Ionic Crystalline Research Articles

Experimental and theoretical evidence for an ionic crystal of ammonia at high pressure

We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterized in both hydrogenated (NH${}_{3}$) and deuterated (ND${}_{3}$) ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy, and x-ray diffraction. The presence of a new strong infrared absorption band centered at 2500 cm${}^{\ensuremath{-}1}$ in NH${}_{3}$ (1900 cm${}^{\ensuremath{-}1}$ in ND${}_{3}$) is in line with previous theoretical predictions regarding the ionization of ammonia molecules into ${{\mathrm{NH}}_{2}}^{\ensuremath{-}}$ and ${{\mathrm{NH}}_{4}}^{+}$ ions. The experimental data suggest the coexistence of two crystalline ionic forms, which our ab initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia appears stable at low temperatures, which contrasts with the behavior of water in which no equivalent crystalline ionic phase has been found.

Wide electrochemical window ionic salt for use in electropositive metal electrodeposition and solid state Li-ion batteries

A stable hydrophobic ionic crystalline solid comprised of the N-propyl-N-methylpiperidinium cation and hexafluorophosphate anion PP13PF6 exhibits a remarkably wide electrochemical window of 7.2 V.

Highly efficient photocatalyst based on Ce doped ZnO nanorods: Controllable synthesis and enhanced photocatalytic activity

Here we successfully report the synthesis of highly efficient Ce doped ZnO nanorods photocatalysts under mild hydrothermal conditions utilizing polyamines triethylenetetraamine (TETA) as the cross-linking agents for the first time. The XRD patterns demonstrate that the ZnO nanorods are highly crystalline and Ce ions were successfully incorporated into the lattice position of ZnO. Presence of Ce3+, Ce4+ and Zn2+ ions cerium doped ZnO nanorods has been confirmed by XPS analysis. SEM images show that with increasing Ce doping, very thin fluffy or porous like structures on smooth ZnO nanorods were appeared. The lattice spacing between the adjacent lattice planes in the HRTEM images is measured to be approximately 0.52nm, confirming the ZnO nanorod grows along [0001] direction. Optical characterizations indicate that the Ce doping can shift the absorption edge of ZnO to the visible range and reduce the band gap. The prepared photocatalysts have been compared with commercial photocatalysts Hombikat UV-100 by the determination of their photonic efficiencies for degradation methylene blue. All Ce doped samples exhibited superior photocatalytic performance than either pure ZnO nanorods or Hombikat UV-100. 0.5% Ce doped ZnO exhibits highest photocatalytic activity among the prepared samples.

Simulations of metastable states near the apex of a force microscope tip interacting with an ionic crystalline surface

Atoms or pairs of ions picked up by probe tips used in dynamic force microscopy (DFM) can be strongly displaced and even hop discontinuously upon approach to the sample surface. The energy barriers for some of those hops are of the right order of magnitude to explain the rise in energy dissipation commonly observed in DFM measurements at room temperature. The systematic computations reported here can explain the infrequent jumps and very low average energy dissipation observed low temperature in a previous DFM study on a KBr(001) sample. Close to the surface we indeed find new states separated by small energy barriers which account for those phenomena. These energy barriers strongly depend on details of the atomic arrangement in the vicinity of the tip apex.

Shaping Calcite Crystals by Means of Comb Polyelectrolytes Having Neutral Hydrophilic Teeth

Comb polyelectrolytes (CPs) having neutral hydrophilic teeth, similar to double hydrophilic block copolymers, are a powerful tool to modify the chemical-physical properties of inorganic crystalline materials. One of their main applications is in concrete technology, where they work as superplasticizers, particle-dispersing agents. Here, CPs, having the same poly(acrylic acid) (PAA) backbone chain and differing in the grafting with methoxy poly(ethylene glycol) chains (MPEG) of two molecular weights, were used to investigate the influence of tooth chains in polymer aggregation and in control on morphology and aggregation of calcite particles. These polymers aggregate, forming interpolymer hydrogen bonds between carboxylic groups and ether oxygen functionalities. The presence of calcium ions in solution further enhances aggregation. Crystallization experiments of calcite in the presence of CPs show that the specificity of interactions between polymers and crystal planes and control on aggregation and size of particles is a function of the content and chain length of the MPEG in the PAA backbone. These parameters limit and can make specific the electrostatic interactions with ionic crystalline planes. Moreover, the mechanism of crystallization, classical or nonclassical, is addressed by the CP structure and concentration. These findings have implications in the understanding of the complex chemical processes associated to concrete superplasticizers action and in the study of the biomineralization processes, where biological comb polyelectrolytes, the acidic glycoproteins, govern formation of calcitic structures.

