Synthesis, crystal structure and properties of a new noncentrosymmetric tetraoxidomolybdate(VI)

Abstract

An attempted exchange of ammonium cations in (NH4)6[Mo7O24]·6H2O by reaction with tris(hydroxymethyl)aminomethane (tris) resulted in the serendipitous formation of a mixed cationic tetraoxidomolybdate(VI) viz. (NH4)(trisH)[MoO4] 1. The spectral and thermal characteristics, solution conductivity study and the single crystal structure of 1 are reported. The presence of the tetrahedral [MoO4]2- unit is confirmed by the characteristic Raman spectrum. Compound 1 crystallizes in the noncentrosymmetric trigonal space group P3c1 with both the unique cations and the [MoO4]2- dianion located on a three-fold axis. The cations and the anion are interlinked with the aid of O–H⋯O, N–H⋯O and C–H⋯O interactions. The strength and number of O⋯H contacts affect the Mo–O bond distances. Compound 1 exhibits a piezoelectric effect, which is comparable with that of α-quartz. Irradiation of 1 with 1064 nm laser light results in a second harmonic generation (SHG) output which is twice that of potassium dihydrogen phosphate (KDP). Thermal decomposition of 1 results in the formation of orthorhombic α-MoO3.