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Formation of contact and multiple cyclic cassiterite twins in SnO₂-based ceramics co-doped with cobalt and niobium oxides.

ABSTRACT: Contact and multiple cyclic twins of cassiterite commonly form in SnO₂-based ceramics when SnO₂ is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO₂ on CoNb₂O₆ and Co₄Nb₂O₉ seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO₂ grain growth; CoNb₂O₆ forms at ∼700°C and Co₄Nb₂O₉ at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO₂ domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb₂O₆ grains can only result in the formation of contact twins, the Co₄Nb₂O₉ grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO₂ grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO₂ grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb⁵⁺ and Co²⁺ ions into the SnO₂ matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

SUBMITTER: Daneu N

PROVIDER: S-EPMC9370213 | biostudies-literature | 2022 Aug

REPOSITORIES: biostudies-literature

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Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides.

Daneu Nina N Dražič Goran G Mazaj Matjaž M Barou Fabrice F Padrón-Navarta José Alberto JA

Acta crystallographica Section B, Structural science, crystal engineering and materials 20220727 Pt 4

Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin gr ...[more]

PMID: 35975835

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Project description:Introduction: Dilutely doped ferroelectric materials are of interest, as engineering these materials by introducing point defects via doping often leads to unique behavior not otherwise achievable in the undoped material. For example, B-site doping with transition metals in barium titanate (BaTiO3, or BTO) creates defect dipoles via oxygen vacancies leading enhanced polarization, strain, and the ability to tune dielectric properties. Though defect dipoles should lead to dielectric property enhancements, the effect of grain size in polycrystalline ferroelectrics such as BTO plays a significant role in those properties as well. Methods: Herein, doped BTO with 1.0% copper (Cu), iron (Fe), or cobalt (Co) was synthesized using traditional solid-state processing to observe the contribution of both defect-dipole formation and grain size on the ferroelectric and dielectric properties. Results and discussion: 1.0% Cu doped BTO showed the highest polarization and strain (9.3 μC/cm2 and 0.1%, respectively) of the three doped BTO samples. While some results, such as the aforementioned electrical properties of the 1.0% Cu doped BTO can be explained by the strong chemical driving force of the Cu atoms to form defect dipoles with oxygen vacancies and copper's consistent +2 valency leading to stable defect-dipole formation (versus the readily mixed valency states of Fe and Co at +2/+3), other properties cannot. For instance, all three Tc values should fall below that of undoped BTO (typically 120°C-135°C), but the Tc of 1.0% Cu BTO actually exceeds that range (139.4°C). Data presented on the average grain size and distribution of grain sizes provides insight allowing us to decouple the effect of defect dipoles and the effect of grain size on properties such as Tc, where the 1.0% Cu BTO was shown to possess the largest overall grains, leading to its increase in Tc. Conclusion/future work: Overall, the 1% Cu BTO possessed the highest polarization, strain, and Tc and is a promising dopant for engineering the performance of the material. This work emphasizes the challenge of extricating one effect (such as defect-dipole formation) from another (grain size modification) inherent to doping polycrystalline BTO.

Dataset Information

Formation of contact and multiple cyclic cassiterite twins in SnO₂-based ceramics co-doped with cobalt and niobium oxides.

Publications

Formation of contact and multiple cyclic cassiterite twins in SnO<sub>2</sub>-based ceramics co-doped with cobalt and niobium oxides.

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