title: 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 K2Ce(PO4)2as 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 (P21/n) phase reversibly transforms to a tetragonal (I41/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 K2Ce(PO4)2. 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 (αV= 3.83 × 10-6K-1) compared to that in ambient monoclinic phase (αV= 41.30 × 10-6K-1). 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 K2Ce(PO4)2have been obtained. Several lattice and external modes of rigid PO4units 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.
authors.affiliation: Homi Bhabha National Institute , Anushakti Nagar, Mumbai 400094, India.
authors.forename: Samatha
authors.initials: S
authors.lastname: Bevara
authors.affiliation: Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research , Kalpakkam 603102, India.
authors.forename: Karuna Kara
authors.initials: KK
authors.lastname: Mishra
authors.forename: Sadeque Jahedkhan
authors.initials: SJ
authors.lastname: Patwe
authors.affiliation: Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research , Kalpakkam 603102, India.
authors.forename: T R
authors.initials: TR
authors.lastname: Ravindran
authors.forename: Mayanak K
authors.initials: MK
authors.lastname: Gupta
authors.forename: Ranjan
authors.initials: R
authors.lastname: Mittal
authors.forename: P Siva Ram
authors.initials: PS
authors.lastname: Krishna
authors.affiliation: Homi Bhabha National Institute , Anushakti Nagar, Mumbai 400094, India.|Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology , Indore 452013, India.
authors.forename: Anil K
authors.initials: AK
authors.lastname: Sinha
authors.affiliation: Homi Bhabha National Institute , Anushakti Nagar, Mumbai 400094, India.
authors.forename: S Nagabhusan
authors.identifier: http://orcid.org/0000-0002-2103-1063
authors.initials: SN
authors.lastname: Achary
authors.affiliation: Homi Bhabha National Institute , Anushakti Nagar, Mumbai 400094, India.
authors.forename: Avesh Kumar
authors.initials: AK
authors.lastname: Tyagi