Jahn-Teller Lattice Distortion Research Articles

Symmetry Breaking and Spin-Orbit Coupling for Individual Vacancy-Induced In-Gap States in MoS2 Monolayers.

Spins confined to point defects in atomically thin semiconductors constitute well-defined atomic-scale quantum systems that are being explored as single-photon emitters and spin qubits. Here, we investigate the in-gap electronic structure of individual sulfur vacancies in molybdenum disulfide (MoS2) monolayers using resonant tunneling scanning probe spectroscopy in the Coulomb blockade regime. Spectroscopic mapping of defect wave functions reveals an interplay of local symmetry breaking by a charge-state-dependent Jahn-Teller lattice distortion that, when combined with strong (≃100 meV) spin-orbit coupling, leads to a locking of an unpaired spin-1/2 magnetic moment to the lattice at low temperature, susceptible to lattice strain. Our results provide new insights into the spin and electronic structure of vacancy-induced in-gap states toward their application as electrically and optically addressable quantum systems.

A multiphase coupling strategy to structurally stable manganese-based oxides cathode for high electrochemical performance sodium ion batteries

Manganese (Mn) based oxides attract extensive attention as cathode materials for sodium (Na) ion batteries (SIBs) due to their high theoretical capacity and low cost. However, the Jahn-Teller distortion caused by the Mn (III) center and the phase transition of P2-O2 at high voltage (>4.0 V) limits its practical applications. In this work, a multiphase coupling scheme involving Na0.7MnO2.05 (P2), Na0.44MnO2 (T), and Na0.91MnO2 (Z) is presented to develop improved and available Mn based oxides cathode for superior electrochemical performance SIBs via solid state method. It is found that the optimal coupling at the molecular and atomic levels can be achieved by adjusting the regulation of morphology and grain size of the multiphase, which possesses larger interlayer spacing and fewer Mn3+ ions to suppress the Jahn-Teller lattice distortion of Mn3+ as well as the phase transition of P2-O2 at high voltage (>4.0 V). As a result, the as-prepared cathode delivers a high discharge specific capacity of 183.28 mA h g−1 at a rate of 0.5 C with a high-capacity retention of 77.2% after 100 cycles. Moreover, it also shows the fastest Na+ diffusion ability and kinetics properties, which is revealed by refined analysis of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves. This study presents a straightforward and scalable approach for developing novel cathode materials based on Mn oxides with excellent performance in sodium-ion batteries (SIBs).

Large piezoelectric response in a Jahn-Teller distorted molecular metal halide

Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d33) or voltage (g33) coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d33*g33. In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d33 and g33. This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl4 (EDABCO = N-ethyl-1,4-diazoniabicyclo[2.2.2]octonium), a molecular piezoelectric with a d33 of 165 pm/V and g33 of ~2110 × 10−3 V m N−1, one that achieved thusly a combined transduction coefficient of 348 × 10−12 m3 J−1. This enables piezoelectric energy harvesting in EDABCO-CuCl4@PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm2 (at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric.

A one-dimensional organic-inorganic hybrid copper(I)-halide with broadband emission

Organic-inorganic perovskite materials have shown excellent performance and strong competitiveness in high-performance photovoltaic and optoelectronic devices. Besides the color-tunable and narrow-band emissions realized in hybrid perovskites, broadband emission from lead-halide perovskites has also attracted strong interest for applications in the next generation of solid-state lighting. Although great progress has been made in lead halide perovskites, a very important aspect that hinders their commercial exploitation is the severe lead toxicity. In this research, a lead-free copper(I)-iodine hybrid was synthesized by a solution evaporation method at room temperature. Single crystal X-ray diffraction shows that this hybrid possesses the chemical formula of C6H5CH2CH2NH3CuI2. The C6H5CH2CH2NH3CuI2 belongs to the monoclinic phase with the C2/c space group, where the edge-sharing [CuI4] tetrahedral chains are separated by organic C6H5CH2CH2NH3+ ions, exhibiting one-dimensional (1D) crystal structure. The optical band-gap of C6H5CH2CH2NH3CuI2 was determined to be 4.02 eV. The photoluminescence (PL) spectrum shows a broadband emission ranging from 380 nm to 780 nm and peaks at 542 nm together with a small peak at 336 nm, which can be attributed to the emission of self-trapped exciton (STE) and free exciton (FE), respectively. The large Stokes shift of 230 nm between the PL and photoluminescence excitation (PLE) as well as the broad emission was ascribed to the Jahn–Teller lattice distortion of [CuI4] tetrahedra. The reduced 1D characteristic facilitates the tetrahedral distortion. PL decay analysis shows that the average lifetime of STE is about 66.9 ns? Temperature and time-dependent PL measurements show that the normal working temperature of this luminescent material should be no higher than 100 °C. Theoretical calculations were performed to better understand this compound. The successful synthesis and preliminary optical investigations on C6H5CH2CH2NH3CuI2 can be of great significance for offering more environmental-benign options in both solid-state lighting and other perovskite optoelectronic devices.

