Investigation of the M\"{o}ssbauer spectrum of RuSr$_2$GdCu$_2$O$_8$ as a function of Temperature shows that there is only one type Ru site

A sample of RuSr$_2$GdCu$_2$O$_8$ was prepared with enriched $^{99}$Ru which allows us to study the temperature dependence of the M\"{o}ssbauer spectrum up 145K. The sample magnetically orders at 138K and has a transition to superconductivity at 8.7K with an onset at $\sim$13K. The spectrum at 4.2K was fit with a single-site fit. The hyperfine field is 59.4K with isomer shift which indicates that the charge state of the Ru ion is close to +5. The strength of the electric quadrupole interaction is 0.36 mm/sec with $\eta=0.2$. This spectrum is essentially identical to that found for a sample prepared with the natural $^{99}$Ru abundance. At 146K, above the magnetic transition temperature, the spectrum is fit with a pure electric quadrupole interaction of the same magnitude as at 4.2K with the same isomer shift.