Correction and integration of solid-angle data from the azimuthally resolving 2D detector at ID06-LVP, ESRF.

The unique diffraction geometry of ESRF beamline ID06-LVP offers continuous static 2D or azimuthally resolving data collections over all accessible solid angles available to the tooling geometry. The system is built around a rotating custom-built Pilatus3 CdTe 900k-W detector from Dectris, in a configuration equivalent to three butted 300k devices. As a non-standard geometry, here the method of alignment, correction and subsequent integration for any data collected over all solid angles accessible, or over any azimuthal range contained therein, are provided and illustrated by parameterizing and extending existing pyFAI routines. At 1° integrated intervals, and typical distances (2.0 m), the system covers an area of near 2.5 m2 (100 Mpx square equivalent), to 0.65 Šresolution, at 53 keV from a total dataset of some 312 Mpx. Standard FWHMs of SRM660a LaB6 vary from 0.005° to 0.01°, depending on beam size, energy and sample dimensions, and are sampled at an elevated rate. The azimuthal range per static frame ranges from <20° to ∼1° over the full range of the detector surface. A full 2θ-intensity data collection at static azimuth takes 1-3 s typically, and can be reduced to ms-1 rates for measurements requiring time-rate determination. A full solid-angle collection can be completed in a minute. Sample detector distances are accessible from 1.6 m to 4.0 m.

Hypervalent hydridosilicate in the Na-Si-H system.

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and in situ synchrotron diffraction studies of reaction mixtures NaH-Si-H2 at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NamSiH(4+m) (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH6 2- complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. In situ studies demonstrated the formation of the double salt Na3[SiH6]H (Na3SiH7, m = 3) containing both octahedral SiH6 2- moieties and hydridic H-. Upon formation at elevated temperatures (>500°C), Na3SiH7 attains a tetragonal structure (P4/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na3SiH7 was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na3SiH7 is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates.

Chemical Reactions and Phase Stabilities in the Si-Te System at High Pressures and High Temperatures.

Chemical reactions and phase stabilities in the Si-Te system at high pressures were explored using in situ angle-dispersive synchrotron powder diffraction in a large-volume multianvil press together with density functional theory-based calculations. Cubic and rhombohedrally distorted clathrates, with the general formula Te8@(Si38Te8) and wide compositional range, preceded by a hexagonal phase with the composition Si0.14Te, are formed for different mixtures of Si and Te as starting materials. Si0.14Te, with the structural formula Te2(Te0.74Si0.26)3(Te0.94Si0.06)3, is the very first chalcogenide with the Mn5Si3-type structure. Silicon sesquitelluride α-Si2Te3 decomposes into a mixture of phases that includes the clathrate and hexagonal phases at high pressures and high temperatures. The higher the pressure, the lower the temperature for the two phases to occur. Regardless of the starting compositions, only the clathrate is quenched to atmospheric conditions, while the hexagonal phase amorphizes on decompression. The rhombohedral clathrates Te8@(Si38Te8) form on quenching of the cubic phases to ambient conditions. There is a high degree of interchangeability of Si and Te not only in the clathrates but also in the Mn5Si3-type structure. The theoretical calculations of enthalpies indicate that the reported decomposition of α-Si2Te3 is energetically favorable over its transformation to another polymorph of the A2X3 type at extreme conditions.

Unconventional Route to High-Pressure and -Temperature Synthesis of GeSn Solid Solutions.

Ge and Sn are unreactive at ambient conditions. Their significant promise for optoelectronic applications is thus largely confined to thin film investigations. We sought to remove barriers to reactivity here by accessing a unique pressure, 10 GPa, where the two elements can adopt the same crystal structure (tetragonal, I41/amd) and exhibit compatible atomic radii. The route to GeSn solid solution, however, even under these directed conditions, is different. Reaction upon heating at 10 GPa occurs between unlike crystal structures (Ge, Fd3m and Sn, I4/mmm), which also have highly incompatible atomic radii. They should not react, but they do. A reconstructive transformation of I4/mmm into the I41/amd solid solution then follows. The new tetragonal GeSn solid solution (I41/amd a = 5.280(1) Å, c = 2.915(1) Å, Z = 4 at 9.9 GPa and 298 K) also constitutes the structural and electronic bridge between 4-fold and newly prepared 8-fold coordinated alloy cubic symmetries. Furthermore, using this high-pressure route, bulk cubic diamond-structured GeSn alloys can now be obtained at ambient pressure. The findings here remove confining conventional criteria on routes to synthesis. This opens innovative avenues to advanced materials development.

