the same layer but distributed discretely from layer to layer. The Rb and Cd atoms alternately occupy sites among the B3O6 rings in the same layer and combine the layers together by Cd-O and Rb-O bonds. The structure comparison among RbCdB3O6, α-BaB2O4 and β-BaB2O4 suggests that RbCdB3O6 may have a large birefringence, which consists with the calculated value (0.06 - 0.07 in the visible region). In addition, the diffuse reflectance spectrum shows that the absorption edge at about 295 nm. The first-principles theoretical calculation shows that the compound is direct-gap semiconductor with band gap of 3.97 eV, which is in good agreement with experimental value. 6. RbCdB3O6 is isomorphic to KCdB3O6. By the comparison bewteen RbCdB3O6 and KCdB3O6, both structures contain a layer of isolated B3O6 groups and one-dimentional CdO5 polyhedra chain. However, the connected atoms are different in the distance between layers and the coordination numbers of oygen atoms. The difference of cations radii leads to the diversities of tunnel size and denity of the compounds. All the changes make the GII value of RbCdB3O6 larger than that of KCdB3O6.