Average Cation Radius Research Articles

Synthesis and microstructure of (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ high-entropy oxides characterized by fluorite structure

In this work, single-phase (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ high-entropy fluorite oxides (HEFOs) were successfully prepared by conventional solid-phase reaction method and crystal structure and microstructure were studied in-depth. Single-phase (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ is formed at 1600 °C for 12 h, and five cationic elements are uniformly distributed without any segregation at the micrometer and nanometer scales. And the XRD patterns with Rietveld refinement reveal the formation of single-phase fluorite structure of (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O2-δ. The SAED patterns on the [0 1‾ 1] band axes are similar to the diffraction patterns of face-centered cubic fluorite structure, and the standard deviation of the cation radius (coordination VIII) distribution of the specimens is 0.1277, which is again identified as forming a single-phase fluorite structure. The bulk specimens are solid compacts free of obvious cracks with randomly distribution porosity at a rare level, and the powder specimens are irregular fragmented shapes with an average particles size of 5.41 μm. The average cation radius and interplanar spacings becomes larger compared with standard fluorite structure of CeO2 due to the cation radius of La3+, Sm3+ and Nd3+ are larger than that of Ce4+, leading to an increase in lattice parameters of the HEFOs. In addition, the concentration of Ce3+ in the Ce cation is only 8.1 %, and the δ-value is calculated to be 0.3081 based on the content of all cations per mole of the substance, which indicates that the composition of the prepared specimen is (Ce0.2Zr0.2La0.2Sm0.2Nd0.2)O1.6919.

Cation disorder and the charge ordering transition temperature in La1/3(Sr1-xCax)2/3FeO3−δ investigated by mechanical spectroscopy

The lattice effect on the charge ordering in La1/3Sr2/3FeO3−δ is addressed by calcium ion substitution. Cation disorder effect due to size differences between A-site La3+, Sr2+, and Ca2+ has been found in La1/3(Sr1-xCax)2/3FeO3−δ perovskites. Below room temperature, a charge ordering transition has been detected by temperature dependent resistivity, differential scanning calorimeter, and mechanical spectroscopy. The charge ordering transition temperature TC shows a clear dependence on the variance of the A-site cation radius distribution. This result shows an important role of local lattice distortions on the charge ordering transition. As a comparison, octahedral titling transitions above room temperature were identified by mechanical spectroscopy, and the tilting transition temperatures are sensitive to the average cation radius. Finally, the implication of the cation disorder effect is discussed by a comparison of the case in manganates.

Size disorder: A descriptor for predicting the single- or dual-phase formation in multi-component rare earth zirconates

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder ( δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy.

Experimentally and Theoretically Revealing the Co-Doping Effects of Ce-Sr in Gd2Zr2O7.

To safely dispose radioactive waste (including, e.g., thorium and radiostrontium), Ce4+ and Sr2+ were chosen as simulated surrogates of α and β waste and were introduced into the Gd3+ site in Gd2Zr2O7 to maintain the average cationic radius and to compensate for charge. A series of Gd2-xSrx/2Cex/2Zr2O7 (0.00 ≤ x ≤ 0.25) compounds were examined by experimental and theoretical calculations to investigate the co-doping effects of α and β waste in a Gd2Zr2O7-based matrix. The effects of Ce4+ and Sr2+ content on the phase, unit cell parameters, active modes, mechanical property, and microstructure were studied systematically. Moreover, the limit of incorporation of Ce4+ and Sr2+ in Gd2Zr2O7 pyrochlore and the lattice parameters were also calculated through virtual crystal approximation theory, and the results were found to well agree with experimental results.

First-Principle Studies of the Structural, Electronic, Mechanical, and Vibrational Properties of Double Carbonates with a Dolomite Structure

Density functional theory with a PBE gradient functional and a dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package are used to calculate crystal structure parameters, electronic and vibrational spectra, elastic constants of rhombohedral double carbonates М1М2 (СО3)2 (М1, М2: Mg, Ca, Mn, Cd, Zn) with a dolomite-type structure. The paper demonstrates the possibility for establishing the linear dependences for lattice constants, interatomic distances, elastic constants and polycrystalline moduli, wave numbers of individual vibrational modes on the radii of cations. The same possibility is shown for the dependences of the parameters of chemical bonds and formation energies on their electronegativities. The lattice parameters increase along with the average cation radius, but the elastic constants and moduli demonstrate their decrease. The highest compressibility of carbonates is predicted in the direction of the c axis, which is consistent with the nature of the chemical bond, where stronger carbon — oxygen bonds are present in the ab plane, and weaker metal — oxygen bonds are in the direction of the c axis. The formation energy of binary carbonates CaMg(CO3)2 produced from solid oxides and gaseous CO2 is equal to -2.82 eV. It is equal to -2.71 eV for CaCd(CO3)2, and -0.054 eV, 0.023 eV for simple carbonates, respectively. The resulting formulas can be used to assess and predict the physical properties of solid solutions of carbonates of variable composition.

