Preparation, crystal structure, thermal studies and optical properties of layered dimethylammonium praseodymium double selenate: [(CH3)2NH2][Pr(H2O) (SeO4)2]

Abstract

Herein, the structure, vibrational and optical properties and thermal stability of [(CH₃)₂NH₂][Pr(H₂O) (SeO₄)₂], a double selenate of dimethylammonium and Pr(III) is reported. The double selenate was crystallized from the aqueous solution under controlled atmosphere and temperature. Crystal structure of the double salt was studied by single crystal X-ray diffraction studies. The compound has monoclinic (P2₁/n) lattice with bicapped trigonal prism [PrO₇(OH₂)], and tetrahedral (CH₃)₂NH₂⁺ and SeO₄^2─ entities as structure building units. The [PrO₇(OH₂)] unit share five of its vertex oxygen with five different selenate groups, one of its edges with a SeO₄^2─ unit and one corner with one water molecule. These units are extended along a-direction as zig-zag chain of composition [Pr(H₂O) (SeO₄)₂]^─. This arrangement also leads to two O-μ bonds with two different SeO₄^2─ units. These chains are linked together through the hydrogen bonds between coordinated water and SeO₄^2─ oxygens forming negatively charged sheet of [Pr(H₂O) (SeO₄)₂]^─. These sheets are stacked along the [010] direction with an interlayer space of about 5.99 Å. The charge neutralization in the unit cell is attained by dimethylammonium ions occupied in between these sheets. The FTIR, Raman spectroscopic investigations indicated the characteristic vibrations of selenate, water, hydrated praseodymium and dimethylammonium groups. The compound was found to be thermally stable up to 373 K, and beyond it collapsed due to loss of coordinated water molecules. Subsequently, the (CH₃)₂NH₂⁺, and SeO₄^2─ ions are lost as (CH₃)₂NH, and SeO₂. Diffused reflectance ultraviolet spectroscopic studies on powdered samples revealed band edge absorption at 4.9 eV. The photoluminescence studies showed the characteristic emission at 390 nm due to charge transfer fluorescence and other characteristics emission from Pr³⁺ ions.