Composite ceramics based on the spinel Mg2TiO4 were prepared by a conventional mixed‐oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO3. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q×f value of around 150–160 THz was obtained for the undoped Mg2TiO4, whereas a reduced Q×f value was observed for an increase in the Co concentration in the system (1−x)Mg2TiO4−xCo2TiO4. Upon doping with 7 mol% CaTiO3, the Q×f value passed through a maximum with increasing Co concentration. Adding ZnO–B2O3 to the composite system based on Co‐doped Mg2TiO4 resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q×f value.

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Dielectric resonator aided sensitivity-enhancing electron paramagnetic resonance was successfully applied to small single crystals of the previously reported metal-organicin a conventional X-band EPR spectrometer at 7 K sample temperature to reveal the nature of mononuclear Cu 2+ ion defect species. We found that these paramagnetic defects are not related to an impurity phase or extraframework species of the parent metal-organic framework material but are formed within the framework. Novel angular resolved single crystal continuous wave electron paramagnetic resonance supported by powder measurements and single crystal X-ray diffraction on this metal-organic framework compound identified defective copper paddle wheel units with one missing Cu 2+ ion as the observed mononuclear paramagnetic species in this compound. The sensitivity enhancement by an estimated factor of 8.6 for the single crystal electron paramagnetic resonance spectroscopy is required to efficiently record the Cu 2+ ion signals in single crystals of typical sizes of 200 × 50 × 50 µm 3 at X-band frequencies. The results demonstrate that conventional electron paramagnetic resonance spectrometers operating at X-band frequencies and equipped with dielectric resonators can successfully be used to perform single crystal studies of these porous, low density materials with very small volume samples at low temperatures.

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Continuous-wave X-band electron paramagnetic resonance with dielectric resonators has successfully been applied to small single crystals of the metal–organic framework HKUST-1 and Cu2.965Zn0.035(btc)2 to investigate the structure of paddle-wheel building blocks with pure Cu/Cu and mixed Cu/Zn pairs. The local paramagnetic Cu2+ ion probes were used to identify the magnetic g and A tensor orientations with respect to the crystal axes. We were able to monitor changes in these tensor orientations by EPR at gas adsorption on MOFs for the first time. We explored the spectral simulations of the spin Hamilton parameters of the single crystals and found results similar to those in previous studies of powder samples, but moreover, the tensor orientations are influenced upon gas adsorption, which is represented by a distinct line broadening effect in the angular resolved single-crystal EPR spectra. The as-synthesized, dehydrated, carbon dioxide-adsorbed, carbon monoxide-adsorbed, methanol-adsorbed, and reactivated states have been analyzed to reveal the magnetic tensor orientations, and the direct coordination of the adsorbed gas to the Cu2+ ions along with consistent, corresponding DFT calculations allows us to predict an improved model for the mixed paddle-wheel structure upon the adsorption of gases to a paddle-wheel based on perturbations of the g and A principal axis orientations. Additionally, we analyzed a reversibly occurring background signal observable not only in Cu2.965Zn0.035(btc)2 but also in pure Cu3(btc)2 at very low temperatures.