ElenaV. Boldyreva, S.V. Goryainov, М.B. Smirnov, H. Ahsbahs, V.V. Chernyshev, H.-P. Weber
Research and education center “Molecular design and ecologically
safe technologies (REC-008)”, Novosibirsk State
University, Institute
of Solid State Chemistry SD RAS,
Institute of Mineralogy SD RAS, Moscow State University, S-Petersbourg State University, Philipps-Universitaet Marburg/Lahn (Germany), ESRF (Grenoble, France)
ul. Kutateladze, 18,
Novosibirsk, 128,
630128 Russia, e-mail: boldyrev@nsu.ru
Effect of hydrostatic pressures up to
8 GPa on the crystals of Na2C2O4 (s.g. P21/c)
was studied in situ in the diamond anvil cells by X-ray powder diffraction
(synchrotron radiation, wavelength=0.7Ĺ, MAR345-detector), Raman spectroscopy
and optical microscopy.
Below 3.8 GPa, maximum compressibility was measured in the direction normal to the layers formed by oxalate anions. Anisotropy of structural distortion under pressure was similar to (but not identical with) that on cooling. At 3.8 GPa a reversible isosymmetric first-order phase transition occurred. The packing of the centers of the oxalate anions was preserved to a large extent, whereas the angles between the planes of the oxalate-ions and the coordination of sodium cations changed jump-wise at the transition point. Linear shifts of the maxima of vibrational bands in the Raman spectra of sodium oxalate were measured with increasing pressure below and above the phase transition point. At the transition point frequency jumps were observed for most vibrational bands. The changes in the Raman spectra with pressure were interpreted on the basis of the analysis of lattice dynamics and of the crystal chemistry of the two polymorphs of sodium oxalate at variable pressure.
Financial support: RFBR (02-03-33358 & 02-05-65313), BRHE
(REC-008), Russian Ministry of Education (Ч0069 “Integration”), Swiss
National Science Foundation (21-45703.95) and the European Synchrotron Radiation Facility (ESRF),
Grenoble (experiment HS-1239). Experimental assistance by the staff of the
Swiss-Norwegian Beamlines at ESRF is gratefully acknowledged.