Residual Electron Density Research Articles

Synthesis, crystal structure and Hirshfeld surface analysis of [Cu(H2 L)2(μ-Cl)CuCl3]·H2O [H2 L = 2-hy-droxy-N'-(propan-2-yl-idene)benzohydrazide

The present study focuses on the synthesis and structural characterization of a novel dinuclear CuII complex, [tri-chlorido-copper(II)]-μ-chlorido-{bis-[2-hy-droxy-N'-(propan-2-yl-idene)benzohydrazide]copper(II)} monohydrate, [Cu2Cl4(C10H12N2O2)2]·H2O or [Cu(H2 L)2(μ-Cl)CuCl3]·H2O [H2 L = 2-hy-droxy-N'-(propan-2-yl-idene)benzohydrazide]. The complex crystallizes in the monoclinic space group P21/n with one mol-ecule of water, which forms inter-actions with the ligands. The first copper ion is penta-coordinated to two benzohydrazine-derived ligands via two nitro-gen and two oxygen atoms, and one bridging chloride, which is also coordinated by the second copper ion alongside three terminal chlorines in a distorted tetra-hedral geometry. The arrangement around the first copper ion exhibits a distorted geometry inter-mediate between trigonal bipyramidal and square pyramidal. In the crystal, chains are formed via inter-molecular inter-actions along the a-axis direction, with subsequent layers constructed through hydrogen-bonding inter-actions parallel to the ac plane, and through slipped π-π stacking inter-actions parallel to the ab plane, resulting in a three-dimensional network. The inter-molecular inter-actions in the crystal structure were qu-anti-fied and analysed using Hirshfeld surface analysis. Residual electron density from disordered methanol mol-ecules in the void space could not be reasonably modelled, thus a solvent mask was applied.

Synthesis and crystal structure of the cluster (Et4N)[(Tp*)MoFe3S3(μ3-NSiMe3)(N3)3

The title compound, tetraethylammonium triazidotri-μ3-sulfido-[μ3-(trimethylsilyl)azanediido][tris(3,5-dimethylpyrazol-1-yl)hydroborato]triiron(+2.33)molybdenum(IV), (C8H20N)[Fe3MoS3(C15H22BN6)(C3H9NSi)(N3)3] or (Et4N)[(Tp*)MoFe3S3(μ3-NSiMe3)(N3)3] [Tp* = tris(3,5-dimethylpyrazol-1-yl)hydroborate(1−)], crystallizes as needle-like black crystals in space group P\overline{1}. In this cluster, the Mo site is in a distorted octahedral coordination model, coordinating three N atoms on the Tp* ligand and three μ3-bridging S atoms in the core. The Fe sites are in a distorted tetrahedral coordination model, coordinating two μ3-bridging S atoms, one μ3-bridging N atom from Me3SiN2−, and another N atom on the terminal azide ligand. This type of heterometallic and heteroleptic single cubane cluster represents a typical example within the Mo–Fe–S cluster family, which may be a good reference for understanding the structure and function of the nitrogenase FeMo cofactor. The residual electron density of disordered solvent molecules in the void space could not be reasonably modeled, thus the SQUEEZE [Spek (2015). Acta Cryst. C71, 9–18] function was applied. The solvent contribution is not included in the reported molecular weight and density.

Toward an understanding of the role of structural organization and point defects in the formation of functional pure and Tm3+-doped (Ca3-xSrx)(VO4)2 for diode-pumped and SRS lasers