High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures, bond characterization, and compressibility of Mg(BH4)2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases of meta-stable I41/amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure. Enthalpy calculation indicates that the ground state of F222 structure proposed by Zhou et al. [2009 Phys. Rev. B 79 212102] will transfer to I41/amd at 0.7 GPa, and then to a P-3m1 structure at 6.3 GPa. The experimental P6122 structure (α-phase) transfers to I41/amd at 1.2 GPa. Furthermore, both I41/amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure. Their theoretical volumetric hydrogen densities reach 146.351 g H2/L and 134.028 g H2/L at ambient pressure, respectively. The calculated phonon dispersion curve shows that the I41/amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa. So the I41/amd phase may be synthesized under high pressure and retained to ambient pressure. Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range. In addition, they each have an anisotropic compressibility. The c axis of these structures is easy to compress. Especially, the c axis and volume of P-3m1 phase are extraordinarily compressible, showing that compression along the c axis can increase the volumetric hydrogen content for both I41/amd and P−3m1 structures.

The microwave ponderomotive effect on ceramic sintering

The evolution of pore structure during microwave sintering of ceramics is investigated based on a model of the non-thermal ponderomotive action of a microwave field in ionic crystalline solids. By means of numerical simulation it is demonstrated that the microwave field facilitates the collapse of faceted pores in the ceramics. Quantitative estimates suggest that the microwave ponderomotive effect can be significant during sintering.

Rare-Earth-doped Laser Materials: Spectroscopy and Laser Properties

ABSTRACTTrivalent rare earth ions in crystalline or fiber hosts are among the most successful of laser materials, but new dopant-host combinations and more detailed understanding of existing materials continue to be needed. This paper presents a few examples from the work of our team at the Army Research Laboratory, highlighting the interrelation between spectroscopic properties and laser behavior. It focuses on bulk solids, though rare-earth-doped fiber lasers are also extremely important. One system discussed is Nd:YAG, particularly concentration quenching in heavily doped ceramic YAG. Spectroscopic properties of Yb:Y2O3 and Yb:Sc2O3 help to elucidate their laser performance. Spectra indicate that Er:YAG is more promising than Er:Sc2O3 for room temperature laser operation, but that the reverse is true for operation at and somewhat above liquid nitrogen temperature.

Quantum Chemical Evaluation of Ionic Nonlinear Optical Chromophores and Crystals Considering the Counteranion Effects

The direction and the magnitude of the first hyperpolarizability are investigated by the finite field (FF) method for organic nonlinear optical ionic chromophores and crystals with an isotropic counteranion, either a point charge or hexafluorophosphate. Simple model compound 4-aminopyridinium and DAPSH (N,N-dimethylamino-N′-phenyl-4-stilbazolium hexafluorophosphate) crystal with the highest quadratic optical nonlinearities measured to date in organic crystals, as well as stilbazolium derivatives having heteroaromatic and/or bulky aromatic rings, are investigated. Calculations indicate that the extrinsic isotropic point-charges or anions almost have no effect on the direction of the intrinsic first hyperpolarizability of the cation chromophores, and induce a limited increase of the magnitude smaller than ∼15% due to their asymmetric distribution with respect to the pyridinium nitrogen atom. This finding enables one to simply approximate the first hyperpolarizability of ionic crystalline systems having 1D i...

Precision measurements in ion traps using slowly moving standing waves

The present paper describes the experimental implementation of a measuring technique employing a slowly moving, near resonant, optical standing wave in the context of trapped ions. It is used to measure several figures of merit that are important for quantum computation in ion traps and which are otherwise not easily obtainable. Our technique is shown to offer high precision, and also in many cases using a much simpler setup than what is normally used. We demonstrate here measurements of i) the distance between two crystalline ions, ii) the Lamb-Dicke parameter, iii) temperature of the ion crystal, and iv) the interferometric stability of a Raman setup. The exact distance between two ions, in units of standing wave periods, is very important for motional entangling gates, and our method offers a practical way of calibrating this distance in the typical lab situation.

