Theoretical and Experimental Study of the Crystal Structures, Lattice Vibrations, and Band Structures of Monazite-Type PbCrO4, PbSeO4, SrCrO4, and SrSeO4

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series

We report diffuse reflectivity measurements in InNbO₄, ScNbO₄, YNbO₄, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO₄ and ScNbO₄ shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO₄, 4.35 eV for LaNbO₄, 4.5 eV for YNbO₄, and 4.73-4.93 eV for SmNbO₄, EuNbO₄, GdNbO₄, DyNbO₄, HoNbO₄, and YbNbO₄. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO₄ to YbNbO₄ and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO₄, CeVO₄, and LaNbO₄ have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.

Thermodynamic Analysis and Experimental Study on the Oxidation of PbX (X = S, Se) Nanostructured Layers

Heat treatment in an oxygen-containing medium is a necessary procedure in the technology of forming photodetectors and emitters based on lead chalcogenides. Lead chalcogenide layers (PbS, PbSe) were prepared via a chemical bath deposition method. Surface oxidation of lead chalcogenide layers was analyzed using X-ray diffraction and Raman spectroscopy methods, and thermodynamic analysis of the oxidation of PbSe and PbS layers was also performed. The calculated phase diagrams from 20 °C to 500 °C showed good agreement with the experimental results. According to the thermodynamic analysis, the oxidation products depend on the initial composition of the layers and temperature of the annealing. In some cases, the formation of a separate metallic phase Pb is possible along with the formation of lead oxide PbO and other oxides. The performed thermodynamic analysis makes it possible to substantiate the two-stage annealing temperature regimes which ensure an increase in the speed of photodetectors.

Hydrothermal synthesis and crystal structure of a novel double-perovskite-type bismuth oxide with 3:1 ordering at the B-site

A low-temperature hydrothermal method was successfully used to synthesize a novel bismuth oxide Ba4Bi3NaO12. Here, NaBiO3·nH2O was used as one of the starting materials. Single-crystal X-ray diffraction revealed the triclinic...

Hydrothermal Synthesis and Crystal Structure of a Novel Bismuth Oxide: (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3

A novel distorted perovskite-type (K_0.2Sr_0.8)(Na_0.01Ca_0.25Bi_0.74)O₃ was prepared by a hydrothermal method using the starting materials NaBiO₃·nH₂O, Sr(OH)₂·8H₂O, Ca(OH)₂, and KOH. Single-crystal X-ray diffraction of the novel compound revealed a GdFeO₃-related structure belonging to the monoclinic system of the space group Cc with the following cell parameters: a = 11.8927 (17) Å, b = 11.8962 (15) Å, c = 8.4002 (10) Å, and β = 90.116 (9)°. The final R-factors were obtained as R ₁ = 0.0354 and wR ₂ = 0.0880 (using all the data). K⁺ and Sr²⁺ ions were distributed at four types of A-sites. On the other hand, four Bi⁵⁺-sites (Bi1, Bi2, Bi3, and Bi4) were occupied by four Ca²⁺ ions (Ca1, Ca2, Ca3, and Ca4), and the first three B-sites were occupied predominantly by Bi⁵⁺ with Na⁺ ions. The forth B-site was occupied predominantly by the Ca²⁺ ion with Bi⁵⁺ ions. Two types of B-sites, thus forming tilted distorted (Na/Ca/Bi)O₆ and (Bi/Ca)O₆ octahedra, have an ordering of 3:1 represented as (K/Sr)₄(Na/Ca/Bi)₃(Bi/Ca)O₁₂. The distorted (Na/Ca/Bi)O₆ and (Ca/Bi)O₆ octahedra formed a perovskite-type network by corner sharing with features closely matching those of a GdFeO₃-type structure. The novel compound is the first example of a perovskite-type bismuth oxide containing only Bi⁵⁺ in a system without a Ba atom and has a unique ordering (3:1) of the B site. The compound showed photocatalytic activity for phenol degradation under visible light irradiation.

High-pressure monoclinic-monoclinic transition in fergusonite-type HoNbO4

In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO₄. Powder x-ray diffraction experiments andab initiodensity-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P2₁/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO₄, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO₄. According toab initiocalculations, the reported phase transition is related to a mechanical instability and a phonon softening.

