Magnetic Ground State Selection and B-site Disorder in the Pyrochlore Magnet Tb2NbScO7

Tb-based cubic pyrochlores have attracted interest due to the exotic, possible quantum spin ice ground state in Tb$_2$Ti$_2$O$_7$. Replacing the homogenous B site of B=Ti$^{4+}$ with a mixture of nonmagnetic ions changes the magnetic ground state and its excitations due in part to the sensitivity of the non Kramers Tb$^{3+}$ ion. Previous results showed that a highly disordered, fluctuating ground state is obtained in isovalent B-site disordered Tb$_2$Ti$_{(2-x)}$Sn$_x$O$_7$. Our current work focuses on the mixed valence B site disordered pyrochlore Tb$_2$NbScO$_7$. In this system, the B-site is occupied by non-magnetic Nb$^{5+}$ and Sc$^{3+}$, for B$^{4+}$ on average. Surprisingly our measurements of a powder sample of Tb$_2$NbScO$_7$ using time-of-flight and triple axis neutron scattering techniques show that this disordered B-site system displays an ordered antiferromagnetic ground state with Tc≈ 1.5 K. Diffuse x-ray scattering measurements on a small single crystal reveals a rich pattern of diffuse scattering due to the underlying B-site structural disorder and concomitant oxygen displacements.We discuss these results in relation to magnetic ground state selection in the Tb pyrochlores with B=Ti,Sn and Ti$_{1-x}$Sn$_{x}$.