In-plane Bond Lengths Research Articles

Phonon Dynamics and Transport Properties of Copper Thiocyanate and Copper Selenocyanate Pseudohalides.

The phonon transport properties of CuSCN and CuSeCN have been investigated using the density functional theory and semiclassical Boltzmann transport theory. The Perdew–Burke–Ernzerhof functional shows an indirect (direct) electronic band gap of 2.18 eV (1.80 eV) for CuSCN (CuSeCN). The calculated phonon band structure shows that both compounds are dynamically stable. The Debye temperature of the acoustic phonons is 122 and 107 K for CuSCN and CuSeCN, respectively. The extended in-plane bond lengths as compared to the out-of-plane bond lengths result in phonon softening and hence, low lattice thermal conductivity. The calculated room temperature in-plane (out-of-plane) lattice thermal conductivity of CuSCN and CuSeCN is 2.39 W/mK (4.51 W/mK) and 1.70 W/mK (3.83 W/mK), respectively. The high phonon scattering rates in CuSeCN give rise to in-plane low lattice thermal conductivities. The room-temperature Grüneisen parameters of CuSCN and CuSeCN are found to be 0.98 and 1.08, respectively.

Switchable orbital polarization and magnetization in strained LaCoO3 films

Strain engineering of epitaxial heterostructures offers opportunities to control the orbital degree of freedom by lifting the degeneracy of ${e}_{g}$ states. Here, we show that the orbital occupation in $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO) films can be switched between two degenerate ${e}_{g}$ bands with epitaxial strain. The orbital polarization of nearly \ensuremath{-}100% (or 100%) is controlled by depleting occupation of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\phantom{\rule{4pt}{0ex}}(\mathrm{or}\phantom{\rule{0.16em}{0ex}}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}})$ orbital entirely in LCO for large compressive (or moderate tensile) strain. The change of electronic configuration associated with the spin-state transition modulates the magnetization of strained LCO films. Under compressive strain, LCO films exhibit a small magnetization without long-range ferromagnetic ordering. With tensile-strain increases, the magnetization of LCO films increases and reaches the maximum value when the bonding angle (Co--O--Co) is close to ${180}^{\ensuremath{\circ}}$ and the in-plane bond length (Co--O) is unstretched. Our results highlight the role of octahedral distortion and spin-state crossover in tailoring the magnetic properties of cobaltite thin films, suggesting an attractive route to deliberately control the orbital polarization that can be tuned to maximize the functionality of oxide heterostructures.

Elastic constants and critical thicknesses of ScGaN and ScAlN

Elastic constants of hexagonal ScxGa1−xN and ScxAl1−xN up to x = 0.375 were calculated using a stress-strain approach. C11, C33, C44, and C66 decreased while C12 and C13 increased slightly with increasing x. The biaxial [0001] Poisson ratios increased from 0.21 for GaN to 0.38 for Sc0.375Ga0.625 N and from 0.22 for AlN to 0.40 for Sc0.375Al0.625N, due to greater u values, in-plane bond lengths and bond ionicities. Subsequently, critical thicknesses for stress relaxation were calculated for ScxAl1−xN/AlN, ScxGa1−xN/GaN, and ScxAl1−xN/GaN heterostructures using an energy balance model. These range from 2 nm for Sc0.375Al0.625N/AlN and Sc0.375Ga0.625N/GaN to infinity for lattice-matched Sc0.18Al0.82N/GaN.

Structural systematics of the [Cu(chelate)3][Y]2series. An interesting crystallographic structural insight involving vibronic coupling and the Jahn–Teller effect (JTE). The syntheses and low temperature crystal structures of tris(2,2′bipyridyl)copper(ii) tetraphenylborate and tris(2,2′bipyridyl)zinc(ii) tetraphenylborate

The crystal structures of [Cu(bipy)(3)][BPh(4)](2), 1, and [Zn(bipy)(3)][BPh(4)](2), have been determined at low temperature. 1 and 2 are closely related, but are not isostructural. Both contain a two-dimensional supramolecular construct (SC) involving a sandwich structure. 1 has a six-coordinate CuN(6) chromophore with a regular elongated octahedral stereochemistry and rhombic in-plane bond lengths. The associated tetragonality value, T, of 1 is 0.8868. 2 involves a six-coordinate octahedral chromophore. Differences between 1 and 2 relate to the tendency of copper(II) complexes to undergo a Jahn-Teller (JT) distortion. The zinc(II) cation feels solely the host site strain, whereas the copper(II) cation also involves vibronic JT type coupling. The copper polyhedron geometry is characterized by both phenomena, with the vibronic interaction dominating. Scatter plot analysis involving the tris-chelate copper(II) series suggests that neither pure Q(theta) or Q(epsilon) components or the a(2u) mode operate in isolation over the entire series. All three operate in combination with varying quantifiable contributions, leading to distortion from the regular tetragonal octahedral stereochemistry.

Defect Analysis of Ionically Compensated Quadruple and Quintuple Perovskite Layered Cuprates with Ti Blocking Layers

This work presents a modified defect model for pure and chemically substituted quadruple and quintuple pure perovskite layered cuprates, using data from the Jonker (thermopower vs ln conductivity) and Brouwer (log conductivity vs log pO2) analyses. The closely matched in-plane bond lengths of the blocking and the active layers in these structures promote ionic compensation rather than electronic compensation upon aliovalent p-type doping. A comprehensive defect model that takes into account all the major defect species in an ionically compensated quadruple or quintuple perovskite provides a better means for understanding the electrical behavior of these novel materials. The presented model allows for the calculation of effective dopant and the oxygen interstitial concentrations in these materials.

Structural instability around Tc observed in Hg-1201 by neutron powder diffraction and EXAFS

The structure of HgBa 2CuO 4+δ is studied as function of temperature by neutron diffraction and Cu-Kedge EXAFS on the same powder sample with Tc=97K. Both techniques reveal the presence of an anomalous behaviour of the Cu O in-plane bond length starting at ≈140K.

Underdoped, optimally doped, and overdoped superconductors TlSr2(R1-xCax)Cu2O7- delta (R=Lu or Tl).

The crystal structures of underdoped, optimally doped, and overdoped TlSr[sub 2]([ital R][sub 1[minus][ital x]]Ca[sub [ital x]])Cu[sub 2]O[sub 7[minus][delta]] ([ital R]=Lu or Tl) are refined from neutron powder-diffraction data. Carrier concentrations in this system are estimated from simple ionic consideration using the refined occupation numbers. The plot of [ital T][sub [ital c]] vs carrier concentration shows the characteristic bell-shaped dependence. The in-plane bond length Cu-O(1) is affected by only the change of carrier concentration and provides a measurement of the doping level, while the apical bond length Cu-O(2) is affected by both the change of carrier concentration and the amount of cation substitution. If we take the effect of cation substitution into account, these bond lengths change linearly'' with carrier concentration over the entire range from the underdoped, through the optimally doped, to the overdoped region.

The graphite-to-diamond transformation

An orbital model for a solid state transformation, the graphite-to-diamond high-pressure reaction, is presented. Using solid state Walsh diagrams we relate this transformation to chemical reactions having orbital symmetry constraints. During the transformation the π-delocalization in the graphite planes is gradually lost, leading to buckling and in-plane bond length elongation as inter-plane bonds form. The pacing of the different components of the reaction coordinate is important. We show that the transformation passes through way points of metallic band structure, which may be related to symmetry dictated level crossings in the Walsh diagrams.