Order–disorder structural tailoring and its effects on the chemical stability of (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic for nuclear waste forms

Series of unequal quantity Nd/Ce co-doped ceramic nuclear waste forms, (Gd, Nd)2(Zr, Ce)2O7, were prepared to tailor its ordered pyrochlore or disordered fluorite structure. The phase transition, microtopography, and elemental composition of the ceramic samples were systematically investigated, especially the effect of order-disorder structure on the chemical stability. It was confirmed that unequal quantity of Nd/Ce could synchronously replace the Gd/Zr-sites of Gd2Zr2O7. And the phase transition of order-disorder structure could be successfully tailored by regulating the average cationic radius ratio of (Gd, Nd)2(Zr, Ce)2O7 series. The elements of Gd, Nd, Zr, and Ce are uniformly distributed in the ordered or disordered structures. The MCC-1 leaching results showed that (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic nuclear waste forms had excellent chemical stability, whose elements' normalized leaching rates were as low as 10−4 -10−7 g‧m−2‧d−1 after 7 days. In particular, the chemical stability of disordered structure was superior to that of ordered structure. It was proposed that the force constant and the closest packing were changed with the structure transformation resulting the chemical stability difference.

Ab initio исследования влияния давления на структуру, электронные и упругие свойства карбонатов щелочных--щелочно-земельных металлов

Density functional theory with a gradient PBE functional and dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package was used to study the effect of pressure on the structural, electronic, and elastic properties of K2Ca(CO3)2, K2Mg(CO3)2, Na2Ca2(CO3)3, Na2Mg(CO3)2. The parameters of the third-order Birch-Murnaghan equation of state are determined, and it is shown that the equilibrium volume and compressibility modulus depend linearly on the average cation radius. Under pressure, the widths of the upper valence bands increase maximally in K2Ca(CO3)2 and minimally in Na2Ca2(CO3)3, while the centers of gravity of the cationic states shift by ~0.2 eV. Elastic constants and polycrystalline moduli are calculated, which increase with decreasing average cation radius. The shear modulus for K2Ca(CO3)2 and K2Mg(CO3)2 decreases with increasing pressure, and this leads to anomalous behavior of the longitudinal and transverse acoustic wave velocities.

Ab initio studies of the effect of pressure on the structure, electronic and elastic properties of carbonates of alkali-alkaline earth metals

Density functional theory with a gradient PBE functional and dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package was used to study the effect of pressure on the structural, electronic, and elastic properties of K2Ca(CO3)2, K2Mg(CO3)2, Na2Ca2(CO3)3, Na2Mg(CO3)2. The parameters of the third-order Birch--Murnaghan equation of state are determined, and it is shown that the equilibrium volume and compressibility modulus depend linearly on the average cation radius. Under pressure, the widths of the upper valence bands increase maximally in K2Ca(CO3)2 and minimally in Na2Ca2(CO3)3, while the centers of gravity of the cationic states shift by ~0.2 eV. Elastic constants and polycrystalline moduli are calculated, which increase with decreasing average cation radius. The shear modulus for K2Ca(CO3)2 and K2Mg(CO3)2 decreases with increasing pressure, and this leads to anomalous behavior of the longitudinal and transverse acoustic wave velocities. Keywords: ab initio calculations, buchliite, eithelite, shortite, equation of state, elastic moduli, density of states, pressure.

An anion-deficient high-entropy fluorite oxide with very low density

In this paper, we explored the possibility of forming single-phase high-entropy fluorite oxides (HEFOs) in the (Ce0.2Zr0.2Ti0.2Sn0.2M0.2)O2-δ (M = Mg, Y, Yb, and Ca) system. The result showed that single-phase high-entropy fluorite can only be obtained from (Ce0.2Zr0.2Ti0.2Sn0.2Ca0.2)O2-δ. Compared with other reported anion-deficient HEFOs, this material has the lowest rare earth element content, the smallest average cationic radius and the lowest theoretical density. The effects of cation radius and starting materials on the formation of single-phase HEFOs are discussed.

