Abstract
The solid-state structure of (Cs,Na)[(Be2Li)Al2Si6O18], a beryl-type material, was studied in-situ on single crystal and powder samples at hydrostatic high pressure conditions (P ≤ 9.8 GPa) as well as under temperature variation (83 ≤ T ≤ 1173 K) by means of Raman spectroscopy, X-ray and synchrotron diffraction. The investigations show an inconspicuous, easily overlooked transformation from R3¯c to non-centrosymmetric R3c space-group symmetry on increasing pressure (Pc ~ 4 GPa) and decreasing temperature (Tc = 312 ± 5 K), as revealed by Raman band splitting as well as discontinuities in the evolution of Raman spectra, thermal expansivity coefficients and anomalous compression behavior. Distortions to acentric symmetry originate from the Cs and Na guest atoms inside the channels parallel the c-axis. The occurrence of diffuse scattering planes in all measured data sets indicates substitutional defects inside the channels following an approximate ~4.6 Å periodicity, without any remarkable ordering between adjacent channels. The determined thermal expansivity follows an inverse behavior compared to other beryl-type structures for the R3c phase. The bulk moduli for the two polymorphs compare, together with the observed compressional anisotropy, to other complex beryl-type framework structures. However, the symmetry reduction at high pressure leads to independent displacements of the framework oxygen positions, and as a consequence, to the increase of the coordination number from [6 + 6] to [6 + 9] for one of the Cs sites.
Highlights
The solid-state properties of microporous framework structures are in the focus of material science since the porosity on a molecular level provides possibilities for various applications, which include functions as microsieves, ion exchangers, catalytic solids, ion and molecule cages or being host materials for nuclear waste disposal e.g. Ref. [1,2,3,4,5]
The present study shows that variation of P and T leads to an apparently second-order displacive phase transition of (Cs,Na)[(Be2Li)Al2Si6O18] from R3c→R3c, which does not influence the structural stability, despite its open-framework structure
The investigations at low T and at the highest P show that deviations from the superordinate framework symmetry originate from displaced atom positions inside the channels, as required for adaption of individual bonds
Summary
The solid-state properties of microporous framework structures are in the focus of material science since the porosity on a molecular level provides possibilities for various applications, which include functions as microsieves, ion exchangers, catalytic solids, ion and molecule cages or being host materials for nuclear waste disposal e.g. Ref. [1,2,3,4,5]. Representing a heterodesmic (strong, covalent bonds) framework, consisting of both tetrahedral [4]T and octahedral [6]M building units, this host structure reveals separated parallel channels, in a strictly one-dimensional channel system (Fig. S1, supplement), and has high capabilities for the fixation of extra-framework guest molecules and suitably sized guest atoms, such as here of cesium ions. The series of recognized beryl-type □1-xMþx[ [4]T13 [6]M2 [4]T26O18](m)y solids [ □ - vacancy, (m) - extra-framework guest molecule] was extended by (Cs,Na,H2O)[(Be2Li) Al2Si6O18], which is known even in nature as the gemstone mineral pezzottaite [13,14,15,16,17,18,19]. The crystallographic investigations [16,18,19]
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