Defect engineering has arisen as a promising approach to tune and optimise the adsorptive performance of metal-organic frameworks. However, the balance between enhanced adsorption and structural stability remains an open question. Here both CO2 adsorption capacity and mechanical stability are calculated for the zirconium-based UiO-66, which is subject to systematic variations of defect scenarios. Modulator-dependence, defect concentration and heterogeneity are explored in isolation. Mechanical stability is shown to be compromised at high pressures where uptake is enhanced with an increase in defect concentration. Nonetheless this reduction in stability is minimised for reo type defects and defects with trifluoroacetate substitution. Finally, heterogeneity and auxeticity may also play a role in overcoming the compromise between adsorption and stability.

We present here the computational chemistry methods our group uses to investigate the physical and chemical properties of nanoporous materials and adsorbed fluids. We highlight the multiple time and length scales at which these properties can be examined and discuss the computational tools relevant to each scale. Furthermore, we include the key points to considerupsides, downsides, and possible pitfallsfor these methods.

PACS 05.30.Rt -Quantum phase transitions PACS 75.10.Jm -Quantized spin models, including quantum spin frustration PACS 03.67.Pp -Quantum error correction and other methods for protection against decoherence Abstract. -We explore the physics of the anisotropic compass model under the influence of perturbing Heisenberg interactions and present the phase diagram with multiple quantum phase transitions. The macroscopic ground state degeneracy of the compass model is lifted in the thermodynamic limit already by infinitesimal Heisenberg coupling, which selects different ground states with Z2 symmetry depending on the sign and size of the coupling constants -then low energy excitations are spin waves, while the compass states reflecting columnar order are separated from them by a macroscopic gap. Nevertheless, nanoscale structures relevant for quantum computation purposes may be tuned such that the compass states are the lowest energy excitations, thereby avoiding decoherence, if a size criterion derived by us is fulfilled.

We study two Kitaev-Heisenberg t-J-like models on a honeycomb lattice, focusing on the zigzag magnetic phase of Na(2)IrO(3), and investigate hole motion by exact diagonalization and variational methods. The spectral functions are quite distinct from those of cuprates and are dominated by large incoherent spectral weight at high energy, almost independent of the microscopic parameters-a universal and generic feature for zigzag magnetic correlations. We explain why quasiparticles at low energy are strongly suppressed in the photoemission spectra and determine an analog of a pseudogap. We point out that the qualitative features of the predominantly incoherent spectra obtained within the two different models for the zigzag phase are similar, and they have a remarkable similarity to recently reported angular resolved photoemission spectra for Na(2)IrO(3).

We explore with exact diagonalization the propagation of a single hole in four magnetic phases of the t-J-like Kitaev-Heisenberg model on a honeycomb lattice: the Néel antiferromagnetic, stripe, zigzag and Kitaev spin-liquid phase. We find coherent propagation of spin-polaron quasiparticles in the antiferromagnetic phase by a similar mechanism as in the t-J model for high-Tc cuprates. In the stripe and zigzag phases clear quasiparticles features appear in spectral functions of those propagators where holes are created and annihilated on one sublattice, while they remain largely hidden in those spectral functions that correspond to photoemission experiments. As the most surprising result, we find a totally incoherent spectral weight distribution for the spectral function of a hole moving in the Kitaev spin-liquid phase in the strong coupling regime relevant for iridates. At intermediate coupling the finite systems calculation reveals a well defined quasiparticle at the Γ point, however, we find that the gapless spin excitations wipe out quasiparticles at finite momenta. Also for this more subtle case we conclude that in the thermodynamic limit the lightly doped Kitaev liquid phase does not support quasiparticle states in the neighborhood of Γ, and therefore yields a non-Fermi liquid, contrary to earlier suggestions based on slave-boson studies. These observations are supported by the presented study of the dynamic spin-structure factor for the Kitaev spin liquid regime.

We study the ground state properties of the Kitaev-Heisenberg model in a magnetic field and explore the evolution of spin correlations in the presence of non-magnetic vacancies. By means of exact diagonalizations, the phase diagram without vacancies is determined as a function of the magnetic field and the ratio between Kitaev and Heisenberg interactions. We show that in the (antiferromagnetic) stripe ordered phase the static susceptibility and its anisotropy can be described by a spin canting mechanism. This accounts as well for the transition to the polarized phase when including quantum fluctuations perturbatively. Effects of spin vacancies depend sensitively on the type of the ground state. In the liquid phase, the magnetization pattern around a single vacancy in a small field is determined, and its spatial anisotropy is related to that of non-zero further neighbor correlations induced by the field and/or Heisenberg interactions. In the stripe phase, the joint effect of a vacancy and a small field breaks the six-fold symmetry of the model and stabilizes a particular stripe pattern. Similar symmetry-breaking effects occur even at zero field due to effective interactions between vacancies. This selection mechanism and intrinsic randomness of vacancy positions may lead to spin-glass behavior.

