Electrifying Fruit Juice: Integrating Applied Electroanalytical Chemistry into the Undergraduate Curriculum.

Electroanalytical chemistry has been advanced through portable devices, providing methods and sensors for the detection of analytes with high sensitivity and accuracy. This subfield of electrochemistry has the potential to be utilized in industry and analytical quality control, in general. This results in an increasing demand for trained personnel, capable of operating benchtop and portable electroanalytical equipment. Although electrochemical techniques are routinely taught in theoretical undergraduate courses, they need to be more often incorporated into experimental didactic activities. Herein, we describe the application of an effective, hands-on, and low-maintenance experiment that can enhance the learning experience of electroanalytical methods and their use in industrial quality control settings. This activity is based on the detection of ascorbic acid (vitamin C) by employing cyclic voltammetry at unmodified glassy carbon electrodes (GCE) in real juice samples. This didactic experiment teaches students about the theoretical concepts of cyclic voltammetry, three-electrode cell setup, chemical reversibility, data treatment, and quantitative analysis. This teaching approach was conducted in a second-year analytical chemistry course and was easily adapted to social distancing measures imposed by the COVID-19 pandemic.

LiNbO3 -Type Polar Antiferromagnet InVO3 Synthesized under High-Pressure Conditions.

The ambient pressure cation disordered InVO3 bixbyite has been predicted to form a GdFeO3 -type perovskite phase under high pressure and high temperature. Contrary to the expectation, InVO3 was found to crystallize in the polar LiNbO3 -type structure with a calculated spontaneous polarization as large as 74 µC cm-2 . Antiferromagnetic coupling of V3+ magnetic moments and a cooperative magnetic ground state below about 10 K coupled with a polar structure suggest an intriguing ground state of the novel LiNbO3 -type high-pressure InVO3 structure.

Understanding the Interplay of Vacancy, Cation, and Charge Ordering in the Tunable Sc2VO5+δ Defect Fluorite System.

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal Sc2VO5+δ defect fluorite superstructure previously thought to be the oxygen precise A3+2B4+O5 phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. Ex situ and in situ powder X-ray and neutron diffraction analyses reveal vanadium disproportionation reactions. The structure-reaction map illustrates the oxygen-dependent competition between the tetragonal cation and anion ordered Sc2VO5+δ and the disordered cubic Sc2VO5+δ' (δ < δ' ≤ 0.5) phases as a function of temperature. Oxidation states and oxide stoichiometries were determined with DC magnetometry and XANES experiments. The tetragonal cation ordered Sc2VO5+δ phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V3+ and V4+ cations occupy octahedral sites, whereas V5+ predominantly occupies a tetrahedral site. The paramagnetic 8g{V3+/4+}4 clusters are isolated by diamagnetic 2cV5+ cations. At temperatures below 500 °C the 8g{V3+/4+}4 clusters can be topotactically fine-tuned with varying V3+/V4+ ratios. Above 600 °C the tetragonal structure oxidizes to the cubic Sc2VO5+δ' fluorite phase-its disordered competitor. The investigation of the cation- and anion-ordered Sc-V-O phases, their formation, and thermal stability is important for the design of low-temperature solid state oxide ion conductors and vacancy structures.

The Highly Frustrated 5d2 Double Perovskite Doppelgängers, SrLaMgReO6 and SrLaLiOsO6. A Comparison including Isostructural La2LiReO6.

The synthesis and characterization of the double perovskite SrLaLiOsO6 is presented. It is isostructural (P21/n) and isoelectronic (5d2) with SrLaMgReO6, which has been reported previously. The cell volumes are the same to within 1.4%: i.e., these perovskites are doppelgängers. In a previous study SrLaMgReO6 showed no sign of spin order to 2 K. New data at lower temperatures disclose a maximum in the dc susceptibility near 1.5 K. As the Curie-Weiss (C-W) temperature (Θ) for this material is -161 K, an enormous frustration index, f ≈ 100, is implied (f = |Θ|/Tord). On the other hand, SrLaLiOsO6 does not follow the C-W law over the investigated susceptibility range, 2-300 K. Fitting with an added temperature independent term (TIP) gives µeff = 1.96 µB, Θ = -102 K, and TIP = 1.01 × 10-3 emu/mol. A clear zero-field-cooled (ZFC), field-cooled (FC) divergence in the dc data occurs at ∼10 K, suggesting a much reduced frustration index, f ≈ 10, relative to SrLaMgReO6. The real part of the ac susceptibility data, χ'max, shows a frequency shift that is consistent with a spin glass ground state according to the Mydosh criterion. Heat capacity data for SrLaLiOsO6 show no sign of a λ peak at 10 K and a linear dependence on temperature below 10 K, also supporting a spin glass ground state. A spin frozen ground state for SrLaMgReO6 could not be established from χ' data due to a much weaker signal. Nonetheless, the 10-fold difference in f between these doppelgänger materials is remarkable. It is possible that the enhanced covalency with the oxide ligands for Os6+ relative to Re5+ plays a major role here. Finally, a comparison with isostructural La2LiReO6 (with a much smaller f ≈ 4) is made and a correlation between the frustration level and the sense of the local distortion of the Re(Os)-O octahedron is pointed out.