Crystal structure and phase transition of TlReO4: a combined experimental and theoretical study

The present work describes a density-functional theory (DFT) study of TlReO₄ in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P2₁/c) is observed at room temperature while two isostructural tetragonal structures (space group I4₁/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic structure has the lowest total energy when compared to the orthorhombic structure, which was originally proposed to be the structure at room temperature, which agrees with our experiments. The structural and vibrational properties of the low- and room-temperature phase of TlReO₄ have been calculated using DFT. Inclusion of van der Waals correction to the standard DFT exchange correlation functional is found to improve the agreement with the observed structural and vibrational properties. The Born effective charge of these phases has also been studied which shows a combination of ionic and covalent nature, resembling metavalent bonding. Calculations of zone-center phonon frequencies lead to the symmetry assignment of previously reported low-temperature Raman modes. We have determined the frequencies of the eight infrared-active, 13 Raman-active and three silent modes of low-temperature TlReO₄ along with 105 infrared-active and 108 Raman-active modes for room-temperature TlReO₄. Phonons of these two phases of TlReO₄ are mainly divided into three regions which are below 150 cm^-1 due to vibration of whole crystal, 250 to 400 cm^-1 due to wagging, scissoring, rocking and twisting and above 900 cm^-1 due to stretching in ReO₄ tetrahedron. The strongest infrared peak is associated to the internal asymmetric stretching of ReO₄ whereas the strongest Raman peak is associated to the internal symmetric stretching of ReO₄. We have also measured the room-temperature Raman spectra of monoclinic TlReO₄ identifying up to 28 modes. This Raman spectrum has been interpreted by comparison with the previously reported Raman frequencies of the low-temperature phase and our calculated Raman frequencies of low- and room-temperature phases of TlReO₄.

PrVO4 under High Pressure: Effects on Structural, Optical, and Electrical Properties

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 PrVO₄ under high-pressure conditions, up to 24 GPa. We have found that PrVO₄ 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 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.

Simple New Method for the Preparation of La(IO3)3 Nanoparticles

We present a cost- and time-efficient method for the controlled preparation of single phase La(IO₃)₃ nanoparticles via a simple soft-chemical route, which takes a matter of hours, thereby providing an alternative to the common hydrothermal method, which takes days. Nanoparticles of pure α-La(IO₃)₃ and pure δ-La(IO₃)₃ were synthesised via the new method depending on the source of iodate ions, thereby demonstrating the versatility of the synthesis route. The crystal structure, nanoparticle size-dispersal, and chemical composition were characterised via angle- and energy-dispersive powder X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy.

Disclosing the behavior under hydrostatic pressure of rhombohedral MgIn2Se4 by means of first-principles calculations

AM2X4 crystalline materials display important technological electronic, optical and magnetic properties that are sensitive to general stress effects. In this paper, the behavior under hydrostatic pressure of the ambient condition rhombohedral phase of MgIn2Se4 is investigated in detail for the first time. We carried out first-principles calculations within the density functional theory framework aimed at determining the pressure-induced polymorphic sequence of this selenide. To accurately evaluate transition pressures at room temperature, thermal corrections have been included after the computation of phonon dispersion curves in potential candidate phases, namely the initial rhombohedral R3[combining macron]m one, inverse and direct spinels, LiTiO2-type and defective I4[combining macron] structures. Only the transition from the R3[combining macron]m to the inverse spinel phase was found to fulfill the thermodynamic and mechanical stability criteria. The direct spinel could appear as metastable if kinetic effects hinder the above transition. Additionally, electronic band structures and chemical bonding properties were analyzed from the outcome of our quantum-mechanical solutions reporting band gap values and ionicity and noncovalent interaction indexes. It is shown that the investigated compound keeps behaving as a semiconductor, loses its van der Waals interactions, and becomes more covalent as hydrostatic pressure is applied.

Influences of hole/electron-lattice coupling on phase transition between λ-Ti3O5and β-Ti3O5

The phase transition between λ-Ti3O5and β-Ti3O5is an intriguing process that can be driven in multiple ways. However, the phase transition has not been reasonably and universally analyzed in atomic-scale, because it is limited by experimental inaccessibility. Here, the nudged elastic band method, crystal orbital Hamiltonian population integral calculation, phonon calculation, and electron (or hole) doping calculation are used to investigate the phase transition between λ-Ti3O5and β-Ti3O5. The atomic displacement mode in the phase transition between the β-Ti3O5and λ-Ti3O5is provided, and a theory that the coupling between the lattice and excited electrons (or holes) is responsible for the phase transition between λ-Ti3O5and β-Ti3O5is established.