We study the energetic stability and structural features of bimetallic metal-organic frameworks. Such heterometallic MOFs, which can result from partial substitutions between two types of cations, can have specific physical or chemical properties used for example in catalysis or gas adsorption. We work here to provide through computational chemistry a microscopic understanding of bimetallic MOFs and the distribution of cations within their structure. We develop a methodology based on a systematic study of possible cation distributions at all cation ratios by means of quantum chemistry calculations at the density functional theory level. We analyze the energies of the resulting bimetallic frameworks and correlate them with various disorder descriptors (functions of the bimetallic framework topology, regardless of exact atomic positions). We apply our methodology to two families of MOFs known for heterometallicity: MOF-5 (with divalent metal ions) and UiO-66 (with tetravalent metal ions). We observe that bimetallicity is overall more favorable for pairs of cations with sizes very close to each other, owing to a charge transfer mechanism inside secondary building units. For cations pairs with significant mutual size difference, metal mixing is globally less favorable; and the energy signifantly correlates with the coordination environment of linkers, determining their ability to adapt the mixing-induced strains. This effect is particularly strong in the UiO-66 family, because of high cluster coordination number.

Multicomponent metal-organic frameworks (MOFs) comprise multiple, structurally diverse linkers fixed into an ordered lattice by metal ions or clusters as secondary building units (SBUs). Here, we show how multicomponent MOFs are ideal platforms for engineering materials with high levels of vacancy defects. First, a new type of quaternary MOF that is built up from two neutral, linear ditopic linkers, a 3-fold-symmetric carboxylate ligand, and a dinuclear paddlewheel SBU was synthesized. This MOF, named MUF-32 (MUF = Massey University Framework), is constructed from dabco, 4,4′-bipyridyl (bipy), 4,4′,4″-nitrilotrisbenzoate (ntb), and zinc(II), and it adopts an ith-d topology. The zinc(II) ions and ntb ligand define an underlying [Zn2(ntb)4/3] sublattice (with pto topology) that is "load bearing" and maintains the structural integrity of the framework. On the other hand, the dabco and bipy ligands are "decorative", and high levels of vacancy defects can be introduced by their partial omission or removal. These defects can be generated by direct synthesis or by postsynthetic modification. The framework structure, crystallinity, and porosity are maintained even when vacancy levels of 80% are reached. Defect healing is possible by introducing free ligands in a solvent-assisted process to restore pristine MUF-32. Computational analysis reveals that the mechanical instability of the [Zn2(ntb)4/3] sublattice sets an upper limit on defect levels in this material. Multicomponent Metal-Organic Frameworks as Defect-Tolerant Materials Seok J. Lee, [a] Celine Doussot, [a] Anthony Baux, [a] Lujia Liu, [a] Geoffrey B. Jameson, [a] Christopher Richardson, [b] Joshua J. Pak, [c] Fabien Trousselet, [d] Francois-Xavier Coudert, [d] Supporting Information PlaceholderABSTRACT: Multicomponent metal-organic frameworks (MOFs) comprise multiple, structurally-diverse linkers fixed into an ordered lattice by metal ions or clusters as secondary building units (SBUs). Here, we show how multicomponent MOFs are ideal platforms for engineering materials with high levels of vacancy defects. First, a new type of quaternary MOF that is built up from two neutral, linear ditopic linkers, a threefold-symmetric carboxylate ligand, and a dinuclear paddlewheel SBU was synthesized. This MOF, named MUF-32 (MUF = Massey University Framework), is constructed from dabco, 4,4'-bipyridyl (bipy), 4,4',4''-nitrilotrisbenzoate (ntb) and zinc(II), and it adopts an ith-d topology. The zinc(III) ions and ntb ligand define an underlying [Zn 2 (ntb) 4/3 ] sublattice (with pto topology) that is 'load bearing' and maintains the structural integrity of the framework. On the other hand, the dabco and bipy ligands are 'decorative' and high levels of vacancy defects can be introduced by their omission. These defects can be generated by direct synthesis or by postsynthetic modification. The framework structure, crystallinity and porosity are maintained even when vacancy levels of 80% are reached. Defect healing is possible by introducing free ligands in a s...