柱撑粘土复合材料的研究进展及其在环境污染物治理方面的应用

以具有片状或层状结构以及较为规整的大孔结构的粘土材料为母体,在其层间引入不同阳离子化合物为柱化剂而合成出性能各异的新型柱撑粘土 (pillared clay)复合材料及其应用是目前环境地球化学研究中备受瞩目的研究前沿之一.对柱撑粘土复合材料的制备进展及其在环境污染治理中的应用进展进行了系统的综述,详细探讨了各种合成影响因素如有机阳离子、无机阳离子以及两者联用柱化剂的选择、复合材料热处理过程、表面酸化预处理等手段对合成柱撑粘土复合材料的制备及性能的影响,同时还对柱撑粘土复合材料在水体和大气环境中毒害有机污染物治理方面的应用进行了比较详细的综述.     

天然沸石在环保中的应用

天然沸石具有良好的离子交换性，吸附性及较好的催化性等性能，且资源丰富、价格低廉，在水处理、空气净化、放射性废物处理和抗菌剂的制备等环境保护与污染治理方面得到了广泛的应用。但目前我国天然沸石的开发利用总体水平还比较低，应加强各种天然沸石的成分，结构和性能研究，大力加强天然沸石的改型改性工艺技术研究，扩大天然沸石在环保中的应用范围。     

Single-crystal Raman spectra of YAlO<Subscript>3</Subscript> and GdAlO<Subscript>3</Subscript>: comparison to several orthorhombic ABO<Subscript>3</Subscript> perovskites

Room-temperature-polarized single-crystal Raman spectra have been measured for both GdAlO₃ and YAlO₃. Both aluminates crystallize in the orthorhombic (Pbnm) perovskite structure. Of the 24 possible Raman modes in 4 symmetries, 20 and 17 modes were observed for gadolinium and yttrium aluminates, respectively. Comparisons of the Raman spectra of these two aluminates to those of 28 other orthorhombic ABO₃ perovskites revealed remarkably similar spectral patterns, regardless of chemistry or valency of the cations. Closer examination of the effect of mass, valencies, and size of the cations on the Raman spectra versus composition revealed that for the observed modes, the A cation plays the dominant role in determining the Raman shift. In particular, the one to two lowest energy modes in each symmetry are determined by cation mass and valency no matter what the chemistry. For some perovskites with common A cations, higher energy modes were also strikingly similar. In particular, the calcium perovskites had almost all A_g modes at the same energies despite the greatly varying B cations. The second to the lowest mode in A_g and B_1g depended only on A cation mass for all perovskites. The volume plays a minor role throughout but is hard to separate from mass effects because the most massive cations are also the largest. However, if the B-cation is common, for example, aluminates or ferrites, the volume has a minor effect on the higher energy modes. These trends were not observed for all perovskites. Notable exceptions were found if a perovskite is near a phase transition or metastable, as found for three manganites. The effect of increased valency of the A cation from 2–4 to 3–3 perovskites expresses itself as relatively larger Raman shifts for the lowest energy modes. Analog studies of MgSiO₃ perovskites should be undertaken with only 2–4 perovskites. The increased understanding for the mode distributions of perovskites allows for better estimates of their thermodynamic properties through vibrational modeling.     

The Raman spectra of several orthorhombic calcium oxide perovskites

We have obtained Raman spectra for a number of orthorhombic perovskites CaBO₃, where B=Ti, Ge, Zr or Sn. The room temperature Raman spectrum of CaTiO₃ was compared with cubic SrTiO₃ to assign first- and second-order features. Partially polarized micro-Raman spectra were obtained for CaTiO₃ perovskite. The CaBO₃ perovskites showed a sequence of increasing complexity in their Raman spectra with increasing degree of orthorhombic distortion from the ideal cubic structure. The spectral changes cannot easily be correlated with changes in chemistry or structure of the perovskite. High temperature micro-Raman spectra for CaGeO₃ perovskite were obtained by laser-heating a 15 μm sample. None of the low frequency Raman modes were soft, but showed only normal anharmonicity up to approximately 700 K.     

