CaF2-type Structure Research Articles

(Invited) New Class of Plasmonic Alloy Nanoparticles

The past 150 years of research on inorganic nanomaterials that exhibit intrinsic localized surface plasmon resonance (LSPR) in the visible region has focused on well-disperse nanoparticles (NPs) composed of coinage metals (Au, Ag, and Cu). Recent theoretical studies on photo-excited electron dynamics in Au NPs have revealed that not only the interband transition but also the screening caused by the vibrations of bound electrons by visible light are important for visible LSPR [1]. However, the correlation between the electronic and crystal structure (as a means to control the screening effect) remains unclear because most alloys based on coinage metals have a face-centred cubic (fcc)-based structure. Here, we show that well-disperse pseudospherical intermetallic Pd-based alloy NPs with a B2 structure and PtIn2 NPs with a C1 (CaF2-type) structure intrinsically exhibit LSPR absorption in the visible region.It is well known that chemical conversion of inorganic NPs via element substitution reactions, such as ion exchange reactions [2] and galvanic replacement reactions, can overcome the difficulties associated with controlling the size, shape, chemical composition, and crystal structure in conventional syntheses [3-5]. Especially, alloying enables us to drastically modulate the electronic properties of metallic NPs. The retained shape of the parent-NPs in element replacement reactions provides an opportunity to obtain non-equilibrium unique structures and even new structures of inorganic NPs. We demonstrate that various Pd-based alloy NPs with a B2 structure were synthesized by the element replacement reactions with Pd–P NPs and showed LSPR in the visible region [6]. This is an alchemy in nanoplasmonics in the sense that the plasmonic properties of group 11 elements are reproduced by using non-group 11 elements. On the other hand, we successfully synthesized pseudospherical PtIn2 intermetallic NPs with a C1 structure as uncommon types of visible plasmonic NPs [7].Simulations of above alloy NPs and fcc-Au NPs indicate that both the reduction in the interband transition and the strong screening by a small number of bound d-electrons are the main origins of the visible plasmonic features of alloy NPs. Our results demonstrate that the composition and crystal structure of the intermetallic NPs of our work determine the interband transition and the screening effect. Our findings dramatically expand the plasmonic NP library and highlight remarkable NPs that are far superior, in terms of physical and chemical properties, to conventional plasmonic NPs.[1] Iida, K.; Noda, M.; Ishihara, K.; Nobusada, K. J. Phys. Chem. A 2014, 118, 11317–11322.[2] Saruyama, M.; Sato, R.; Teranishi, T. Acc. Chem. Res. 2021, 54, 765–775.[3] Wu, H.-L.; Sato, R.; Teranishi, T. et al., Science 2016, 351, 1306–1310.[4] Li, Z.; Saruyama, M.; Asaka, T.; Tatetsu, Y.; Teranishi, T. Science 2021, 373, 332–337.[5] Matsumoto, K.; Sato, R.; Tatetsu, Y.; Teranishi, T. et al., Nat. Commun. 2022, 13, 1047.[6] Sato, R.; Takekuma, H.; Teranishi T. et al., to be submitted.[7] Takekuma, H.; Sato, R.; Iida, K.; Teranishi, T. et al., submitted.

Correction to: Metal–insulator transition in the high pressure cubic CaF2-type structure of CrO2

Correction to: Metal–insulator transition in the high pressure cubic CaF2-type structure of CrO2

N-Heterocyclic Thione-Protected Ag4 Tetrahedra and Ag8 Cubes Cocrystallized in a Single Crystal.

Polynuclear silver clusters have attracted intensive attention in the academic community owing to their rich physicochemical properties. The development of thione-protected silver clusters has been lagging behind the well-explored thiolate-protected silver-sulfide clusters. Herein, we report two N-heterocyclic thione-protected silver clusters: [Ag4(2-TBI)6(SO4)3]2- (Ag4) and [Br@Ag8(2-TBI)12(SO4)2]3+ (Ag8) (2-TBI = 2-thiobenzimidazol), which cocrystallize to form cluster-based molecular crystals with a CaF2-type structure. The cocrystal shows high thermal stability in air. Notably, the two cluster-based layers are alternately assembled to exhibit a unique k-vector-differential crystallographic arrangement. This work may lay a foundation for synthesis of atomically precise and stable silver clusters using readily available N-heterocyclic thione ligands.

