Perovskite Polymorph Research Articles

Epitaxially stabilized growth of orthorhombic LuScO3 thin films

Metastable lutetium scandate (LuScO3) thin films with an orthorhombic perovskite structure have been prepared by molecular-beam epitaxy and pulsed-laser deposition on NdGaO3(110) and DyScO3(110) substrates. Stoichiometry and crystallinity were investigated using Rutherford backscattering spectrometry/channeling, x-ray diffraction, and transmission electron microscopy. The results indicate that LuScO3, which normally only exists as a solid solution of Sc2O3 and Lu2O3 with the cubic bixbyite structure, can be grown in the orthorhombically distorted perovskite structure. Rocking curves as narrow as 0.05° were achieved. A critical film thickness of approximately 200nm for the epitaxially stabilized perovskite polymorph of LuScO3 on NdGaO3(110) substrates was determined.

Electronic structure and magnetic properties of cubic and hexagonalSrMnO3

$\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is a rare example of a compound having both a cubic (high-temperature) and a hexagonal (low-temperature) perovskite polymorph. While the former is built from corner-sharing $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra only, the latter contains corner-sharing confacial bioctahedral ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities along the $c$ axis. The electronic and magnetic structures of both polymorphs are investigated by density functional theory. Both the cubic and the hexagonal polymorphs are insulators at $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but with quite different band gaps (0.3 vs $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). The hexagonal ground state shows antiferromagnetic coupling both within the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities and between the Mn ions in the corner-sharing octahedra. The lowest energy cubic configuration is found to be $G$-type antiferromagnetic and is $260\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per formula unit higher in energy than the hexagonal ground-state structure. While the bonding interactions involving Sr are found to be mainly ionic, there is a significant covalent contribution to the Mn-O bond. This covalency is very important for the stabilization of the hexagonal structure compared to the cubic polymorph. Two additional factors that stabilize hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ relative to the cubic polymorph are identified. (i) the Mn atoms in the face-sharing octahedra are displaced along the $c$ axis by about $0.012\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ from the center of the octahedra. (ii) The charge transfer giving lower charges for the oxygen in the face-sharing triangle compared to the corner-sharing oxygen. The latter effect results in a contraction of the oxygen triangle in the shared face. This negatively charged oxygen triangle effectively shields the repulsive interaction between the manganese atoms in the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ dimer and facilitates the short Mn-Mn distance present in hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$. Hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is more compressible than cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$, owing to the more open structure. The calculated bulk modulus for hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is in good agreement with the high-pressure powder x-ray diffraction measurements also reported in the present paper.

Crystal and Magnetic Structure of the Orthorhombic Perovskite YbMnO3

The perovskite polymorph of YbMnO3 has been revisited. The orthorhombic phase is obtained through a high-pressure (HP) reconstructive transformation from the hexagonal sol−gel-synthesized YbMnO3 by means of HP annealing at 5 GPa and 1100 °C. Neutron powder diffraction shows that the HP form crystallizes in the orthorhombic space group Pbnm. At 298 K, the refined values for lattice parameters a, b, and c are respectively 5.2208(3), 5.8033(3), and 7.3053(4) A. The average Mn−O−Mn angle is 141.9°, indicative of a strong tilting of the MnO6 octahedra. Magnetic susceptibility measurements indicate antiferromagnetic order of Mn3+ spins at TN = 43 K and of Yb3+ spins at 4 K. The magnetic structure is commensurate Ay with an ordered moment 3.45(5) μB at 9 K. The magnetic susceptibility obeys the Curie−Weiss law, χ = C/(T − θ), in two temperature regions, that is, above 240 K and 6−30 K. The obtained effective paramagnetic moments μeff are 6.37 μB and 4.83 μB for the two temperature ranges, respectively. The forme...

High-pressure phases ofFeTiO3from first principles

The structural, elastic, and electronic properties of the high-pressure $\mathrm{Fe}\mathrm{Ti}{\mathrm{O}}_{3}$ polymorphs and their relative phase stability under pressure are investigated using hybrid exchange density functional theory. For the currently known phases the computed structural and elastic parameters are in good agreement with those observed, as is the predicted stability of the phases. The transformation of the perovskite polymorph to a new high-pressure polymorph with space group $Cmcm$ is predicted to occur at 44 GPa at 0 K.

