Low-permittivity MgP2O6 and NiP2O6 microwave dielectric ceramics: crystal structure, properties, and P–V–L bond theory analysis

Phase-pure MgP 2 O 6 and NiP 2 O 6 metaphosphate ceramics were synthesized via solid-state reaction. Both crystallize in a monoclinic C2/c structure, with NiP 2 O 6 exhibiting a contracted unit cell due to the smaller ionic radius of Ni 2+ compared to Mg 2+ . The optimum sintering temperature for MgP 2 O 6 was 975 °C, yielding ε r  = 5.1, Q × f  = 45,300 GHz, τ f  = −14.7 ppm/°C, while for NiP 2 O 6 it was 925 °C, yielding ε r  = 5.4, Q × f  = 66,100 GHz, τ f  = −8.7 ppm/°C. The slightly higher ε r of NiP 2 O 6 stems from higher bond ionicity and polarizability density. The superior Q × f is attributed to a higher packing fraction, larger lattice energies, and weaker phonon damping, as evidenced by a narrower Raman FWHM. The improved τ f arises from stronger A–O and P–O bond energies, resisting thermal vibrations. These results indicate MgP 2 O 6 and NiP 2 O 6 as promising low-permittivity microwave ceramics and suggest that A-site cation engineering provides a promising proof-of-concept for tailoring dielectric properties in metaphosphates.