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Equation of State and Amorphization of Ca9R(VO4)7 (R = La, Nd, Gd): A Combined High-Pressure X‑ray Diffraction and Raman Spectroscopy Study
journal contribution
posted on 2018-10-11, 14:06 authored by Katarzyna M. Kosyl, Wojciech Paszkowicz, Olga Ermakova, Damian Wlodarczyk, Andrzej Suchocki, Roman Minikayev, Jaroslaw Z. Domagala, Alexei N. Shekhovtsov, Miron Kosmyna, Catalin Popescu, François FauthCa9R(VO4)7 (R = rare earth) multicomponent
oxides of a whitlockite-related structure are under consideration
for applications in optoelectronics. In this work, the Czochralski-grown
Ca9R(VO4)7 crystals were investigated
as a function of pressure by powder X-ray diffraction and single-crystal
Raman spectroscopy. The diffraction experiments were performed at
the ALBA synchrotron under pressures ranging up to 9.22(5), 10.7(1),
and 8.55(5) GPa for R = La, Nd, and Gd, respectively, to determine
the third order equation of state (EOS) parameters. Fitting of the Birch–Murnaghan
EOS provided the isothermal bulk moduli K0 = 63(4), 63(2), and 61(5) GPa for these three orthovanadates. These
values are apparently lower than that reported for structurally related
tricalcium vanadate Ca3(VO4)2. The
compressibility anisotropy was observed; the lattice is markedly stiffer
in [001] than in [100] direction. For Ca9Nd(VO4)7, the variation of the diffractograms just above 10
GPa provides an indication on the beginning of amorphization process;
during pressure release the whitlockite-like structure is recovered. Raman spectroscopy measurements
for single crystals of the above-mentioned rare-earth vanadates and
Ca9Y(VO4)7 were performed (the maximum
pressures achieved were 16.3(1), 21.2(1), 15.3(1), and 18.6(1) GPa
for R = Y, La, Nd, and Gd, respectively). These measurements reveal
a partially reversible phase transition interpreted as amorphization,
with an onset at the pressure of ∼9–10 GPa, characterized
by broadening of the peaks and their shift to lower energies.
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Raman spectroscopy measurementspowder X-ray diffractionEOSwhitlockite-like structurebulk moduli K 0amorphization process10 GPawhitlockite-related structureRaman Spectroscopy Studypressure releaseorder equationALBA synchrotronGdmulticomponent oxidessingle-crystal Raman spectroscopyNdphase transitionrare-earth vanadates7 crystalsVO 4compressibility anisotropytricalcium vanadateLadiffraction experiments
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