Structural characterization of a new high-pressure phase of GaAsO4

Polymorphism and Phase Stability of Hydrated Magnesium Carbonate Nesquehonite MgCO3·3H2O: Negative Axial Compressibility and Thermal Expansion in a Cementitious Material

The P-T phase diagram of the hydrated magnesium carbonate nesquehonite (MgCO3·3H2O) has not been reported in the literature. In this paper, we present a joint experimental and computational study of the phase stability and structural behavior of this cementitious material at high-pressure and high-temperature conditions using in situ single-crystal and synchrotron powder X-ray diffraction measurements in resistive-heated diamond anvil cells plus density functional theory calculations. Our results show that nesquehonite undergoes two pressure-induced phase transitions at 2.4 (HP1) and 4.0 GPa (HP2) at ambient temperature. We have found negative axial compressibility and thermal expansivity values, likely related to the directionality of the hydrogen bonds. The equations of state of the different phases have been determined. All the room-temperature compression effects were reversible. Heating experiments at 0.7 GPa show a first temperature-induced decomposition at 115 °C, probably into magnesite and a MgCO3·4H2O phase.

Compressibility and Phase Stability of Iron-Rich Ankerite

The structure of the naturally occurring, iron-rich mineral Ca1.08(6)Mg0.24(2)Fe0.64(4)Mn0.04(1)(CO3)2 ankerite was studied in a joint experimental and computational study. Synchrotron X-ray powder diffraction measurements up to 20 GPa were complemented by density functional theory calculations. The rhombohedral ankerite structure is stable under compression up to 12 GPa. A third-order Birch–Murnaghan equation of state yields V0 = 328.2(3) Å3, bulk modulus B0 = 89(4) GPa, and its first-pressure derivative B’0 = 5.3(8)—values which are in good agreement with those obtained in our calculations for an ideal CaFe(CO3)2 ankerite composition. At 12 GPa, the iron-rich ankerite structure undergoes a reversible phase transition that could be a consequence of increasingly non-hydrostatic conditions above 10 GPa. The high-pressure phase could not be characterized. DFT calculations were used to explore the relative stability of several potential high-pressure phases (dolomite-II-, dolomite-III- and dolomite-V-type structures), and suggest that the dolomite-V phase is the thermodynamically stable phase above 5 GPa. A novel high-pressure polymorph more stable than the dolomite-III-type phase for ideal CaFe(CO3)2 ankerite was also proposed. This high-pressure phase consists of Fe and Ca atoms in sevenfold and ninefold coordination, respectively, while carbonate groups remain in a trigonal planar configuration. This phase could be a candidate structure for dense carbonates in other compositional systems.

M3+(H2AsO4)(H2As2O7) (M3+ = Al, Ga) and In2(H2AsO4)2(H2As2O7)2: a new layer structure type and a new framework structure type containing the rare H2As2O72- group

The crystal structures of hydrothermally synthesized aluminium dihydrogen arsenate(V) dihydrogen diarsenate(V), Al(H₂AsO₄)(H₂As₂O₇), gallium dihydrogen arsenate(V) dihydrogen diarsenate(V), Ga(H₂AsO₄)(H₂As₂O₇), and diindium bis[dihydrogen arsenate(V)] bis[dihydrogen diarsenate(V)], In₂(H₂AsO₄)₂(H₂As₂O₇)₂, were determined from single-crystal X-ray diffraction data collected at room temperature. The first two compounds are representatives of a novel sheet structure type, whereas the third compound crystallizes in a novel framework structure. In all three structures, the basic building units are M³⁺O₆ octahedra (M = Al, Ga, In) that are connected via one H₂AsO₄^- and two H₂As₂O₇^2- groups into chains, and further via H₂As₂O₇^2- groups into layers. In Al/Ga(H₂AsO₄)(H₂As₂O₇), these layers are interconnected by weak-to-medium-strong hydrogen bonds. In In₂(H₂AsO₄)₂(H₂As₂O₇)₂, the H₂As₂O₇^2- groups link the chains in three dimensions, thus creating a framework topology, which is reinforced by weak-to-medium-strong hydrogen bonds. The three title arsenates represent the first compounds containing both H₂AsO₄^- and H₂As₂O₇^2- groups.

Gallium fluoroarsenates

Six new phases in the gallium-fluoride-arsenate system have been synthesised hydrofluorothermally using a fluoride-rich medium and "HAsF6" (HF : AsF5) as a reactant. RbGaF3(H2AsO4), KGaF(H2AsO4) and [piperazine-H2]2[Ga2F8(HAsO4)]·H2O have one dimensional structures, [DABCO-H2]2[Ga4F7O2H(AsO4)2]·4H2O consists of two dimensionally connected polyhedral layers, while GaF(AsO3[OH,F])2 and (NH4)3Ga4F9(AsO4)2 both have three-dimensionally connected polyhedral frameworks.

Atomistic modeling of the low-temperature atom-beam deposition of magnesium fluoride

We model the deposition and growth of MgF(2) on a sapphire substrate as it occurs in a low-temperature atom-beam-deposition experiment. In the experiment, an (X-ray) amorphous film of MgF(2) is obtained at low temperatures of 170-180 K, and upon heating, this transforms to the expected rutile phase via the CaCl(2)-type structure. We confirm this from our simulations and propose a mechanism for this transformation. The growth process is analyzed as a function of the synthesis parameters, which include the substrate temperature, deposition rate of clusters, and types of clusters deposited. Upon annealing an initially amorphous deposit, we observe the formation of two competing nanocrystalline modifications during this process, which exhibit the CaCl(2) and CdI(2) structure types, respectively. We argue that this joint growth of the two nanocrystalline polymorphs stabilizes the kinetically unstable CaCl(2)-type structure on the macroscopic level long enough to be observed in the experiment.

Six-fold-coordinated phosphorus by oxygen in AlPO4 quartz homeotype under high pressure

AlPO4 belongs to the berlinite quartz homeotype family, which has been the subject of intense high-pressure research triggered by the supposed existence of reversible pressure-induced amorphization. X-ray diffraction experiments, complemented with ab initio calculations, demonstrate the existence of two high-pressure crystalline polymorphs and show that AlPO4 shares the same two-stage densification mechanism as silica. In the first step, a compact hexagonal sublattice of oxygen atoms is formed. In the second step, the cations redistribute in the interstices giving rise to a monoclinic distorted CaCl2 phase. The most outstanding feature of the phase is that phosphorous becomes six-fold coordinated by oxygen, adopting a configuration unknown so far in solid-state science. This finding opens possibilities in the high-pressure chemistry of phosphorus. The close relationship of AlPO4 with silica suggests the existence of completely unexplored families of compounds analogous to those of six-fold-coordinated silicates but based on PO6.