Effect of cobalt doping on structural parameters, cation distribution and magnetic properties of nickel ferrite nanocrystals

Here, thermal heat treatment method has been used for the synthesis of PVP capped nickel ferrite (NiFe2O4) and cobalt doped nickel (Ni0.5Co0.5Fe2O4) spinel ferrite nanocrystals with the aim of investigating the effect of Co doping on the structural and magnetic properties of NiFe2O4 nanocrystals. XRD analysis confirms the formation of a pure form of FCC nanocrystals of spinel ferrite type, where the average crystallite size has been found to be approximately 17 nm and 19 nm respectively for nickel ferrite (NF) and cobalt doped nickel ferrite (CNF) from XRD. Again, HRTEM study gives the average size for NF and CNF as 20 nm and 25 nm respectively. Further, SAED pattern indicates the polycrystallinity of both the sample, whereas EDX analysis confirms the presence of expected elements in the sample. From XRD data analysis different structural parameters such as lattice spacing, lattice constant, tetrahedral and octahedral ionic radii, oxygen position parameter, average cation radius per molecule, ionic packing coefficients (Pa, Pb), the degree of ionic packing (α), vacancy parameter (β), hopping lengths, and bond lengths etc. have been calculated for both nickel and cobalt doped nickel ferrite nanocrystals. Further, obtained cation distribution for NF and CNF nanocrystals from their XRD pattern analysis shows huge change in the distribution as a result of doping. In addition, FTIR spectral analysis also confirms the phase purity of both the samples. Magnetic characterization shows the improvement of different magnetic parameters due to cobalt doping on nickel, thereby making CNF nanocrystals suitable for different application purposes.

Effect of A-site cationic radius on ceramic La0.67−xDyxSr0.33MnO3 prepared by sol–gel technique

La0.67−xDyxSr0.33MnO3 (x = 0.1, 0.12, 0.15, 0.20, 0.25, 0.30, 0.32) ceramics were prepared by sol–gel method. The samples were characterized by scanning electron microscopy, X-ray diffractometry, and resistance–temperature test (ρ–T) method. The experimental results showed that all of the samples were composed of a single phase with the orthorhombic perovskite structure and Pnma space group. The samples had high density, the average size of the grain decreased from 11.83 μm to 7.24 μm, while the number of grain boundaries increased. The one reason why resistance (ρmax) increases was supposed to be the boundary scattering enhancement, while the temperature corresponding to the peak resistivity (TP) decreases. The average cation radius of the A-site decreased as the amount/concentration of doping with Dy3+ increased, changing the bond length and bond angle. Meanwhile, the double exchange was suppressed, this being another and more important reason why the resistance increased and the TP decreased. The metal–insulator transition temperature wide degree (ΔT) and resistivity (ρ) are the important factors in the temperature coefficient of resistance. In the competition of the two factors, the ρ occupied an advantageous place, therefore the electrical properties of the materials can be further optimized by adjusting the average cation radius of the A-site.

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites.

Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)x[HC(NH2)2](1-x)PbI3 = GUAxFA(1-x)PbI3 and ternary CsyGUAxFA(1-y-x)PbI3 mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA+-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs+ can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Impact of Ag doping on the structural, surface morphologic and electrical properties of La0.625(Ca0.285Sr0.09)MnO3 polycrystalline ceramics

In this study, Ag-doped La0.625(Ca0.285Sr0.09)MnO3:Agz (LCSMO:Agz, z = 0, 0.1, 0.2, 0.3, and 0.4) polycrystalline ceramics were prepared by sol-gel method, then characterized by various techniques. X-ray diffraction (XRD) showed that all bulk prepared specimens crystalized in LCSMO phase with orthogonal cubic structure (space group of Pnma) and without impurities. Scanning electron microscopy (SEM) revealed good compactness with fewer holes. Average grain sizes and quantity of grain boundaries (GBs) correlated with Ag doping amounts. As z content rose, resistivity dropped, and both peaks temperature coefficient of resistance (TCR) and magnetoresistance (MR) first increased and then decreased. Several previous studies indicated that Ag doping amount could affect TCR and MR peaks. At z = 0.3, TCR peaks reached maxima of 10.27% (289.56 K), at z = 0.1, MR peaks reached maxima of 15.63% (293.08 K). As z increased from 0 to 0.4, the temperature of peak TCR (Tk) was adjusted to room temperature from 289.56 to 294.22 K. TCR peak of La0.625(Ca0.285Sr0.09)MnO3:Agz material was improved at room temperature when compared to values reported in the literature. This could be related to changes in tolerance factor (τ) and average A-site cationic radius (rA). Overall, LCSMO:Agz ceramics look promising for infrared detectors and colossal magnetoresistance (CMR) radiant heating instruments at room temperature.

