4 research outputs found
Pressure and Temperature Dependent Structural Studies on Hollandite Type Ferrotitanate and Crystal Structure of a High Pressure Phase
The
structural stability and phase transition behavior of tetragonal (<i>I</i>4/<i>m</i>) hollandite type K<sub>2</sub>Fe<sub>2</sub>Ti<sub>6</sub>O<sub>16</sub> have been investigated by <i>in situ</i> high pressure X-ray diffraction using synchrotron
radiation and a diamond anvil cell as well as by variable temperature
powder neutron and X-ray diffraction. The tetragonal phase is found
to be stable in a wider range of temperatures, while it reversibly
transforms to a monoclinic (<i>I</i>2/<i>m</i>) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced
phase transition occurs with only a marginal change in structural
arrangements. The unit cell parameters of ambient (t) and high pressure
(m) phases can be related as <i>a</i><sub><i>m</i></sub> ∼ <i>a</i><sub><i>t</i></sub>, <i>b</i><sub><i>m</i></sub> ∼ <i>c</i><sub><i>t</i></sub>, and <i>c</i><sub><i>m</i></sub> ∼ <i>b</i><sub><i>t</i></sub>. The pressure evolution of the unit cell parameters indicates
anisotropic compression with β<sub>a</sub> = β<sub>b</sub> ≥ β<sub>c</sub> in the tetragonal phase and becomes
more anisotropic with β<sub>a</sub> ≪ β<sub>b</sub> < β<sub>c</sub> in the monoclinic phase. The pressure–volume
equations of state of both phases have been obtained by second order
Birch–Murnaghan equations of state, and the bulk moduli are
122 and 127 GPa for tetragonal and monoclinic phases, respectively.
The temperature dependent unit cell parameters show nearly isotropic
expansion, with marginally higher expansion along the <i>c</i>-axis compared to the <i>a-</i> and <i>b-</i>axes. The tetragonal to monoclinic phase transition occurs with a
reduction of unit cell volume of about 1.1% while the reduction of
unit cell volume up to 6 K is only about 0.6%. The fitting of temperature
dependent unit cell volume by using the Einstein model of phonons
indicates the Einstein temperature is about 266(18) K
Phase Transformation, Vibrational and Electronic Properties of K<sub>2</sub>Ce(PO<sub>4</sub>)<sub>2</sub>: A Combined Experimental and Theoretical Study
Herein we report
the high-temperature crystal chemistry of K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> 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 (<i>P</i>2<sub>1</sub><i>/n</i>) phase reversibly transforms to a tetragonal
(<i>I</i>4<sub>1</sub><i>/amd</i>) structure at
higher temperature. Also, from the experimental and theoretical calculations,
a possible existence of an orthorhombic (<i>Imma</i>) structure
with almost zero orthorhombicity is predicted which is closely related
to tetragonal K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub>. 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<sub>V</sub> = 3.83 × 10<sup>–6</sup> K<sup>–1</sup>) compared to that in ambient monoclinic phase
(α<sub>V</sub> = 41.30 × 10<sup>–6</sup> K<sup>–1</sup>). 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 K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> have been obtained. Several lattice
and external modes of rigid PO<sub>4</sub> 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
Phase Transformation, Vibrational and Electronic Properties of K<sub>2</sub>Ce(PO<sub>4</sub>)<sub>2</sub>: A Combined Experimental and Theoretical Study
Herein we report
the high-temperature crystal chemistry of K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> 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 (<i>P</i>2<sub>1</sub><i>/n</i>) phase reversibly transforms to a tetragonal
(<i>I</i>4<sub>1</sub><i>/amd</i>) structure at
higher temperature. Also, from the experimental and theoretical calculations,
a possible existence of an orthorhombic (<i>Imma</i>) structure
with almost zero orthorhombicity is predicted which is closely related
to tetragonal K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub>. 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<sub>V</sub> = 3.83 × 10<sup>–6</sup> K<sup>–1</sup>) compared to that in ambient monoclinic phase
(α<sub>V</sub> = 41.30 × 10<sup>–6</sup> K<sup>–1</sup>). 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 K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> have been obtained. Several lattice
and external modes of rigid PO<sub>4</sub> 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
Phase Transformation, Vibrational and Electronic Properties of K<sub>2</sub>Ce(PO<sub>4</sub>)<sub>2</sub>: A Combined Experimental and Theoretical Study
Herein we report
the high-temperature crystal chemistry of K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> 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 (<i>P</i>2<sub>1</sub><i>/n</i>) phase reversibly transforms to a tetragonal
(<i>I</i>4<sub>1</sub><i>/amd</i>) structure at
higher temperature. Also, from the experimental and theoretical calculations,
a possible existence of an orthorhombic (<i>Imma</i>) structure
with almost zero orthorhombicity is predicted which is closely related
to tetragonal K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub>. 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<sub>V</sub> = 3.83 × 10<sup>–6</sup> K<sup>–1</sup>) compared to that in ambient monoclinic phase
(α<sub>V</sub> = 41.30 × 10<sup>–6</sup> K<sup>–1</sup>). 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 K<sub>2</sub>CeÂ(PO<sub>4</sub>)<sub>2</sub> have been obtained. Several lattice
and external modes of rigid PO<sub>4</sub> 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