New Insights into Phase Separation of Cerium Hydrides under Pressure.

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH2, CeH2.5, and CeH3. Under pressure, the experimental CaF2-type structure of CeH2 is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (β-UH3 type) structure of CeH3 above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH3 moves through the following sequence of phases: Pm3®n (β-UH3 type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I41/amd structure of CeH2.5 has the similar decomposition as that of CeH2. Finding this previously unreported pressure-induced decomposition of CeH2 will pave the way for investigations on the nature of hydrogen–metal interactions.

Local order in high-entropy alloys and associated deuterides – a total scattering and Reverse Monte Carlo study

Many of the materials properties of high-entropy alloys (HEAs), like increased hardness, reduced thermal and electrical conductivity, and interesting hydrogen storage properties, are proposed to be related to local lattice distortions of the crystal structure due to the significant size differences between the elements of the alloy. However, direct evidence of this effect is very limited in the literature, and it therefore remains a hypothesis. This work presents a detailed assessment of the local lattice distortion in three body-centered cubic (bcc) HEAs TiVNb, TiVZrNb and TiVZrNbHf with varying atomic size differences using total scattering measurements and Reverse Monte Carlo structure modelling. The analysis indicates that the amount of local lattice distortion in the alloys increases with the elemental size difference in the alloy. The amount of lattice distortion is relieved when dideuterides with CaF2-type structures (Fm3¯m) are formed from the bcc (Im3¯m) HEAs. Analyses of the local environments around the deuterium atoms reveal an interesting correlation between the valence-electron concentration (VEC) of the nearest-neighbour metals and the stability of tetrahedral interstices with respect to deuterium occupation. Moreover, there is a tendency towards Ti/Nb short-range order in TiVNbD5.7 where the mixing entropy is lowest. In TiVZrNbHfD10, about 6 % of the deuterium atoms are displaced from the tetrahedral interstices with smaller volumes to octahedral interstices.

Enhanced Thermoelectric Performances Driven by High-Pressure Phase Transition of Mg2Sn Compound

The ground state properties of the Mg2Sn compound under seven different crystal structures were studied using a first principles approach based on the full-potential augmented plane wave method FP-LAPW within the GGA-PBEsol approximation. We fixed the cubic structure of CaF2-type as zero pressure phase and we have considered the orthorhombic PbCl2-type, hexagonal Ni2In-type, tetragonal Al2Cu-type, cubic MgCu2-type, hexagonal MgZn2-type and dihexagonal MgNi2-type as candidates structures for the high-pressure phases. We used Gibbs’ equation to examine these high-pressure structures and compare their enthalpies. Thus, it appears that the Mg2Sn compound undergoes three pressure-induced phase transitions. First, it is a transition from the CaF2-type cubic structure to the orthorhombic PbCl2-type structure; second, the transition from the PbCl2-type structure to the hexagonal Ni2In-type structure; and third, the transition will take place toward the cubic structure MgCu2-type which should be stable. In addition, to further examine the thermoelectric performances of each stable phase, we combined the results calculated for the electronic properties by the TB-mBJ approximation with the semi-classical Boltzmann theory using the BoltzTraP code. Our key result is that Mg2Sn compound with cubic MgCu2-type structure has a high value of figure of merit (ZTe) compared to the cubic CaF2-type structure, which could promise it as an excellent candidate for potential thermoelectric applications at high pressure.

