High-Pressure Phase Transitions in Rubidium and Caesium Hydroxides† Cite This: Phys

PCCP View Article Online PAPER View Journal | View Issue High-pressure phase transitions in rubidium and caesium hydroxides† Cite this: Phys. Chem. Chem. Phys., 2016, 18,16527 Andreas Hermann* A computational investigation of the high-pressure phase sequence of the heaviest alkali hydroxides, RbOH and CsOH, shows that the phase diagram of both compounds is richer than hitherto thought. First-principles calculations suggest, based on energetics and comparisons to experimental diffraction and spectroscopy signatures, that the high-pressure phase RbOH-VI, stable above 6 GPa in experiment, should be assigned the KOH-VI structure type, and features localised hydrogen-bonded (OH)4 units. Meanwhile, a new high-pressure phase CsOH-VII is predicted to be stable above 10 GPa in an Received 11th May 2016, isosymmetric phase transition that, like RbOH-VI, marks the transition from layered to three-dimensional Accepted 1st June 2016 network structures under increased compression. Both new phases highlight an unexpected flexi- DOI: 10.1039/c6cp03203f bility of hydrogen bond network formation in a series of compounds that seemingly only vary in Creative Commons Attribution 3.0 Unported Licence. the cation size, and potential consequences for similar systems, such as water-carrying minerals, are www.rsc.org/pccp discussed briefly. 1 Introduction modifications in these compounds, and such transitions (below P = 1 GPa) have indeed been found in all alkali hydroxides and Alkali hydroxides, chemical composition MOH with M an alkali deuteroxides.13–16 The compounds’ responses to increased den- metal from lithium to caesium, are intriguing compounds at sity (especially of the hydrogen bond network) are of particular the interface of ionic and hydrogen bonding. Their crystal interest. In a sense, alkali hydroxides represent the simplest This article is licensed under a structures at high temperatures are driven by ionic interactions models of hydrous minerals. Those are complex phases within + À between M and OH : just below the melting point, all alkali the MgO–H2O–SiO2 and Al2O3–H2O–SiO2 phase diagrams, and hydroxides – bar lithium hydroxide, LiOH – form a cubic ‘‘rotator’’ are important carriers and storage materials for water in Earth’s À 17–19 Open Access Article. Published on 02 June 2016. Downloaded 10/4/2021 2:57:47 AM. phase in the sodium chloride structure with freely rotating OH mantle. Water in these minerals is mainly present in groups on one lattice site, and alkali cations on the other.1–5 hydroxyl form, and can drastically alter the minerals’ physical Around room temperature, hydrogen bonds between the hydroxyl properties. The ladder of interactions that drive the formation groups are stabilised, which break the cubic symmetry and and properties of hydrous minerals (ionic, covalent, and hydro- result in monoclinic, orthorhombic, or tetragonal phases.6,7 gen bonding) is present in the alkali hydroxides as well, and The respective structures are – again, except for LiOH – proton- studies of the behaviour of the latter under pressure can give us disordered, and low-temperature proton-ordered modifications valuable insight into the behaviour of compressed hydrogen- have been found in almost all protonated and deuterated com- bonded networks. pounds (except for NaOH).8–11 Despite these commonalities, Experimental high-pressure studies of the alkali hydroxides microscopic details of the respective compounds’ structures have been performed up to P = 15 GPa in some of these com- differ. For instance, the low-temperature modification of pounds, close to mantle transition zone conditions. Phase transi- RbOH/D has one-dimensional OHÁÁÁOH chains in overall ferro- tions beyond the presumed low-temperature modifications, electric order, while KOH/D and CsOH/D show antiferroelectric resulting in denser non-layered phases, have been reported: order.12 Fig. S1 in the ESI† provides a schematic overview over NaOH-V, stable above 0.8 GPa, with bifurcated hydrogen this phase sequence. bonds;16 KOH-VI, stable above 6.5 GPa, and RbOH-VI, stable It has been argued that moderate compression at room above 6.3 GPa, which were both described as taking up a ‘‘anti- temperature could induce transitions to the low-temperature NaOH’’ structure, though it was not made clear what that structure is.15 Recently, first-principles calculations together with neutron powder diffraction were able to identify the KOH-VI Centre for Science at Extreme Conditions and SUPA, School of Physics and Astronomy, The University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, structure as having an unusual hydrogen-bond network topology, 20 EH9 3FD, UK. E-mail: [email protected] with localised (OH)4 units in a matrix of potassium cations. † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp03203f Likewise, the high-pressure phase LiOH-III, stable above 0.7 GPa, This journal is © the Owner Societies 2016 Phys. Chem. Chem. Phys., 2016, 18, 16527--16534 | 16527 View Article Online Paper PCCP was shown