Structure, thermal and optical properties of hydrogen bond stabilized hybrid rare-earth selenates: [(CH3)3NH][Ln(H2O)4(SeO4)2], (Ln = Pr, Nd)

Abstract

Herein the structures, thermal stabilities and optical properties of hydrated hybrid double selenate of trimethylammonium and Ln(III), as [(CH3)3NH][Ln(H2O)4(SeO4)2]; for Ln = Pr, Nd, (named as PrTMAS and NdTMAS) are reported. Both the hydrated double selenates were crystalized from aqueous solution under controlled atmosphere and temperature, and their crystal structures were determined by single crystal X-ray diffraction. Both the Pr3+ and Nd3+ compounds are isostructural and have triclinic (space group P-1) lattices with distorted cubical [LnO4(OH2)4], and tetrahedral (CH3)3NH+ and SeO42─ entities as structure building units. The [LnO4(OH2)4] and SeO42─ units are linked together through oxygen atoms forming chains of [Ln(H2O)4(SeO4)2]─ and these chains are linked together by cyclic, acyclic and bifurcated hydrogen bonds. The charge neutralization in the unit cell is attained by trimethylammonium ions occupied in between the chains. The FTIR and Raman spectroscopic investigations indicated the characteristic vibrations of selenate, water, hydrated praseodymium and trimethylammonium groups. The photoluminescence spectra of PrTMAS revealed intense emission near red side of visible spectrum centred at 661 nm upon excitation with 350 nm whereas that of NdTMAS shows blue-green emission on excitation with 400 nm. The diffused reflectance spectra of PrTMAS and NdTMAS compounds indicated band edges absorption at 4.4 and 5.0 eV, respectively. The compounds were found to be stable at room temperature, but with increasing temperature, the water molecules are lost where the lattices collapse, and subsequently the SeO42─ ions are lost as gaseous SeO2.