Highly luminescent Cit/CaF2: Ce3+, Tb3+ nanoparticles and detection of Cu2+ ions

Terbium doped calcium fluoride (CaF2:Tb3+) and cerium co-doped CaF2:Tb3+ (CaF2:Ce3+, Tb3+) nanoparticles were synthesized by the hydrothermal method by using citric acid (Cit) as surfactant. The as-synthesized nanoparticles have good dispersibility in water to form an aqueous transparent dispersion at a concentration of 20 mg/mL. The nanoparticles have the CaF2-type cubic structures. Photoluminescence (PL) investigation showed that energy transfer can be occurred effectively from Ce3+ ions to Tb3+ ions in Cit/CaF2: Ce3+, Tb3+ nanoparticles. And the PL intensity of Cit/CaF2: Ce3+, Tb3+ nanoparticles is very strong at a low concentration of 1 mg/mL, which is 16-fold as high as that of Cit/CaF2: Tb3+ nanoparticles. The strong green emission from Cit/CaF2: Ce3+, Tb3+ nanoparticles can selectively be quenched by Cu2+ ions in comparison to other metal ions and its detection limit is 1.02 × 10−5 M in an aqueous solution. The PL quenching is reversible by the addition of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) with the recovery of almost 80% of the original PL. The results revealed that Cit/CaF2: Ce3+, Tb3+ nanoparticles could find a promising application for the detection of Cu2+ ions in an aqueous solution.

Synthesis and characterization of Magnesium-Iron-Cobalt complex hydrides

The formation, structure and deuterium desorption properties of Mg2FexCo(1−x)Dy (0 ≤ x ≤ 1 and 5 ≤ y ≤ 6) complex hydrides were investigated. The synthesis was carried out by reactive ball milling, using a mixture of powders of the parent elements in D2 atmosphere. The formation of quaternary deuterides was identified from Rietveld refinements of powder X-Ray diffraction and powder neutron diffraction patterns, and from infrared attenuated total reflectance analysis. It was observed that the crystal structure of deuterides depends on the transition metal fraction. For Co-rich compositions, i.e. up to x = 0.1, hydrides have the tetragonal distorted CaF2-type structure (space group P4/nmm) of Mg2CoD5 at room temperature. For Fe-rich compositions, i.e. x ≥ 0.5, a cubic hydride is observed, with the same K2PtCl6-type structure (space group Fmbar{{bf{3}}}m) as Mg2FeD6 and as Mg2CoD5 at high temperatures. For x = 0.3, both the cubic and the tetragonal deuterides are detected. Differential scanning calorimetry coupled with thermogravimetric and temperature programmed desorption analyses show rather similar deuterium desorption properties for all samples, without significant changes as a function of composition. Finally, hydrogen sorption experiments performed for Mg2Fe0.5Co0.5H5.5 at 30 bar of H2 and 673 K showed reversible reactions, with good kinetic for both absorption and desorption of hydrogen.

Mg-containing multi-principal element alloys for hydrogen storage: A study of the MgTiNbCr0.5Mn0.5Ni0.5 and Mg0.68TiNbNi0.55 compositions

Recently, there has been growing interest in multi-principal element alloys for hydrogen storage. However, most of the papers published so far report compositions based only on transition metal elements, which limit the gravimetric storage capacities due to their densities. Since Mg is a low-density element promising for hydrogen storage, the study of Mg-containing multi-principal element compositions is opportune. In the present work, we report for the first time the structural characterization and hydrogen storage properties of the A2B type MgTiNbCr0.5Mn0.5Ni0.5 alloy and its derivative Mg0.68TiNbNi0.55 alloy. These Mg-containing multi-principal element alloys form major BCC phase (W-type, Im3¯m) and major FCC hydride (MH2 with CaF2-type structure) when synthesized by mechanical alloying (MA) and reactive milling (RM), respectively. Hydrogen is desorbed from both RM samples in two steps, with some overlap, from different hydrides formed during synthesis. The microstructure of the Mg0.68TiNbNi0.55 composition is more homogeneous (less secondary phases), but both alloys present a total gravimetric capacity of around 1.6 wt% H2.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 14: Concentration Dependence of the Defect Structure of Nonstoichiometric Phases Sr1 – xNdxF2 + x As Grown (x = 0.10, 0.25, 0.40, 0.50)

The defect structure of as grown Sr1 – xNdxF2 + x (x = 0.10–0.50) crystals grown from a melt by directional solidification under identical conditions has been studied by X-ray diffraction. All crystals belong to the CaF2-type structure (sp. gr. $$Fm\bar {3}m$$ ). The change in the type of structural defect cluster with a change in the NdF3 content, which occurs in some range of x rather than stepwise, was traced for the first time in the entire homogeneity range of fluorite nonstoichiometric phase Sr1 – xNdxF2 + x. Clusters of two types coexist in crystals with х = 0.10 and 0.25: tetrahedral [Sr4 – nNdnF26] and cuboctahedral [Sr14 – nNdnF64 + n]. With a further increase in the NdF3 content in the Sr1 – xNdxF2 + x structure, a transition from two types of clusters to only one type occurs, and only tetrahedral clusters are formed in crystals with х > 0.40.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 13: Crystal Structure of SrF2 and Concentration Dependence of the Defect Structure of Nonstoichiometric Phase Sr1 – xLaxF2 + x As Grown (x = 0.11, 0.20, 0.32, 0.37, 0.47)

The defect structure of as-grown SrF2 and nonstoichiometric phases Sr1 – xLaxF2 + x (x = 0.11, 0.20, 0.32, 0.37, 0.47) single crystals, grown from a melt under identical conditions, has been studied by X-ray diffraction analysis. All crystals belong to the CaF2-type structure, sp. gr. $$Fm\bar {3}m$$ . Deficit of fluorine anions is found in the 8c site in SrF2. Interstitial anions are not visualized in SrF2 in difference electron-density maps. The Sr1 – xLaxF2 + x phases exhibit the presence of vacancies in the main anion motif and interstitial fluorine ions of three types: in two sites 32f (w, w, w) with different coordinates w and in one site 4b. A model of the defect structure of Sr1 – xLaxF2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La3+ are grouped into clusters of the [Sr1 – nLanF26] tetrahedral configuration. Calculations based on structural data revealed that the average number of La3+ ions per cluster linearly increases from 2.6 to 3.13 with an increase in the LaF3 concentration. The average crystal volume corresponding to one cluster decreases from 1170.6(3) to 336.1(5) A3. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) A3, passing through a minimum in the composition with x = 0.32, which is similar to that of congruently melting phase, and then increases to 2.44(9) A3 at х = 0.47. Dynamic thermal displacement of matrix anions in Sr1 – xLaxF2 + x is observed in the [111] direction towards the cubic void center in the anion sublattice. Therefore, according to the mechanism of electrical conductivity, anion jumps are most likely in this direction.

First principles studies of the structural stability and lattice dynamics for rare earth hydridies REHx (RE = Sm, Gd, Tb, Dy, Ho, Tm, x = 2, 3) under pressure

In the paper, the structures, pressure-induced structural transitions (from cubic to hexagonal), lattice dynamics and the possible transition mechanisms for rare earth dihydrides REH2 and trihydrides REH3 (RE = Sm, Gd, Tb, Dy, Ho, and Tm) are investigated comparatively and systematically using first-principles calculations. The transition pressures from the cubic CaF2-type structure to the hexagonal REB2-type structure for REH2 (RE = Sm, Gd, Tb, Dy, Ho, Tm) and the transition pressures from the cubic BiF3-type structure to the hexagonal LaF3-type structure for REH3 (RE = Sm, Gd, Tb, Dy, Tm) are predicted reasonably. It is concluded that the two kinds of structural phase transitions are not sensitive to RE 4f electrons. In the two kinds of structural phase transitions under pressure, we note that the ‘‘2.1 Å rule’’ (the purposed minimum HH distance for the stable metal hydirde at 0 K and 0 GPa) doesn't work. While lattice dynamics show that the dynamical stable pressure ranges for the cubic REH2 (RE = Sm Gd, Tb, Dy, Ho, Tm) are about in 0–52, 0–60, 0–70,0-87, 0–94 and 0–108 GPa, respectively. Ones of the cubic REH3 (RE = Sm, Gd, Tb, Dy, Tm) are around 17–147, 31–182, 13–200, 16–224 and 14–280 GPa, respectively. In contrast to the corresponding structural transition pressure as well as the lattice dynamics of the hexagonal structure under high pressure, the phonon modes softening behaviors under pressure might be responsible for the pressure-induced structural phase transition in the cubic REH2, but not in the cubic REH3.

