Crystal Structures, Optical Behavior, and Magnetic Properties in Hydrated Lanthanide Iron Sulfates

Single crystals of LnFe(SO₄)₃(H₂O)₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm; compounds <b>1-11</b>) and LnFe(SO₄)₃(H₂O) (Ln = Tm, Yb, Lu; compounds <b>12-14</b>) were synthesized under hydrothermal conditions. Single-crystal X-ray diffraction (SCXRD) analysis revealed that the dihydrated compounds (<b>1-11</b>) crystallize in centrosymmetric (CS) structures, with the lanthanide ions adopting eight-coordinate geometries. In contrast, the monohydrated compounds (<b>12-14</b>) exhibit noncentrosymmetric (NCS) structures, where the lanthanide ions are seven-coordinated. Vibrating sample magnetometry (VSM) confirmed that compounds <b>2</b>, <b>4</b>, and <b>7-11</b> are paramagnetic below 400 K, with compound <b>8</b> displaying the highest magnetic susceptibility. Compounds <b>1</b>, <b>5</b>, <b>6</b>, <b>13</b>, and <b>14</b> show a sharp increase in magnetic susceptibility at Néel temperatures (<i>T</i>_N) of approximately 72, 76, 70, 58, and 56 K, respectively, indicating antiferromagnetic ordering. High-temperature magnetic susceptibility measurements further support the presence of antiferromagnetic transitions in these compounds. Second harmonic generation (SHG) measurements showed that the noncentrosymmetric Yb (compound <b>13</b>) and Lu (compound <b>14</b>) compounds exhibit SHG intensities of 0.3× and 1.6× that of potassium dihydrogen phosphate (KDP), respectively.