纳米矿物及其环境效应

纳米矿物作为连接原子/分子和块体矿物材料的桥梁,在建立矿物微观反应机制和宏观现象的研究中具有重要的意义.随着纳米地质学的迅速发展,纳米矿物在地表环境中的分布、存在形式及其反应活性引起了越来越多关注.综述了天然环境中常见的纳米矿物的成因、存在方式、特殊的尺寸效应、团聚行为、生物/非生物界面反应的分子机制,及其对地表环境和元素生物地球化学循环的影响;着重介绍了具有重要环境意义的纳米矿物与其对应的大尺寸矿物颗粒在吸附行为、溶解速率、团聚状态、催化活性、界面电子传递效率等方面的差异.对于纳米矿物与其对应的宏观矿物晶体之间差异的研究,有助于全面认识矿物对各种地质过程的作用,对于推动地球科学向更加微观和深入的方向发展具有极其重要的意义.      

Oxide perovskites: pressure derivatives of the bulk and shear moduli

The pressure derivatives of elastic moduli (∂M/∂P; M=K_S and G) for a suite of polycrystalline oxide perovskites (2 titanates, 1 stannate and 2 aluminates) have been measured up to 3 GPa using the ultrasonic interferometry method combined with a buffer rod technique. Two empirical systematic relationships (∂G/∂P vs K_S/G and ∂K_S/∂P vs K_S (/ρ)^1/3) have been used to investigate the elasticity systematics of this suite of perovskites and to estimate ∂M/∂P of MgSiO₃ perovskite. The pressure derivatives ∂G/∂P and ∂K_S/∂P for this suite of perovskites scatter between well-defined linear trends for the rutile, rocksalt and spinel structures. The more diffuse trends observed for the perovskites might reflect greater flexibility in the response of its corner-connected octahedral framework structure to changing pressure. The pressure derivatives of the elastic moduli for MgSiO₃ perovskite estimated by the “perovskite bands” are ∂G/∂P=1.6–2.2 and ∂K_S/∂P=3.9–4.2. Received: 13 November 1997 / Revised, accepted: 31 August 1998     

Oxygen deficient perovskites in the system CaSiO3–CaAlO2.5 and implications for the Earth’s interior

Oxygen deficient perovskites of the system CaSiO₃–CaAlO_2.5 have been synthesised at high-pressure and -temperature conditions relevant to the Earth’s transition zone in order to investigate their stabilities in the Earth’s mantle and determine structural properties associated with vacancy incorporation. Two polysomes of thermodynamically stable defect perovskites with Ca(Al_0.4Si_0.6)O_2.8 and Ca(Al_0.5Si_0.5)O_2.75 stoichiometry have been identified. The ordering of oxygen defects into pseudo-cubic (111) layers results in well-ordered ten- or eightfold superstructures, respectively. At all other compositions examined, a metastable formation of perovskites has been observed instead, which are assumed to grow initially disordered. These are now characterised by tiny domains, formed due to subsequent ordering of vacancies along various pseudo-cubic {111} layers. Both ordered defect perovskites show a large P–T stability field ranging from about 9–18 GPa and 4–12 GPa, respectively. Microstructural TEM analyses revealed the presence of growth and ferroelastic twins, which indicate a phase transition from rhombohedral to monoclinic symmetry during quenching. Electron energy loss spectroscopy of Si and Al K edges point at the presence of tetrahedral, octahedral and maybe some pentacoordinated silicon, whereas aluminium is predominantly octahedrally coordinated with minor fractions in lower coordination. Observed properties are interpreted in terms of a new structural model, explaining the observed phase transition and formation of different twin laws as well as giving reasons for the development of such large superstructures. With respect to phase relations of the transition zone, the potential occurrence of such defect perovskites in the Earth’s interior is discussed.     