Whitlockite-type polycrystalline and single-crystal (Ca3−xSrx)(VO4)2 solid solutions were successfully obtained by solid-state synthesis and Czochralski method, respectively. Synchrotron powder X-ray diffraction allowed to reveal actual compositions of polycrystalline (Ca3−xSrx)(VO4)2 solid solutions with х = 0.3, 0.54, 0.9, 1.2, 1.35 refined by Rietveld method, which are slightly different from the initial ones. Synchrotron X-ray single-crystal diffraction analysis of Czochralski-grown undoped (Ca1.8Sr1.2)(VO4)2 (CSVO) revealed its actual composition (Ca2.982(11)Sr0.031)(VО4)2. Difference between initial and refined compositions is analyzed and discussed. In the CSVO structure, Sr2+ ions partially occupy monocapped Ca2 and two-capped Ca3 trigonal prismatic sites and Ca2+ ions are in the Ca5 and Ca5A sites, forming two independent strongly distorted tetrahedral Ca5O4 and octahedral Ca5AO6 oxygen environments. Synchrotron X-ray single-crystal diffraction analysis together with X-ray absorption spectroscopic study of (Сa2.4Sr0.6)(VO4)2 doped with 0.5 and 1.0 wt% Tm2O3 (СSVO:0.5Tm and CSVO:1.0Tm) allowed to establish the absence of cations in the Ca5A site, the localization of Sr2+ ions in the Ca2 and Ca3 sites and additionally in octahedral Ca4 site together with Tm3+ ions, forming an individual eight-coordinated environment, in the refined compositions (Ca2.776(7)Sr0.210(4)Tm3+0.008(1))(VO4)2 and (Ca2.855(7)Sr0.140(7)Tm3+0.009(1)□0.022(7))(VO4)2, respectively, with vacancies (marked by a square,□) in the two-capped Ca1 trigonal prismatic site in the CSVO:1.0Tm structure. Additional interstitial oxygen, participating in the formation of the Tm3+ environment, found in the residual electron density, together with vacancies in the Ca1 site provides electroneutrality to CSVO:0.5Tm and CSVO:1.0Tm crystals. Several structural models of the coordination environment of Tm3+ ions were analyzed and the most correct one was proposed.

Structural characterization of a new samarium–sodium heterometallic coordination polymer

Lanthanide-containing materials are of interest in the field of crystal engineering because of their unique properties and distinct structure types. In this context, a new samarium–sodium heterometallic coordination polymer, poly[tetrakis(μ2-2-formyl-6-methoxyphenolato)samarium(III)sodium(I)], {[SmNa(C8H7O3)4]·solvent} n (Sm-1), was synthesized and crystallized via slow evaporation from a mixture of ethanol and acetonitrile. The compound features alternating SmIII and NaI ions, which are linked by ortho-vanillin (o-vanillin) ligands to form a mono-periodic chain-like coordination polymer. The chains propagate along the [001] direction. Residual electron density of disordered solvent molecules in the void space could not be reasonably modeled, thus the SQUEEZE function was applied. The structural, vibrational, and optical properties are reported.

Structural complexity in the apparently simple crystal structure of Be2Ru.

The structural features of the hexagonal layered crystal structure of Be2Ru (a = 5.7508(3) Å, c = 3.0044(2) Å, space group P -62m) were investigated by single crystal X-ray diffraction and transmission electron microscopy (TEM). The residual electron density and high-resolution TEM images show that the real structure can be described as an intergrowth of the main hexagonal matrix of the Fe2P type with minor orthorhombic inclusions of its stacking variants. Such atomic arrangement is stabilized by the charge transfer from Be to Ru and by a system of polar three- and four-atomic bonds involving both components. The calculated electronic density of states (DOS) of Be2Ru revealed, contrarily to typical intermetallic compounds, a pseudo gap (dip) in the vicinity of the Fermi level. The temperature dependence of the electrical resistivity of Be2Ru shows metal behavior in agreement with the non-zero DOS at the Fermi level.

A mixed phosphine sulfide/selenide structure as an instructional example for how to evaluate the quality of a model.

This paper compares variations on a structure model derived from an X-ray diffraction data set from a solid solution of chalcogenide derivatives of cis-1,2-bis-(di-phenyl-phosphan-yl)ethyl-ene, namely, 1,2-(ethene-1,2-di-yl)bis-(di-phenyl-phoshpine sulfide/selenide), C26H22P2S1.13Se0.87. A sequence of processes are presented to ascertain the composition of the crystal, along with strategies for which aspects of the model to inspect to ensure a chemically and crystallographically realistic structure. Criteria include mis-matches between F obs 2 and F calc 2, plots of |F obs| vs |F calc|, residual electron density, checkCIF alerts, pitfalls of the OMIT command used to suppress ill-fitting data, comparative size of displacement ellipsoids, and critical inspection of inter-atomic distances. Since the structure is quite small, solves easily, and presents a number of readily expressible refinement concepts, we feel that it would make a straightforward and concise instructional piece for students learning how to determine if their model provides the best fit for the data and show students how to critically assess their structures.

