Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic Pr2Ti2O7

Little is so far known about the magnetism of the A(2)B(2)O(7) monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for r(A)/r(B) > 1.78. We show that high quality monoclinic Pr2Ti2O7 single crystals with a three-dimensional network of non-Kramers Pr3+ ions that interact through edge-sharing super-exchange interactions, form a singlet ground state quantum paramagnet that does not undergo any magnetic phase transitions down to at least 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite Pr2Ti2O7 were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of Pr2Ti2+xO(7) (-0.16 <= x <= 0.16) showed that deviations from the Pr2Ti2O7 stoichiometry lead to secondary phases of related, structures including the perovskite phase Pr2/3TiO3 and the orthorhombic phases Pr4Ti9O24 and Pr2TiO5. No indications of site disordering (stuffing and anti-stuffing) or vacancy defects were observed in the Pr2Ti2O7 majority phase. A procedure for growth of high-structural-quality, stoichiometric single crystals of Pr2Ti2O7 by the traveling solvent floating zone (TSFZ) method is reported. Thermo-magnetic measurements of single-crystalline Pr2Ti2O7 reveal an isolated singlet ground state that we associate with the low symmetry crystal electric field environments that split the (2J+1=9)-fold degenerate spin-orbital multiplets of the four differently coordinated Pr3+ ions into 36 isolated singlets resulting in an anisotropic temperature independent van-Vleck susceptibility at low T. A small isotropic Curie term is associated with 0.96(2)% non-interacting Pr4+ impurities.