High-pressure characterization of multifunctional CrVO4

The structural stability and physical properties of CrVO4under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, andab initiocalculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

Hydrothermal Synthesis and Crystal Structure of a Mixed-Valence Bismuthate, Na3Bi3O8

Four types of bismuth oxides, Na₃Bi₃O₈, NaBiO₃, α-Bi₂O₃, and ε-Bi₂O₃, were obtained by hydrothermal reactions using NaBiO₃·nH₂O in NaOH solution. The crystal structure of a new phase (Na₃ Bi³⁺)Bi₂⁵⁺O₈ ((Na_0.75Bi_0.25)₂BiO₄) was determined by using single crystal X-ray diffraction data, and this compound was found to show a Na₂MnCl₄-related structure with a monoclinic system (space group, Pm) with the following lattice parameters: a = 5.990 (2) Å, b = 3.335 (2) Å, c = 10.108 (2) Å, and β = 91.08 (3)°. The final R-factors R₁ and wR₂ were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi³⁺ and Bi⁵⁺, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi⁵⁺ (Bi1 and Bi2) fully occupied the distorted octahedral sites and three Bi³⁺ (Bi3, Bi4, and Bi5) were statistically distributed at the split sites with Na⁺ (Na3, Na4, and Na5). The Na6 site is fully occupied. The distorted Bi⁵⁺O₆ octahedra formed one-dimensional chains via edge-sharing along the b-axis, with the chains held by Bi³⁺/Na⁺ split sites. The structural feature except for the split distribution of Bi³⁺/Na⁺ was classified as a Na₂MnCl₄-type structure. DFT calculations based on a model discounting the split distribution of Bi³⁺/Na⁺ indicated that Bi 6s and O 2p orbitals form sp hybridization at the conduction band. This new mixed valence bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation. In addition to Na₃Bi₃O₈, the hydrothermal reaction using NaBiO₃·nH₂O in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO₃ and two polymorphs of bismuth oxides with monoclinic (α-Bi₂O₃) and orthorhombic (ε-Bi₂O₃) structures, depending on the reaction temperature and NaOH concentration.

Towards materials discovery: assays for screening and study of chemical interactions of novel corrosion inhibitors in solution and coatings

Using chemical design, computational modelling, structure–activity and structure–property relationships, with high-throughput solution and coating assays we can rapidly identify prime corrosion inhibitor candidates for further detailed evaluation.

Optical and magnetic properties of Gd1-x Sr x CrO3 (0 ⩽ x ⩽ 0.15)

Optical and magnetic properties of Gd_1-x Sr _x CrO₃ (0  ⩽  x  ⩽  0.15) polycrystalline samples have been investigated to identify the role of Sr²⁺ ion in GdCrO₃ compound. Quantitative structural analysis using Rietveld refinement suggests the formation of single orthorhombic phase up to x  =  0.1 composition, and in addition, a secondary monazite-type structure of SrCrO₄ arises for x  =  0.15 composition. Raman and Fourier transform infrared spectra also confirm the presence of SrCrO₄ component for higher concentration of Sr²⁺ ion. Mixed valence state of Cr such as Cr³⁺ and Cr⁴⁺ is obtained from x-ray photoelectron spectroscopy. The presence of Cr⁴⁺ ion induces a reduction of optical band gap with increase of Sr content. The unoccupied states originate from Cr⁴⁺ modify d-d transitions. Larger cell volume and breaking of magnetic exchange pathways through nonmagnetic element (Sr) weaken the strength of interaction and consequently lower Néel and spin reorientation temperatures.

A High-Pressure Investigation of the Synthetic Analogue of Chalcomenite, CuSeO3∙2H2O

Synthetic chalcomenite-type cupric selenite CuSeO3∙2H2O has been studied at room temperature under compression up to pressures of 8 GPa by means of single-crystal X-ray diffraction, Raman spectroscopy, and density-functional theory. According to X-ray diffraction, the orthorhombic phase undergoes an isostructural phase transition at 4.0(5) GPa with the thermodynamic character being first-order. This conclusion is supported by Raman spectroscopy studies that have detected the phase transition at 4.5(2) GPa and by the first-principles computing simulations. The structure solution at different pressures has provided information on the change with pressure of unit–cell parameters as well as on the bond and polyhedral compressibility. A Birch–Murnaghan equation of state has been fitted to the unit–cell volume data. We found that chalcomenite is highly compressible with a bulk modulus of 42–49 GPa. The possible mechanism driving changes in the crystal structure is discussed, being the behavior of CuSeO3∙2H2O mainly dominated by the large compressibility of the coordination polyhedron of Cu. On top of that, an assignation of Raman modes is proposed based upon density-functional theory and the pressure dependence of Raman modes discussed. Finally, the pressure dependence of phonon frequencies experimentally determined is also reported.

