Structural phase transition of Sr₂CuO₂(CO₃)

The structural phase transition of Sr₂CuO₂(CO₃) has been studied by X-ray and neutron powder diffraction and phonon Raman scattering. The crystal structure at room temperature is tetragonal with space group of I4 (S₄²) and the lattice parameters a = 7.8045(1) A ̊ and c = 14.993(1) A ̊. The structural phase-transition temperature determined by X-ray powder diffraction is found at 490 K. The structure above this transition temperature is tetragonal P42₁2 (D₄²), and the lattice parameters are a = 5.54364(8) A ̊ and c = 7.53823(11) A ̊ at 573 K. The neutron-diffraction data are refined by a Rietveld analysis and these structural parameters for below and above the transition temperature are presented. Raman spectra of Sr₂CuO₂(CO₃) have been measured in the temperature region between 5 K and 593 K. In the temperature region between 5 K and 483 K, a new extra mode was not observed, but the peak intensity of the strongest mode at 480 cm^-1 was decreased from room temperature to 373 K. Above 490 K, a new peak has been observed at 160 cm^-1 and the weak peaks which are observed at room temperature disappear. Judging from the facts that the spectral shape and line width originating from the CO₃ slab change at the transition temperature but a soft mode has not been observed, the type of this phase transition is order-disorder due to the change of the CO₃ alignments.