## Abstract

Spin-frustrated polyoxometalates, K_{11}H[(VO)_{3}(SbW _{9}O_{33})_{2}]·27H_{2}O (1) and K _{12}[(VO)_{3}(BiW_{9}O_{33}) _{2}]·29H_{2}O (2), containing approximately equilateral and isosceles (VO)_{3}^{6+}-triangles (V^{IV}⋯ V^{IV} separation of 5.4-5.5 Å) sandwiched by two diamagnetic α-B nonatungstate ligands ([SbW_{9}O_{33}]^{9-} and [BiW_{9}O_{33}]^{9-}) with approximate D _{3h} symmetry, are found to show magnetization jumps with distinct hysteresis for the S = 1/2 ↔ S = 3/2 level crossing under fast sweeping pulsed magnetic fields (∼10^{3} T/s) at T ≤ 0.5 K. This unusual phenomenon is attributed to the theoretical prediction of half step magnetization, which is expected for an antiferromagnetic spin triangle with antisymmetrical Dzyaloshinky-Moriya interaction. The degeneracy of the S = 1/2 states for 1 is removed by slightly lower symmetry effects of triangular structure for 2. The calorimetry of 1 and 2 shows the heat capacity anomaly at 2 ≤ T ≤ 20 K which is associated with a thermal excitation from the S = 1/2 ground states to the S = 3/2 state at zero field. Zero-field splitting energies (5-7 K) between S = 1/2 and S = 3/2 states for 1 and 2, readily estimated by the level-crossing field for the magnetization, allow us to measure the hyperfine-structural 22 lines due to three equivalent I = 7/2 ^{51}V nuclei, the fine-structural triplet line of the S = 3/2 excited state, and the g anisotropy on the high-frequency ESR spectra. The spin-frustrated (VO) _{3}^{6+}-triangle for 1 and 2 is a good model of the magnetization between pure quantum states S = 1/2 and 3/2 and provides a new class of single-molecule magnets.

Original language | English |
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Pages (from-to) | 8150-8157 |

Number of pages | 8 |

Journal | Inorganic Chemistry |

Volume | 43 |

Issue number | 25 |

DOIs | |

Publication status | Published - 2004 Dec 13 |

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