Theoretical Study On Magnetic Properties Of The 3d-Transition-metal-substituted Polyoxometalates Using The DFT-BS Method

Molecule magnets have quite different characteristics compared with the traditional magnetic materials, which have possibilities to be made ferromagnetic dopes. They can form homogeneous magnetic film, having the characteristics of high magnetic capacity, light relative density, various structures, easy to composite and shape, etc, which have extensive application future in the material field of information storage, mobile telephone communication and spaceflight. Polyoxometalates (POMs) are a class of inorganic compounds that possess a remarkable degree of molecular and electronic tunabilities. The ability of the POM framework to accommodate one or several magnetic centers at specific positions enables the design of novel molecular materials combining useful electric and magnetic properties. Furthermore, the POM matrix may be considered a diamagnetic host encapsulating and thereby isolating magnetic clusters of transition metals. These characteristics make these complexes ideal models for studying the magnetic exchange interactions in clusters of definite topologies. The main method in experiment study is fitting the experimental data to a theoretical model on the basis of thermal dependence of magnetic susceptibilities, and obtains magnetic coupling constants and other relative magnetic parameters. This method excessively depend on the experimental data and the measurement error potentially results in that the physical models of no big differences can not be distinguished by the obtained data. In addition, the property is obtained as macroscopic quantity in experiment and usually a macroscopic quantity cannot be directly converted to a microscopic one, therefore, calculation of magnetic coupling constants and comparison of the results with corresponding experimental results are of importance in establishing the structure-property relationship and estimating the amount of such effects on molecular magnetic exchange interactions.In this thesis, we have performed systematic theoretical research of electronic and magnetic properties of transition-metal-substituted POM derivatives by using density functional theory combined with broken-symmetry approach (DFT-BS) scheme. Molecular geometries, energies of different spin states, spin density distribution and magnetic orbitals of these systems are obtained by theoretical calculation. Combined with various theoretical models of magnetism, we discuss the reasons influenced on magnetic exchange effectiveness, These results may establish the basis of structure-property relationships and provide a valuable theoretical support to experiment that would be helpful in the design and synthesis of new, interesting POM-based molecular magnetic materials with anticipated magnetic coupling interactions. Our work has been focus on the following four aspects: 1. The magnetic exchange interactions for the dimanganese(II)-substituted complexes with the heteropolymolyanion, [Mn^II₂(Xn+Mo₉O₃₃)₂]_2^(n-10)- (X = P^V(I), As^V(II) and Se^VI(III)), are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) method. The calculated negative J values indicate that antiferromagnetic exchange interactions exist in these complexes. The change of the heteroatom X via P^V–As^V–Se^VI effects the core geometry composed of dimanganese and bridge oxygen, consequently, decreases the magnetic exchange interactions between the magnetic centers. Our results suggest that the heteroatoms(X) allow the fine tuning of the magnetic parameters of I?III. The effectiveness of the superexchange pathways contributing to the molecular antiferromagnetism is analyzed in detail by using HoffmannΔ-model, which shows that the magnetic coupling in complexes I, II and III originates from the similar superexchange mechanisms and the Mn(d_yz)–O_b(p)–Mn(d_yz) superexchange pathway is dominant.2. The magnetic exchange interactions for the different-dinuclear Keggin-type POMs [M(H₂O)XW₁₁O₃₉]^n- (I: X = Co^II, M = Fe^III; II: X = Fe^III, M = Co^II; III: X = Co^III, M = Co^II) are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) method. The calculated negative J values are in accord with the experimental value, which indicate that antiferromagnetic exchange interactions exist in these complexes. The order of absolute value of J is: |J(I)| < |J(II)| < |J(III)|, indicating the increase of coupling interaction via I–II–III. The interchange of X and M result in the increase of spin densities on bridge Ob and the magnetic exchange interaction via the X–Ob–M superexchange pathway become stronger in II than in I. With the change of the heteroatom X via Fe^III(II)→Co^III(III), the spin densities on bridge oxygen atoms O_b and O_b2(O^′_b2) increase. Furthermore, from the comparison of BS magnetic orbitals in II and III we find the overlap of relative orbitals in III increases. Consequently, a stronger antiferromagnetic coupling arises in III than in II.3. The tricopper(II)-substituted sandwich-type polyoxotungstates [Cu₃(H₂O)₃(α- XW₉O₃₃)₂]^12- (X = As^III, Sb^III) are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) method. The magnetic exchange interactions of Cu₃(II) cluster which encapsulate in the heteropolyanion are discussed. Our calculated results show that antiferromagnetic exchange interactions exist in the complex and the antiferromagnetic coupling constant (J) is in reasonable agreement with the experimental value. The change of the heteroatom X via As→Sb effects the structure of Cu₃(II) and relative oxygen atoms: elongates the distances of Cu···Cu, Cu–O_b, W–O_b and W–O_eq, reduces the effectiveness of the superexchange pathways, consequently, decreases the magnitude of the antiferromagnetic coupling constant J. Spin density and magnetic orbital distribution indicate spin exchange coupling takes place by an indirect pathway involving two tungsten and three oxygen atoms of each SbW₉O₃₃ fragment.4. The magnetic coupling constant (J) of sandwich-type polyoxotungstates [Cu₄(H₂O)₂(GeW₉O₃₄)₂]^12- are investigated by using DFT-BS method combined with nonprojected scheme. The calculated ratio J₂/J₁ is in reasonable agreement with the experimental value, and the single-determinant state energies E_BS2 and E_BS4 are lower than other states, which indicates the studied system is of spin-ground in (0, 0, 0) and (1, 0, 1) states. The calculated negative J values indicate that antiferromagnetic exchange interactions exist in the complex. Spin density and magnetic orbital distribution on magnetic center Cu and bridge O atoms indicate the antiferromagnetic coupling between Cu₂···Cu₄ corresponding to J₂ is stronger than the antiferromagnetic couplings corresponding to J₁. The coupling exchange originates from the Cu–O_b–Cu superexchange pathways and the spin density distribution on O atoms is a determinate reason that affects magnetic coupling exchange.5. The nonlinear third-order polarizabilities of novel sandwichlike clusters [Al₄MAl₄]^n- (n = 0-2, M = Ti, V and Cr) are investigated by employing time-dependent density functional theory combined with sum-over-states method (TDDFT-SOS). Furthermore, we design a complex [Al₄Cr₂Al₄] including two transition-metals and explore the potential magnetic properties. The results show that these complexes possess remarkably large third-order static polarizability, and change of a metal centre has a great influence on the third-order nonlinear optical (NLO) properties. The calculated third-order polarizability follows: [Al₄CrAl₄] > [Al₄VAl₄]- > [Al₄TiAl₄]^2-. Analysis of the main contributions to the third-order polarizability suggests that charge transfer (Al₄^2-→M) along the z-axis direction plays a key role in the nonlinear optical response. The calculated magnetic coupling exchange constant of [Al₄Cr₂Al₄] is -39 cm^-1, indicating antiferromagnetic properties in the complex.