Effects of Ga substitution for Al and cation vacancy on microwave dielectric properties of spinel-structured Co1-3z(Al1-xGax)2+zO4 ceramics

The microwave dielectric properties and cation distribution of spinel-structured Co1-3z(Al1-xGax)(2+z)O-4 (z = 0-0.25) ceramics, where z represents the concentration of the cation vacancy, were characterized in this study. Ga substitution for Al in the Co(Al1-xGax)(2)O-4 ceramics was effective in increasing the quality factor (Q . f) values from 27,000 to 103,000 GHz, making the Co2+, Al3+, and Ga3+ cations randomly occupy both the tetrahedral and octahedral sites to form an intermediate spinel. A similar result was also obtained by the introduction of the cation vacancy for the Co1-3zAl2+zO4 (x = 0) ceramic and the highest Q . f value of 120,000 GHz was obtained for z = 0.25 at which the degree of inversion, y, was estimated to be 0.5 from Al-27 NMR spectra. Further improvement in Q . f value was achieved by the introduction of cation vacancy and Ga substitution for Al in Co-0.25(Al1-xGax)(2.5)O-4 ceramics. Ga-71 NMR spectra showed that Ga3+ cations occupied both the tetrahedral and octahedral sites, while a single peak was observed from Al-27 NMR spectra, implying that Al3+ cations occupied only the octahedral site. The preferential site occupancy of Al in the an octahedral site led to the formation of ordered spinel structure with the space group P4(3)32 and the Q . f value of the Co-0.25(Al0.4Ga0.6)(2.5)O-4 (z = 0.25 and x = 0.6) ceramic was 170,000 GHz. (C) 2017 The Japan Society of Applied Physics