An Unexpected Cubic Symmetry in Group†IV Alloys Prepared Using Pressure and Temperature.

The cubic diamond (Fd 3 ‾ m) group IVA element Si has been the material driver of the electronics industry since its inception. We report synthesis of a new cubic (Im 3 ‾ m) group IVA material, a GeSn solid solution, upon heating Ge and Sn at pressures from 13 to 28 GPa using double-sided diamond anvil laser-heating and large volume press methods. Both methods were coupled with in situ angle dispersive X-ray diffraction characterization. The new material substantially enriches the seminal group IVA alloy materials landscape by introducing an eightfold coordinated cubic symmetry, which markedly expands on the conventional tetrahedrally coordinated cubic one. This cubic solid solution is formed, despite Ge never adopting the Im 3 ‾ m symmetry, melting inhibiting subsequent Im 3 ‾ m formation and reactant Ge and Sn having unlike crystal structures and atomic radii at all these pressures. This is hence achieved without adherence to conventional formation criteria and routes to synthesis. This advance creates fertile avenues for new materials development.

Na3FeH7 and Na3CoH6: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis.

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH-TM-H2 at p ≈ 10 GPa. Na3FeH7 and Na3CoH6 feature pentagonal bipyramidal FeH73- and octahedral CoH63- 18-electron complexes, respectively. At high pressure, high temperature (300 < T ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics. Upon cooling, subtle changes in the diffraction patterns evidence reversible and rapid phase transitions associated with ordering of the complexes. During decompression, Na3FeH7 and Na3CoH6 transform to tetragonal and orthorhombic low pressure forms, respectively, which can be retained at ambient pressure. The discovery of Na3FeH7 and Na3CoH6 establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

Na-Ni-H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH5]3- Versus the Perovskite NaNiH3.

The Na-Ni-H system was investigated by in situ synchrotron diffraction studies of reaction mixtures NaH-Ni-H2 at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions Na3NiH5 and NaNiH3, where Ni attains the oxidation state II, is demonstrated. Upon heating at ∼5 GPa, face-centered cubic (fcc) Na3NiH5 forms above 430 °C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 °C to an orthorhombic (Cmcm) form. Upon pressure release, Na3NiH5 further transforms into its recoverable Pnma form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. Na3NiH5 features previously unknown square pyramidal 18-electron complexes NiH5 3-. In the high temperature fcc form, metal atoms are arranged as in the Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature Cmcm form, in which NiH5 3- complexes are ordered. It is considerably rearranged in the low pressure Pnma form. Experiments at 10 GPa showed an initial formation of fcc Na3NiH5 followed by the addition of the perovskite hydride NaNiH3, in which Ni(II) attains an octahedral environment by H atoms. NaNiH3 is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of Na3NiH5 = NaNiH3 + 2 NaH is enthalpically favored at all pressures, suggesting that Na3NiH5 is metastable and its formation is kinetically favored. Ni-H bonding in metallic NaNiH3 is considered covalent, as in electron precise Na3NiH5, but delocalized in the polyanion [NiH3]-.

Exploring the Mg-Cr-H System at High Pressure and Temperature via in Situ Synchrotron Diffraction.