Electronic structure and fine structural features of the air-grown UNxOy on nitrogen-rich uranium nitride

Oxide formation on surface of nitrogen-rich uranium nitride film/particles was investigated using X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), aberration-corrected transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with electron energy-loss spectroscopy (EELS). XPS and AES studies indicated that the oxidized layer on UN2-x film is ternary compound uranium oxynitride (UNxOy) in 5–10 nm thickness. TEM/HAADF-STEM and EELS studies revealed the UNxOy crystallizes in the FCC CaF2-type structure with the lattice parameter close to the CaF2-type UN2-x matrix. The work can provide further information to the oxidation mechanism of uranium nitride.

Single-crystal structure refinement of YbF2 with a remark about YbH2

Abstract Transparent-yellow single crystals of YbF2 were obtained as only crystalline product from the solid-state reaction of Yb and teflon designed to yield ‘Yb3C3F2’ in addition to some amorphous black material. The first single-crystal structure determination of YbF2 (cubic space group Fm3̅m, CaF2-type structure, a=559.46(16) pm; R1=1.2%, wR2=3.2%) was the starting point to compare isostructural binary fluorides MF2 and hydrides MH2 (M=Ca, Yb, Eu, Sr and Ba) exhibiting an as-yet unexplained small volume per formula unit for YbH2.

Bonding Structures of ZrHx Thin Films by X-ray Spectroscopy

The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 {\AA} in the hexagonal closest-packed metal (alpha-phase) to 3.395 {\AA} in the understoichiometric delta-ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk delta-ZrH2. For yet lower hydrogen contents, the structures are mixed alpha and delta-phases, while sufficient hydrogen loading (x>1) yields a pure {\delta}-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.

Electronic properties and bonding in ZrHx thin films investigated by valence-band x-ray photoelectron spectroscopy

The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x=0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at% are investigated by 4d valence band, shallow 4p core-level and 3d core-level X-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity towards the H 1s - Zr 4d hybridization region at about 6 eV below the Fermi level. For low hydrogen content (x=0.15, 0.30), the films consist of a superposition of hexagonal closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type structure) phases, while for x=1.16, the film form single phase ZrHx that largely resembles that of stoichiometric delta-ZrH2 phase. We show that the cubic delta-ZrHx phase is metastable as thin film up to x=1.16 while for higher H-contents, the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge-transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as function of hydrogen content is discussed in relation to the charge-transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

Theoretical investigation of solid solution states of Ti1−xVxH2

The solution states of metal atoms in Ti1−xVxH2, the stability of ordered ground-state structures, and the effect of metal-atom configurations on the bonding strength of H atoms are investigated by combining the cluster expansion method with first-principles calculations. Formation energies of all possible configurations of metal atoms in CaF2-type structures with up to sixteen metal atoms calculated using an optimized set of clusters are reported. Nine ground-state structures with different compositions are identified, most of which are found to have layered structures containing V bilayers. Analysis of the component clusters suggests that, while pair clusters contribute to formation of the layered structures, the higher-body clusters contribute more to stabilization of V bilayers than Ti bilayers. Based on calculations of short-range order parameters for all configurations of composition x = 1/2, low-energy structures including the layered structures appear to have a tendency for phase separation at the atomic scale. Order-disorder transition temperatures of the ground-state structures are estimated from Monte Carlo simulations to be less than 460 K. Comparison of lattice constants with experimental values as well as the low transition temperatures suggests that disordered solutions will be predominantly formed under typical synthesis conditions. In addition, we examine the stability of H atoms in different coordination environments and find that the metal-atom ordering does not greatly affect the local bonding between H and surrounding metal atoms.

Half-Metallic Properties of the Mn2Te Telluride by a First-Principles Study

The electronic band structure, magnetic properties, and half-metallic properties of the Mn2Te compound in the C1b-type, CaF2-type, and Ni2In-type structures have been studied by using the full-potential linearized augmented plane wave (FPLAPW) method within the generalized gradient approximation (GGA). The ferrimagnetic ground state of the Mn2Te compound is the most stable with the C1b-type structure. The value of total magnetic moment, 2 μ B/f.u. at the equilibrium lattice constant a 0= 5.92 A, agrees with the Slater–Pauling rule for the C1b-type Mn2Te compound. The Mn2Te compound with the C1b-type structure is found to be a half-metallic ferrimagnet with a half-metallic band gap of 1.19 eV. Half metallicity is kept for a broad range of lattice constants below 6.13 A. The Curie temperature for the C1b-type structure is calculated to be 1231 K in the mean field approximation (MFA).

First principles prediction of a new high-pressure phase and transport properties of Mg2Si

We have investigated the structural properties of seven different structure types of Mg2Si which include the cubic CaF2, orthorhombic PbCl2, hexagonal Ni2In, tetragonal Al2Cu, Laves phase (cubic MgCu2), hexagonal MgZn2 and dihexagonal MgNi2 type of structures, using a full potential linearized augmented plane wave method as implemented in WIEN2k within the framework of density functional theory. The exchange–correlation potential is treated by the new form of generalized gradient approximation (GGA-PBEsol). In total energy calculations it is clearly seen that cubic CaF2-type structure is stable at ambient conditions, and it undergoes a first-order phase transition to orthorhombic PbCl2-type, then to the hexagonal Ni2In-type structure and finally to the cubic Laves phase MgCu2-type. A new structure type is predicted to be stable at high pressure. Moreover, we intend to combine the electronic structure calculations performed by mean of generalized gradient approximation and modified Becke–Johnson potential with Boltzmann transport theory as incorporated in BoltzTraP code to interpret and predict the thermoelectric performance of each stable phase as a function of the chemical potential at various temperatures. We find a high thermoelectric thermopower values in cubic CaF2-type structure that could promise an excellent candidate for potential thermoelectric applications.

Theoretical Study of Phase Separation of Scandium Hydrides under High Pressure

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of three scandium hydrides: ScH, ScH2, and ScH3. For the monohydride, ScH, we predict several phases to be more stable at 1 atm than the previously suggested rock-salt structure, in particular one of P42/mmc symmetry. The NaCl-type structure for ScH takes over at 10 GPa and dominates over a wide pressure range until it is replaced by a Cmcm structure around 265 GPa. Under pressure, the experimental P = 1 atm CaF2-type structure of ScH2 should transform to a C2/m structure around 65 GPa, which then is likely to disproportionate to NaCl-type ScH and face-centered cubic ScH3 above 72 GPa. According to theory, as the pressure is elevated, ScH3 moves through the following sequence of phases: P63 → Fm3̅m → P63/mmc(YH3-type) → Cmcm; the corresponding transition pressures are calculated to be 29, 360, and 483 GPa, respectively. The predicted disproportionation tendencies of ScH2 are fascinating: stable to decomposition to ScH and ScH3 at low pressures, it should begin to disproportionate near 72 GPa. However, the process is predicted to reverse at still higher pressures (above 300 GPa). We also find ScH to be stable to disproportionation to Sc and ScH2 above ∼25 GPa. The three hydrides are metallic, except for (at low pressures) ScH3.