This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH3)4N]2MnX4 (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P21/c monoclinic structure describes precisely the [(CH3)4N]2MnCl4 crystal in the 0.1-6 GPa range, prior to crystal decomposition and amorphization, while [(CH3)4N]2MnBr4 can be described by a Pmcn orthorhombic structure in its stability pressure range of 0-3 GPa. These materials are attractive systems for pressure studies since they are readily compressible through the weak interaction between organic/inorganic [(CH3)4N+/MnX42-] tetrahedra through hydrogen bonds and contrast with the small compressibility of both tetrahedra. Here we determine the equation-of-state (EOS) of each crystal and compare it with the corresponding local EOS of the MnX42- and (CH3)4N+ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the crystal compressibility, respectively, in both chloride and bromide. The variations of the Mn-Cl bond distance obtained by extended X-ray absorption fine structure and the frequency of the totally symmetric ν1(A1) Raman mode of MnCl42- with pressure in [(CH3)4N]2MnCl4 allowed us to determine the associated Grüneisen parameter (γloc = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Grüneisen parameters and corresponding variations of the intramolecular Mn-Br and C-N bond distances of MnBr42- (γloc = 1.45) and (CH3)4N+ (γloc = 3.0) in [(CH3)4N]2MnBr4.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry