論文の公開元へPentacyanidonitrosylmolybdate-based magnetic compounds with nonlinear optical properties
概要
1. Introduction
Cyanido-bridged bimetal assemblies are good candidates for molecule-based magnets with various functionalities, such as high Curie temperature, photomagnetism and humidity-controled magnetism. Magnetization-induced second harmonic generation (MSHG), which is a cross-effect between non-centrosymmetric structure and magnetic ordering, is an interesting functionality of magnetic materials. In our laboratory, we have reported some non-centrosymmetric ferromagnets showing MSHG, including RbMn[Fe(CN)6] and [Fe(4-bromopyridine)4]2[Nb(CN)8]·2H2O, which consist of hexacyanidometallate and octacyanidometallate ions, respectively.
Pentacyanidonitrosylmolybdate, [MoI(CN)5(NO)]3−, is an attractive building block to construct cyanido-bridged metal assemblies showing high magnetic ordering temperature (TC) because a non-zero spin (S = 1/2) on a diffused 4d orbital of MoI can most likely causes strong superexchange interaction through the cyanide bridge. In addition, pentacyanidonitrosyl- molybdate with one nitrosyl and five cyanides has an advantage in constructing of a non- centrosymmetric structure due to the lower symmetry than that of hexacyanidometallate bearing six cyanides. Thus, using pentacyanidonitrosylmolybdate, we can expect non-centrosymmetric cyanido-bridged metal assemblies exhibiting long-range magnetic ordering. In this work, I report non-centrosymmetric cyanido-bridged bimetal assemblies composed of pentacyanidonitrosylmolybdate, polar one-dimensional magnetic chains of [Ln(dmf)6][Mo(CN)5(NO)] (Ln = Gd (1), Eu (2), Ce (3), Pr (4), Nd (5), Sm (6), Tb (7), Dy (8),Ho (9), Er (10)) showing second harmonic generation (SHG), and a non-centrosymmetric ferrimagnet, Cs1.26Mn0.87[Mo(CN)5(NO)]·0.6H2O (11) exhibiting SHG and MSHG effects.
2. One-dimensional cyanido-bridged magnetic chains showing second harmonic generation
The target compounds were synthesized by mixing (PPh4) 3[MoI(CN)5(NO)]·2H2O salt (PPh4+= tetraphenylphosphonium) and LnIII(NO3)3·nH2O (Ln = Gd, Eu, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er) in N,N-dimethylformamide (dmf) solution under an argon atmosphere. Elemental analyses revealed the chemical formula as [Ln(dmf)6][Mo(CN)5(NO)] (Ln = Gd (1), Eu (2), Ce (3), Pr (4), Nd (5), Sm (6), Tb (7), Dy (8), Ho (9), Er (10)). Crystal structures of 1–10 have been determined by single crystal X-ray diffraction. 1–10 crystalized in orthorhombic structures with a non- centrosymmetric space group of Pna21 and have an isomorphic structure. In the crystal structure of 1, a GdIII site is coordinated by two nitrogen atoms of cyanide groups and six oxygen atoms of dmf molecules. The coordination geometry around the GdIII site is an eight-coordinated square antiprism and that of the MoI site is a six-coordinated octahedron. The MoI and GdIII sites are bridged by cyanide groups, resulting in a one-dimensional chain structure. Directions of nitrosyl groups are aligned to the c-axis direction producing the non-centrosymmetric crystal structure of 1. Non-centrosymmetric materials are expected to show SHG. When the powder form sample of 1 was irradiated with a femto-second pulsed laser with the wavelength of 1064 nm, 1 emitted 532- nm light due to SHG. An effective SHG susceptibility of 1 is ca. 0.1 times as large as that of KH2PO4 (KDP) reference material. The room temperature product of the molar magnetic susceptibility and temperature of 1 is 8.1 K cm3 mol−1, which is close to 8.2 K cm3 mol−1 calculated from one GdIII (SGd = 7/2, gGd = 2) and one MoI (SMo = 1/2, gMo = 2). To confirm the presence of magnetic coupling, the χMT-T plot is fitted by using a Seiden’s model, considering the intra-chain magnetic interaction JGdMo between classical spins of GdIII and quantum spins of MoI. From the result of the fitting, the JGdMo value was estimated to be −1.02(5) cm−1, indicating an antiferromagnetic interaction between GdIII and MoI. Magnetic measurement for 2 indicates that magnetic spins of MoI are isolated by the diamagnetic EuIII centers at 2 K.
3. Magnetization-induced second harmonic generation in a pentacyanidonitrosyl- molybdate-based bimetal assembly
The presented compound was obtained as yellow powder by adding an aqueous solution of MnCl2·4H2O and CsCl to an aqueous solution containing Cs3[MoI(CN)5(NO)] and CsCl under an argon atmosphere. A chemical formula of this compound is determined as Cs1.26Mn0.87[Mo(CN)5(NO)]·0.6H2O (11). Powder X-ray diffraction and Rietveld analysis indicate that 11 has a non-centrosymmetric crystal structure of F–43m space group. Mn and Mo centers are bridged by cyanide and nitrosyl ligands to form a three-dimensional cubic structure. Cs+ cations exist in two different interstitial sites with different occupancies (Cs1: 66%, Cs2: 60%). The difference of the occupancies of Cs+ cations generates the non-centrosymmetric structure of 11. Magnetic measurement shows that 11 exhibits ferrimagnetism with a TC of 65 K (Figure 4). SHG measurement was performed by using a 1064-nm nanosecond pulsed laser. At room temperature, 11 emitted the 532-nm light due to the SHG effect and the effective SHG susceptibility was 5×10−11 esu. The temperature dependent SH intensity followed the temperature dependent magnetic susceptibility under an external field of 5000 Oe. In the electric-dipole approximation, SH polarization (PSH) is given by the following equation, PSH=ε0χSHEE, where ε0,E and χSH represent the permittivity of vacuum, electric field of fundamental light and SH susceptibility tensor, respectively. From the symmetry of–43m point group, nonzero elements of the crystal term of SH susceptibility tensor are χxyzcry, χ cry, and χzxycry (χxyzcry=χ cry=χzxycry). Furthermore, the magnetic ordering below TC produces a magnetic term (χmag). A magnetic point group of the present compound magnetized along the Mo–CN–Mn direction belongs to–4m2. In this case, χxyzmag, χyxzmag, and χzxymag are nonzero. Below TC, SH susceptibility is given by χSH=χcry+χmag, that is, the SH light was caused by both of the crystal term and magnetic term. These results concluded that the SHG effect MSHG effects were observed due to the non- centrosymmetric crystal structure with the magnetic ordering.
4. Conclusion
In this work, I report two types of the non-centrosymmetric pentacyanidonitrosylmolybdate-based bimetal assemblies, [Ln(dmf)6][Mo(CN)5(NO)] Ln = Gd (1), Eu (2), Ce (3), Pr (4), Nd (5), Sm(6), Tb (7), Dy (8), Ho (9), Er (10)) and Cs1.26Mn0.87[Mo(CN)5(NO)]·0.6H2O (11). 1–10 have one-dimensional chain structures with spontaneous electric polarization and exhibit a SHG effect due to the non-centrosymmetric crystal structure. These are the first examples of the single crystalline sample of pentacyanidonitrosylmolybdate-based bimetal assemblies that are characterized by single crystal X-ray analysis. 11 is a non-centrosymmetric ferrimagnet showing the SHG and MSHG effects due to the cross-effect between magnetic ordering and non-linear optical effect. The results presented in this work suggest that pentacyanidonitrosylmolybdate can be a good building block for construction of non-centrosymmetric magnets.
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