title: Bulk and molecular compressibilities of organic-inorganic hybrids [(CH₃)₄N]₂MnX₄ (X = Cl, Br); role of intermolecular interactions.
abstract: 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 (γ(loc) = 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⁻ (γ(loc) = 1.45) and (CH₃)₄N⁺ (γ(loc) = 3.0) in [(CH₃)₄N]₂MnBr₄.
authors.affiliation: MALTA Consolider Team, Earth Science and Condensed Matter Physics Department (DCITIMAC), Facultad de Ciencias and ‡MALTA Consolider Team, Applied Physics Department, University of Cantabria , 39005 Santander, Spain.
authors.forename: Jose Antonio
authors.initials: JA
authors.lastname: Barreda-Argüeso
authors.forename: Lucie
authors.initials: L
authors.lastname: Nataf
authors.forename: Yamilet
authors.initials: Y
authors.lastname: Rodríguez-Lazcano
authors.forename: Fernando
authors.initials: F
authors.lastname: Aguado
authors.forename: Jesús
authors.initials: J
authors.lastname: González
authors.forename: Rafael
authors.initials: R
authors.lastname: Valiente
authors.forename: Fernando
authors.initials: F
authors.lastname: Rodríguez
authors.forename: Heribert
authors.initials: H
authors.lastname: Wilhelm
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Jephcoat