Structural, magnetic and dielectric properties of lanthanum substituted Mn0.5Zn0.5Fe2O4

Mn0.5Zn0.5LaxFe2-xO4 (x = 0.0, 0.025, 0.050, 0.075 and 0.10) ferrites have been synthesized using coprecipitation method. Effect of substitution of La3+ on structural, magnetic and dielectric properties of Mn0.5Zn0.5Fe2O4 was studied. Structural properties have been investigated using X-ray diffraction (XRD) and morphological by transmission electron microscopy (TEM). XRD analysis revealed the formation of cubic spinel structure. Different structural parameters like lattice parameter, crystallite size, interionic distances, average cationic radii, bond angles and bond lengths have been calculated and compared with various theoretically predicted parameters. TEM images confirmed that particle size decreases with La content. Magnetic behavior of synthesized samples was studied employing vibrating sample magnetometer (VSM). It was noted that as x increased from 0 to 0.1, maximum magnetization showed decreasing trend from 34.57 emu/gm to 15.34 emu/gm (noted at 15000 G) and Curie temperature from 193 °C to 147 °C at constant field of 500 G. Real (є ̍) and imaginary part (є ̎) of dielectric constant, dielectric loss (tan δ), ac conductivity (σac), real (Z ̍) and imaginary part (Z ̎) of impedance were studied with frequency (0.1 Hz–1 MHz) and composition. є̍, є ̎, tan δ and σac were observed to decrease with substitution of La3+ ions. This variation can be attributed to the hopping of electrons between Fe2+ ions and Fe3+ ions at octahedral sites. Cole-cole plot showed a single semicircular arc for Mn0.5Zn0.5LaxFe2-xO4 clearly indicating the dominant effect of grain boundary in conduction.

Crystal structure of calcium vanadate-phosphate fluoride

Apatite-type materials AI4AII6(BO4)6X2 have two unique cations sites AI and AII, which can host large mono-, di- tri- and tetra-valent cations. The average cation radii will affect the twist angle and lattice constants. However, there are few reports on the influence of B site substitutions on the twist angle and lattice parameters. It is believed that the lattice constant variation as a function of B site substitutions may not follow the same twist-angle model as proposed for A site. This work reports our results on the crystal chemistry of synthetic apatite Ca10(VxP1−xO4)6F2 obtained through the crystal structure characterization using Rietveld refinement and high-resolution transmission electron microscopy. The quantification of vanadium/phosphorus partitioning in BO4 tetrahedra showed that equilibrium with more than 70% substitution of phosphorous by vanadium was difficult to achieve unless longer annealing (about 1 week at 900 °C) was employed. In comparison with the apatites with different ionic radii at AI and AII sites, Ca10(VxP1−xO4)6F2 apatites with different ionic radii at B site show little twist angle variation for the whole series, which indicates that the dilation of unit cell constants is mainly because of the expansions of BO4 tetrahedra when A site cation is fixed.

Microstrain-assisted polymorphic phase transitions in (Eu1−x La x )2O3

Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.

Compositional control of tunnel features in hollandite-based ceramics: structure and stability of (Ba,Cs)1.33(Zn,Ti)8O16

The impact of composition on the tunnel features of hollandite materials for the purpose of radioactive cesium (Cs) immobilization was evaluated. The barium (Ba) to cesium (Cs) ratio was varied in the tunnel sites referred to as the A-site of the hollandite structure. Zinc (Zn) was substituted for titanium (Ti) on the B-site to achieve the targeted stoichiometry with a general formula of BaxCsyZnx+y/2Ti8−x−y/2O16 (0 < x < 1.33; 0 < y <1.33). The tunnel cross-section depended on the average A-site cation radius, while the tunnel length depended on the average B-site cation radius. Substitution of Cs resulted in a phase transition from a monoclinic to a tetragonal structure and an increase in unit cell volume of 1.8% across the compositional range. Cs loss due to thermal evaporation was found to decrease in compositions with higher Cs content. The enthalpies of formation from binary oxides of Zn-doped hollandite measured using high-temperature oxide melt solution calorimetry were strongly negative, indicating thermodynamic stability with respect to their parent oxides. The formation enthalpies became more negative, indicating hollandite formation is more energetically favorable, when Cs was substituted for Ba across the range of Zn-doped compositions investigated in this study. Compositions with high Cs content exhibited lower melting points of approximately 80 °C. In addition, high Cs content materials exhibited a significant reduction in Cs release from the solid to liquid phase by leaching or aqueous corrosion as compared to low Cs content materials. These property changes would be beneficial for applications in radioactive cesium immobilization in a multi-phase ceramic by allowing for decreased processing temperatures and higher cesium weight loadings. More broadly, these results establish the link between composition, structural symmetry, and thermodynamic stability for tunnel structured ceramics with implications in the design of new energy conversion and storage materials.

