Bulk And Molecular Compressibilities Of Organic-Inorganic Hybrids [(Ch3)(4)N](2)Mnx4 (X = Cl, Br); Role Of Intermolecular Interactions

This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH3)(4)N](2)MnX4 (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P2(1)/c monoclinic structure describes precisely the [(CH3)(4)N](2)MnCl4 crystal in the 0.1-6 GPa range, prior to crystal decomposition and amorphization, while [(CH3)(4)N](2)MnBr4 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)(4)N+/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)(4)N+ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the 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 nu(1)(A(1)) Raman mode of MnCl42- with pressure in [(CH3)(4)N](2)MnCl4 allowed us to determine the associated Gruneisen parameter (gamma(loc) = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Gruneisen parameters and corresponding variations of the intramolecular Mn-Br and C-N bond distances of MnBr42- (gamma(loc) = 1.45) and (CH3)(4)N+ (gamma(loc) = 3.0) in [(CH3)(4)N](2)MnBr4.