  Cr^5+ doped K_3NbO_8, considered to be useful as a electron spin qubit, has
been investigated by pulsed X-band (~9.7 GHz) and 240 GHz electron paramagnetic
resonance and electron nuclear double resonance (ENDOR). Comparison of the low
temperature electronic spin-lattice relaxation rate 1/T_1 at 9.7 and 240 GHz
shows that it is 250 times faster at 240 GHz than at X-band. On the other hand,
the spin-spin relaxation rate 1/T_2 appears largely frequency independent and
is very likely related to the superhyperfine (SHF) coupling of the Cr^5+
electron with the surrounding potassium and niobium nuclei. This coupling was
investigated by HYSCORE at 9.7 GHz and pulsed Mims ENDOR at 240 GHz. The high
frequency and field enabled us to unambiguously measure the hyperfine and
quadrupole couplings of the 39K in spite of its small magnetic moment. We find
that the largest 39K SHF coupling is positive, with 0.522 MHz and 0.20 MHz as
its isotropic and dipolar parts respectively. 93Nb ENDOR was dominantly due to
its quadrapolar interaction, with a coupling of about 0.8 MHz, and a SHF
coupling of about 0.08 MHz. The significance of these data to spin qubit
studies is pointed out.