Structural Competition and Reactivity of Rare-Earth Oxide Phases in Y xPr2- xO3 (0.05 ≤ x ≤ 0.80).

We report, for the first time, members of the Y xPr2- xO3 system with non-bixbyite or defect fluorite structures. The synthesis, structure, phase transitions, and high temperature reactivity of the trigonal A-type and monoclinic B-type structures are reported along with those of the cubic C-type phase (bixbyite). Combined powder X-ray and neutron diffraction Rietveld refinements are used to report structural details of all three reported phases. Phase transitions are investigated, showing a clear dependence on average cation size. Using neutron diffraction, phase transitions are followed in situ, revealing that all high temperature phases are quenchable. In-situ powder X-ray diffraction experiments in flowing oxygen allow insights into mechanistic details of redox processes in the reported phases. In contrast to the C-type cubic bixbyite, the trigonal A-type and monoclinic B-type structures do not allow for topotactic oxygen uptake, displaying instead a phase transition to either the bixbyite C-type capable of accommodating additional oxide anions or the direct oxidation to the cubic defect fluorite structure. The findings reported here agree with the accepted lanthanide sesquioxide phase diagrams and provide exceptional control of phases. The work is important for the prediction of structures, and the synthetic control needed for rational design of functional materials.

Structure Evolution and Reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.0) System.

Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that directly convert chemical energy of fuels into electricity with high efficiency. Because of their fuel flexibility, low emissions, high conversion efficiency, no moving parts, and quiet operation, they are considered as a promising energy conversion technology for low carbon future needs. Solid-state oxide and proton conducting electrolytes play a crucial role in improving the performance and market acceptability of SOFCs. Defect fluorite phases are some of the most promising fast oxide ion conductors for use as electrolytes in SOFCs. We report the synthesis, structure, phase diagram, and high-temperature reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.00) oxide defect model system. For all Sc(2- x)V xO3.0 phases with x ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for x ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < x < 1.68. Structural details obtained from the simultaneous Rietveld refinements using powder neutron and X-ray diffraction data are reported for the bixbyite phases, demonstrating a slight V3+ preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the Sc(2- x)V xO3.0 phases. In all cases ScVO4 was found as a final product, accompanied by Sc2O3 for x < 1.0 and V2O5 when x > 1.0; however, the oxidative pathway varied greatly throughout the series. Comments are made on different synthesis strategies, including the effect on crystallinity, reaction times, rate-limiting steps, and reaction pathways. This work provides insight into the mechanisms of solid-state reactions and strategic guidelines for targeted materials synthesis.

Order/Disorder and in Situ Oxide Defect Control in the Bixbyite Phase YPrO3+δ (0 ≤ δ < 0.5).

The YPrO3+δ system is a nearly ideal model system for the investigation of oxide defect creation and annihilation in oxide ion conductor related phases with potential applications as solid state electrolytes in solid oxide fuel cells. The formation, structure, high temperature reactivity, and magnetic susceptibility of phase pure YPrO3+δ (0 ≤ δ ≤ 0.46) are reported. The topotactic reduction and oxidation of the YPrO3+δ system was investigated by powder X-ray in situ diffraction experiments and revealed bixbyite structures (space group: Ia3̅) throughout the series. Combined neutron and X-ray data clearly show oxygen uptake and removal. The research provides a detailed picture of the Y(3+)/Pr(3+)/Pr(4+) sublattice evolution in response to the redox chemistry. Upon oxidation, cation site splitting is observed where the cation in the ((1)/4, (1)/4, (1)/4) position migrates along the body diagonal to the (x, x, x) position. Any oxygen in excess of YPrO3.0 is located in the additional 16c site without depopulating the original 48e site. The in situ X-ray diffraction data and thermal gravimetric analysis have revealed the reversible topotactic redox reactivity at low temperatures (below 425 °C) for all compositions from YPrO3 to YPrO3.46. Magnetic susceptibility studies were utilized in order to further confirm praseodymium oxidation states. The linear relation between the cubic unit cell parameter and oxygen content allows for the straightforward determination of oxygen stoichiometry from X-ray diffraction data. The different synthesis strategies reported here are rationalized with the structural details and the reactivity of YPrO3+δ phases and provide guidelines for the targeted synthesis of these functional materials.

Topotactic Solid-State Metal Hydride Reductions of Sr2MnO4.

We report novel details regarding the reactivity and mechanism of the solid-state topotactic reduction of Sr2MnO4 using a series of solid-state metal hydrides. Comprehensive details describing the active reducing species are reported and comments on the reductive mechanism are provided, where it is shown that more than one electron is being donated by H(-). Commonly used solid-state hydrides LiH, NaH, and CaH2, were characterized in terms of reducing power. In addition the unexplored solid-state hydrides MgH2, SrH2, and BaH2 are evaluated as potential solid-state reductants and characterized in terms of their reductive reactivities. These 6 group I and II metal hydrides show the following trend in terms of reactivity: MgH2 < SrH2 < LiH ≈ CaH2 ≈ BaH2 < NaH. The order of the reductants are discussed in terms of metal electronegativity and bond strengths. NaH and the novel use of SrH2 allowed for targeted synthesis of reduced Sr2MnO(4-x) (0 ≤ x ≤ 0.37) phases. The enhanced control during synthesis demonstrated by this soft chemistry approach has allowed for a more comprehensive and systematic evaluation of Sr2MnO(4-x) phases than previously reported phases prepared by high temperature methods. Sr2MnO3.63(1) has for the first time been shown to be monoclinic by powder X-ray diffraction and the oxidative monoclinic to tetragonal transition occurs at 450 °C.

Platinum uptake and Ba2CePtO6 formation during in situ BaCe(1-x)M(x)O(3-δ) (M = La, In) formation.

We report the formation, structures, temperature-dependent phase transitions, and high-temperature reactivity of the potential proton and oxide ion conductors BaCe(1-x)M(x)O3 (M(3+) = In(3+), La(3+)). The present in situ diffraction studies show oxidative platinum uptake at temperatures as low as 950 °C into BaCeO3, forming the cubic Ba2CePtO6 double perovskite. The transient B-site double perovskite expels platinum at around 1200-1250 °C. Platinum oxidation via BaCeO3 is investigated by in situ powder X-ray and neutron diffraction experiments in various atmospheres. Doped BaCe(1-x)M(x)O3 phases show the formation of Ba2CePtO6 without incorporating the M(3+) dopant. Oxidative platinum uptake is also observed during the synthesis of BaCeO3 on platinum metal. We report the reaction pathway for the low-temperature oxidative formation of Ba2CePtO6 and the subsequent liberation of platinum for the barium cerate system. The findings are supported by ambient-temperature X-ray diffraction, in situ powder X-ray, and powder neutron diffraction as well as XPS.

Absence of magnetic ordering in the spin liquid candidate Ca3Cu2GeV2O12.

Typically, quantum spin liquid candidates can be found in materials with a combination of geometrical frustration along with low spin. Due to its spin of S = 1/2 the copper (II) ion is often present in the discussion on spin liquid candidates. The solid state compound Ca3Cu2GeV2O12 is a material that crystallizes in the garnet structure (s.g. #230, Ia-3d), where 3D frustration is known to occur. Heat capacity has shown a lack of magnetic ordering down to 0.35 K, confirmed with low temperature neutron diffraction to 0.07 K. This system displays a Weiss temperature of -0.93(1) K indicating net antiferromagnetic interactions and significant J 1-J 2 competition causing frustration. Using both neutron and x-ray diffraction along with heat capacity and magnetometry, the work presented here shows Ca3Cu2GeV2O12 has potential as a new spin liquid candidate.

Lack of a threefold rotation axis in α-Fe2O3 and α-Cr2O3 crystals.

The crystal structure of α-Fe2O3 and α-Cr2O3 is usually described with the corundum-type trigonal crystal structure based on the space group R3¯c. There are, however, some observations of the magnetic ordering of both α-Fe2O3 and α-Cr2O3 that are incompatible with the trigonal symmetry. We show experimental evidence based on X-ray powder diffraction and supported by transmission electron microscopy that the symmetry of the crystal structure of both α-Fe2O3 and α-Cr2O3 is monoclinic and it is described with the space group C2/c (derived from R3¯c by removing the threefold rotation axis). The magnetic orderings of α-Fe2O3 and α-Cr2O3 are compatible with the magnetic space groups C2/c and C2/c', respectively. These findings are in agreement with the idea from Curie [(1894), J. Phys. 3, 393-415] that the dissymmetry of the magnetic ordering should be associated with a dissymmetry of the crystal structure.