Exchange coupling transformations in Cu (II) heterospin complexes of “breathing crystals” under structural phase transitions

Abstract

Family of "breathing crystals" is the polymer-chain complexes of Cu(hfac) 2 with nitroxides. The polymer chains consist of one-, two-or three-spin clusters. The "breathing crystals" experience simultaneous magnetic and Jahn-Teller type structural phase transitions with change of total cluster spin and drastic change of bond lengths (ca. 10-12%). For the first time the intra-cluster magnetic couplings in "breathing crystals" have been calculated both by band structure methods GGA+U and hybrid DFT (B3LYP and PBE0) for the isolated exchange clusters. The temperature dependence of the magnetic coupling constant was calculated for two polymer-chain compounds of the "breathing crystal" family -C 21 H 19 CuF 12 N 4 O 6 with the chains containing two-spin clusters and C 22 H 21 CuF 12 N 4 O 6 with the chains of alternating three-spin clusters and one-spin sites. It was found that adding a Hubbard-like parameter not only to the copper 3d electrons but also to the oxygen 2p electrons (GGA+U d +U p approach) results in an improved description of exchange coupling in the "breathing crystal" compounds. At the same time treatment of the isolated clusters by a large basis hybrid DFT with high computational cost provides a similar quality fit of the experimental magneto-chemical data as that for the GGA+U d +U p band structure calculation scheme. Our calculations also showed that in spite of the abrupt transformation of the magnetic coupling constant under the phase transition, the band gap in the "breathing crystals" remains about the same value with temperature decrease. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Light‐Induced Spin Transitions in Copper‐Nitroxide‐Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations

“…Unlike the previously reported periodic calculations on breathing crystals, in our calculations the experimental space group for each system is employed ( P for compounds 2 and 3 in both temperatures, P 2 1 / c and C 2/ c for compound 1 at low and room temperatures, respectively), with unit cells of 552, 170 and 162 atoms for compounds 1, 2 and 3 respectively. Hence, our calculations take into account the interchain interactions, neglected in the works by Morozov et al . as a consequence of the reduced crystal symmetry used in their calculations.…”

“…Comparing with the results obtained for Cu(hfac) 2 L Bu ⋅ 0.5 C 8 H 18 ( 2 ) complex from DDCI calculations on the spin triad ( J =−145.3 cm −1 and 8.7 cm −1 for the LT and HT phases, respectively, using the quartet CASSCF(3/3) molecular orbitals in the configuration interaction expansion), the J constants at PBE+ U d + U p level are those in better agreement with the DDCI ones, although always larger in amplitude for both phases. This overestimation is a well‐known feature of the DFT‐based evaluations of the magnetic coupling constants, extensively discussed in the past for many magnetic binuclear compounds, and also found for the family of Cu(hfac) 2 L R breathing crystals . The interchain interaction is antiferromagnetic in nature and of the same order of magnitude as the J value within the spin triads at high temperature.…”

Methane dehydrogenation on Cu and Ni surfaces with low and moderate oxygen coverage

“…The DFT + U method incorporates additional energy factors for delocalization of the d‐orbitals. The formalism of Dudarev et al with the U‐J values of 7 eV and 6 eV was used for Cu and Ni, respectively, which have shown to yield results that are in good agreement in terms of lattice parameters and magnetization with those obtained from experiments.…”

Theoretical Analysis of Materials, used in Energy Storage Applications: the Quest for Robust and Accurate Computational Methodologies

“…It should be noted that relatively few references on the works, describing such techniques (e. g. DFT+ U + J method) can be found in the literature to date. Moreover, even less papers include actual application of these methods in their studies . Clearly more tests on application of thus extended DFT+ U method is needed in the future.…”

“…(7). However, within the recent years, realization is growing that interactions between the electrons with the opposite spins should also be corrected . We presented such correction in the following form: …”