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GaGeTe is a layered topological semimetal that has been recently found to exist in at least two different polytypes, $\alpha$-GaGeTe ($R\bar{3}m$) and $\beta$-GaGeTe ($P6_3 mc$). Here we report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of these two polytypes at high pressure. Both polytypes show anisotropic compressibility and two phase transitions, above 7 and 15 GPa, respectively, as confirmed by XRD and Raman spectroscopy measurements. Although the nature of the high-pressure phases is not confirmed, comparison with other chalcogenides and total-energy calculations allow us to propose possible high-pressure phases for both polytypes with an increase in coordination for Ga and Ge atoms from 4 to 6. In particular, the simplification of the X-ray patterns for both polytypes above 15 GPa suggests a transition to a structure of relatively higher symmetry than the original one. This result is consistent with the rocksalt-like high-pressure phases observed in parent III-VI semiconductors, such as GaTe, GaSe, and InSe. Pressure-induced amorphization is observed upon pressure release. The electronic band structures of $\alpha$-GaGeTe and $\beta$-GaGeTe and their pressure dependence also show similarities to III-VI semiconductors, thus suggesting that the germanene-like sublayer induces a semimetallic character in both GaGeTe polytypes. Above 3 GPa, both polytypes lose their topological features, due to the opening of the direct band gap, while the reduction of the interlayer space increases the thermal conductivity at high pressure., Comment: Main manuscript: 47 pages, 12 figures, 4 tables; ESI: 20 pages, 12 figures, 9 tables

SbPO4 is a complex monoclinic layered material characterized by a strong activity of the non-bonding lone electron pair (LEP) of Sb. The strong cation LEP leads to the formation of layers piled up along the a-axis and linked by weak Sb-O electrostatic interactions. In fact, Sb is 4-fold coordination with O similar to what occurs with the P-O coordination, despite the large difference of ionic radii and electronegativity between both elements. Here we report a joint experimental and theoretical study of the structural and vibrational properties of SbPO4 at high pressure. We show that SbPO4 is not only one of the most compressible phosphates but also one of the most compressible compounds of the ABO4 family. Moreover, it has a considerable anisotropic compression behavior with the largest compression occurring along a direction close to a-axis and governed by the compression of the LEP and the weak inter-layer Sb-O bonds. The strong compression along the a-axis leads to a subtle modification of the monoclinic crystal structure above 3 GPa leading from a 2D to a 3D material. Moreover, the onset of a reversible pressure-induced phase transition is observed above 9 GPa, which is completed above 20 GPa. We propose that the high-pressure phase is a triclinic distortion of the original monoclinic phase. The understanding of the compression mechanism of SbPO4 can aid in understanding the importance of the ion intercalation and catalytic properties of this layered compound., Comment: Main text - 43 pages Supplementary Information - 13 pages

High-pressure X-ray diffraction studies on nanocrystals of the pseudo-vaterite-type borates GdBO$_3$ and YBO$_3$ are herein reported up to 17.4(2) and 13.4(2) GPa respectively. The subsequent determination of the room-temperature pressure-volume equations of state is presented and discussed in the context of contemporary publications which contradict the findings of this work. In particular, the isothermal bulk moduli of GdBO$_3$ and YBO$_3$ are found to be 170(13) and 163(13) GPa respectively, almost 50% smaller than recent findings. Our experimental results provide an accurate revision of the high-pressure compressibility behaviour of GdBO$_3$ and YBO$_3$ which is consistent with the known systematics in isomorphic borates and previous ab initio calculations. Finally we discuss how experimental/analytical errors could have led to unreliable conclusions reported elsewhere.

In pursue of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO$_4$ under high pressure conditions, up until 24 GPa. We have found that PrVO$_4$ undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO$_4$-I--type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. Monazite phase is thus a metastable polymorph of PrVO$_4$. The monazite-BaWO$_4$-II transition is found to be reversible instead and occurs with a similar volume change. Here we report and discuss the axial and bulk compressibility of all phases. We also compare our results with those for other rare-earth orthovanadates. Finally, by means of optical-absorption experiments and resistivity measurements we determined the effect of pressure on the electronic properties of PrVO$_4$. We found that the zircon-monazite transition produces a collapse of the band gap and an abrupt decrease of the resistivity. The physical reasons for this behavior are discussed. Density-functional-theory simulations support our conclusions., Comment: 34 pages,19 Figures (Including Supporting Information). Supporting Information. Submitted to Inorganic Chemistry

