Structure\nand Magnetic Properties of a Family of Two-Leg\nSpin Ladder Compounds Ba<sub>2</sub>RE<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> (RE = Pr, Nd, and Gd–Ho)

Compared with the intensive investigation on 3d transition\nmetal\n(TM)-based spin ladder compounds, less attention has been paid to\nthe ones constructed by rare earth (RE) ions. Herein, we report a\nfamily of RE-based spin ladder compounds Ba<sub>2</sub>RE<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> (RE = Pr, Nd, Gd–Ho) crystallized\ninto a monoclinic structure with the space group <i>C</i>2/<i>c</i>. The RE ions are arranged on a two-leg spin\nladder motif along the <i>b</i>-axis, where the rung and\nleg exchange interactions are bridged via RE–O–RE pathways\nand RE–O–Ge–O–RE routes, respectively.\nMoreover, the much shorter rung distance in RE<sub>2</sub>O<sub>12</sub> dimer units than the leg distance suggests that Ba<sub>2</sub>RE<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> compounds are strong-rung spin\nladder systems. All of the synthesized Ba<sub>2</sub>RE<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> (RE = Pr, Nd, Gd–Ho) compounds\nexhibit dominant antiferromagnetic (AFM) interactions and the absence\nof magnetic order down to 1.8 K. Among the family members, Ba<sub>2</sub>Dy<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> can be described\nby <i>J</i><sub>eff</sub> = 1/2 Kramers doublet states,\nand the low-temperature specific heat indicates the coexistence of\nthe spin-dimerized state with a broad maximum at ∼2.4 K and\nlong-range AFM order with <i>T</i><sub>N</sub> = 0.81 K.\nThis family of Ba<sub>2</sub>RE<sub>2</sub>Ge<sub>4</sub>O<sub>13</sub> compounds thereby provides a rare platform to investigate the novel\nspin ladder physics constructed by 4f electrons.