Raman microscopy study of basic aluminum sulfate Part II Raman microscopy at 77 K

Abstract

The tridecameric aluminum polymer [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ was prepared by forced hydrolysis of Al³⁺ up to an OH/Al molar ratio of 2.2. Upon addition of sulfate the tridecamer crystallised as the monoclinic basic aluminum sulfate Na_0.1[AlO₄Al₁₂(OH)₂₄(H₂O)₁₂](SO₄)_3.55. These crystals have been studied using Raman microscopy at 300 and 77 K and compared to Na₂SO₄·xH₂O and Al₂(SO₄)₃·xH₂O. The Raman spectrum of basic aluminum sulfate is dominated by two broad bands, which are assigned to the ν₂ and ν₄SO₄ ²⁻ triplets at 446, 459 and 496 cm⁻¹ and 572, 614 and 630 cm⁻¹. The ν₁ is observed as a single band at 990 cm⁻¹, partly overlapped by the ν₃ triplet at 979, 1009 and 1053 cm⁻¹ of the sulfate group in the Al₁₃ sulfate structure. Furthermore the band at 726 cm⁻¹ is assigned to an Al–O mode of the polymerised Al–O–Al bonds in the Al₁₃ Keggin structure. For the first time the OH-stretching region of the basic aluminum sulfate has been reported. The 77 K spectrum shows the presence of 3 crystal water bands at 3035, 3138 and 3256 cm⁻¹ accompanied by 3 Al–H₂O bands at 3354, 3418 and 3498 cm⁻¹ and 4 Al-OH bands at 3533, 3584, 3671 and 3697 cm⁻¹.