Investigation of the Orbital Ordering Transition in La$_4$Ru$_2$O$_{10}$ using the Mossbauer Effect*

There is a structural phase transition in La$_4$Ru$_2$O$_{10}$ from a triclinic phase starting at 140K to a monoclinic phase which is complete by 190K. This is a accompanied by the development of a local moment from $\mu_{eff} \simeq 0.4\mu_B$ to $\mu_{eff} \simeq 2.5\mu_B$ which leads to the identification of this transition with orbital ordering. The Mossbauer Effect(ME) has been measured from 4.2K to 196K in a sample prepared with enriched $^{99}$Ru(97\%). The ME spectrum is fit with two sites consistent with the triclinic structure. The spectra for the two sites are characterized by a quadrupole splitting(QS) and an isomer shift(IS): QS$_1$=0.51mm/s and I$_1$=-0.27mm/s and QS$_2$=0.38mm/s and IS$_2$=-0.32mm/s. At 171K the sample is mostly in the monoclinic phase which has a single-site ME spectrum with QS=0.38mm/s and IS=-0.31mm/s. The less symmetric site with the larger QS has disappeared. Throughout the transition the IS is consistent with a +4 charge state for the Ru site. The Debye temperature is 307K.