Understanding doping effects on P2 NaxMn1−yMyO2 ( M=Li , Mg, Al, Ti, V, Cr, Fe, Co, Ni) cathode materials for Na-ion batteries

The P2-layered oxide ${\mathrm{NaMnO}}_{2}$ is known as a cheap and high-capacity material for secondary batteries, but has been limited in application due to the large Jahn-Teller lattice distortion. Through the highly accurate hybrid functional method (HSE06), we computationally evaluate the doping influences on the lattice distortions, stabilities, electronic structures, redox potentials, and diffusion mechanisms. Our calculations indicate that dopants not only reduce the lattice distortion degree, especially for Li, Mg, Ti, and V cases, but also increase the stability of the structure, implying the dopants would alleviate the Jahn-Teller lattice distortion. At low Na concentrations, the Li dopant preferably diffuses out of the ${\mathrm{MnO}}_{2}$ layer, but hardly moves to the Na layer, suggesting that P2-layered oxides can prevent the dopant's migrations during the Na extractions. At full Na concentrations, all the considered dopants, except for Ti and V, have a small effect on the redox potential. The effect on the diffusion mechanism is described through the diffusion of a Na ion--polaron complex near the dopant's environments. Fe, Mg, Ti, and Cr dopants can hinder the Na ion-polaron complex diffusion with significantly higher activation energies, respectively, while the Al dopant almost remains the activation energy as well as the perfect structure. However, Li, V, Co, and Ni dopants benefit from such complex diffusion with much lower activation energy so the ion diffusivities increase significantly. It is found that the doping influence on the activation energy for Na ion diffusion is associated with the $M$--O bond change and charges of the neighboring dopants.

Proton-Induced Disproportionation of Jahn-Teller-Active Transition-Metal Ions in Oxides Due to Electronically Driven Lattice Instability.

The interfacial chemical reactivity of Jahn-Teller-active transition-metal oxides remains an enigmatic area, often leading to uncontrollable phase transformations in the oxide-based technological applications. In particular, the higher tendency of unwanted transition-metal-ion dissolution and side-reactions in Jahn-Teller-active oxides is accompanied by performance degradation in many electrochemical systems, for example, lithium-ion batteries. We show here that the fundamental electronic structure instability that leads to Jahn-Teller (lattice) distortion in an octahedral ligand field is the active chemical driving force for the spontaneous disproportionation, phase transformation, and metal-ion dissolution in transition-metal oxides upon exposure to protons. On the basis of electronic structure analyses and 18O isotope labeling, we present a mechanism comprising a coupled acid-base/redox reaction that leads to a proton-induced disproportionation of Jahn-Teller-active transition-metal ions, as exemplified by the broad classes of respective oxides.