Temporal electric field and electron density behavior of a He plasma jet with pulse width closing to pulse duration

Previously, it was observed that the plasma plume appears with three regimes (dark regime next to the nozzle, bright regime in the middle, and dim regime on the right) where the regime next to the nozzle is dark when the pulse width of the applied voltage is close to the pulse duration. In this paper, to understand such observation, the spatial and temporal resolved electron density and electric field in the three regimes are measured through the Thomson scattering and electric field-induced second harmonic method, respectively. It is found that, in the dark regime next to the nozzle, the electric field is relatively low, and it has a peak value of about 10 kV/cm, but the electron density is high, and it has a peak value of about 4.2 × 1020 m−3. So, the dark regime is like a conductive channel. On the other hand, for the bright regime, the electric field is much higher, and it has a peak value of about 17 kV/cm. However, the electron density is significantly lower than that in the dark regime, and its peak value is only about 1020 m−3. Even in the dim regime, the electric field is higher than that in the dark regime, and it has a peak value of about 13 kV/cm. Because electron temperature is related to the electric field, the results obtained in the paper indicate that the brightness of the plasma plume at different regimes is mainly decided by the electron temperature rather than the electron density. Finally, when a trace amount of O2 is mixed into working gas He, the dark regime disappears, and this is believed to be due to the fast attachment of electron to O2, which results in the decrease in the residual electron density in the regime next to the nozzle.

Stepwise development of atmospheric pressure plasma jet driven by bursts of high-voltage nanosecond pulses at multi-tens MHz

This study employs the bursts of high-voltage nanosecond pulses at multi-tens MHz to drive the helium atmospheric pressure plasma jet. Such bursts are obtained by modulating a high-voltage nanosecond pulse based on the wave reflections in a coaxial cable. The development processes and mechanisms of the plasma jet are analyzed in detail based on the discharge waveforms, discharge images, gas temperature, electron density, and axial electric field. Because the time interval between adjacent pulses is much shorter than the characteristic plasma decay time, the discharge channel driven by the first pulse still has high residual electron density and conductivity when the second pulse arrives. The first discharge channel serves as an extension of the high-voltage electrode. In this case, the second discharge starts at the end of the first discharge channel and continues to propagate forward. Driven by the bursts of high-voltage nanosecond pulses, the stepwise propagation of a guided streamer along the plasma jet is observed. The characteristic of the stepwise development of the guided streamer is stable and repeatable under the same condition and does not change at different helium flow rates if the flow is laminar. Reducing the cable length results in a higher equivalent pulse frequency in the bursts and significantly increases the plasma jet length. However, an excessively high frequency will cause a rise in gas temperature and pressure fluctuation in helium flow, resulting in a reduction in the length of the laminar region and an unstable discharge.

Crystal structure of mevalonate 3,5-bisphosphate decarboxylase reveals insight into the evolution of decarboxylases in the mevalonate metabolic pathways

Mevalonate 3,5-bisphosphate decarboxylase is involved in the recently discovered Thermoplasma-type mevalonate pathway. The enzyme catalyzes the elimination of the 3-phosphate group from mevalonate 3,5-bisphosphate as well as concomitant decarboxylation of the substrate. This entire reaction of the enzyme resembles the latter half-reactions of its homologs, diphosphomevalonate decarboxylase and phosphomevalonate decarboxylase, which also catalyze ATP-dependent phosphorylation of the 3-hydroxyl group of their substrates. However, the crystal structure of mevalonate 3,5-bisphosphate decarboxylase and the structural reasons of the difference between reactions catalyzed by the enzyme and its homologs are unknown. In this study, we determined the X-ray crystal structure of mevalonate 3,5-bisphosphate decarboxylase from Picrophilus torridus, a thermoacidophilic archaeon of the order Thermoplasmatales. Structural and mutational analysis demonstrated the importance of a conserved aspartate residue for enzyme activity. In addition, although crystallization was performed in the absence of substrate or ligands, residual electron density having the shape of a fatty acid was observed at a position overlapping the ATP-binding site of the homologous enzyme, diphosphomevalonate decarboxylase. This finding is in agreement with the expected evolutionary route from phosphomevalonate decarboxylase (ATP-dependent) to mevalonate 3,5-bisphosphate decarboxylase (ATP-independent) through the loss of kinase activity. We found that the binding of geranylgeranyl diphosphate, an intermediate of the archeal isoprenoid biosynthesis pathway, evoked significant activation of mevalonate 3,5-bisphosphate decarboxylase, and several mutations at the putative geranylgeranyl diphosphate–binding site impaired this activation, suggesting the physiological importance of ligand binding as well as a possible novel regulatory system employed by the Thermoplasma-type mevalonate pathway.