Synthesis, Structures, and Physical Properties of Yttrium-Doped Strontium Manganese Oxide Films

AbstractCubic strontium manganese oxide is an end-member of the colossal magnetoresistive (CMR) family of manganese-based perovskites, Ln1-xAExMnO3. Because normal synthesis conditions lead to the 4-H hexagonal polymorph, high-pressure conditions are typically used to obtain the cubic perovskite polymorph. In this work, we describe the synthesis and structural/physical characterization of the cubic perovskite form of the high-alkaline-earth containing phases of Y1-xSrxMnO3 (x ≥ 0.7) as epitaxial thin films. Thin films of various stoichiometries were grown on single-crystal perovskite substrates SrTiO3, NdGaO3, and LaAlO3 using pulsed laser deposition. After optimizing deposition conditions, the perovskite polymorph is obtained using PLD at 800°C and 10-100 mTorr O2 for x=1, 0.9, 0.8, and 0.7, as demonstrated by x-ray diffraction. Epitaxial growth was determined to be cube-on-cube. Electrical property measurements demonstrated insulating behavior and no metal-insulator transition or magnetoresistive behavior, similar to related stable compounds.

A novel approach for identifying and synthesizing highdielectric materials

Modern telecommunications require materials with high dielectric constants (κ). The number of suitable elements ultimately limits one approach to the discovery of new materials, targeting compositions with high atomic polarizabilities (α). By decreasing the molar volume of compositions with high α, however, we anticipated dramatic increases in κ and demonstrated that this approach works. The quenched high-pressure perovskite polymorph of Na2MTeO6 (M = Ti, Sn) showed a twofold increase in κ, compared to the ilmenite form. This result suggested the highest values of κ occur for compositions with high α, which form quenchable compounds at high pressures and temperatures.

Pressure-induced phase transformation and amorphization in Ca 3Fe 2Si 3O 12: a high pressure X-ray diffraction study

Pure andradite (Ca 3Fe 2Si 3O 12) was synthesized at 1150°C and 3 GPa, and then compressed to pressure above 21 GPa, and heated to about 1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After temperature quenching in situ high pressure X-ray diffraction studies were carried out. The results show that andradite has transformed to a cubic perovskite type polymorph with a=3.460(4) Å. Upon decompression, in situ X-ray observations reveal that perovskite phase has transformed to an amorphous phase. The density of the perovskite polymorph of Ca 3Fe 2Si 3O 12 is about 13.6% greater than that of an isochemical composition at 21 GPa, and ferric iron can replace Ca 2+ and Si 4+ in the perovskite structure (2Fe 3+=Si 4++Ca 2+), giving a chemical formula: (Ca,Fe)(Si,Fe)O 3. Combining with the electron hopping mechanism between Fe 2+ and Fe 3+ in semiconductivity materials, this new Fe 3+-rich perovskite polymorph may provide new data to clarify the electrical conductivity of the lower mantle, based on the mineralogical model with magnesiuwustite (Mw) and perovskite (Pv).

Incorporation of ferric iron in CaSiO3 perovskite at high pressure

AbstractSynthetic andradite (Ca3Fe2Si3O12) has been compressed to loading pressures >21 GPa and heated to ∼1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After quenching to room temperature, X-ray diffraction of the sample, still held at 21 GPa, showed that andradite had transformed to a cubic perovskite type polymorph with a = 3.460(4) Å. Upon decompression the perovskite phase transformed into an amorphous phase. The density of the perovskite polymorph (Ca3Fe2Si3O12) is ∼13.6% greater than that of isochemical andradite at 21 GPa. Ferric iron replaces Ca2+ and Si4+ in the perovskite structure (Fe3+ + Fe3+ = Si4+ + Ca2+), giving a formula unit: (Ca,Fe3+)(Si,Fe3+)O3. The new Fe3+-rich Ca-perovskite may provide new insight into the controls on the electrical conductivity of the lower mantle.

First observation of FeGeO 3-perovskite under high pressure

Pressure-induced phase transition from high pressure clinopyroxene type FeGeO 3 to its perovskite polymorph has been observed for the first time. X-ray measurements were conducted at 40 GPa and at room temperature. The material is indexed on an orthohombic lattice, with unit cell parameters a = 4.93 ± 0.02 A ̊ , b = 5.06 ± 0.06 A ̊ , c = 6.66 ± 0.03 A ̊ and V = 166.1 ± 0.8 A ̊ 3 at 40 GPa. The pressure-induced transition to FeGeO 3-perovskite occurs during room temperature compression of the high pressure clinopyroxene type of FeGeO 3. Furthermore, FeGeO 3-perovskite was transformed into the lithium niobate structure during decompression. This phase was indexed on a hexagonal lattice, with unit cell parameters a = 5.05 ± 0.04 A ̊ , c = 14.2 ± 0.2 A ̊ and V = 313.6 ± 1.2 A ̊ 3 at ambient pressure.

Structural modifications of CaGeO 3 -wollastonite under room temperature pressurization

In-situ X-ray diffraction measurements of CaGeO3-wollastonite at high pressure at room temperature have been performed using a diamond anvil cell with an X-ray source. A new structural modification of CaGeO3-wollastonite is observed at about 6GPa and the characteristic reflections of the high pressure form are preserved on decompression to an ambient pressure. A rhodonite-like structure is proposed as a high pressure form from the crystal chemical consideration. The rhodonite-like phase is further transformed into a perovskite-form at about 15 GPa. The rhodonite-like-form of CaGeO3 seems not to be a stable phase from the heating experiments under high pressures. The metastable transition path from the wollastonite to the perovskite polymorph through the rhodonite-like structure is kinetically favored under room temperature pressurization. No pressure-induced amorphization is observed during the transition into the perovskite-form, although the transition is accompanied by the coordination change of Ge atoms from fourfold to sixfold.

Alpe Arami: A Peridotite Massif from Depths of More Than 300 Kilometers

The abundance of FeTiO 3 and chromite precipitates in olivine of the Alpe Arami peridotite massif, Switzerland, requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers. Three previously unknown crystal structures of FeTiO 3 were identified that indicate that the originally exsolved phase was the high-pressure perovskite polymorph of ilmenite, implying a minimum depth of origin of 300 kilometers. These observations, coupled with the unique lattice preferred orientation of olivine, further suggest that the original composition of olivine may reflect that of a precursor phase, perhaps wadsleyite, that is stable only at depths greater than 400 kilometers.

Calorimetry of phase transition and melting in silicates

High temperature reaction calorimetry is used to study phase transitions and melting reactions in silicates. Two types of calorimeters are used in our laboratory: a custom built Calvet-type twin microcalorimeter for solution, drop-solution, and transposed-temperature-drop calorimetry near 700 °C and a commercial Setaram HT-1500 calorimeter for transposed-temperature-drop and step-scan measurements at 600–1500 °C. The two types of calorimeters are complementary in their capabilities and together can be used to tailor a calorimetric experiment to the exact conditions needed for a specific reaction. Metastable phases quenched from ultra-high pressure or prepared as glasses, thin films, or gels present special challenges in determining their energetics. A new differential drop-solution calorimetric method has been developed. A sample contained in a capsule made of the lead borate glass used as calorimetric solvent is dropped into one side of a twin microcalorimeter at the same time as an empty capsule is dropped on the other side. The increased sensitivity and accuracy of this method has allowed the determination of the enthalpy of formation of the high pressure perovskite polymorph of MgSiO 3 using a single 5 mg sample. The enthalpy of the reaction MgSiO 3 (orthopyroxene) = MgSiO 3 (perovskite) is 110 ± 5 kJ/mol; that of the ilmenite-perovskite transition is 59 ± 8 kJ/mol. These values support phase equilibrium studies which indicate that reactions forming MgSiO 3-rich perovskite at the 670 km discontinuity in the earth's mantle generally have negative P-T slopes. The energy systematics of pyroxene, garnet, ilmenite, and perovskite structures in silicates and germanates have been determined. The step-scan and transposed-temperature-drop methods also are giving new information on nonquenchable phase transitions, including those involving rapid order-disorder reactions, at 600–1500 °C. The spinels MgAl 2O 4 and NiAl 2O 4, and the pseudobrookite MgTi 2O 5 each show complex cation positional order-disorder. The orientation transition in CaCO 3 (calcite) near 987 °C has been characterized by in situ calorimetry, analyzed in terms of Landau-type order parameters, and applied to understanding apparent curvature in measured calcite-aragonite phase equilibria. Melting and crystallization reactions in silicates at 1100–1500 °C have been measured directly by using the Setaram HT-1500 calorimeter in step scanning mode. The heat capacities of silicate liquids have also been measured. Results for CaMgSi 2O 6 (diopside) agree well with previous thermochemical data. Crystallization runs provide kinetic as well as thermodynamic information. The ternary system CaMgSi 2O 6-NaAlSi 3O 8-CaAl 2Si 2O 8 (diopside-albite-anorthite), as a model for igneous rocks, is being studied in detail by this method.

CdGeO3-phase transformations at high pressure and temperature and structural refinement of the perovskite polymorph

Four polymorphs of CdGeO3 were synthesized at high temperatures (600 ∼ 1200° C) and high pressures up to 12 GPa. The pyroxenoid phase synthesized under ambient pressure transforms to garnet, ilmenite and perovskite phases with increasing pressure. The phase boundary of ilmenite-perovskite had a slightly negative P-T slope in contrast to the positive P-T slopes of the pyroxenoid-garnet and garnet-ilmenite transition boundaries. CdGeO3III has the ilmenite structure with hexagonal lattice parameters, a=5.098 A and c =14.883 A. The c/a ratio of 2.919 is greater than that of any other ilmenite. CdGeO3IV has a distorted perovskite structure with orthorhombic lattice parameters a = 5.209 A, b = 5.253 A and c = 7.434 A. Synthesis of a CdGeO3IV single crystal was successful and structural refinement revealed that the structure is isomorphic to GdFeO3 with the space group Pbnm. The increase of density with the CdGeO3III→CdGeO3IV transformation is the largest (9.8%) for any ilmenite-perovskite transition studied so far.

Elasticity of aluminate, titanate, stannate and germanate compounds with the perovskite structure

Ultrasonic data for the velocities of a large number of perovskite-structure compounds have been determined as a a function of pressure to 6 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 100 kbar in solid-media devices: ScAlO 3, GdAlO 3, SmAlO 3, EuAlO 3, YAlO 3, CdTiO 3, CdSnO 3, CaSnO 3 and CaGeO 3. The elasticity data for these orthorhombic and cubic perovskites define systematic patterns on bulk modulus ( K S)-volume ( V O) and bulk sound velocity ( υ φ —mean atomic weight ( M) diagrams which are insensitiv to the details of cation chemistry and crystallographic structure. These isostructural trends are used to estimate K S = 2.5 ± 0.3 Mbar and υ φ = 7.9 ± 0.4 km/s for the perovskite polymorph of MgSiO 3. On a Birch diagram of veloc vs. density, the perovskite data define linear trends which lead to erroneous estimates of velocity for MgSiO 3 unless specific account is taken of ionic radius effects in isomorphic substitutions.

Synthesis of a perovskite-type polymorph of CaSiO 3

Synthetic crystalline (wollastonite) and glass forms of CaSiO 3 have been compressed to loading pressures above 160 kbar and heated to about 1500° C by a laser in a diamond-anvil cell. After cooling, an X-ray diffraction study carried out whilst the sample was maintained at high pressure revealed that it had transformed to a cubic perovskite-type 3olymorph with a = 3.485 ± 0.008A˚. After release of pressure, however, the sample showed a mixture of glass plus a few weak lines corresponding to ε-CaSiO 3 which is thus interpreted as a retrogressive transition product. The density of the perovskite polymorph of CaSiO 3 is about 9.2% greater than that of an isochemical mixture of CaO + SiO 2 (stishovite) at about 160 kbar.