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In the pursuit 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 PrVO4under high-pressure conditions, up to 24 GPa. We have found that PrVO4undergoes 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 BaWO4-II type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. The monazite phase is thus a metastable polymorph of PrVO4. The monazite–BaWO4-II transition is found instead to be reversible 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 PrVO4. We found that the zircon–monazite transition produces a collapse of the band gap and an abrupt decrease in the resistivity. The physical reasons for this behavior are discussed. Density functional theory simulations support our conclusions.

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 Ia3̅ (C-type) Tb2O3undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic C2/m(B-type) phase at ∼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3̅m1 (A-type) phase at ∼12GPa. Thus, Tb2O3is found to follow the well-known C → B → A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb. Our ab initiotheoretical 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, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase. Finally, we extract the bulk moduli and the Raman-active mode 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 as well as with new calculations for selected 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 and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals.

High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4into the high-pressure phases of its parent binary compounds (α-Sb2Te3and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4is framed within the extended orbital radii map of BA2Te4compounds, so our results pave the way to understand the pressure behavior and stability ranges of other “natural van der Waals” compounds with similar stoichiometry.

SbPO4is a complex monoclinic layered material characterized by a strong activity of the nonbonding lone electron pair (LEP) of Sb. The strong cation LEP leads to the formation of layers piled up along the aaxis and linked by weak Sb–O electrostatic interactions. In fact, Sb has 4-fold coordination with O similarly to what occurs with the P–O coordination, despite the large difference in ionic radii and electronegativity between both elements. Here we report a joint experimental and theoretical study of the structural and vibrational properties of SbPO4at high pressure. We show that SbPO4is not only one of the most compressible phosphates but also one of the most compressible compounds of the ABO4family. Moreover, it has a considerable anisotropic compression behavior, with the largest compression occurring along a direction close to the aaxis and governed by the compression of the LEP and the weak interlayer Sb–O bonds. The strong compression along the aaxis 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 SbPO4can aid to improve the ion intercalation and catalytic properties of this layered compound.