Intramolecular and Lattice Dynamics in V_6-n^IVV_n^VO₇(OCH₃)₁₂Crystal

Abstract

Multi-nuclear mixed-valence clusters V IV 4 V V 2 O 7 (OCH 3 ) 12 were studied by X-band EPR in the temperature range 4.2-300 K. An isotropic exchange interactions between four V IV ions with individual spin Si = 1/2 determine the energy levels structure of the compound with the total spin states S = 0, 1, and 2, which are doubled and split due to the extra electron transfer. The spin-Hamiltonian approach was used for the analysis of the temperature dependences of the EPR spectra parameters and the cluster dynamics. Two types of the electron transfer are assumed: the single jump transfer leading to the splitting of the total spin states by intervals comparable in magnitude with the exchange parameter J ≈ 100−150 cm −1 and the double jump one resulting in dynamics. The dependence of the transition rates ν tr on the energy of the total spin states was observed. In particular, in the range 300-220 K the ν tr ≈ 0.

Exchange Interactions and Electron Delocalization in the Mixed-Valence Cluster V4IVV2VO7(OC2H5)12

“…However, the fine structure of these spin states (as well as the hyperfine structure) is not observed, and instead, signals of two types appear: a single symmetrical signal with line width ∆B ∼ (20-47) mT, g factor values 1.97-1.99 and a broad extended signal whose shape is temperature dependent. As we described in the previous paragraph and showed in Figure 2, in the temperature range of 293-140 K, the broad signal looks like an unresolved spectrum and then it vanishes to appear again in the narrow temperature interval of ∼40-30 K and is observed down to 4.2 K. Taking into regard the results for the [V 4 IV V 2 V O 7 (OCH 3 ) 12 ] cluster, 40,41 to the unresolved fine structure of the EPR spectrum of the S ) 2 state. The absence of resolution of this spectrum at high temperatures and its changes with the temperature lowering lead to a conclusion that certain dynamic processes have to take place, resulting in the averaging of the expected fine structure spectrum.…”

“…exchange). A remarkable feature of the [V 4 IV V 2 V O 7 (OC 2 H 5 ) 12 ] cluster as well as of the [V 4 IV V 2 V O 7 (OCH 3 ) 12 ] cluster studied earlier 40,41 is the fact that both narrow (from S ) 1 states) and broader (from S ) 2 state) EPR signals are simultaneously observed in the whole temperature range down to liquid helium temperatures, with the intensity of the total spectrum being a sum of the signals from S ) 1 and S ) 2 increasing on temperature decreasing to 4.2 K. This shows that the S ) 1 and S ) 2 states are the lowest and have comparable energy. We should mention that the correlation diagram shown in Figure 8 permits the S ) 1 and S ) 2 states to be in close vicinity when J < 0 and J 1 /|J| ≈ -1; however, these states are not the lowest states in this case.…”

“…The temperature-dependent magnetic data quite often can provide insight into the pattern of the cluster energy levels. Electron paramagnetic resonance (EPR) is also known to be a very useful instrument of magnetic clusters investigation, giving information on both isotropic and anisotropic exchange interactions − and cluster dynamics. − We have studied the mixed-valence cluster [V 4 IV V 2 V O 7 (OCH 3 ) 12 ] using EPR and confirmed the decisive influence of the electron transfers on the dynamics of the electron system and structure characteristics of the cluster molecule. , The present paper reports the EPR study of [V 4 IV V 2 V O 7 (OC 2 H 5 ) 12 ], the ethoxo cluster from the above-mentioned group of oxo−vanadium compounds.…”

“…[37][38][39] We have studied the mixed-valence cluster [V 4 IV V 2 V O 7 (OCH 3 ) 12 ] using EPR and confirmed the decisive influence of the electron transfers on the dynamics of the electron system and structure characteristics of the cluster molecule. 40,41 The present paper reports the EPR study of [V 4 IV V 2 V O 7 (OC 2 H 5 ) 12 ], the ethoxo cluster from the abovementioned group of oxo-vanadium compounds.…”