High-Pressure Phase Relations and Crystal Structures of Postspinel

2 days ago - A number of experimental studies have shown that most spinel (Sp)-type A2+B23+O4 compounds transform at high pressure to CaFe2O4 (CF)- ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV2O4, FeV2O4, and MnCr2O4: Crystal Chemistry of AB2O4 Postspinel Compounds Takayuki Ishii,*,†,§ Tsubasa Sakai,† Hiroshi Kojitani,† Daisuke Mori,†,∥ Yoshiyuki Inaguma,† Yoshitaka Matsushita,‡ Kazunari Yamaura,‡ and Masaki Akaogi† †

Department of Chemistry, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

‡

ABSTRACT: We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr2O4. At 1200−1800 °C, MgV2O4 Sp decomposes at 4−7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2O3, and they react at 10−15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2O4 Sp transforms to CT-type FeV2O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000−1400 °C. Rietveld refinements of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B3+O6 octahedra (B3+ = V3+ and Cr3+) running parallel to one of orthorhombic cell axes. A relatively large A2+ cation (A2+ = Mg2+, Fe2+, and Mn2+) occupies a tunnelshaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen−A2+ cation distances of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eightcoordinated) sites for the A2+ cations, respectively. A relationship between cation sizes of VIIIA2+ and VIB3+ and crystal structures (CF- and CT-types) of A2+B23+O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+B23+O4 crystallize in wide ionic radius ranges of 0.9−1.4 Å for VIIIA2+ and 0.55−1.1 Å for VIB3+, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIIIA2+ and 0.6−0.65 Å for VIB3+. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A2+ cation than that of CF-type.

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Teller active cations such as Mn3+, resulting in a lower symmetry than that of CT-type. Materials with CF, CT and CM structure types have been widely studied both in geoscience and materials science because of their distinctive structures as described below. In geoscience, they can be host phases for relatively large cations (Na+, K+, and Ca2+) and trivalent cations (Al3+, Cr3+, and Fe3+) in the Earth’s deep mantle, because the postspinel structures have a tunnel-shaped space surrounded by B3+O6 octahedra which can accommodate the large cations. The postspinel transitions in MgAl2O4, which is a major component of Al-rich phases in the deep mantle, have received special attention. MgAl2O4 spinel transforms to CF and CT structure types at ∼25 and ∼40 GPa, respectively.3−9 However, because the transition pressures are relatively high, phase stabilities and physical properties of the postspinel phases of MgAl2O4 are not fully understood, especially those of CT-type structure. In materials science,

INTRODUCTION

A number of experimental studies have shown that most spinel (Sp)-type A2+B23+O4 compounds transform at high pressure to CaFe2O4 (CF)-, CaTi2O4 (CT)-, and CaMn2O4 (CM)-type structures.1,2 We call this type of high-pressure transition a “postspinel transition” in this paper. The CF-, CT-, and CMtype structure types belong to space groups Pnma (No. 62), Cmcm (No. 63), and Pbcm (No. 57), respectively. Figure 1 shows a structural comparison of the three postspinel phases. These structures consist of double chains of edge-sharing BO6 octahedra running parallel to one of the orthorhombic cell axes. The four double chains form a tunnel-like space in which A2+ cations are accommodated. The CF-, CT-, and CM-type structures can be distinguished by characteristics of the BO6octahedral frameworks. The tunnel space in the CF structure type has a glide plane parallel to the a axis, whereas that in the CT structure type has a mirror plane parallel to b axis. The CM structure type is essentially the same as CT type, but the octahedra in the CM structure type are distorted by Jahn− © XXXX American Chemical Society

Received: March 27, 2018

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DOI: 10.1021/acs.inorgchem.8b00810 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry VIII

Mn2+ than those of VIIIMg2+ and VIIIFe2+ and the fact that CdCr2O4 and CaCr2O4, with VIIICd2+ and VIIICa2+ having larger ionic radii than VIIIMn2+, have the CF structure type. We have discovered three new postspinel phases and have conducted structure refinements. We have also determined high-pressure, high-temperature phase relations for MgV2O4, FeV2O4, and MnCr2O4. Finally, combining our new data with available previous data on the AB2O4 postspinel phases, we propose a relationship between the structural types of AB2O4 postspinel phases and ionic radii of VIIIA- and VIB-site cations.