First-Principles Prediction of Enhanced Magnetic Anisotropy of α″-Phase Fe₁₆N₂ With B and C Impurities

Intrinsic magnetic properties and uniaxial magnetic anisotropy (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) of α"-phase ordered Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloys have been investigated through first-principles calculations. Being in agreement with experimental results, the lattice parameters, magnetic moments, and K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> of α"-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> have been determined. Small addition of B or C dopant atoms enhances tetragonal distortion and K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> up to 0.75 MJ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for both Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.75</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.75</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> compounds, which is more than 20% larger compared with that (0.6 MJ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) of α"-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Furthermore, the presence of B and C reduces magnetic moments of its neighboring Fe atoms at the 4e and 8h sites, resulting in reduced magnetization, due to less electron transfer from Fe to B and C than N. The underlying mechanism for the enhancement in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> is discussed in connection with the Jahn-Teller lattice distortion, atom-decomposed magnetocrystalline anisotropy, and orbital magnetism.

Spin-Orbital Glass Transition in a Model of a Frustrated Pyrochlore Magnet without Quenched Disorder.

We show theoretically that spin and orbital degrees of freedom in the pyrochlore oxide Y_{2}Mo_{2}O_{7}, which is free of quenched disorder, can exhibit a simultaneous glass transition, working as dynamical disorder for each other. The interplay of spins and orbitals is mediated by the Jahn-Teller lattice distortion that selects the choice of orbitals, which then generates variant spin exchange interactions ranging from ferromagnetic to antiferromagnetic ones. Our MonteCarlo simulations detect the power-law divergence of the relaxation times and the negative divergence of both the magnetic and dielectric nonlinear susceptibilities, resolving the long-standing puzzle on the origin of the disorder-free spin glass.

Jahn–Teller Lattice Distortions and Asymmetric Electron Density Distribution in the Structure of TmB12 Dodecaboride in the Temperature Range of 85–293 K

The structure of single-crystal dodecaboride TmB12, which possess metallic properties, has been studied by X-ray diffraction at nine temperatures in the range of 85–293 K. Jahn–Teller distortions of the cubic lattice are revealed. They are found to cause an asymmetric electron density distribution in crystal. Static components of atomic displacements are selected, and characteristic Einstein and Debye temperatures are determined for thulium and boron atoms, respectively. Temperature dependences of interatomic distances are obtained. The problem of localizing conduction electrons by means of X-ray diffraction analysis is stated.

Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites

Using a two-band double-exchange model with Jahn-Teller lattice distortions and super-exchange interactions, supplemented by quenched disorder, at electron density $n=0.65$, we explicitly demonstrate the coexistence of the $n$ = 1/2-type ($\pi, \pi$) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature $T_{\rm c}$ in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters with decreasing the temperature down to $T_{\rm c}$. The ferromagnetic nanoclusters start to grow and merge, and the volume fraction of the charge-ordered nanoclusters decreases below $T_{\rm c}$, leading to the sharp drop in the resistivity. By applying a small external magnetic field $h$, we show that the resistivity above $T_{\rm c}$ increases, as compared with the case when $h=0$, a fact which further confirms the coexistence of the charge-ordered and the ferromagnetic nanoclusters. In addition, we show that the volume fraction of the charge-ordered nanoclusters decreases with increasing the bandwidth and consequently the resistivity hump diminishes for large bandwidth manganites, in good qualitative agreement with experiments. The obtained insights from our calculations provide a complete pathway to understand the phase competition in CMR manganites.

Magnetic reconstructions in B-site doped manganites

The magnetic nature of the B-site dopants controls the magnetic phases in B-site doped manganites RE1−xAExMn1−ηBηO3. Different B-site dopants of equal valence, doped into the same reference manganite, lead to different magnetic phases at low temperature, which cannot be explained using the valence change scenario. We focus on trivalent B-site dopants in CE-CO-OO-I (CE-type antiferromagnet; CO: charge order; OO: orbital order; I: Insulator) manganites at half-filling x = 0.50 to study the role of magnetic interactions between the B-site dopants and the neighboring Mn-sites by using a two-orbital double-exchange model including super-exchange interactions, Jahn-Teller lattice distortions, and substitutional disorder in two dimensions. We show that the magnetic reconstructions around the B-site dopants due to the modified double-exchange and super-exchange interactions control the phase competition in B-site doped manganites.

Role of 5f electrons in the structural stability of light actinide (Th-U) mononitrides under pressure.

