Defect-related local chemical environments and microwave dielectric response in Li/Te-co-doped CaWO4

Li/Te co-doped CaWO 4 ceramics with Li:Te molar ratios of 1:0.5, 1:0.75 and 1:1 was prepared to examine composition- and processing-dependent structural evolution and microwave dielectric properties. The Li:Te = 1:1 series was optimized at 890 °C, whereas the Li:Te = 1:0.5 and 1:0.75 series were optimized at 910 °C. XRD and Rietveld refinement, Raman spectroscopy, EPR, XPS, GCIB depth-profile XPS, ICP-OES and electron microscopy were used to correlate phase assemblage, local bonding, chemical-state signals, retained Li, microstructure and dielectric response. Under the respective optimized conditions, the Li:Te = 1:0.75, x = 0.04 ceramic exhibited the highest Q× f value of 89,613 GHz, while the Li:Te = 1:1, x = 0.04 ceramic showed the least negative τf value of −32.06 ppm/°C. GCIB depth-profile XPS revealed composition-dependent depth evolution of the relative Te 4 + - and Te 6+ -related signals, and ICP-OES confirmed systematic differences in total retained Li. In the Li:Te = 1:1 series, electron microscopy identified a Te/O-enriched amorphous interfacial region. Together, these results show that the Li:Te ratio and sintering condition jointly influence local structure, densification behaviour and microwave dielectric properties. The work provides a basis for optimizing the balance between low dielectric loss and temperature stability in CaWO 4 -based ceramics. Furthermore, a preliminary Ag co-firing test further indicates chemical compatibility under the tested condition.