Lattice dynamics of the phases I, II, and III of sodium hyperoxide NaO2

Abstract

AbstractThe lattice vibrations of NaO2, single crystals in the disordered pyrite, the ordered pyrite, and the marcasite phase are studied using far‐infrared reflection spectroscopy and inelastic neutron scattering. The far‐infrared spectra of all the different phases exhibit a high‐frequency group of bands centred at about 240 cm−1 and a low‐frequency group centred at about 100 cm−1. This similarity can be explained by the fact that the phase transitions primarily involve a mere change in the orientationsl arrangement of the O molecule ions. This leads to a relation between certain zone centre and zone boundary phonons in the corresponding Brillouin zones. The high‐frequency group corresponds in essence to the NaCl reststrahlen bands whereas the low‐frequency bands arise from the particular orientation pattern of the molecule ions demanding a primitive cell that is larger than the primitive NaCl‐type cell. The occurrence of the low‐frequency band in the disordered pyrite phase can be interpreted as a precursor effect of the transition into the ordered pyrite phase. The Raman‐ and infrared‐active phonons of the marcasite structure are analyzed on the basis of a simple short‐range force model.