Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Złotnik, Sebastian; Sahu, Sulata K.; Navrotsky, Alexandra; Vilarinho, Paula M.

doi:10.1002/chem.201405666

Cited by 25 publications

(13 citation statements)

References 30 publications

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“…depicts the relation of the experimental enthalpies of formation, ΔHf ox , of a number of perovskites as a function of s. Compounds from the present study are included (with KTaO3 taken from Zlotnik et al54 ), together with other perovskites from group III-III, II-IV and I-V (and additionally mixed II-IV and I-V). The III-III group is represented by the lanthanides 60LuFeO3, the II-IV group by the titanates and zirconates 62 : SrTiO3, CaTiO3, BaTiO3, PbTiO3, SrZrO3, CaZrO3, BaZrO3 and PbZrO3, and the mixed II-IV and I-V group by (Na1-xSrx)(Nb1-xTix)O330 .…”

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confidence: 99%

Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

et al. 2015

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Lead free niobates and tantalates currently form one of the most promising groups of ferroelectrics, piezoelectrics and related materials, with important applications for the next generation of lead free sensors, actuators and microelectromechanical systems (MEMs). In view of their importance, the enthalpies of formation from binary oxide components at 25 °C, measured by high temperature oxide melt solution calorimetry of a set of alkali tantalates and niobates with perovskite-like structures, LiTaO3, LiNbO3, NaTaO3, NaNbO3 and KNbO3 are reported to be -93.kJ/mol for LiTaO3, LiNbO3, NaTaO3, NaNbO3 and KNbO3, respectively. The surface energies of nanocrystalline perovskites of these alkali tantalates and niobates were experimentally determined for the first time by calorimetry. The energies of the hydrated surface are 1.04 ± 0.34, 1.21 ± 0.78, 1.58 ± 0.29, 2.16 ± 0.57 and 2.95 ± 0.59 J/m 2 for LiTaO3, LiNbO3, NaTaO3, NaNbO3 and KNbO3, respectively. The stability of the lead-free perovskites of I-V type is discussed based on their tolerance factor and acid-base chemistry. The formation enthalpy becomes more exothermic (higher thermodynamic stability) and surface energy increases (greater destabilization for a given particle size) with increase in ionic radius of the A-site cation (Li, Na and K) and with increasing tolerance factor. These correlations provide key insights into how lead free niobates and tantalates behave during synthesis and processing;i.e they explain, for example, why KNbO3 and KTaO3 nanoparticles will be thermodynamically more reactive than their Li and Na counterparts. This understanding will facilitate the development of optimized processing techniques and applications.

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Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

et al. 2015

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“…In addition, according to our pioneer thermodynamic studies using oxide melt solution calorimetry, the enthalpy of formation of perovskite from oxides becomes less exothermic from pyrochlore phases, thus indicating a less stable structure with respect to the constituent oxides. The decomposition enthalpy of K 2 Ta 2 O 6 to KTaO 3 indicates that pyrochlore is energetically more stable than perovskite, and also confirms that pyrochlore is the low-temperature phase [31].…”

Section: Introductionmentioning

confidence: 69%

Nonstoichiometry Role on the Properties of Quantum-Paraelectric Ceramics

Tkach¹,

Vilarinho²

2020

Structure Processing Properties Relationships in Stoichiometric and Nonstoichiometric Oxides

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Among the lead-free perovskite-structure materials, strontium titanate (SrTiO 3-ST) and potassium tantalate (KTaO 3-KT), pure or modified, are of particular importance. They are both quantum paraelectrics with high dielectric permittivity and low losses that can find application in tunable microwave devices due to a dependence of the permittivity on the electric field. Factors as Sr/Ti and K/ Ta ratio in ST and KT ceramics, respectively, can alter the defect chemistry of these materials and affect the microstructure. Therefore, if properly understood, cation stoichiometry variation may be intentionally used to tailor the electrical response of electroceramics. The scientific and technological importance of the stoichiometry variation in ST and KT ceramics is reviewed and compared in this chapter. The differences in crystallographic phase assemblage, grain size, and dielectric properties are described in detail. Although sharing crystal chemical similarities, the effect of the stoichiometry is markedly different. Even if the variation of Sr/Ti and K/Ta ratios did not change the quantum-paraelectric nature of ST and KT, Sr excess impedes the grain growth and decreases the dielectric permittivity in ST ceramics, while K excess promotes the grain growth and increases the dielectric permittivity in KT ceramics.

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“…Supplementary Materials: Thermochemical Cycles (with oxide drop solution enthalpies taken from previous studies) [27], values for hydrated (non-normalized for water) enthalpies of dissolution, thermogravimetric analyses, and energy dispersive x-ray spectra are available as Supplementary Information.…”

Section: Determination Of Drop Solution Enthalpiesmentioning

confidence: 99%

Thermochemical Measurements of Alkali Cation Association to Hexatantalate

Sures¹,

Nagabhushana²,

Navrotsky³

et al. 2018

Preprint

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Ion association is an important process in aqueous dissolution, precipitation, and crystallization of ionic inorganic, organic, and biological materials. Polyoxometalates (POMs) are good model compounds for understanding the complex relationships between lattice energy, ion-pairing in solution, and salt solubility. Here we perform calorimetric measurements to elucidate trends in cluster stability, lattice energy, and ion-pairing behavior studies of simple hexatantalate salts in neat water, parent hydroxide solutions, and molybdate melts, extending previous studies on the isostructural hexaniobates. High temperature calorimetry of alkali salts of hexatantalate reveals that the enthalpies of formation from oxides of the K, Rb, and Cs salts are more similar to each other than they are for their niobate analogues and that the tantalate cluster is energetically less stable than hexaniobate. Aqueous dissolution calorimetry reveals that the cesium salt of hexatantalate has a similar concentration dependence on its dissolution enthalpy to that of hexaniobate. However, unlike rubidium hexaniobate, rubidium hexatantalate also exhibits increased concentration dependence, indicating that hextantalate can undergo increased ion-pairing with alkali salts other than cesium, despite the dilute environments studied. Dissolution enthalpies of POM salts in the parent alkali hydroxides shows that protonation of clusters stabilizes lattices even more than the strongly associating heavy alkalis do. Additionally, neither weak nor strong lattice ion associations necessarily correlates with respectively high or low aqueous solubility. These studies illuminate the importance of considering ion-pairing among the interrelated processes in the aqueous dissolution of ionic salts, that can be extended to serving as a model of cation association to metal oxide surfaces.

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Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Cited by 25 publications

References 30 publications

Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

Nonstoichiometry Role on the Properties of Quantum-Paraelectric Ceramics

Thermochemical Measurements of Alkali Cation Association to Hexatantalate

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