Synthesis, Crystal Structure from Single-Crystal and Powder X-ray Diffraction Data, and Thermal Behavior of Mixed Potassium Lanthanide Squarates: Thermal Transformations of Layered [Ln(H 2 O) 6 ]K(H 2 C 4 O 4 )(C 4 O 4 ) 2 into Pillared LnK(C 4 O 4 ) 2 (Ln = Y, La, Gd, Er)