Low Mantle Perovskite: Solid Solution,Spin State of Iron and Water Solubility

Silicate perovskites((Mg, Fe)SiO 3 and CaS iO 3) are believed to be the major constituent minerals in the lower mantle. The phase relation, solid solution, spin state of iron and water solubility related to the lower mantle perovskite are of great effect on the geodynamics of the Earth's interior and on ore mineralization. Previous studies indicate that a large amount of iron coupled with aluminum can incorporate into magnesium perovskite, but this is discordant with the disproportionation of(Mg,Fe)SiO 3 perovskite into iron-free MgS i O3 perovskite and hexagonal phase(Mg0.6Fe0.4)SiO 3 in the Earth's lower mantle. MnS iO 3 is the first chemical component confirmed to form wide range solid solution with Ca SiO 3 perovskite and complete solid solution with MgS i O3 perovskite at the P-T conditions in the lower mantle, and addition of Mn Si O3 will strongly affects the mutual solubility between Mg Si O3 and CaS iO 3. The spin state of iron is deeply depends on the site occupation of the Fe3+or Fe2+, the synthesis and the annealing conditions of the sample. It seems that the spin state of Fe2+ in the lower mantle perovskite can be settled as high spin, however, the existence of intermediate spin or low spin state of Fe2+ in perovskite has not been clarified. Moreover, different results have also been reported for the spin state of Fe3+ in perovskite. The water solubility of the lower mantle perovskite is related with its composition. In pure Mg SiO 3 perovskite, only less than 500 ppm water was reported. Al–Mg Si O3 perovskite or Al–Fe–MgS iO 3 perovskite in the lower mantle accommodates water of 1100 to 1800 ppm. Further experiments are necessary to clarify the detailed conditions for perovskite solid solution, to reliably analyze the valence and spin states of iron in the coexisting iron-bearing phases, and to compare the water solubility of different phases at different layers for deeply understanding the geodynamics of the Earth's interior and ore mineralization.     

High-pressure perovskites on the join CaTiO3-FeTiO3

An exploratory high-pressure study of the join CaTiO₃-FeTiO₃ has uncovered two intermediate perovskites with the compositions CaFe₃Ti₄O₁₂ and CaFeTi₂O₆. These perovskites have ordering of Ca²⁺ and Fe²⁺ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe²⁺ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe₃Ti₄O₁₂, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn₇O₁₂. Fe²⁺ is in a square-planar A site (similar to Mn³⁺ in NaMn₇O₁₂) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi₂O₆ crystallizes in a unique tetragonal double perovskite structure (SG P4₂/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO₃ recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe₃Ti₄O₁₂ is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.     

Evolution of the distortion of perovskites under pressure: An EXAFS study of BaZrO3, SrZrO3 and CaGeO3

We have investigated the evolution of the distortion of several oxide perovskites with increasing pressure, using EXAFS in the diamond anvil cell. Cubic perovskite BaZrO₃ remains cubic up to 52 GPa. Orthorhombic perovskite CaGeO₃ becomes less distorted as pressure increases, becomes tetragonal at about 12 GPa and evolves toward cubic structure, still not obtained at 23 GPa. The distortion of orthorhombic perovskite SrZrO₃ first increases with pressure up to 8 GPa, then decreases until the perovskite becomes cubic at 25 GPa. The results are interpreted in terms of a systematics, relating the distortion to the ratio f of the volumes of the AO₁₂ dodecahedron and the BO₆ octahedron, and to the compressibilities of the polyhedra. For cubic perovskites, f=5, which may correspond to a situation where the compressibilities of octahedra and dodecahedra are equal.The behavior of SrZrO₃ offers a clue to predict the evolution of the distortion of MgSiO₃ at lower mantle pressures. It is suggested that the increase in distortion experimentally observed at lower pressures should stop above about 10 GPa, and the distortion decrease until the perovskite undergoes ferroelastic transitions to tetragonal and cubic phases, at pressures possibly below the pressure at the core-mantle boundary.     

Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir

We have determined the partitioning of a wide range of trace elements between silicate melts and CaSiO₃ and MgSiO₃ perovskites using both laser ablation-ICPMS and ion microprobe techniques. Our results show that, with the exception of Sc, Zr, and Hf, all trace elements we considered are incompatible in MgSiO₃ perovskite, from highly incompatible for U, Th, Ba, La, Sr and monovalent elements to slightly incompatible for heavy rare earth elements. MgSiO₃ perovskite-melt partition coefficients increase slightly with Al content in the perovskite. These observations contrast strongly with partitioning between CaSiO₃ perovskite and silicate melts. In the latter case, all rare earth elements are clearly compatible as are U and Th. Our data also suggest that, contrary to pressure and temperature, melt composition can significantly affect CaSiO₃ perovskite-melt partitioning; partition coefficients for rare earth elements and U and Th increase with decreasing CaO melt content. The presence of ∼0.4 wt% water in melt makes little difference, however. Partitioning of trace elements into the large site of both MgSiO₃ and CaSiO₃ perovskites follows the near-parabolic dependence on ionic radius predicted from the lattice strain model. The peaks of the parabolae are much higher for the CaSiO₃ phase, perhaps suggesting that the mechanisms of charge compensation for heterovalent substitution are different in the two cases. Our partitioning data have been used to assess the potential effect of perovskite fractionation into the lower mantle during early Earth history. Crystallisation of less than 8% of a mixture of CaSiO₃ and MgSiO₃ perovskites could have led to a ‘layer’ enriched in U and Th without disturbing the chondritic pattern of refractory lithophile elements in the primitive upper mantle. The resultant reservoir could have high Sm/Nd, U/Pb, Sr/Rb, Lu/Hf ratios similar to the HIMU component of ocean island basalts, but would not balance the observed depletion of the primitive upper mantle in Si and Nb.     