Putting the Squeeze on Lead Chromate Nanorods

We have studied by means of X-ray diffraction and Raman spectroscopy the high-pressure behavior of PbCrO₄ nanorods. We have found that these nanorods follow a distinctive structural sequence that differs from that of bulk PbCrO₄. In particular, a phase transition from a monoclinic monazite-type PbCrO₄ to a novel monoclinic AgMnO₄-type polymorph has been discovered at 8.5 GPa. The crystal structure, Raman-active phonons, and compressibility of this novel high-pressure phase are reported for the first time. The experimental findings are supported by ab initio calculations that provide information not only on structural and vibrational properties of AgMnO₄-type PbCrO₄ but also on the electronic properties. The discovered phase transition triggers a band gap collapse and a subsequent metallization at 44.2 GPa, which has not been observed in bulk PbCrO₄. This suggests that nanoengineering can be a useful strategy to drive metallization under compression.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties

We have investigated the high-pressure behavior of PbCrO₄. In particular, we have probed the existence of structural transitions under high pressure (at 4.5 GPa) by single-crystal X-ray diffraction and density functional theory calculations. The structural sequence of PbCrO₄ is different than previously determined. Specifically, we have established that PbCrO₄, under pressure, displays a monoclinic-tetragonal phase transition, with no intermediate phases between the low-pressure monoclinic monazite structure (space group P2₁/ n) and the high-pressure tetragonal structure. The crystal structure of the high-pressure polymorph is, for the first time, undoubtedly determined to a tetragonal scheelite-type structure (space group I4₁/ a) with unit-cell parameters a = 5.1102(3) Å and c = 12.213(3) Å. These findings have been used for a reinterpretation of previously published Raman and optical-absorption results. Information of calculated infrared-active phonons will be also provided. In addition, the pressure dependence of the unit-cell parameters, atomic positions, bond distances, and polyhedral coordination are discussed. The softest and stiffest direction of compression for monazite-type PbCrO₄ are also reported. Finally, the theoretical pressure dependence of infrared-active modes is given, for the first time, for both polymorphs.

Effect of High Pressure on the Crystal Structure and Vibrational Properties of Olivine-Type LiNiPO4

In this work, we present an experimental and theoretical study of the effects of high pressure and high temperature on the structural properties of olivine-type LiNiPO₄. This compound is part of an interesting class of materials primarily studied for their potential use as electrodes in lithium-ion batteries. We found that the original olivine structure (α-phase) is stable up to ∼40 GPa. Above this pressure, the onset of a new phase is observed, as put in evidence by the X-ray diffraction (XRD) experiments. The structural refinement shows that the new phase (known as β-phase) belongs to space group Cmcm. At room temperature, the two phases coexist at least up to 50 GPa. A complete conversion to the β-phase was only obtained at high-pressure and high-temperature conditions (973 K, 6.5 GPa), as confirmed by both XRD and Raman spectroscopy. Ab initio calculations support the same structural sequence. The need of high-temperature conditions to obtain the complete transformation of the α-phase into the β-phase is indicative of the existence of a kinetic barrier for the phase transition. Here, we report the evolution of crystallographic parameters as a function of pressure for both phases, comparing them with the theoretical predictions. We also discuss the influence of pressure on the polyhedral units and report room-temperature equations of state. The dependence of the Raman phonons of both phases on pressure is also studied, assigning to each phonon its respective symmetry by comparison with the results of the ab initio simulations.

Crystal Structure, Thermal Behavior, and Photocatalytic Activity of NaBiO3· nH2O

The crystal structure of NaBiO₃· nH₂O was refined using synchrotron powder X-ray diffraction and was assigned to a trigonal unit cell (space group P3̅) consisting of layered structures formed by edge-sharing BiO₆ octahedra and consisting of an interlayer composed of water molecules sandwiched between two layers of sodium atoms, perpendicular to the c axis. An intermediate phase was observed during the dehydration of the hydrated compound. Density of state calculations showed hybridization of the Bi 6s and O 2p orbitals at the bottom of the conduction bands for both the hydrated and the dehydrated phases, which narrows the band gap and promotes their photocatalytic activity in the visible region.

High-pressure structural and vibrational properties of monazite-type BiPO4, LaPO4, CePO4, and PrPO4

Monazite-type BiPO₄, LaPO₄, CePO₄, and PrPO₄ have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us with information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure

We report optical absorption and luminescence measurements in pure and trivalent neodymium (Nd³⁺) doped LaVO₄ crystals up to 25 GPa. We also present the theoretical framework to accurately explain the observed experimental results.