Decision problem with high residual electron density on the metal atom

It is shown that replacing the experimental electron density with the theoretical one on the terbium atom describes the experimental topology in the coordination sphere with high accuracy. Such a replacement is fundamentally important in the case of high residual electron density on a heavy metal atom.

On the transfer of theoretical multipole parameters for restoring static electron density and revealing and treating atomic anharmonic motion. Features of chemical bonding in crystals of an isocyanuric acid derivative

The crystal and electronic structure of an isocyanuric acid derivative was studied by high-resolution single-crystal X-ray diffraction within the Hansen–Coppens multipole formalism. The observed deformation electron density shows signs of thermal smearing. The experimental picture meaningfully assigned to the consequences of unmodelled anharmonic atomic motion. Straightforward simultaneous refinement of all parameters, including Gram–Charlier coefficients, resulted in more significant distortion of apparent static electron density, even though the residual density became significantly flatter and more featureless. Further, the method of transferring multipole parameters from the model refined against theoretical structure factors as an initial guess was employed, followed by the subsequent block refinement of Gram–Charlier coefficients and the other parameters. This procedure allowed us to appropriately distinguish static electron density from the contaminant smearing effects of insufficiently accounted atomic motion. In particular, some covalent bonds and the weak π...π interaction between isocyanurate moieties were studied via the mutual penetration of atomic-like kinetic and electrostatic potential φ-basins with complementary atomic ρ-basins. Further, local electronic temperature was applied as an advanced descriptor for both covalent bonds and noncovalent interactions. Total probability density function (PDF) of nuclear displacement showed virtually no negative regions close to and around the atomic nuclei. The distribution of anharmonic PDF to a certain extent matched the residual electron density from the multipole model before anharmonic refinement. No signs of disordering of the sulfonyl group hidden in the modelled anharmonic motion were found in the PDF.

Partial Order–Disorder Transformation of Interpenetrated Porous Coordination Polymers

Controlling interpenetration and flexibility behaviors is intriguing and fundamental for porous coordination polymers. We report exceptional interpenetration behaviors involving controllable partia...

On the upper limit of laser intensity attainable in nonideal vacuum

The upper limit of the laser field strength in a perfect vacuum is usually considered as the Schwinger field, corresponding to ∼ 10 29 W / cm 2 . We investigate such limitations under realistic nonideal vacuum conditions and find that intensity suppression appears starting from 10 25 W / cm 2 , showing an upper threshold at 10 26 W / cm 2 level if the residual electron density in chamber surpasses 10 9 cm − 3 . This is because the presence of residual electrons triggers the avalanche of quantum electrodynamics cascade that creates copious electron and positron pairs. The leptons are further trapped within the driving laser field due to radiation reaction, which significantly depletes the laser energy. The relationship between the attainable intensity and the vacuity is given according to particle-in-cell simulations and theoretical analysis. These results answer a critical problem on the achievable light intensity based on present vacuum conditions and provide a guideline for future hundreds of petawatt class laser development.

Synthesis and structure of 4-{[(E)-(7-meth-oxy-1,3-benzodioxol-5-yl)methyl-idene]amino}-1,5-dimethyl-2-phenyl-2,3-di-hydro-1H-pyrazol-3-one.

In the title compound, C20H19N3O4, the dihedral angles between the central pyrazole ring and the pendant phenyl and substituted benzene rings are 50.95 (8) and 3.25 (12)°, respectively, and an intra-molecular C-H⋯O link generates an S(6) ring. The benzodioxolyl ring adopts a shallow envelope conformation with the methyl-ene C atom as the flap. In the crystal, the mol-ecules are linked by non-classical C-H⋯O inter-actions, which generate a three-dimensional network. Solvent-accessible voids run down the c-axis direction and the residual electron density in these voids was modelled during the refinement process using the SQUEEZE algorithm [Spek (2015 ▸). Acta Cryst. C71, 9-18] within the structural checking program PLATON.