We report a joint experimental and theoretical investigation of the high-pressure structural and vibrational properties of terbium sesquioxide (Tb2O3). Powder x-ray diffraction and Raman scattering measurements show that cubic (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic (B-type) phase at ~7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal (A-type) phase at ~12 GPa. Thus, Tb2O3 is found to follow the well-known C-B-A phase transition found in other cubic rare-earth sesquioxides with cations of larger atomic mass than Tb. Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results. Moreover, Raman-active modes of the three phases have been monitored as a function of pressure and lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C-type phase as well as to make a tentative assignment of the symmetry of most vibrational modes in the B- and A-type phases. Finally, we extract the bulk moduli and the Raman-active modes frequencies together with their pressure coefficients for the three phases of Tb2O3. These results are thoroughly compared and discussed in relation to those reported for rare-earth and other related sesquioxides. It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations., Comment: Supplementary Material is attached below the main manuscript

This joint experimental and theoretical study of the structural, vibrational and electrical properties of rhombohedral SnSb2Te4 at high pressure unveils the internal mechanisms of its compression. The equation of state and the internal polyhedral compressibility, the symmetry and behavior of the Raman-active modes and the electrical behavior of this topological insulator under compression have been discussed and compared with the parent binary alpha-Sb2Te3 and SnTe compounds and with related ternary compounds. Our X-ray diffraction and Raman measurements together with theoretical calculations, which include topological electron density and electronic localization function analysis, evidence the presence of an isostructural phase transition around 2 GPa and a Fermi resonance around 3.5 GPa. The Raman spectrum of SnSb2Te4 shows vibrational modes that are forbidden in rocksalt SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Additionally, since SnSb2Te4 is an incipient metal, like its parent binary compounds, we establish a new criterion to identify the recently proposed metavalent bonding in complex materials when different bond characters coexist in the system. Finally, SnSb2Te4 exhibits a pressure-induced decomposition into the high-pressure phases of its parent binary compounds above 7 GPa, which is supported by an analysis of their formation enthalpies. We have framed the behavior of SnSb2Te4 within the extended orbital radii map of BA2Te4 compounds, which paves the way to understand the pressure behavior and stability ranges of other layered van der Waals-type compounds with similar stoichiometry.

The compression of arsenolite (cubic As2O3) has been studied from a joint experimental and theoretical point of view. Experimental X-ray diffraction and Raman scattering measurements of this molecular solid at high pressures with different pressure-transmitting media have been interpreted with the help of ab initio calculations. Our results confirm arsenolite as one of the softest minerals in absence of hydrogen bonding and provide evidence for helium trapping above 3 GPa between adamantane-type As4O6 cages, thus leading to a new compound with stoichiometry He2As4O6. Helium trapping alters all properties of arsenolite. In particular, pressure-induced amorphization, which occurs in pure arsenolite above 15 GPa, is impeded when He is trapped between the As4O6 cages; thus resulting in a mechanical stability of He2As4O6 beyond 30 GPa. Our work paves the way for the modification of the properties of other molecular solids by compression depending on their ability to trap relatively small atomic or molecular species and form new compounds. Furthermore, our work suggests that compression of molecular solids with noble gases as helium could result in unexpected results compared to other pressure-transmitting media., Comment: 15 pages, 4 figures

Contradictory results of high-pressure studies in cubic bixbyite-type indium oxide (c-In2O3) at room temperature (RT) have motivated us to perform high-pressure powder x-ray diffraction and Raman scattering measurements in this material. On increasing pressure c-In2O3 undergoes a transition to the Rh2O3-II structure. On decreasing pressure Rh2O3-II-type In2O3 undergoes a transition to a previously unknown phase which is isostructural to Rh2O3-III. On further decrease of pressure, another phase transition to corundum-type In2O3, which is metastable at room conditions, is observed. Recompression of metastable corundum-type In2O3 shows that the Rh2O3-III phase is the post-corundum phase. Our results are supported by theoretical ab initio calculations which show that the Rh2O3-III phase could be present in other sesquioxides, thus leading to a revision of the pressure-temperature phase diagrams of sesquioxides., Comment: 14 pages, 7 figures, 2 tables

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging, and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.