Characterisation of the rare cadmium chromate pigment in a 19th century tube colour by Raman, FTIR, X-ray and EPR

In an investigation of the artists' materials used by P. S. Krøyer the contents of the tube colours found in Krøyer's painting cabinet were examined. In most cases, the results of the pigment analyses were as expected based on our knowledge of artists' colours used in the late 1800s and early 1900s. However, in one of the tube colours labelled "Jaune de Cadmium Citron" (cadmium lemon yellow) an extremely rare cadmium chromate pigment was found. The pigment was analysed and characterised by Raman microscopy (MRS), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), powder X-ray diffraction (PXRD), single-crystal X-ray crystallography, and electron paramagnetic resonance (EPR) spectroscopy. Cadmium chromate was synthesised by precipitation from an aqueous solution of cadmium nitrate and potassium chromate, and the resulting yellow crystals proved identical to the pigment found in the tube colour "Jaune de Cadmium Citron". The structure determined by single-crystal X-ray diffraction identified the pigment as 2CdCrO₄·KOH·H₂O or more accurately as KCd₂(CrO₄)₂(H₃O₂) illustrating the μ-H₃O₂^- species. The yellow colour of the paint sample taken from the tube had a greenish hue, which became even more prominent upon storage and drying. EPR analysis of the sample showed the presence of paramagnetic degradation products containing Cr(III) and Cr(V).

A novel Z-scheme BiPO4-Bi2O2(OH)(NO3) heterojunction structured hybrid for synergistic photocatalysis

Photocatalysis has been gaining a growing popularity in water treatment, and their engineered applications inspire the development of effective photocatalyst materials. To develop photocatalyst that is effective for degradation of organic pollutants, we fabricate a novel direct solid Z-scheme BiPO₄-Bi₂O₂(OH)(NO₃) (BPO-BHN) heterojunction structured hybrid. The results demonstrate an enhanced photocatalytic activity of BPO-BHN to produce OH radicals, according to diffuse reflectance spectroscopy (DRS), electron spin-resonance resonance (ESR), photoelectrochemical measurements, and theoretical calculation results. The BPO-BHN is shown to greatly promote the degradation of 2,4-dichlorophenol (2,4-DCP) under ultraviolet light. On the basis of pseudo-first-order kinetics, the apparent degradation rate constant (k_app) of 0.050 min^-1 obtained for BPO-BHN is approximately 3.33 and 12.5 times of that for individual BPO (k_app = 0.015 min^-1) and BHN (k_app = 0.004 min^-1), respectively. This suggests a virtually synergistic photocatalysis of BPO and BHN when they form a direct solid Z-scheme heterojunction structure, which is favorable for improving UV-light harvesting, hole/electron separation and oxidizing capability. In particular, as a novel non-linear optical (NLO) material, the BHN plays a significant role in the formation of Z-scheme structure for its unique ability of capturing photo-electrons from BPO by high-potential C⁺ face in valence band. This study provides a proof-of-concept strategy to develop more effective photocatalysts for degradation of organic pollutants in water.

ScVO4 under non-hydrostatic compression: a new metastable polymorph

The high-pressure (HP) behaviour of scandium vanadate (ScVO₄) is investigated under non-hydrostatic compression. The compound is studied by means of synchrotron-based powder x-ray diffraction (XRD) and optical-absorption techniques. The occurrence of a non-reversible phase transition is detected. The transition is from the zircon structure to the fergusonite-type structure and takes place around 6 GPa with nearly 10% volume discontinuity. XRD measurements on the pressure cycled sample confirm for the first time that the fergusonite-type ScVO₄ can be recovered as the metastable phase at ambient conditions. Raman spectroscopic measurements verify the metastable phase to be of a fergusonite-type phase. Theoretical calculations also corroborate the experimental findings. The fergusonite phase is found to be stiffer than the ambient-pressure zircon phase, as indicated by the observed experimental and theoretical bulk moduli. The optical properties and lattice-dynamics calculation of the fergusonite ScVO₄ are discussed. At ambient pressure the band gap of the zircon (fergusonite)-type ScVO₄ is 2.75 eV (2.3 eV). This fact suggests that the novel metastable polymorph of ScVO₄ can have applications in green technologies; for instance, it can be used as photocatalytic material for hydrogen production by water splitting.

New pressure-induced polymorphic transitions of anhydrous magnesium sulfate

Discovery of phase transitions in MgSO₄ under compression. The high-pressure phases involve a coordination number increase for magnesium.

Structural and electronic properties of the PbCrO4 chrome yellow pigment and of its light sensitive sulfate-substituted compounds

Chrome Yellows (CY) are a family of synthetic pigments of formula (PbCr_(1−x)S_xO₄) used by van Gogh. We investigate structure/property relations in CY by first-principles methods, providing insight into their possible degradation mechanisms.