HAR, TAAM and BODD refinements of model crystal structures using Cu Kα and Mo Kα X-ray diffraction data

The Independent Atom Model (IAM) of electron density is used in routine X-ray data analysis. However, this model does not give a quantitative description of the electron-density distribution. A better model that allows for modelling of aspherical charge density deformations is introduced by the Hansen–Coppens variant of the multipole model of electron density. However, the application of this model requires crystals of excellent quality and high-resolution XRD data which are quite often difficult criteria to fulfil. Therefore, Mo Kα and Cu Kα data of three model compounds (tricyclic imide, xylitol and methyluracil) were refined using IAM and new methods which enabled the refinement and reconstruction of charge density based on the Cu Kα data. These methods were the Bond-Oriented Deformation Density (BODD) model, Hirshfeld Atom Refinement (HAR) and the Transferable Aspherical Atom Model (TAAM). The final results were compared to the model obtained from neutron diffraction experiments. Our results demonstrated not only that Cu Kα data may be refined using BODD, HAR and TAAM methods, but also revealed systematic errors arising from the use of Cu Kα data. These errors were a result of the limited information in the low-resolution data set that manifested as higher values for the anisotropic displacement parameters (ADPs) and smaller maxima and minima of the residual electron density for the Cu Kα data compared to the Mo Kα data. Notably, these systematic errors were much less significant than those found for the IAM. Therefore, the application of BODD, HAR and TAAM on Cu Kα data has a more significant influence on the final results of refinement than for the Mo Kα data.

Discharge characteristics and mechanism of plasma plume generated by atmospheric pulsed discharge

Atmospheric pressure plasma plume generated by pulsed discharge is studied by experimental diagnostics and numerical simulations. It is found that the plasma plume is generated in the rising phase of pulse voltage, in which a plasma bullet propagates toward the ground electrode at a speed on the order of 10<sup>4</sup> m/s. It is also found that the electric field in the vicinity of the plasma bullet reaches 10<sup>6</sup> V/m, indicating that the formation of plasma bullet can be attributed to the localized enhanced electric field, which will be enhanced near to the grounded electrode. The spatiotemporal evolution of electron density in the discharge reveals that the residual electron density remains after the plasma bullet has passed through, which explains the tailing phenomenon of plasma bullet. The enhanced electron generation rate at the head of plasma bullet corresponds to the localized enhanced electric field, which explains the generation mechanism of plasma bullet. This study of the characteristics and mechanism of plasma bullet provides a theoretical basis for developing the atmospheric plasma plume generated by pulsed discharge.

Dependence of phantom current in a metal vapor laser on electrode geometry

It is observed that the discharge current in a copper vapor laser rises prior to the voltage breakdown. This current, which appears almost 50 ns before the discharge breakdown and is equivalent to almost 70% of the peak current at the time of breakdown, is termed the phantom current. It is reported that by reducing the residual electron density (phantom current) in a metal vapor laser (MVL), its performance can be significantly improved. Metallic pins are used in the electrode to facilitate discharge in the long discharge tube. Besides augmenting the discharge during the initial stage, it has a significant impact on the laser performance. In this paper, we find out the correlation between the phantom current and the electrode pin geometry and then present the optimum electrode geometry for improved laser performance. It was observed that the number of pins in the electrode affects the localized electric field in the nearby region and plays a dominant role in the quantum of the phantom current in the discharge tube of the laser. The MVL system was tested with electrodes that had a different number of pins (zero, eight and 36) and it was found that the phantom current is minimum when the electrode has zero pins and highest with an eight-pin electrode.

Bis(μ4-adamantane-1,3-di-carboxyl-ato-1κO 1:2κO 1':3κO 3:4κO 3')octa-carbonyl-1κ2 C,2κ2 C,3κ2 C,4κ2 C-tetra-kis-[tris-(4-methyl-phen-yl)phosphane]-1κP,2κP,3κP,4κP-tetra-osmium(I)(2 Os-Os).

The title complex, [{Os2(CO)4(C21H21P)2}2(C12H14O4)2], is a centrosymmetric mol-ecular loop consisting of two Os-Os sawhorse units linked by two adamantane di-carboxyl-ato bridges. It was synthesized by the microwave-mediated reaction between Os3(CO)12 and adamantane-1,3-di-carb-oxy-lic acid. In contrast to the related complex [{Os2(CO)6}2(μ4-adamantane-1,3-di-acetate)2], the metal-metal axes within each mol-ecule are oriented parallel rather than perpendicular to one another. The crystal structure exhibits cavities that contain residual electron density peaks, but it was not possible to unambiguously identify the solvent therein. The contribution of the disordered solvent mol-ecules to the scattering was removed using the SQUEEZE (Spek (2015 ▸). Acta Cryst. C71, 9-18) routine in PLATON [Spek (2020 ▸). Acta Cryst. E76, 1-11]. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

The Crystal Structures and Raman Spectra of Three New Hydrothermally Synthesized Copper–Zinc–Oxotellurates(IV)

Abstract. Three new copper–zinc–tellurites, Zn4Cu(TeIVO3)4Cl2, Cu2Zn2(TeIVO3)2(SO4)(OH)2·H2O and Cu2Zn(TeIVO4)(SO4)·H2O (henceforth I, II and III), were synthesized under mild hydrothermal conditions (473 K, in Teflon‐lined steel vessels). They were characterized in detail by a combination of crystal‐structure determination (using single‐crystal X‐ray diffraction data), single‐crystal micro‐Raman spectroscopy and chemical analyses (energy‐dispersive X‐ray spectroscopy in a scanning electron microscope). Each compound crystallizes in a new structure type, and additionally, II and III represent the first two ever reported copper–zinc–tellurite–sulfates. I [systematic name: tetrazinc copper(II) tetrakis‐oxotellurate(IV) dichloride] is triclinic, P1, and forms a framework structure based on ZnO6 and ZnO5Cl octahedra, linked into sheets connected via Jahn–Teller‐distorted CuO4Cl2 octahedra, with TeIVO3 trigonal pyramids and TeIV2O6 dimers (composed of two edge‐sharing TeIVO4 disphenoids) filling the remaining space. II [dicopper(II) dizinc bis‐oxotellurate(IV) oxosulfate(VI) bis‐hydroxide monohydrate] is trigonal, R3m, with a simonkolleite‐like framework. Distinct layers formed from (Cu,Zn)φ6 (φ = O, OH) octahedra and TeIVO3 trigonal pyramids extend parallel to (001) and sandwich disordered SO42– anions and H2O groups. III [dicopper(II) zinc oxotellurate(IV) oxosulfate(VI) hydrate] is orthorhombic, Pnma, and also has a layered structure [extending parallel to (100)]. Positively charged layers of composition [Cu2ZnTeIVO4]2+ (containing Te as TeIVO4 disphenoids) alternate with SO42– anions and H2O groups in the interlayer space. Stacking disorder caused by the order‐disorder nature of the crystal structure is reflected by the presence of residual electron density in difference‐Fourier maps and the structure was refined as an overlay of two stacking possibilities.

Comparison and evaluation of a bottom-up GPS-RO electron density retrieval for D and E regions using radar observations and models

The lack of reliable global E-region electron density or conductivity measurements has hampered a detailed and quantitative understanding of the E-region electrodynamic processes. The characterization of the global E-region ionospheric irregularities as a function of local time, longitude, and season remains a great challenge. In this study, we evaluate the electron densities retrieved by Wu (2018) in the D and E regions using ground-based radar data and empirical models. Unlike in the conventional top-down retrieval methodology, the new approach avoids the Abel weighting function that carries substantial sensitivity to the electron density residuals from the F-region even after they have been corrected. The new technique uses a bottom-up approach in which the F-region contributions in the excess phase are more effectively removed compared to the Abel inversion, and hence more accurate electron density retrievals in D and E regions are determined. The results are compared and evaluated with two low and mid-latitude incoherent scatter radar observations — the Arecibo Observatory’s 430 MHz radar, located in Puerto Rico (18∘N, 67∘W) and the Millstone Hill Observatory’s 440 MHz radar, located in Massachusetts (42∘N, 72∘W). In addition, the results are also compared with two models — the International Reference Ionosphere model (IRI-2016) and the Faraday International Reference Ionosphere model (FIRI-2018) at these two locations. The comparison of electron density height profiles from GPS-RO retrievals at the two locations show good agreement with radar measurements, as well as the FIRI-2018 model predictions, especially at ∼ 85–100 km. The seasonal and diurnal climatology comparisons of GPS-RO retrieved electron density show good agreement with radar observations at both locations, and also reflect the influence of the solar cycle on E-region electron density. It is noticed that the GPS-RO background electron density at Millstone Hill increases by ∼ 20% at around 105 km from solar minimum to the maximum during cycle 24 which is in agreement with radar observations and FIRI-2018 model predictions.