Pressure induced structural sequences and their mechanism for light actinide (Th-U) mononitrides were studied as a function of 5f-electron number using first-principles total energy and electronic structure calculations. Zero pressure lattice constants, bulk module and C11 elastic module vary systematically with 5f-electron number implying its direct role on crystal binding. There is a critical 5f-electron number below which the system makes B1-B2 and above it B1-R3̄m-B2 structural sequence under pressure. Also, the B1-B2 transition pressure increases with increasing 5f-electron number whereas an opposite trend is obtained for the B1-R3̄m transition pressure. The ascending of N p anti-bonding states through the Fermi level at high pressure is responsible for the structural instability of the system. Above the critical 5f-electron number in the system a narrow 5f-band occurs very close to the Fermi level which allows the system to lower its symmetry via band Jahn-Teller type lattice distortion and the system undergoes a B1-R3̄m phase transition. However, below the critical 5f-electron number this mechanism is not favorable due to a lack of sufficient 5f-state occupancy and thus the system undergoes a B1-B2 phase transition like other ionic solids.

Magneto-optical properties of nano-crystalline cubic and tetragonal copper ferrite thin films

Copper ferrite, CuFe2O4, films can be stabilized in cubic or tetragonal phases depending on the deposition conditions and post-deposition heat treatment. The difference is explained by redistribution of cations over particular crystal sites and Jahn-Teller distortion of the spinel lattice. Comparative magneto-optical study of the copper ferrite thin films with cubic and tetragonal symmetry is presented. It is shown that the Faraday rotation and magnetic circular dichroism spectra represent a sensitive tool for examination of cation distribution of magnetic Cu2+ and Fe3+ ions in tetrahedral and octahedral crystal sites of the spinel lattice.

Magnetic Properties and Mössbauer spectroscopy of NdFe1-xMnxO3

The effect of particle reduction to nanoscale size and substitution of Mn3+ ions for Fe3+ ions on the crystal structure, lattice dynamics, Mössbauer spectra and magnetic properties in NdFe1-xMnxO3 compounds have been studied. X-ray powder diffraction and Raman spectroscopy revealed that the Jahn-Teller distortion of lattice dominates for samples with x > 0.4. The average particle size of NdFeO3 nanoparticles (NAP) increases with annealing at 600°C from about 15 nm to 25 nm. The presence of superparamagnetic particles was indicated by Mössbauer measurements in NdFeO3 NAP system. Sextets in NdFe1-xMnxO3 Mössbauer spectra can be modelled with several local environments of Fe3+ induced by substitution. The reduction of dimensionality and the substitution induce a decrease of the Néel temperature TN1 from 691 K to 544 K for NAP or to 356 K for x = 0.4, however the temperature of spin reorientation TSR increases with substitution. The saturated magnetization obtained at 1.9 K increases and ferromagnetic component is removed below TSR with the substitution.

Dynamics of Mn3+ in off-stoichiometric LiMn1.5Ni0.5O4

LiMn1.5Ni0.5O4 is investigated by anelastic spectroscopy in order to study the manganese dynamics. An intense thermally activated peak, detectable only in off-stoichiometric samples, is attributed to a polaronic conduction due to an electron transfer from Mn3+ to a Mn4+, and the consequent motion of the Jahn–Teller lattice distortion. In order to have a good fit of the experimental data, the Jonscher model for ionic dynamics is adopted for the first time in this spinel structure. A value of about 0.3eV for the energy barrier of the polaronic conduction is obtained.

Interfacial magnetism in manganite superlattices

We use a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long-range Coulomb interactions to investigate the origin of magnetically disordered interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in FM/AFI superlattices. The induced magnetic moment in the AFI layer varies nonmonotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this nonmonotonic behavior which has a one-to-one correspondence with the magnetization of the FM interface. The obtained insights provide a basis for improving the tunneling magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead layer in the FM manganite.

