PHYSICAL REVIEVf 8 VOLUME 2$, NUMBER 9 High-pressure x-ray study of Cu20 and Ag20 A. Werner and H. D. Hochheimer (Received 19 November 1981; revised manuscript received 29 January 1982) Pressure-induced phase transitions have been found in Cu20 and Ag20 by high-pressure x-ray diffraction technique at room temperature. Cu20 transforms from the cuprite to a hexagonal structure at a pressure of 10 GPa. This hexagonal form changes in the pressure range from 13 to 18 GPa into another hexagonal form, with a CdC12-type structure. Up to 24 GPa, the highest pressure generated, no decomposition of Cu20 into Cu and CuO was observed. Ag20 transforms at 0. 4 GPa from the cuprite structure to a hexagonal structure which is identical with that found above 10 GPa in Cu20. Extended p-T phase diagrams of Cu20 and Ag20 are presented using the results of the x-ray study and, in the case of Cu20, additional results of an optical investigation. I. INTRODUCTION Cu20 and Ag20 have a cubic structure with high symmetry (space group 0»'= Pn3m— ) 'This . cuprite structure, displayed in Fig. 1, can be considered as two interpenetrating lattices: one is fcc occupied by the metal atoms and the other one is bcc occupied by the 0 atoms. This rather open structure suggests, at higher pressures, phase transitions to denser struc- tures. Ultrasonic measurements' of Cu20 at pres- sures up to 0. 3 GPa give a negative pressure depen- dence of the elastic constant C44. This indicates that the stability of the cuprite lattice decreases as the pressure increases. The first x-ray measurements of Cu2O at pres- sures up to 16 GPa gave evidence of two phase tran- sitions, one at 5 GPa (phase I ~phase Ia) and a second one at 12 GPa (phase Ia ~phase D). At pressures above 15 GPa a decomposition of Cu2O into CuO and Cu was reported. The results of a room-temperature x-ray study of Cu20, presented in this paper, confirm the existence of phase II, but a transition pressure of 10 GPa instead of 12 GPa. In addition, our measurements show that there is no in- dication for a phase transition around 5 GPa, i.e. , FIG. 1. Different plots of the cuprite structure show (a) the bcc lattice formed by the 0 atoms (open circles) and (b} the fcc lattice formed by the metal atoms Cu or Ag (closed circles). phase Ia does not exist, On the other hand, we find a continuous transition from phase II to a new phase which was not observed before, Furthermore, there is no evidence for a decomposition of Cu20 up to 24 GPa, the highest pressure generated in this study. In the case of Ag20 volumetric measurements show a drastic decrease of volume around 0. 4 GPa. Our x-ray measurements confirm a phase transition at this pressure. The structure of the new phase is identical with that of phase II in Cu2O. Up to 29 GPa no further phase transition was observed. In this paper we present for the first time detailed descriptions of the high-pressure modifications of the cuprite structure and extended p-T phase diagrams of Cu20 and Ag20. II. EXPERIMENTAL Fine po~dered samples of Cu20 were prepared from batches grown in our institute as well as from a larger batch from the group of Professor Carabatos, whereas Ag20 was bought as a fine powder from "Ventron" w&th a purity of 99,999'/o. The Cu20 crys- tals were grown in both cases by oxidation of crystal- line copper by the grain-growth method. ' High pressure was generated in a gasketed diamond anvil cell, 6 using a 4:1 mixture of methanol and ethanol as well as silicon grease as pressure medium, The pressure was determined by the well-known ruby fluorescence technique. ' X-ray diffraction patterns were taken at room temperature in the energy- dispersive mode. The optical studies of the phase transition of Cu20 were carried out with a microscope in connection with a cryostat for cooling down the diamond cell to liquid-nitrogen temperature. In this setup the pres- sure was determined again by the ruby fluorescence 1982 The American Physical Society