The complex transition metal hydride Mg3CrH8 has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H7]5-. Here, we investigated the formation of Mg3CrH8 by in situ studies of reaction mixtures of 3MgH2-Cr-H2 at 5 GPa. The formation of the known orthorhombic form (o-Mg3CrH8) was noticed at temperatures above 635 °C, albeit at a relatively slow rate. At temperatures around 750 °C a high temperature phase formed rapidly, which upon slow cooling converted into o-Mg3CrH8. The phase transition at high pressures occurred reversibly at ∼735 °C upon heating and at ∼675 °C upon slow cooling. Upon rapid cooling, a monoclinic polymorph (m-Mg3CrH8) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg3CrH8 was found to crystallize in P21/n space group (a = 5.128 Å, b = 16.482 Å, c = 4.805 Å, ß = 90.27°). Its structure elucidation from high resolution synchrotron powder diffraction data was aided by first-principles DFT calculations. Like the orthorhombic polymorph, m-Mg3CrH8 contains pentagonal bipyramidal complexes [CrH7]5- and interstitial H-. The arrangement of metal atoms and interstitial H- resembles closely that of the high pressure orthorhombic form of Mg3MnH7. This suggests similar principles of formation and stabilization of hydrido complexes at high pressure and temperature conditions in the Mg-Cr-H and Mg-Mn-H systems. Calculated enthalpy versus pressure relations predict o-Mg3CrH8 being more stable than m-Mg3CrH8 by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg3CrH8 is very similar to that of o-Mg3CrH8. The stable 18-electron complex [CrH7]5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.

Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype.

Hexagonal Si allotropes are expected to enhance light absorption in the visible range as compared to common cubic Si with diamond structure. Therefore, synthesis of these materials is crucial for the development of Si-based optoelectronics. In this work, we combine in situ high-pressure high-temperature synthesis and vacuum heating to obtain hexagonal Si. High pressure is one of the most promising routes to stabilize these allotropes. It allows one to obtain large-volume nanostructured ingots by a sequence of direct solid-solid transformations, ensuring high-purity samples for detailed characterization. Thanks to our synthesis approach, we provide the first evidence of a polycrystalline bulk sample of hexagonal Si. Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking) in contrast with Si-2H (AB stacking) proposed previously. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.

Unraveling Hidden Mg-Mn-H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction.

The Mg-Mn-H system was investigated by in situ high pressure studies of reaction mixtures MgH2-Mn-H2. The formation conditions of two complex hydrides with composition Mg3MnH7 were established. Previously known hexagonal Mg3MnH7 (h-Mg3MnH7) formed at pressures 1.5-2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg3MnH7) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H6]5- complexes and interstitial H-. Interstitial H- is located in trigonal bipyramidal and square pyramidal interstices formed by Mg2+ ions in h- and o-Mg3MnH7, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg3MnH7 (m-Mg3MnH7). The structure elucidation of o- and m-Mg3MnH7 was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg3MnH7 to be more stable than h-Mg3MnH7 above 4.3 GPa. Phonon calculations revealed o-Mg3MnH7 to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg3MnH7 on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg3MnH7 is very similar. The stable 18-electron complex [MnH6]5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of in situ investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides.

Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways.

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

An Alternative Route to Pentavalent Postperovskite.

Two different high-pressure and -temperature synthetic routes have been used to produce only the second-known pentavalent CaIrO3-type oxide. Postperovskite NaOsO3 has been prepared from GdFeO3-type perovskite NaOsO3 at 16 GPa and 1135 K. Furthermore, it has also been synthesized at the considerably lower pressure of 6 GPa and 1100 K from a precursor of hexavalent Na2OsO4 and nominally pentavalent KSbO3-like phases. The latter synthetic pathway offers a new lower-pressure route to the postperovskite form, one that completely foregoes any perovskite precursor or intermediate. This work suggests that postperovskite can be obtained in other compounds and chemistries where generalized rules based on the perovskite structure may not apply or where no perovskite is known. One more obvious consequence of our second route is that perovskite formation may even mask and hinder other less extreme chemical pathways to postperovskite phases.

The large volume press facility at ID06 beamline of the European synchrotron radiation facility as a High Pressure-High Temperature deformation apparatus.

We report here the newly developed deformation setup offered by the 20MN (2000T) multi-anvil press newly installed at sector 7 of the European synchrotron radiation facility, on the ID06 beamline. The press is a Deformation-DIA (D-DIA) type apparatus, and different sets of primary anvils can be used for deformation experiments, from 6 mm to 3 mm truncations, according to the target pressure needed. Pressure and temperature calibrations and gradients show that the central zone of the assemblies is stable. Positions of differential RAMs are controlled with a sub-micron precision allowing strain rate from 10(-4) to 10(-6) s(-1). Moreover, changing differential RAM velocity is immediately visible on sample, making faster reaching of steady state. Lattice stresses are determined by the shifting of diffraction peak with azimuth angle using a linear detector covering typically a 10° solid-angle in 2θ mounted on rotation perpendicular to the beam. Acquisition of diffraction pattern, at a typical energy of 55 keV, is less than a minute to cover the whole azimuth-2θ space. Azimuth and d-spacing resolution are respectively better than 1° and 10(-3) Å making it possible to quantify lattice stresses with a precision of ±20 MPa (for silicates, which have typically high values of elastic properties), in pure or simple shear deformation measurements. These mechanical data are used to build fully constrained flow laws by varying P-T-σ-ε̇ conditions with the aim to better understanding the rheology of Earth's mantle. Finally, through texture analysis, it is also possible to determine lattice preferred orientation during deformation by quantifying diffraction peak intensity variation with azimuth angle. This press is therefore included as one of the few apparatus that can perform such experiments combining with synchrotron radiation.

Perovskite to postperovskite transition in NaFeF3.

The GdFeO3-type perovskite NaFeF3 transforms to CaIrO3-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the p-V data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO3-type MgSiO3 at high pressure and temperature. Theoretical studies using density functional theory at the GGA + U level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott-Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets.

Structures of dolomite at ultrahigh pressure and their influence on the deep carbon cycle.

Carbon-bearing solids, fluids, and melts in the Earth's deep interior may play an important role in the long-term carbon cycle. Here we apply synchrotron X-ray single crystal micro-diffraction techniques to identify and characterize the high-pressure polymorphs of dolomite. Dolomite-II, observed above 17 GPa, is triclinic, and its structure is topologically related to CaCO(3)-II. It transforms above 35 GPa to dolomite-III, also triclinic, which features carbon in [3 + 1] coordination at the highest pressures investigated (60 GPa). The structure is therefore representative of an intermediate between the low-pressure carbonates and the predicted ultra-high pressure carbonates, with carbon in tetrahedral coordination. Dolomite-III does not decompose up to the melting point (2,600 K at 43 GPa) and its thermodynamic stability demonstrates that this complex phase can transport carbon to depths of at least up to 1,700 km. Dolomite-III, therefore, is a likely occurring phase in areas containing recycled crustal slabs, which are more oxidized and Ca-enriched than the primitive lower mantle. Indeed, these phases may play an important role as carbon carriers in the whole mantle carbon cycling. As such, they are expected to participate in the fundamental petrological processes which, through carbon-bearing fluids and carbonate melts, will return carbon back to the Earth's surface.

The structural behaviour of LaF(3) at high pressures.

We have performed in situ X-ray diffraction, Raman scattering, pseudosymmetry analysis and quantum mechanical atomistic simulations of the structure and behaviour of LaF(3) at high pressure, up to and exceeding the phase transition pressure reported for the I4/mmm structure and Cmma symmetry proposed previously. We observe that the structure of LaF(3)-II is best described as being of anti-Cu(3)Ti-type (oP8, Pmmn, SG59), which is closely related to the I4/mmm structure obtained by simulation, through notional distortion, and is evidently similar to the Cmma symmetry by comparison of published diffraction data. We demonstrate that the models are also related to each other, and can be derived through pseudosymmetry searches. LaF(3)-II does not undergo further phase transitions before at least 60 GPa, and none are expected before 1 Mbar. The similarity between the anti-Cu(3)Ti-type and the I4/mmm structure models and our in situ diffraction data, supports the transition mechanisms derived from atomistic model simulations for a generic REF(3) transition to the post-tysonite phase (where RE = La, Ce, Pr, Nd).

Portable multi-anvil device for in situ angle-dispersive synchrotron diffraction measurements at high pressure and temperature.

A recently developed portable multi-anvil device for in situ angle-dispersive synchrotron diffraction studies at pressures up to 25 GPa and temperatures up to 2000 K is described. The system consists of a 450 ton V7 Paris-Edinburgh press combined with a Stony Brook ;T-cup' multi-anvil stage. Technical developments of the various modifications that were made to the initial device in order to adapt the latter to angular-dispersive X-ray diffraction experiments are fully described, followed by a presentation of some results obtained for various systems, which demonstrate the power of this technique and its potential for crystallographic studies. Such a compact large-volume set-up has a total mass of only 100 kg and can be readily used on most synchrotron radiation facilities. In particular, several advantages of this new set-up compared with conventional multi-anvil cells are discussed. Possibilities of extension of the (P,T) accessible domain and adaptation of this device to other in situ measurements are given.

Establishing the structure of GeS(2) at high pressures and temperatures: a combined approach using x-ray and neutron diffraction.

The change in structure of glassy GeS(2) with pressure increasing to [Formula: see text] at ambient temperature was explored by using in situ neutron and x-ray diffraction. Under ambient conditions, the glass structure is made from a mixture of corner- and edge-sharing Ge(S(1/2))(4) tetrahedra where 47(5)% of the Ge atoms are involved in edge-sharing configurations. The network formed by these tetrahedra orders on an intermediate range as manifested by the appearance of a pronounced first sharp diffraction peak in the measured total structure factors at a scattering vector k = 1.02(2) Å(-1) which has a large contribution from Ge-Ge correlations. The intermediate range order breaks down when the pressure on the glass increases above ≈2 GPa but there does not appear to be a significant alteration of the Ge-S coordination number or corresponding bond length with increasing density. The results for the glass are consistent with a densification mechanism in which there is a replacement of edge-sharing by corner-sharing Ge centred tetrahedral motifs and/or a reduction in the Ge-[Formula: see text]-Ge bond angle between corner-sharing tetrahedral motifs with increasing pressure. The change in structure with increasing temperature at a pressure of [Formula: see text] was also investigated by means of in situ x-ray diffraction as the glass crystallized and then liquefied. At 5.2(1) GPa and 828(50) K the system forms a tetragonal crystal, with space group [Formula: see text] and cell parameters a = b = 4.97704(12) and c = 9.5355(4) Å, wherein corner-sharing Ge(S(1/2))(4) tetrahedra pack to form a dense three-dimensional network. A method is described for correcting x-ray diffraction data taken in situ under high pressure, high temperature conditions for a cylindrical sample, container and gasket geometry with a parallel incident beam and with a scattered beam that is defined using an oscillating radial collimator. A method is also outlined for obtaining coordination numbers from direct integration of the peaks in measured x-ray total pair distribution functions.

Phase separation, crystallization and polyamorphism in the Y(2)O(3)-Al(2)O(3) system.

A detailed study of glass formation from aerodynamically levitated liquids in the (Y(2)O(3))(x)(Al(2)O(3))(1-x) system for the composition range 0.21≤x≤0.41 was undertaken by using pyrometric, optical imaging and x-ray diffraction methods. Homogeneous and clear single-phase glasses were produced over the composition range [Formula: see text]. For Y(2)O(3)-rich compositions ([Formula: see text]), cloudy materials were produced which contain inclusions of crystalline yttrium aluminium garnet (YAG) of diameter up to 40 µm in a glassy matrix. For Y(2)O(3)-poor compositions around x = 0.24, cloudy materials were also produced, but it was not possible to deduce whether this resulted from (i) sub-micron inclusions of a nano-crystalline or glassy material in a glassy matrix or (ii) a glass formed by spinodal decomposition. For x = 0.21, however, the sample cloudiness results from crystallization into at least two phases comprising yttrium aluminium perovskite and alumina. The associated pyrometric cooling curve shows slow recalescence events with a continuous and slow evolution of excess heat which contrasts with the sharp recalescence events observed for the crystallization of YAG at compositions near x = 0.375. The materials that are the most likely candidates for demonstrating homogeneous nucleation of a second liquid phase occur around x = 0.25, which corresponds to the limit for formation of a continuous random network of corner-shared AlO(4) tetrahedra.