Effect of Ca-doping on electrical properties of La0.46Sm0.21Sr0.33-xCaxMnO3 ceramics prepared by sol-gel technique

La0.46Sm0.21Sr0.33−xCaxMnO3 (x = 0.00, 0.05, 0.10, 0.15, and 0.20) ceramics synthesized by Pechini sol–gel method were investigated by X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectroscopy. Temperature dependence of resistivity (R-T) was measured using the standard four-probe method. All the samples had single phases of orthorhombic perovskite crystal structure. By substituting Sr ion with Ca ion, the samples achieved high crystalline quality, density, and chemical uniformity. After the substitution, the resistivity of samples decreased, and the metal–insulator (M–I) transition temperature (TP) shifted to a lower temperature, making the M–I transition sharper. Therefore, the temperature coefficient of resistance increased. Moreover, the substitution decreased the A-site average cationic radius , thus decreasing the Mn–O–Mn bond angle. This weakens the double exchange between Mn3+ and Mn4+.

Investigation of the local structures and transport properties of quaternary molten alkali chloride systems by MD simulations for liquid metal batteries

When selecting a suitable molten salt electrolyte system for liquid metal batteries (LMBs), it is crucial to have an accurate understanding of the local structures and transport properties of molten salt mixtures. In this study, molecular dynamics simulations have been employed to calculate such properties of (Li, Na, K, Cs)Cl and (Li, K, Rb, Cs)Cl quaternary molten alkali chloride systems at 700–1000 K using the Born–Mayer–Huggins potential. The results show that the density and shear viscosity of the molten salt electrolyte increase when the average cation radius increases, while the self-diffusion coefficient and ionic conductivity decrease. Increasing the temperature in LMBs can enhance the diffusibility and electroconductivity of the molten salt electrolyte and reduce the density and shear viscosity to a certain extent. Overall, the calculated results provide a useful reference for the selection of molten salt electrolytes with reasonable composition to optimize the electrochemical performance of LMBs. The molten salt mixtures as the electrolytes have a crucial effect on the performances of LMBs. The transport properties such as ionic conductivity, shear viscosity and self-diffusion capacity of quaternary molten salt systems (Li, Na, K, Cs)Cl and (Li, K, Rb, Cs)Cl have been calculated by MD simulation, and the simulation results can provide guidance for the researches of LMBs in the future.

The structure of P21/c (Ca0.2Co0.8)CoSi2O6 pyroxene and the C2/c–P21/c phase transition in natural and synthetic Ca–Mg–Fe2+ pyroxenes

ABSTRACTA P21/c synthetic (Ca0.2Co0.8)CoSi2O6 pyroxene was synthesized by slow cooling from melt at high pressure. Single crystals suitable for X-ray diffraction were obtained and refined. The results were compared to those of C2/c pyroxenes along the series CaCoSi2O6–Co2Si2O6. Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2O6–Fe2Si2O6 and CaMgSi2O6–Mg2Si2O6 pyroxenes, both at an average and local level are apparent.The results, examined together with two new refinements of pigeonite in the ureilites ALHA77257 and RKPA80239 and with a set of natural and synthetic C2/c and P21/c pyroxenes, show that the average cation radius in the M2 site is the driving force for the phase transition from C2/c to P21/c. The longest M2–O3 distances and the O3–O3–O3 angles follow the same trend, dictated only by the ionic radius in M2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M1–O, M2–O1 and M2–O2 distances are unaffected by the transition and are determined only by the ionic radius of the bonding cation. The structural changes between the ionic radius and the high temperature C2/c and P21/c transitions are similar, and different to the high-pressure transition.Analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 Å. Increasing strain stabilizes the P21/c structure to a higher temperature and larger cation radius.Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the mineral pigeonite, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling.