Rhombohedral Polymorph Research Articles

Theoretical investigation of the thermal conductivity of Ga2O3 polymorphs

Gallium oxide (Ga2O3) is a promising thermal preserving and heat-insulating material; understanding the thermal properties is important to improve its performance in technological applications. The thermal conductivities of Ga2O3 polymorphs labeled as α, β, δ, and ε are computed via the Boltzmann phonon transport equation (BTE) employing first-principles techniques. The lattice thermal conductivity tensor k of Ga2O3 for temperatures ranging from 50 K to 1000 K is derived using the second and third-order interatomic force constants (IFCs) for the potential based on a generalized gradient approximation (GGA), as well as the phonon dispersion relation, projected density of states (PDOS), and phonon group velocities. The results agree with the observed experimental values of rhombohedral polymorph β-Ga2O3 and with the previously calculated results of the other phases. At room temperature, the predicted thermal conductivity of the δ-Ga2O3 phase is 15.6 W/(m•K). By breaking down k into mode contributions, it is projected that the optical phonons contribute significantly to the lattice thermal conductivity because of a peculiar phonon dispersion relation.

Exploring the ambient metastability of yttrium substituted Bi2O3 electrolyte materials

The phase stability of (Bi1-xYx)2O3 (0.1 ≤ x ≤ 0.275) solid state electrolytes synthesized using the sol-gel method was investigated over prolonged periods under ambient conditions. The cubic polymorph was found to be metastable under ambient conditions, particularly for 0.1 ≤ x ≤ 0.15 where significant cubic-to-tetragonal transformation occurred. Spontaneous minor formation of both the rhombohedral polymorph and a bismuth subcarbonate phase was evident across the doping range but with varied reproducibility. Aging was significantly more prevalent for samples that had only been calcined (at 450 °C) as compared to those that were subsequently annealed (at 750 °C). The more highly substituted annealed materials (0.25 ≤ x ≤ 0.275) were initially phase pure cubic and far more stable, yet some rhombohedral and bismuth subcarbonate phase formation was also seen. Although the initial source of carbon which leads to the formation of the subcarbonate phase is not known at this stage, several suggestions are made. Pelletisation with additional sintering, as well as annealing powder samples under an inert atmosphere, was also shown to enhance the long-term phase stability characteristics of the cubic phase. This is the first longer term (over 2 years) ambient phase aging study of substituted bismuth oxides.

The pressure-stabilized polymorph of indium triiodide.

A layered rhombohedral polymorph of indium(III) triiodide is synthesized at high pressure and temperature. The unit cell symmetry and approximate dimensions are determined by single crystal X-ray diffraction. Its R3̄ crystal structure, with a = 7.214 Å, and c = 20.47 Å, is refined by the Rietveld method on powder X-ray diffraction data. The crystal structure is based on InI6 octahedra sharing edges to form honeycomb lattice layers, though with considerable stacking defects. Different from ambient pressure InI3, which has a monoclinic molecular structure and a light-yellow color, high pressure InI3 is layered and has an orange color. The band gaps of both the monoclinic and rhombohedral variants of InI3 are estimated from diffuse reflectance measurements.

Carbonates of alkaline earth metals: Impact of the cation size on strengths of interionic interactions and polymorphic preferences

The energies of all types of pair-wise interionic interactions between atoms of the representative (MCO3)13 clusters sampled from X-ray structures of rhombohedral magnesite (M = Mg) and calcite (M = Ca), on one hand, and orthorhombic aragonite (M = Ca) and cerussite (M = Pb), on the other hand, are evaluated theoretically to ascertain how the size of a divalent metal cation governs the preferable existence of rhombohedral or (and) orthorhombic polymorph (polymorphs) at ambient temperature and pressure. The carbonates studied and their clusters are considered as those consisting of (i) columns of carbonate anions (CAs) separated by trios of cations, (ii) ribbons and layers of CAs, and (iii) rods and layers of cations. For each cluster, coordination energies of six CAs by a cation and six cations by a CA are evaluated. Energies of interactions of C and O atoms of columns and ribbons with cations and energies of inter-anionic C···O, O···O, and C···C interactions in column and ribbons of clusters are compared. Strengths of cation – cation repulsions are also evaluated. Analyses of the interaction energies, interionic distances in the carbonates studied, and thermodynamic properties of carbonates of alkaline earth metals allow one to understand how the size of a particular divalent metal cation affects relative thermodynamic stabilities and the very existence of its rhombohedral or (and) orthorhombic crystalline phase (phases).

The T2 structure of polycrystalline cubic human insulin.

The polymorphism of human insulin upon pH variation was characterized via X-ray powder diffraction, employing a crystallization protocol previously established for co-crystallization with phenolic derivatives. Two distinct rhombohedral (R3) polymorphs and one cubic (I213) polymorph were identified with increasing pH, corresponding to the T6, T3R3f and T2 conformations of insulin, respectively. The structure of the cubic T2 polymorph was determined via multi-profile stereochemically restrained Rietveld refinement at 2.7 Å resolution. This constitutes the first cubic insulin structure to be determined from crystals grown in the presence of zinc ions, although no zinc binding was observed. The differences of the polycrystalline variant from other cubic insulin structures, as well as the nature of the pH-driven phase transitions, are discussed in detail.

Evaluation of Structural and Functional Properties of La0.6Sr0.4MnO3 Perovskite Prepared by the Fast Solution Combustion Approach

A series of La0.6Sr0.4MnO3 (LSM) perovskite was made using the rapid solution combustion method, which was calcined by varying the temperatures. In order to determine how the calcination temperature affected the nanopowders produced and calcined at various temperatures, their microstructural, morphological, compositional, optical, and electrical properties were analyzed using corresponding characterization tools. The XRD results showed the coexistence of the rhombohedral polymorphs R-3c and Pm-3m for the perovskite phase under a calcination temperature of 1400 °C, which were eliminated with increased calcination temperature. The average grain size was found to increase with increasing calcination temperature. The EDS analysis showed better agreement of the stoichiometry with the theoretical composition. The apparent porosity decreased with increasing temperature due to the coalescence of sintering pores. The sample obtained after calcination at 1500 °C showed 10.3% porosity. The hardness also improved with increasing calcination temperature and reached a maximum value of 0.4 GPa, which matched the bulk density. A similar trend was observed in the resistivity studies as a function of temperature, and all the samples exhibited a low resistivity of ~1.4 Ω·cm in the temperature range of 500–600 °C. The optical characterization showed broad absorption at 560–660 nm and bandwidth values between 3.70 and 3.95 eV, according to the applied heat treatment.

Biphasic BiFeO3 ceramics based on rhombohedral and tetragonal polymorphs

ABSTRACT From a new perspective, polymorphism is now often considered as a functional property that extends the applications of many materials. BiFeO3 is still an interesting material from both fundamental and applied points of view. A unique characteristic of BiFeO3 film, the polymorphism was stabilised only using the epitaxial strain caused by the substrate of the film. Here we report the hydrothermal synthesis of biphasic BiFeO3 ceramics using low NaOH concentration and temperature. The ability of BiFeO3 ceramics to morph into tetragonal and rhombohedral allotropic phases was demonstrated, confirming the theoretical prediction that the polymorphism can be stabilised even in powder form. Based on our experimental study NaOH(aq) is proposed as responsible for the strain on the structure of the rhombohedral phase. In addition, the in-depth characterisation of this biphasic ceramic opens up new opportunities for the technological applications of BiFeO3 material.

On the mechanism of Mn(II)-doping in Scandia stabilized zirconia electrolytes

Cubic Scandia-stabilized zirconia (ScSZ) is an attractive electrolyte material for solid oxide cells due to its significant ionic conductivity, provided that the phase transition to its rhombohedral polymorph upon cooling is suppressed. The latter is achieved with addition of a secondary co-dopant, albeit it may be at the detriment of its ionic conductivity Here, we thoroughly investigate how MnO2 (0.5–10 mol%) as a co-dopant impacts on the sinterability, thermal expansion, crystal structure and ionic conductivity of ZrO2 doped with 10 mol% Scandia (10ScSZ), and we provide new insight on the chemistry of dissolved manganese in the fluorite lattice. Reactive sintering of 2 mol% MnO2 mixed with 10ScSZ enables to produce dense electrolyte with significant reduction of the peak sintering temperature and stabilisation of the cubic structure down to room temperature. Combined density functional theory and X-ray photoelectron spectroscopy analyses reveal that manganese predominantly enters the structure as Mn2+ during reactive sintering, with a prevalence of higher valence states at the surface and grain boundaries. The highest oxide ion conductivity is achieved for 2 mol% doped 10ScSZ (120 mScm−1 at 800 °C) and it decreases with increasing Mn concentration. For all compositions, the bulk conductivity remains independent of pO2 – corroborating a limited electronic conductivity contribution from Mn-doping. The grain boundary conductivity is found to decrease with sintering time and pO2, which is attributed to the chemistry and concentration of segregated manganese at the surface and grain boundaries, yielding depletion of oxygen vacancies in the space charge layer.

Polymorphism of boron phosphide: theoretical investigation and experimental assessment

Stable crystal structures of wurtzite and recently discovered rhombohedral polymorphs of boron phosphide were obtained based on crystal chemistry rationale and unconstrained geometry optimization calculations within density functional theory.

Electrochemical Ion Exchange in Nasicon Based Solid Electrolytes

Solid electrolytes form the crux of the all-solid-state batteries. Garnets, sulfides, and NASICONs are some of the most promising solid electrolyte material families. An ideal solid electrolyte should have higher ionic conductivity, higher mechanical strength, chemically and electrochemically stable with Li-anode and cathode, and environmentally benign. In the search for a reliable solid electrolyte, researchers are revisiting NASICON material families which have a wide compositional variety [1-2]. Properties of these materials can be easily tuned with substitution and replacement of cations and anions. In this work, we are exploring AM1M2(XO4)3 (where M1=Mn, M2=Zr, X=P) NASICONs for Li-ion based ASSBs. Rhombohedral polymorph of this composition provides the highest ionic conductivity. However, the synthesis of a Li-based NASICON in this composition is not a straightforward through conventional methods.In this work, we have investigated the capability of Na1+xMnx/2Zr2-x/2(PO4)3 (NMZP), which crystallizes in R-3C space group, being able to conduct Li-ions. NMZP was synthesized from x=0.5 to x=2. Neutron diffraction revealed that there are two main sites for Na-ions in this structure denoted by M1 and M2. Rietveld refinement analysis shows a monotonic increase of Na occupancy and Na-O bond length increase in the M2 site with an increase in x suggesting higher concentration of mobile Na in the M2 site (Figure 1a and b). Electrochemical impedance spectroscopy (EIS) analysis revealed that there is an increase in ionic conductivity from x=0.5 to x=1.5. The ionic conductivity of compositions with x=1.5 and x=2 is approximately the same which can be attributed to vacancy-charge-imbalance. Galvanostatic plating and stripping were carried out on symmetric Na | NMZP | Na, and Li | NMZP | Li cells at 70 ⁰C. A stable polarization profile was observed for both cells at various current densities (20, 40, and 80 µA cm-2 for Na | NMZP | Na and 40, 60, and 80 µA cm-2 for Li | NMZP | Li cell) (Figure 1c). Stable polarization observed for Li | NMZP | Li suggests that Na-ions in M2 sites can be readily exchanged with Li-ion from electrodes. Additionally, Na-ions should be pushed out of the structure and platted on the Li-counter electrode. Post-mortem analysis Li-foil from Li|NMZP|Li cell was carried out with SEM after 120 hours of cycling. Li-metal foil showed the presence of Na-metal plating on Li-electrode validating this phenomenon [3]. To further corroborate our findings that NMZP can conduct Li-ions readily, post-mortem XPS analysis of Li-foil and NMZP pellet from Li | NMZP | Li cell was carried out after 120 hours of cycling. Linear scanning map of Li-metal shows the presence of Na metal from 0.15 to 2.07 at% showcasing electroplated Na out of NMZP structure. Furthermore, XPS depth profiling of the cycled NMZP pellet shows the presence of Li within the structure (225 nm depth). These experiments prove the dual-ion conduction capability of NMZP type of NASICON materials. The proof-of-concept work showcased here opens exciting avenues of material combinations which can be leveraged for both Na and Li all-solid-state batteries. References Zhou et al., Small Methods 2020, 4, 2000764.Gao et al., J. Am. Chem. Soc. 2018, 140, 51, 18192–18199Parejiya et al., ACS Energy Lett. 2021, 6, 429−436 Figure 1

Phase transformation and hydrogen storage properties of LaY2Ni10.5 superlattice alloy with single Gd2Co7-type or Ce2Ni7-type structure

Abstract The structural evolution of as-cast LaY2Ni10.5 multi-phase alloy with the annealing temperature has been investigated, and the single-phase alloy with rhombohedral Gd2Co7-type or hexagonal Ce2Ni7-type structure was firstly prepared. It is found that the rhombohedral structure is more stable than the hexagonal polymorph at the low temperature, which is reversed at the high annealing temperature. Structural refinement indicates that Y atoms preferably substitute La in the [A2B4] subunit of both hexagonal and rhombohedral superlattice structures. Compared with the rhombohedral polymorph, the hexagonal phase possesses a relatively smaller volume of [A2B4] subunit and thus demonstrates higher structural stability during the hydrogen charging-discharging cycling. This work offers the guideline for the preparation process and composition optimization of single-phase rare earth-based A2B7-type alloys for nickel-metal hydride battery applications.

HI-Light: A Glass-Waveguide-Based "Shell-and-Tube" Photothermal Reactor Platform for Converting CO2 to Fuels.

SummaryIn this work, we introduce HI-Light, a surface-engineered glass-waveguide-based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO2 hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y, catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol gcat−1 hr−1 at 300°C and atmospheric pressure. This is 20 times higher conversion rate per unit catalyst mass per unit time beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

Structural and optical properties of α-Fe2O3 nanoparticles realized by simple thermal decomposition route

Herein, we present the synthesis and thermal treatment effect of Hematite (α-Fe2O3). Hematite nanoparticles were prepared by repeated calcination of a precursor obtained from Fe(NO3)3.9H2O and Hexamethylenetetramine (HMTA). Thermal treatment of the precursor at 400 °C for 4 h, resulted in the formation of Fe3O4 nanoparticles, which on repeated calcination at 400 °C, 500 °C and 600 °C, resulted in α-Fe2O3 nanoparticles. Synthesized samples were characterized by using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy and UV–vis spectroscopy techniques. XRD result reveals the formation of cubic Fe3O4 phase at 400 °C, which is transformed into rhombohedral α-Fe2O3 polymorphs on repeated calcination. Crystallinity of the α-Fe2O3 is enhanced on repeated calcination up to 500 °C after that it gets slightly amorphized at 600 °C. Raman and FTIR results further corroborate the XRD result. Optical band gap of most crystalline α-Fe2O3, as estimated from UV–vis results, is 1.87 eV. Urbach energy of the samples has been calculated from absorption data, which shows the minimum value for most crystalline material. These findings demonstrate the close relation between structure and optical properties of α-Fe2O3.

Polymorphism in a Multicomponent Crystal System of Trimesic Acid and t-Butylamine

Polymorphism in a multicomponent crystal system of trimesic acid (TMA) and t-butylamine (TBA) with stoichiometry (TMA)2(TBA)3 is reported, with the discovery and structural characterization of two polymorphs with rhombohedral and triclinic symmetries. In each polymorph, the TBA molecules exist as protonated cations, and the two independent TMA molecules are deprotonated by loss of a total of three protons (the specific protonation state of each TMA anion depends on the interpretation of hydrogen-bond disorder in the crystal structure). Both structures are based on sheets of TMA anions arranged in a hydrogen-bonded honeycomb network; these sheets are essentially planar in the rhombohedral polymorph but corrugated in the triclinic polymorph. The TBA cations are linked by hydrogen bonding to the TMA anions in this network, with the t-butyl groups occupying the void space within the honeycomb network and the regions of space between adjacent sheets. Periodic DFT-D calculations suggest that the rhombohedral polymorph is more energetically stable than the triclinic polymorph.

Enhanced magneto-capacitance in Sr2+ modified BiFeO3–PbTiO3 solid solutions

Polycrystalline ceramics of 0.7BiFeO3-0.3 Pb(1-x)SrxTiO3 compositions with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were successfully synthesized by solid state reaction method. The structural, microstructural, ferroelectric, magnetic, dielectric and magnetocapacitive properties have been investigated. X-Ray diffraction analysis using Rietveld refinement revealed the existence of multiphase region with rhombohedral (R3c) and tetragonal (P4mm) polymorphs for x = 0, 0.2, 0.4, 0.6 & 0.8 and single phase with cubic crystallinity for x = 1.0 with Pm3‾m space group. SEM micrographs depicted dense microstructure with non-uniform grains. Except x = 0, pinched ferroelectric loops have been observed for x = 0.2, 0.4, 0.6, 0.8 and 1.0 with low remnant polarization. Sr2+ substitution enhanced the magnetization by suppressing antiferromagnetic spin cycloid of BiFeO3 and the maximum value of remnant was found to be 9.98 memu/g for x = 1.0. Room temperature dielectric studies revealed that the dielectric constant decreases with increasing Sr2+ content in the composition. The magnetoelectric coupling was measured through observing the relative change in dielectric constant induced by applied magnetic field (Magnetocapacitance: MC). The maximum value of magnetocapacitance was found to be 0.88% for the composition with x = 0.6. The magnetic field induced fractional dielectric constant can also be expressed as MC ~ γM2 using Ginzburg–Landau theory and the linear fitting gave the value of positive value of magnetoelectric interaction constant, γ = 3.79, 2.08 and 1.76 for x = 0, 0.06 and 1.0, respectively.

HI-Light: A Glass Waveguide Based 'Shell-and-Tube' Photothermal Reactor Platform for Converting CO <sub>2</sub> to Fuels

In this work, we introduce HI-Light, a surface-engineered glass waveguide based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO2 hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol gcat−1 h−1 at 300°C and atmospheric pressure. This is 20 times higher conversion rate beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions, and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

In-situ high temperature micro-Raman investigation of annealing behavior of high-pressure phases of Si

Among the 13 polymorphic phases of Si, the ambient temperature stable body-centered cubic (bc8) and rhombohedral (r8) polymorphs have gained significant interest due to their attractive optical and electronic properties suitable for photovoltaic applications. Though ex situ methods were extensively employed previously to understand the pressure-induced phase transformation kinetics of Si, the limited number of available in situ studies has significantly improved the knowledge in this field and clarified uncertainties. Similarly, in this article, we attempt to understand the thermal annealing behavior of nanoindentation-induced r8 and bc8 phases of Si and their volume dependence using in situ high temperature micro-Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). A spherical diamond indenter of ∼20-μm radius was chosen to indent diamond cubic (dc) Si (100) at different peak loads (Pmax) to create different volumes of high-pressure phases. The Raman spectra, Raman imaging, and XTEM of the pre- and postannealed indents confirm complete annealing of r8/bc8 phases at 200 ± 10 °C, irrespective of the volume of indents. In contrast to the previous ex situ studies, no signature of the presence of the hexagonal diamond (hd)-Si phase was found at elevated temperatures and the overall transformation observed is directly from r8 → polycrystalline dc-Si and bc8 → polycrystalline dc-Si rather than through other metastable phases such as Si-XIII/hd-Si. The present systematic in situ study provides evidence for a few earlier predictions and clarifies ambiguities involved in understanding the annealing behavior and transformation pathways of two high-pressure phases of Si at elevated temperatures.