in calculations (analysing neutron diffraction patterns, 3 Results and discussion vibrational and energetic properties) to be a new structure type 3.1 RbOH and CsOH at atmospheric pressure with linear OHÁÁÁOH chains. Calculations also predicted a high-pressure phase LiOH-IV (above 17 GPa) that is structurally The low-temperature phases of RbOH and CsOH, denoted RbOH- identical to NaOH-V.21 IVb and CsOH-IVc respectively, are very similar: in one inter- Here, the thread of theoretical studies of the compressed pretation, they feature kinked one-dimensional OHÁÁÁOH chains alkali hydroxides is continued, and the high-pressure phase running through channels of a cation matrix, see Fig. 1. Ortho- transitions exhibited by RbOH and CsOH are examined. It is rhombic RbOH-IVb, space group Cmc21, has ferroelectrically suggested, on grounds of energetics as well as comparisons to ordered chains, while CsOH-IVc, space group P212121,hasanti- energy-dispersive X-ray (EDX) and spectroscopic data, that ferroelectrically ordered chains (in Fig. 1(b), these point along RbOH-VI has the same structure as KOH-VI, thus also featuring the c axis, alternately into and out of the paper plane). A very common alternative view of both structures is that they are localised (OH)4 units; and that CsOH undergoes an isosymmetric phase transition to a high-pressure phase at 10 GPa, which has layered compounds, as schematically shown in Fig. 1 (c): (Rb,Cs)O5 not been detected in previous experiments. polyhedra form layers of edge-sharing square planar pyramids, with their bases connected to adjacent layers through hydrogen bonds. The topology shown in Fig. 1(c) is the same in RbOH-IVb and CsOH-IVc, the two phases only differ in the relative orienta- 2 Computational methodology tions of the inter-layer hydrogen bonds. To search for potential high-pressure structures, ground-state In the present calculations, the structural parameters of both crystal structure searches were performed for both RbOH RbOH-IVb and CsOH-IVc, as determined by neutron powder and CsOH, with four formula units per unit cell and at 10 and diffraction at atmospheric pressure are reproduced very well, 20 GPa, respectively. The former is significantly above the onset see Table 1. Lattice constants are mostly in agreement by 0.1 Å, and atomic positions by 0.02 in relative coordinates. Creative Commons Attribution 3.0 Unported Licence. of RbOH-VI, reported at 6.3 GPa, and the search unit cells correspond to the unit cell size deduced from the EDX data. They would also be able to describe any structure with one or 3.2 High-pressure phase evolution in RbOH two formula units per cell. Over 1200 structures were optimized In Fig. 2, the ground state enthalpies of formation (relative to in each structure search. RbOH-IVb) of various RbOH phases are plotted as function of The evolutionary algorithm approach as implemented in pressure. The figure includes known alkali hydroxide phases the XTALOPT package was used in conjunction with scalar (found at atmospheric or higher pressure) as well as results from relativistic density functional total energy calculations using crystal structure searches at P = 10 GPa. Naturally, the atomistic the Vasp package.22,23 This combined computational approach models here are restricted to proton-ordered phases. This article is licensed under a has proven very fruitful in recent years in computational studies The calculations correctly reproduce RbOH-IVb as the ground of high-pressure systems, including hydrogen-rich and hydrogen- state of RbOH at low pressures. Only slightly higher in enthalpy bonded compounds.24–28 Candidate structures from the crystal is the CsOH-IVc structure type that, as discussed above, only Open Access Article. Published on 02 June 2016. Downloaded 10/4/2021 2:57:47 AM. structure searches, as well as other structures known in the differs in the anti-ferroelectric order of the OH chains, but is alkali hydroxides, were optimized across the pressure range otherwise very similar. At P = 10 GPa, two structures very close in P = 1 atm to 20 GPa. Density functional calculations employed enthalpy are found to become more stable than RbOH-IVb. Both the PBE exchange–correlation functional to describe the are tetragonal, of I4/m and P4/mnc symmetry, and emerged from electron–electron interaction and projector augmented wave the crystal structure searches. These are the same structures that (PAW) data sets to model the electron–ion interaction,29,30 were found as high-pressure candidates in potassium hydroxide using Rb(4s24p65s1) Cs(5s25p66s1) O(2s22p4) and H(1s1) valence spaces, with cutoff radii of 2.5, 2.5, 1.52, and 1.1 aB, respectively. Spin-polarisation and spin–orbit effects were not included, as all systems studied here are effectively closed-shell, wide-gap insulators. A plane wave basis cutoff of 600 eV and regular k-grids of density 20/ÅÀ1 to sample reciprocal space were used to achieve convergence of total energies, and geometries were optimised until remaining force components were below 1 meV ÅÀ1.

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