Coulomb interactions in charged fluids

The use of Ewald summation schemes for calculating long-range Coulomb interactions, originally applied to ionic crystalline solids, is a very common practice in molecular simulations of charged fluids at present. Such a choice imposes an artificial periodicity which is generally absent in the liquid state. In this paper we propose a simple analytical O(N(2)) method which is based on Gauss's law for computing exactly the Coulomb interaction between charged particles in a simulation box, when it is averaged over all possible orientations of a surrounding infinite lattice. This method mitigates the periodicity typical of crystalline systems and it is suitable for numerical studies of ionic liquids, charged molecular fluids, and colloidal systems with Monte Carlo and molecular dynamics simulations.

1,2-Bis[N-(N′-alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals

A new class of organic ionic plastic crystals was discovered. A series of alkylene 1,2-bis[N-(N′-alkylimidazolium)] salts with Br− and PF6− anions was prepared and most of the ethylene (C2) linked compounds undergo multiple solid–solid phase transitions as determined by differential scanning calorimetry (DSC). The salts with longer spacers (C3 or C4) between the imidazolium units do not display any solid–solid transitions. The PF6− salts with terminal C10 and C12n-alkyl moieties are “classical” organic ionic plastic crystals by Timmermans' definition; they have low ΔSf (11 J K−1 mol−1 for C10 and 12 J K−1 mol−1 for C12). Additionally, the C2 linked bis-imidazolium salts with n-butyl, n-hexyl, n-heptyl and n-octyl termini all undergo two, three or four solid state transitions and exhibit ΔSf values in the range of 36–49 J K−1 mol−1, which are similar to those of other known ionic plastic crystalline materials. These materials were additionally characterized via ionic conductivity and solid state NMR. These ionic plastic crystals are presumably single-ion conductors, but the ionic conductivities appear to be too low for practical applications. The activation energy for conduction decreases as these compounds are heated through each solid–solid transition. The lack of any change in solid state 2H NMR spectra with temperature indicates that there is no change in phenyl ring flipping, suggesting no change in the imidazolium local environment. A resolution of this apparent dichotomy is perhaps that the counterions reside with the ethylene spacers between imidazolium moieties.

COOPER PAIR BREAKING

An overview is given of a number of pair-breaking interactions in superconductors. They have in common that they violate a symmetry of the pair state. In most cases pairs are formed from time reversed single-particle states, a noticeable exception being antiferromagnetic superconductors. When time reversibility is broken by an interaction acting on the electrons, the time evolution of the time-reversal operator plays an important role. Depending on whether it is nonergodic or ergodic, we deal with pair weakening or pair breaking. Numerous different interactions are analyzed and discussed. Unifying features of different pair-breaking cases are pointed out. Special attention is paid to the Zeeman effect and to scattering centers with low-energy excitations. The Kondo effect and crystalline field split rare-earth ions belong in that category. Modifications caused by strongly anisotropic pair states are pointed out. There is strong evidence that in some cases intra-atomic excitations lead to pair formation rather than pair breaking for which an explanation is provided.

Mechanical and Dynamic Properties of N,N,N,N-Tetramethylammonium Dicyanamide Plastic Crystal

Elastic constants and relaxation of N,N,N,N-tetramethylammonium (TMA) dicyanamide (DCA) plastic crystal was investigated as a function of temperature using a quantum chemistry based, dipole polarizable force field. Simulations predict the [TMA][DCA] crystal structure and dimensions in good agreement with experiment. Ion-counterion spatial distributions are used to understand the local environment and ion pairing of both ions in crystalline and liquid phases. The rotational dynamics of ions in the crystalline system are thoroughly explored. Arrest of the DCA rotational degrees of freedom is associated with the experimentally observed solid-solid phase transitions.

Electrically active centers of capture of charge carriers and electret effect in polymeric dielectrics and mica

The results of an experimental investigation of the electret effect and electrically active centers of capture in polymeric dielectrics modified by radiation and subjected to deformation (X-ray radiation of electric gas barrier discharge using the EGBD technique), as well as mica after δCo60 radiation and heat treatment, have been presented. It has been shown that electrically active centers of capture in polymeric electrets are polar C-H bonds and, in mica crystals, these centers are radiation defects of K+-OH− ionic crystalline sublattice. It was shown that the process of the formation of polymeric and mica electrets is described by the wave-current model of polarization.

Preparation and Properties of P-Type Transparent Conductive NiO Films

The NiO-Cu composite films with various Cu contents of 0 – 18.17 at.% are deposited on glass substrate. An ultra high electrical resistivity (ρ) is obtained and cannot be detected by four point probe measurement when the Cu contents in the films are lower than 6.97 at.%. The ρ value is reduced significantly to 35.8 Ω-cm as Cu content is increased to 9.18 at.%, and it further decreases to 0.02 Ω-cm when the Cu content further increases to 18.17 at.%. The Hall measurement for all Cu-doped NiO films show p-type conduction. In addition, the transmittance of NiO films also decreases continuously from 96 % to 43 % as Cu content increases from 0 to 18.17 at.%. The XRD patterns of Cu-doped NiO films only appear NiO peaks and the crystallinity of NiO films becomes worse as Cu content is added to above 6.97 at.%. Large amount of lattice sites of Ni2+ ions in NiO crystalline are replaced by the Cu+ ions that leads to p-type conduction and result in the degradation of crystallinity for NiO-Cu composite films having higher Cu content.

Molecular dynamics simulations of N,N,N,N-tetramethylammonium dicyanamide plastic crystal and liquid using a polarizable force field

A quantum chemistry based, dipole polarizable force field has been used to simulate the N,N,N,N-tetramethylammonium (TMA) dicyanamide (DCA) ionic salt, in both plastic crystalline and liquid phases. Simulations predicted the [TMA][DCA] crystal structure and dimensions in good agreement with experiment. Ion-counterion spatial distributions are used to understand the local environment and ion pairing of both ions in crystalline and liquid phases. The rotational dynamics of ions in the crystalline system are thoroughly explored. Arrest of the DCA rotational degrees of freedom was associated with the experimentally observed solid-solid phase transitions. The self-diffusion coefficient and conductivity were calculated for the liquid state; however no net ion diffusion is noted in the pristine crystalline state. Introduction of ion vacancy at 0.3% concentration is found to be sufficient to enable ion diffusive behavior and conduction at 425 K in the crystalline state, with good agreement found between the experimental and simulated conductivity.

Peculiarities of Er3+ photoluminescence in halogen-doped amorphous silica

The spectra and photoluminescence kinetics of Er3+ ions embedded in amorphous fluorine- and chlorine-doped silica matrices synthesized by surface-plasma chemical vapor deposition were investigated at 27–300 K. Luminescence was excited with an Ar+ laser at a wavelength of 514.5 nm and with a diode laser at a wavelength of 975 nm. Narrow and well-expressed components of Stark sublevels with a small contribution of inhomogeneous broadening intrinsic to Er3+ ions in crystalline rather than amorphous matrices were revealed and identified in photoluminescence spectra. The structure of Stark sublevels was well-resolved at low temperatures. The presence of the well-resolved Stark structure in spectra was indicative of stable anion complex formation in Er3+ environment presumably associated with halogen incorporation. This environment was formed at a stage of the low-temperature plasma-chemical synthesis and was destroyed at glass fusion.

Size‐Selective Sorption of Small Organic Molecules in One‐Dimensional Channels of an Ionic Crystalline Organic–Inorganic Hybrid Compound Stabilized by π–π Interactions

Size‐Selective Sorption of Small Organic Molecules in One‐Dimensional Channels of an Ionic Crystalline Organic–Inorganic Hybrid Compound Stabilized by π–π Interactions

Size‐Selective Sorption of Small Organic Molecules in One‐Dimensional Channels of an Ionic Crystalline Organic–Inorganic Hybrid Compound Stabilized by π–π Interactions

Very selective: The cross-sectional area of the narrowest channel opening in an organic–inorganic porous ionic crystal that possesses one-dimensional channels is around 30 Å2. The compound sorbs molecules such as propane, 1-propanol, and 1,2-dichloroethane, which have cross-sectional areas smaller than the channel opening, while n-butane, 1-butanol, and 1,2-dichloropropane are excluded (see picture).