High-pressure Raman spectroscopic studies on orthophosphates Ba 3(PO 4) 2 and Sr 3(PO 4) 2 (original) (raw)
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High-pressure studies of SrNi3(P2O7)2 pyrophosphate by Raman spectroscopy and X-ray diffraction
Journal of Molecular Structure, 2006
High-pressure investigations of SrNi 3 (P 2 O 7 ) 2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy and X-ray synchrotron radiation source. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30A g C30B g for SrNi 3 (P 2 O 7 ) 2 , of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi 3 (P 2 O 7 ) 2 pyrophosphate to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, SrNi 3 (P 2 O 7 ) 2 , compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The new high-pressure phase persists metastably down to 1 atm. upon release of pressure. We also measured the pressure dependence of the lattice parameters of the polycrystalline SrNi 3 (P 2 O 7 ) 2 sample up to a pressure of 48 GPa. The compressibility of SrNi 3 (P 2 O 7 ) 2 along the b-axis is larger than that along the a-axis. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to a tetragonal structure started. The bulk modulus of SrNi 3 (P 2 O 7 ) 2 (P2 1 /c) is 124G2 GPa with a pressure derivative of 4.37G0.12. q
Mechanism of hardening for the surface phosphates under external high pressure
2007
Mechanism by which Zn/Fe methaphosphate material is transformed to glassy meta-phosphate is partially understood. The two decades of intensive study demonstrates that Zn and Fe ions participate to cross-link network under friction, hardening the phosphate. In the present work we consider our (in progress) spectroscopy study of zinc and iron phosphates under the influence external high pressure to determine Zn ion change coordination from tetrahedral to octahedral (or hexahedral) structure. The standard equipment is the optical high pressure cell with diamond (DAC). The DAC is assembled and then vibrational or electronic spectra are collected by mounting the cell in an infrared, Raman, EXAFS or UV-visible spectrometer. Transition metal atoms with d orbital have flexible coordination numbers, for example zinc acts as a cross-linking agent increasing hardness, by changing coordination from tetrahedral to octahedral. Perhaps the external pressure effect on the [Zn–(O-P-)4 ] complex caus...
High-pressure Raman spectra of Sr-substituted γ-Ca3−xSrx(PO4)2
High Pressure Research, 2011
By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of Sr-substituted γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) were carried out up to 30.7, 32.4 and 31.4 GPa, respectively. No pressure-induced phase transformation was observed in the present studies. The behaviors of the phosphate modes in γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) below 14.4 GPa were quantitatively analyzed since a methanol:ethanol:water (16:3:1) mixture was used as the pressure medium in DAC. The Raman wavenumbers of all PO4 vibrations increased linearly and continuously with pressure for γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2). With the increasing content of Sr, the corresponding pressure coefficients α changed, and the v i0 shifts to higher wavenumbers except that for ν2 bending vibration.
Theoretical and experimental study of the structural stability of TbPO_{4} at high pressures
Physical Review B, 2010
We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at ϳ9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon→ monazite→ scheelite→ SrUO 4 -type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO 4 -type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO 4 .
Journal of Molecular Structure, 2005
The effect of high external pressures on the Raman spectra of four metal pyrophosphate dihydrates, KMHP 2 O 7 $2H 2 O [MZCo(II), Mn(II), Zn(II) and Mg(II)], has been investigated by diamond-anvil cell, Raman microspectroscopy for pressures up to w42 kbar. At w29 kbar, the n asym POP band of the Co(II) compound disappears completely and irreversibly, suggesting the occurrence of a pressure-driven structural transformation or possibly even the first stages of amorphization. No such dramatic spectral changes were observed for the other three metal pyrophosphates. The previously published crystallographic data for the unit cells of the Co(II) and Mn(II) salts indicate that the former is more densely packed and, therefore, more likely to be affected by the application of high pressures than is the latter, as is observed. The vibrational modes of all four compounds shift gradually to higher wavenumbers with increasing pressure and the measured pressure dependences range from 0.08 to 1.10 cm K1 kbar K1 .
Determination of the high-pressure crystal structure of BaWO4 and PbWO4
Physical Review B, 2006
We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in BaWO4 and PbWO4 at pressures of up to 56 GPa and 24 GPa, respectively. BaWO4 is found to undergo a pressure-driven phase transition at 7.1 GPa from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in PbWO4 at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO4-II and PbWO4-III (space group P21/n). We have also performed ab initio total energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is however metastable and can only occur if the transition to the P21/n phases were kinetically inhibited. Our experiments in BaWO4 indicate that it becomes amorphous beyond 47 GPa.
Boron phosphide under pressure: In situ study by Raman scattering and X-ray diffraction
Journal of Applied Physics, 2014
Cubic boron phosphide BP has been studied in situ by X-ray diffraction and Raman scattering up to 55 GPa at 300 K in a diamond anvil cell. The bulk modulus of B 0 = 174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state data, allowed us to estimate the Grüneisen parameters of the TO and LO modes of zinc-blende structure, γ G TO = 1.16 and γ G LO = 1.04, just like in the case of other A III B V diamond-like phases, for which γ G TO > γ G LO 1. We also established that the pressure dependence of the effective electro-optical constant is responsible for a strong change in relative intensities of the TO and LO modes from I TO /I LO ~0.25 at 0.1 MPa to I TO /I LO ~2.5 at 45 GPa, for which we also find excellent agreement between experiment and theory.
Inorganica Chimica Acta, 2007
The Raman spectra of crystalline bis(pyrophosphato)tetrazinc(II) and-manganese(II) decahydrates, M 4 (P 2 O 7) 2 AE 10H 2 O (M = Zn and Mn), have been investigated at room temperature. The Mn(II) salt readily underwent photodecomposition upon extended exposure to 785-nm laser irradiation and so could not be examined under high pressures in a diamond-anvil cell. The Zn(II) salt was much more stable and was studied successfully at pressures up to $35 kbar. The pressure dependences of the d s PO 2 , m s POP, m as POP, m s PO 2 , and m as PO 2 internal modes of the pyrophosphate group and of an external H 2 O mode indicated the occurrence of a pressure-induced phase transition in the 15-20 kbar region. Further confirmation of this structural change came from the appearance of four initially forbidden modes in the 19-26 kbar range. This phase transition proved to be reversible upon release of the applied pressure. The pressure dependences before and after the phase transition were À0.22 to 0.66 and 0.06 to 0.33 cm À1 /kbar, respectively.
On the high-pressure phase transition in
The European Physical Journal B, 1998
X-ray diffraction (XRD) experiments have been carried out on quartz-like α−GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a = 7.306Å, b = 5.887Å and c = 5.124Å.