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 cr ystal structure at room temperature is tetragonal with space group of <I(4)over bar> (S-4(2)) and the lattice parameters a = 7.8045(1) Angst rom and c = 14.993(1) Angstrom. The structural phase-transition temper ature determined by X-ray powder diffraction is found at 490 K. The st ructure above this transition temperature is tetragonal P42(1)2 (D-2(4 )), and the lattice parameters are a = 5.54364(8) Angstrom and c = 7.5 3823(11) Angstrom at 573 K. The neutron-diffraction data are refined b y a Rietveld analysis and these structural parameters for below and ab ove 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 ne w peak has been observed at 160 cm(-1) and the weak peaks which are ob served 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 t he CO3 alignments.