Polymorph selection towards photocatalytic gaseous CO2 hydrogenation

Titanium dioxide is the only known material that can enable gas-phase CO2 photocatalysis in its anatase and rutile polymorphic forms. Materials engineering of polymorphism provides a useful strategy for optimizing the performance metrics of a photocatalyst. In this paper, it is shown that the less well known rhombohedral polymorph of indium sesquioxide, like its well-documented cubic polymorph, is a CO2 hydrogenation photocatalyst for the production of CH3OH and CO. Significantly, the rhombohedral polymorph exhibits higher activity, superior stability and improved selectivity towards CH3OH over CO. These gains in catalyst performance originate in the enhanced acidity and basicity of surface frustrated Lewis pairs in the rhombohedral form.

Impact of the coupled substitution of CaCO3 and CaSO4·2H2O on chloride binding behavior in hydrated C3A pastes

In this study, the impact of CaCO3 and CaSO4·2H2O on chloride binding in hydrated blended C3A pastes was investigated. The chloride binding capacity and pH value were obtained through equilibration method, and the phase assemblages were evaluated using XRD and Raman spectroscopy. The results showed that the exposure of the hydrated blended C3A pastes to NaCl solutions caused an increase of pH as a result of the chloride binding by the AFm phases. Both CaCO3 and CaSO4·2H2O led to the decrease of chloride binding capacity and pH value, and CaCO3 was more obvious than CaSO4·2H2O on reducing chloride binding capacity. The greater the CaCO3 content was, the more obvious the reduction was, while most reduction occurred for the CaCO3/C3A molar ratios less than 0.5. Especially, coupled incorporation of CaCO3 and CaSO4·2H2O caused a significant reduction in chloride binding capacity and pH, with binding capacity of ettringite < carboaluminate hydrates < monosulfoaluminate < C3AH6. The chloride-bearing AFm phases formed after exposure were solid solutions which presented two monoclinic and rhombohedral polymorphs at ambient temperature, and the polymorphs were dependent on the composition of blended C3A pastes and chloride concentrations. CaCO3 whether with CaSO4·2H2O or not led to the solid solutions formation with carbonate-AFm as end member. While CaSO4·2H2O showed no effect on the constitution of solid solutions, but led to the transformation of polymorphs.

Comparative Study of Electrical Conduction and Oxygen Diffusion in the Rhombohedral and Bixbyite Ln6MoO12 (Ln = Er, Tm, Yb) Polymorphs.

Electrical conduction and oxygen diffusion mobility in the bixbyite ( Ia3̅) and rhombohedral ( R3̅) polymorphs of the Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ, δ1, δ2; δ1 > δ2) heavy lanthanide molybdates, belonging to new, previously unexplored classes of potential mixed (ionic-electronic) conductors, have been studied in the range of 200-900 °C. The oxygen self-diffusion coefficient in bixbyite ( Ia3̅) Yb6MoO12-δ phase estimated by the temperature-programmed heteroexchange with C18O2 was shown to be much higher than that for rhombohedral ( R3̅) RI (with large oxygen deficiency) and ( R3̅) RII (with small oxygen deficiency) Ln6MoO12-Δ (Ln = Tm, Yb; Δ = δ1; δ1 > δ2) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er6MoO12-δ, Tm6MoO12-δ, and Yb6MoO12-δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200-500 °C. Oxygen mobility data for the rhombohedral Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) phases RI and RII indicate that the oxygen in these phases exhibits mobility at much higher temperatures, such as those above 600-700 °C. Accordingly, below 600-700 °C they have predominantly electronic conductivity. As shown by total conductivity study of Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites ( Ia3̅) and rhombohedral phases Ln6MoO12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) ( R3̅) in dry and wet air, the proton conductivity contribution exists only in Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites up to 450-600 °C and decreases with a decreasing of the lanthanide ionic radius. The obtained data on the mobility of oxygen and the presence of proton contribution in bixbyites in the 300-600 °C temperature range make it possible to confirm unequivocally that Ln6MoO12-δ (Ln = Er, Tm, Yb) bixbyites are mixed electron-proton conductors at these temperatures.