Lattice effects on electron resonance of chromium(III) complexes. Second-neighbor effects

Dynamic Nuclear Polarization of 1H, 13C, and 59Co in a Tris(ethylenediamine)cobalt(III) Crystalline Lattice Doped with Cr(III)

“…While the occurrence of a nonvanishing ZFS in such a system is necessarily caused by the distortion from hexagonal symmetry, 104 its variation with temperature is less trivial to explain; however it can be generally attributed to coupling of the paramagnetic ion to temperature-induced lattice vibrations. 105,106 A significant temperature dependence in D has been thoroughly documented with experiment and theory on systems such as MgO:Cr 3+ and ruby, and has been attributed to lattice expansion and electron−phonon interactions. 107−111 However, in these cases the absolute value of D was much larger than in our system and increased only slightly in magnitude with increasing temperature (for Cr 3+ in ruby, the low-temperature value is |D| ≈ 5.7 GHz, and the value increases by only ∼90 MHz over the range of 600 K).…”

“…While D reaches a plateau value of 840 MHz at temperatures below 20 K, its magnitude decreases almost linearly with increasing temperature above 40 K, reaching a value of ∼186 MHz at 320 K; the axial ZFS tensor symmetry is conserved throughout the whole accessible temperature range. While the occurrence of a nonvanishing ZFS in such a system is necessarily caused by the distortion from hexagonal symmetry, its variation with temperature is less trivial to explain; however it can be generally attributed to coupling of the paramagnetic ion to temperature-induced lattice vibrations. , A significant temperature dependence in D has been thoroughly documented with experiment and theory on systems such as MgO:Cr 3+ and ruby, and has been attributed to lattice expansion and electron–phonon interactions. − However, in these cases the absolute value of D was much larger than in our system and increased only slightly in magnitude with increasing temperature (for Cr 3+ in ruby, the low-temperature value is | D | ≈ 5.7 GHz, and the value increases by only ∼90 MHz over the range of 600 K) . Misra et al investigated the EPR properties of [Cr­(H 2 O) 6 ] 3+ in guanidinium aluminum sulfate hexahydrate and reported values of D = −1164 and −892 MHz for the two different Cr 3+ lattice sites at 1.6 K, which decreased in absolute value to D = −732 and −585 MHz, respectively, at 298 K. These values were later confirmed theoretically by Pan et al and explained in terms of a trigonal distortion of the hexagonal Cr 3+ site due to a slight mismatch of the ionic radii of Al 3+ (0.535 Å) and Cr 3+ (0.615 Å), which causes an elongated distortion around the Cr 3+ .…”

Synthesis and characterization of new paramagnetic tetraaryl derivatives of chromium and molybdenum

“…The value of the orthorhombicity parameter g for 1 is Table 4 Relevant electronic parameters derived from the analyses of the EPR spectra of polycrystalline samples of the homoleptic polychloroaryl Cr III (d 3 Table 4). Pronounced effects in the EPR parameters had already been reported to occur in systems with S > 1/2 (as is the case of the Cr III ion) caused by small changes in the surroundings of the paramagnetic center, which include lattice effects derived from the nature of the counterion [26]. Also the temperature is known to induce modifications in the ZFS contribution through the spin coupling to the lattice vibrations [27].…”