Improving electrochemical performance of LiCoPO4 via Mn substitution

We report a Mn substitution strategy to tailor the morphology, size, surface of LiCo1–xMnxPO4 (x = 0·2, 0·5 and 0·8) materials using a facile organic acid mediated hydrothermal approach. The results show that the Mn substitution plays a profound role in reducing the particle size, stabilising the surface, and improving the diffusivity of LiCo1–xMnxPO4 materials, thus leading to much enhanced electrochemical performance compared with LiCoPO4. However, when excessive Mn (e.g. x = 0·8) is present in the olivine crystal, the performance of LiCo1–xMnxPO4 degrades, possibly due to Jahn–Teller lattice distortion. As a result, the LiCo0·5Mn0·5PO4 displays the best electrochemical performance in terms of capacity delivery, cyclability and rate capability. Therefore, the Mn substitution in LiCoPO4 could be an efficient way to achieve high energy density and long cycle life for high voltage materials.

Electronic and magnetic reconstructions in manganite superlattices

We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. The magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density n = 0.6. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.

Complex state found in the colossal magnetoresistance regime of models for manganites

The colossal magnetoresistance (CMR) effect of manganites is widely believed to be caused by the competition between a ferromagnetic (FM) metallic state induced by the double-exchange mechanism and an insulator with complex spin, charge, and orbital order. Recent computational studies in small clusters have indeed reported a CMR precisely near the frontier between those two states at a realistic hole density x = 1/4. However, the detailed characteristics of the competing insulator were not fully understood in those previous investigations. This insulator is expected to display special properties that lead to the CMR; otherwise any competition between ferromagnetic and antiferromagnetic states would induce such an effect, which is not the case experimentally. In this report, the competing insulator at electronic density x = 1/4 and in the CMR regime is studied in detail using the double-exchange two-orbital model with Jahn-Teller lattice distortions on two-dimensional clusters, employing a careful large-scale cooling down process in the Monte Carlo simulations to avoid being trapped in metastable states. Our investigations show that this competing insulator has an unexpected complex structure, involving diagonal stripes with alternating regions displaying FM and CE-like order. The level of complexity of this new state even surpasses that of themore » recently unveiled spin-orthogonal-stripe states and their associated high degeneracy. This new state complements the long-standing scenario of phase separation, since the alternating FM-CE pattern appears even in the clean limit. The present and recent investigations are also in agreement with the many glassy characteristics of the CMR state found experimentally, due to the high degeneracy of the insulating states involved in the process. Results for the spin-structure factor of the new states are also here provided to facilitate the analysis of neutron scattering experiments for these materials.« less

Optical characterization of spin-charge-orbital orders inPr(Sr1−yCay)2Mn2O7

Bilayered-structure manganites $\text{Pr}{({\text{Sr}}_{1\ensuremath{-}y}{\text{Ca}}_{y})}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$ show the competing spin-charge-orbital orders; the ground state is the spin CE-type charge-orbital ordered state for $y&gt;0.4$ and the spin-A-type ferroic ${x}^{2}\text{\ensuremath{-}}{y}^{2}$ orbital ordered state for $y&lt;0.4$. For the two representative crystals of $y=0.9$ and $y=0$, we have investigated the optical conductivity and Raman spectra to probe the charge, spin, and lattice dynamics of the respective phases. For $y=0.9$, the charge-orbital ordered states are characterized by strongly activated Raman phonon bands corresponding to the modulated breathing and Jahn-Teller lattice distortions, as observed commonly for other charge-orbital ordered manganites. As the unique features of this bilayered manganite, however, we found that the infrared-active phonon mode can also show up in the Raman spectra in the lower-temperature charge-orbital ordered state, confirming the breaking of centrosymmetry to produce the spontaneous electric polarization. The other feature to be noted is the appearance of the two-magnon excitation band of the CE-type spin order in the Raman spectra which estimates the energy scale of spin exchange energies. For $y=0$, on the other hand, we have observed a charge gap in the optical conductivity spectrum which becomes narrowed with the evolution of the A-type (in-plane ferromagnetic) spin order but remains finite (0.15 eV) at the ground state. The persistence of the charge gap is ascribed to the vertical charge ordering or charge-density wave (CDW) that was previously verified by diffuse x-ray scattering. Such charge-order or CDW formation also activates additional infrared phonon modes at 26 and 35 meV, whose spectral intensity appears to be enhanced by resonance with the underlying electronic continuum.