ESR Studies of BO32− Ions in Potassium Borate Ceramics

BO₃²⁻ ions have been observed and identified by ESR and diffuse reflectivity measurements in a series of potassium borate ceramics. The spin‐Hamiltonian parameters for these ions in the ceramic were investigated as a function of temperature from 4.2 to 300°K. At 77°K, the parameters are found to be: $A_1\text{\hspace{0.17em}=\hspace{0.17em}7.3(2)}\mathrm{G}{\mathrm{,\; A}}_2\text{\hspace{0.17em}=\hspace{0.17em}7.6(2)}\mathrm{G}{\mathrm{,\; A}}_3\text{\hspace{0.17em}=\hspace{0.17em}12.2(2)}\mathrm{G}{\mathrm{,\; g}}_1{\text{\hspace{0.17em}=\hspace{0.17em}2.0143(2), g}}_2{\text{\hspace{0.17em}=\hspace{0.17em}2.0128(2), g}}_3\text{\hspace{0.17em}=\hspace{0.17em}2.0091(2)}$ . Useful trends in the isoelectronic series of paramagnetic ions BO₃²⁻, CO₃⁻, NO₃ are examined. Using the observed hyperfine coupling constants and general molecular orbitals for the unpaired spin consistent with a 23‐electron, AB₃‐type molecule ($a_2\mathrm{\prime ,\; e\prime }$ , or $\mathrm{e″}$ orbitals), the $s$ density on the central atom is found to be small and negative for all three species. The $p$ density is also found to be small and negative when $a_2\prime$ or $\mathrm{e″}$ orbitals are assumed, but may be positive when an $\mathrm{e\prime }$ orbital is assumed. If the orbital for the unpaired spin is approximately $a_2\prime$ , then the $g$ tensors for all three isoelectronic ions are in agreement with planar distortions of the AB₃‐type molecules with these distortions increasing as one goes down the series from NO₃ to CO₃⁻ to BO₃²⁻.