High-pressure Raman spectroscopic study of Mn2GeO4, Ca2GeO4, Ca2SiO4, and CaMgGeO4 olivines

We present a Raman spectroscopic study of the structural modifications of several olivines at high pressures and ambient temperature. At high pressures, the following modifications in the Raman spectra are observed: 1)?in Mn₂GeO₄, between 6.7 and 8.6?GPa the appearance of weak bands at 560 and 860?cm^?1; between 10.6 and 23?GPa, the progressive replacement of the olivine spectrum by the spectrum of a crystalline high pressure phase; upon decompression, the inverse sequence of transformations is observed with some hysteresis in the transformation pressures; this sequence may be interpreted as the progressive transformation of the olivine to a spinelloid where Ge tetrahedra are polymerized, and then to a partially inverse spinel; 2)?in Ca₂SiO₄, the olivine transforms to larnite between 1.9 and 2.1?GPa; larnite is observed up to the maximum pressure of 24?GPa and it can partially back-transform to olivine during decompression; 3)?in Ca₂GeO₄, the olivine transforms to a new structure between 6.8 and 8?GPa; the vibrational frequencies of the new phase suggest that the phase transition involves an increase of the Ca coordination number and that Ge tetrahedra are isolated; this high pressure phase is observed up to the maximum pressure of 11?GPa; during decompression, it transforms to a disordered phase below 5?GPa; 4)?in CaMgGeO₄, no significant modification of the olivine spectrum is observed up to 15?GPa; between 16 and 26?GPa, broadening of some peaks and the appearance of a weak broad feature at 700–900?cm^?1 suggests a progressive amorphization of the structure; near 27?GPa, amorphization is complete and an amorphous phase is quenched down to ambient pressure; this unique behaviour is interpreted as the result of the incompatibilities in the high pressure behaviour of the Ca and Mg sublattices in the olivine structure.     

Schultenit,PbHAsO4, und PbHPO4: Synthesen und Kristallstrukturen nebst einer Diskussion zur Symmetrie

Zusammenfassung Schultenit, PbHAsO₄ [a=4,859(1) Å,b=6,756(1) Å,c=5,843(1) Å, =95,40(1)°] und PbHPO₄ [a=4,6838(3) Å,b=6,6451(2) Å,c=5,7817(3) Å, =97,138(4)°] sind isotyp und kristallisieren monoklin. Für beide Verbindungen war bei einer Temperatur vonT _c312 K der Übergang von RaumgruppeP _c nachP2/c bekannt. Der Strukturtyp von Schultenit ist durch das Vorliegen einer kurzen Wasserstoffbrückenbindung zwischen zwei in RaumgruppeP2/c über ein Symmetriezentrum ineinander überführbarer XO₄-Tetraeder charakterisiert. Die O–H...O-Bindungslänge beträgt in beiden Verbindungen übereinstimmend 2,46 Å. Mit Hilfe von Röntgen-Einkristallstrukturuntersuchungen konnte gezeigt werden, daß dieser Übergang vonPc nachP2/c offensichtlich nur auf einer Ordnung des H-Atoms beruht, während alle anderen Atome auch bei Zimmertemperatur innerhalb des Fehlers eine zentrosymmetrische Atomanordnung aufweisen.

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Schultenite, PbHAsO₄, and PbHOP₄: Syntheses and crystal structures with a discussion on their symmetry

Summary Schultenite PbHAsO₄ [a=4.859(1) Å,b=6.756(1) Å,c=5,843(1) Å, =95.40(1)°] and PbHPO₄ [a=4,6838(3) Å,b=6,6451(2) Å,c=5.7817(3) Å =97.138(4)°] are isotypic and crystallize monoclinic. For both compounds a transition from space groupPc toP2/c has been described atT _c312 K. The structure type of schultenite is characterized, by a short hydrogen bond between two XO₄ tetrahedra which are combined by a center of symmetry in space groupP2/c. The O–H...O bond length is for both these compounds 2.46 Å. Based on X-ray single crystal structure refinements it has been shown, that the transition fromPc toP2/c is obviously caused only by an ordering of the H atom; all the other atoms are also at room temperature centrosymmetrically arranged within limits of error.

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Hern Prof.Dr.K.Komarek zum 60.Geburtstag gewidmet

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