Phase Transformation, Vibrational and Electronic Properties of K₂Ce(PO₄)₂: A Combined Experimental and Theoretical Study

Abstract

Herein we report the high-temperature crystal chemistry of KCe(PO) as observed from a joint in situ variable-temperature X-ray diffraction (XRD) and Raman spectroscopy as well as ab initio density functional theory (DFT) calculations. These studies revealed that the ambient-temperature monoclinic (P2/n) phase reversibly transforms to a tetragonal (I4/amd) structure at higher temperature. Also, from the experimental and theoretical calculations, a possible existence of an orthorhombic (Imma) structure with almost zero orthorhombicity is predicted which is closely related to tetragonal KCe(PO). The high-temperature tetragonal phase reverts back to ambient monoclinic phase at much lower temperature in the cooling cycle compared to that observed at the heating cycle. XRD studies revealed the transition is accompanied by volume expansion of about 14.4%. The lower packing density of the high-temperature phase is reflected in its significantly lower thermal expansion coefficient (α = 3.83 × 10 K) compared to that in ambient monoclinic phase (α = 41.30 × 10 K). The coexistences of low- and high-temperature phases, large volume discontinuity in transition, and large hysteresis of transition temperature in heating and cooling cycles, as well as drastically different structural arrangement are in accordance with the first-order reconstructive nature of the transition. Temperature-dependent Raman spectra indicate significant changes around 783 K attributable to the phase transition. In situ low-temperature XRD, neutron diffraction, and Raman spectroscopic studies revealed no structural transition below ambient temperature. Raman mode frequencies, temperature coefficients, and reduced temperature coefficients for both monoclinic and tetragonal phases of KCe(PO) have been obtained. Several lattice and external modes of rigid PO units are found to be strongly anharmonic. The observed phase transition and structures as well as vibrational properties of both ambient- and high-temperature phases were complimented by DFT calculations. The optical absorption studies on monoclinic phase indicated a band gap of about 2.46 eV. The electronic structure calculations on ambient-temperature monoclinic and high-temperature phases were also carried out.

Electrosteric colloidal stabilization for obtaining SrTiO3/TiO2 heterojunction: Microstructural evolution in the interface and photonics properties

“…Since the monoclinic phase comes from the original lattice type of a distorted cubic system [67] it is supported the growth of this phase on the interface with the SrTiO 3 cubic phase. Several works [19,20,68] reported the phase transitions between different levels of symmetry (cubic, tetragonal and monoclinic), and emphasize that among the factors for the transition are the temperature increase in the interface that entails in entropy variation, serving as the driving force for the transition of phase. Goutenoire et al [69] evaluating the transition from monoclinic to cubic phase in the La 2 Mo 2 O 9 system, reported that the higher symmetry is obtained by oxygen diffusion in the crystalline lattice according to the temperature variation.…”

“…However, these works are related to single phase materials and not for heterojunctions with interface formed with different chemical compounds, phases and crystalline lattices tensions. It has been reported in several works that physical chemistry parameters deriving from growth surface between materials [18] and from synthesis procedure [19,20] can promote changes in crystalline phase stability, and hence changes on energy emission behavior. Different synthesis have been proposed in order to modulate the heterostructure properties, but, traditional methods are generally complex [10,14,21,22].…”

A Density-Functional Theory Structural Database for Discovery of Novel Actinide Waste Forms

“…The calculations for the complex waste compounds studied here yield 0 K representative formation enthalpies such that they are metastable with respect to their simpler constituent oxides, lying above the 0 K thermodynamic convex hull. ,,, Thermal entropic contributions are important for these materials and can naturally influence the relative stability of crystal structures, affecting ranking of each candidate structure. ,, While including temperature-dependent contributions could potentially change our understanding of the relative stabilities, adding thermal contributions requires several phonon calculations per structure. , Although these calculations are possible for individual studies, , the length of time to carry out these calculations on a database scale would require significant computational resources that are not available. Even the use of approximate thermal corrections would require an extensive number of calculations for each candidate crystal structure and, as such, were not attempted. , We are currently evaluating approximate methods to add thermal contributions to our calculations such that we can obtain Gibbs energies for subsets of modeled compounds and thus predict possible reaction pathways for novel waste form materials. , …”

“…The calculations for the complex waste compounds studied here yield 0 K representative formation enthalpies such that they are metastable with respect to their simpler constituent oxides, lying above the 0 K thermodynamic convex hull. 5,33,48,65 Thermal entropic contributions are important for these materials and can naturally influence the relative stability of crystal structures, affecting ranking of each candidate structure. 32,33,36 While including temperaturedependent contributions could potentially change our understanding of the relative stabilities, adding thermal contributions requires several phonon calculations per structure.…”

“…5,33,48,65 Thermal entropic contributions are important for these materials and can naturally influence the relative stability of crystal structures, affecting ranking of each candidate structure. 32,33,36 While including temperaturedependent contributions could potentially change our understanding of the relative stabilities, adding thermal contributions requires several phonon calculations per structure. 66,67 Although these calculations are possible for individual studies, 16,65 the length of time to carry out these calculations on a database scale would require significant computational resources that are not available.…”