Solid solution behaviour of CaSiO<Subscript>3</Subscript> and MgSiO<Subscript>3</Subscript> perovskites

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO₃ and CaSiO₃ perovskites were calculated for pressures and temperatures of the Earth’s lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO₃ at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO₃ is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO₃ perovskite completely in MgSiO₃ perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO₃ perovskite might completely dissolve in MgSiO₃ perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO₃ is increased by other major mantle constituents such as Fe and Al.     

High-pressure Raman studies and heat capacity measurements on the MgSiO<Subscript>3</Subscript> analogue CaIr<Subscript>0.5</Subscript>Pt<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Raman spectroscopy and heat capacity measurements have been used to study the post-perovskite phase of CaIr_0.5Pt_0.5O₃, recovered from synthesis at a pressure of 15 GPa. Laser heating CaIr_0.5Pt_0.5O₃ to 1,900 K at 60 GPa produces a new perovskite phase which is not recoverable and reverts to the post-perovskite polymorph between 20 and 9 GPa on decompression. This implies that Pt-rich CaIr_1−xPt_xO₃ perovskites including the end member CaPtO₃ cannot easily be recovered to ambient pressure from high P–T synthesis. We estimate an increase in the thermodynamic Grüneisen parameter across the post-perovskite to perovskite transition of 34%, of similar magnitude to those for (Mg,Fe)SiO₃ and MgGeO₃, suggesting that CaIr_0.5Pt_0.5O₃ is a promising analogue for experimental studies of the competition in energetics between perovskite and post-perovskite phases of magnesium silicates in Earth’s lowermost mantle. Low-temperature heat capacity measurements show that CaIrO₃ has a significant Sommerfeld coefficient of 11.7 mJ/mol K² and an entropy change of only 1.1% of Rln2 at the 108 K Curie transition, consistent with the near-itinerant electron magnetism. Heat capacity results for post-perovskite CaIr_0.5Rh_0.5O₃ are also reported.     

High-pressure phase behavior of MnTiO<Subscript>3</Subscript>: decomposition of perovskite into MnO and MnTi<Subscript>2</Subscript>O<Subscript>5</Subscript>

The phase relations and compression behavior of MnTiO₃ perovskite were examined using a laser-heated diamond-anvil cell, X-ray diffraction, and analytical transmission electron microscopy. The results show that MnTiO₃ perovskite becomes unstable and decomposes into MnO and orthorhombic MnTi₂O₅ phases at above 38 GPa and high temperature. This is the first example of ABO₃ perovskite decomposing into AO + AB₂O₅ phases at high pressure. The compression behavior of volume, axes, and the tilting angle of TiO₆ octahedron of MnTiO₃ perovskite are consistent with those of other A²⁺B⁴⁺O₃ perovskites, although no such decomposition was observed in other perovskites. FeTiO₃ is also known to decompose into two phases, instead of transforming into the CaIrO₃-type post-perovskite phase and we argue that one of the reasons for the peculiar behavior of titanate is the weak covalency of the Ti–O chemical bonds.     

含钛蒙脱石多孔材料的研究进展

含钛蒙脱石多孔材料是近几年来很受国内外矿物学者关注的矿物材料之一，具有很好的吸附性能和光催化性能，在石油催化裂解和污水处理等领域作为吸附剂、催化剂和催化载体材料等有很好的应用前景。本文对近年来国内外此材料的先进制备工艺，表征手段进行归纳总结，并介绍该材料在石油化工和环保领域中的应用。     

铁细菌胞外多聚物对铁矿物的调控形成及其环境意义

环境介质溶液中铁的水解作用和稳定化作用主要受铁细菌及其代谢有机物质的影响。铁细菌普遍存在于自然环境中,可利用低价铁源为自身生长所需能量。铁细菌胞外有机物的主要组分如多糖和蛋白质等可与铁结合,并通过氧化或沉淀作用使铁稳定、沉积而形成铁矿物;此外铁细菌胞外多聚物可催化铁的氧化或促进铁的聚集。这些生物成因铁矿物因具有良好的表面吸附与氧化还原等化学活性,及有效固定环境中的重金属、放射性核素和催化降解有机污染物的良好环境属性,在环境生物矿物材料和环境治理研究领域被日益重视。故本文基于铁细菌及其胞外多聚物对铁矿物矿化形成的重要调控作用,介绍了环境中存在的铁细菌及其生物矿化特征,重点阐述了铁细菌胞外多聚物(组分、结构及特性)及其在铁矿物矿化过程中的作用,同时对铁细菌胞外多聚物及生物成因铁矿物的环境意义进行了概述。     

Optical Absorption Spectra of (Mg,Fe)SiO3 Silicate Perovskites

Electronic absorption spectra have been measured at room temperature and pressure for polycrystalline samples of (Mg, Fe)SiO₃ silicate perovskites synthesized by multi-anvil device. One strong near-infrared band at about 7000 cm^-1 and several weak bands in the visible region were found. The near-infrared band at 7000 cm^-1 is assigned to a spin-allowed transition of Fe²⁺ at the 8–12 coordinated site in perovskite. However, definite assignments of the weak bands in the visible region are difficult because of their low intensities and the scattering effect at the gain boundaries. Crystal field calculations for Fe²⁺ at different sites in perovskite have been carried out based on the crystal structure data. The results agree with the assignment of Fe²⁺ to the 8–12 coordinated site in perovskite. Crystal field stabilization energy of Fe²⁺ with coordination number of 8 in perovskite is 3332 cm^-1 which is small compared to the octahedral site of magnesiowüstite (4320 cm^-1), another important lower-mantle mineral.     

High-pressure transformations of ilmenite to perovskite, and lithium niobate to perovskite in zinc germanate

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO₃. ZnGeO₃ ilmenite transformed into perovskite at 30.0 GPa and 1300±150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite–perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO₆ octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO₆ octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO₆ octahedra for other germanate perovskites is also discussed.     

Structure and crystal chemistry of perovskite-type MgSiO3

Synthetic clinoenstatite (MgSiO₃) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure. The unit cell dimensions and density of perovskite-type MgSiO₃ at ambient conditions (1 bar, 25°C) are a=4.780(1) Å, b=4.933(1) Å, c=6.902(1) Å, V=162.75 ų, and ρ=4.098(1) g/cm³. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO₂)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO₃ composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO₃ or NiSiO₃ perovskite.     

High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates: systematics in enthalpies of formation of A2+B4+O3 perovskites

Phase transitions in MgGeO₃ and ZnGeO₃ were examined up to 26 GPa and 2,073 K to determine ilmenite–perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite–perovskite boundaries have negative slopes and are expressed as P(GPa)=38.4–0.0082T(K) and P(GPa)=27.4−0.0032T(K), respectively, for MgGeO₃ and ZnGeO₃. Enthalpies of SrGeO₃ polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO₃ pseudowollasonite–walstromite and walstromite–perovskite transitions at 298 K were determined to be 6.0±8.6 and 48.9±5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO₃, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (ΔH _f°) of SrGeO₃ perovskite from the constituent oxides at 298 K was determined to be −73.6±5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite–perovskite transition boundaries in MgGeO₃ and ZnGeO₃ and the boundary of formation of SrSiO₃ perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The ΔH _f° of MgGeO₃, ZnGeO₃ and SrSiO₃ perovskites from constituent oxides were 10.2±4.5, 33.8±7.2 and −3.0±2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A²⁺B⁴⁺O₃ perovskites stable at both atmospheric and high pressures to explore the relationship between ΔH _f° and ionic radii of eightfold coordinated A²⁺ (R _A) and sixfold coordinated B⁴⁺ (R _B) cations. The results show that enthalpy of formation of A²⁺B⁴⁺O₃ perovskite increases with decreasing R _A and R _B. The relationship between the enthalpy of formation and tolerance factor ( R _o: O²⁻ radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A²⁺ and B⁴⁺ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A²⁺ and B⁴⁺ radii indicates a systematic change with equienthalpy curves. These relationships of ΔH _f° with R _A and R _B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.