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EXPERIMENTAL DETAILS

Starting Materials. Black Sp-type MgV2O4 and FeV2O4 were prepared as starting materials for the high-pressure and hightemperature experiments. MgV2O4 Sp was synthesized by heating a mixture of MgO + V2O3 with 1:1 molar ratio at 1000 °C for 24 h in an evacuated, sealed silica tube. Green MnCr2O4 Sp was also prepared as the starting material from a mixture of MnO2 and Cr2O3 with 1:1 molar ratio at 1200 °C for 24 h under controlled oxygen fugacity using a mixture of CO2, H2, and Ar with volume ratios of 1:2:3, respectively. Both samples were confirmed to be single phases by powder X-ray diffraction (XRD). Lattice parameters of Sp-type MgV2O4 and MnCr2O4 were determined by powder XRD as a = 8.4189(4) Å and a = 8.4380(4) Å, respectively, and are in good agreement with those from refs 22 and 23, respectively. FeV2O4 Sp was made by heating a mixture of Fe2O3 and V2O3 at 1100 °C for 24 h in a mixture of CO2, H2, and Ar with volume ratios of 3:2:5, respectively. XRD measurement showed that the synthesized FeV2O4 sample contained about 5 wt % V2O3 as impurity. For the starting material of high P−T experiments, 5 wt % iron metal powder was added to the above FeV2O4 Sp sample to prevent oxidization of ferrous iron during the runs. The lattice parameter of Sp-type FeV2O4 synthesized above was determined by powder XRD to be a = 8.4570(8) Å, which is consistent with that given in ref 24. The agreement of the lattice parameters of our synthesized Sp-type MgV2O4, FeV2O4, and MnCr2O4 with those of published cell parameters of the stoichiometric Sp phases suggests that our synthesized samples had stoichiometric compositions. High-Pressure, High-Temperature Experiments. High-pressure, high-temperature experiments were conducted with a Kawai-type 6-8 multianvil high-pressure apparatus (KMA) at Gakushuin University. Phase relations in MgV2O4, FeV2O4, and MnCr2O4 under high pressure and high temperature were determined with tungsten carbide anvils of 5 mm truncated edge length. A 5 wt % Cr2O3-doped MgO octahedron with a 10 mm edge length was used as the pressure medium. A cylindrical rhenium heater was set in the central part of the octahedral pressure medium. A tubular Au75Pd25 or Pt capsule was used to pack a powdered starting material. The capsule was put in the Re heater which was electrically insulated from the capsule with a thin BN sleeve. For thermal insulation, a LaCrO3 sleeve was put outside of the heater, and LaCrO3 end-plugs were placed at both ends of the BN sleeve. Temperature was monitored at the central part of the outer-surface of the furnace with a Pt/Pt−13%Rh thermocouple. No correction was made on the pressure effect on the thermocouple emf. In each run, the sample was compressed to a target pressure at an almost constant rate (1 ton/min) at room temperature, and then was heated to a target temperature at a rate of ∼100 °C/min. The sample was kept at 3.5−21 GPa and 1000−1800 °C for 30−120 min, then quenched under pressure by turning electrical power off, and slowly decompressed to ambient conditions. High-pressure, high-temperature syntheses of the postspinel phases of MgV2O4, FeV2O4, and MnCr2O4 for Rietveld analyses were conducted using tungsten carbide anvils with 2.5 mm truncation, in combination with a pressure medium of 5 wt % Cr2O3-doped MgO octahedron with 7.0 mm edge length. The cell assembly was similar to those for the phase-relation experiments. A Au75Pd25 capsule was chosen for all the synthesis runs. The samples of MnCr2O4 and FeV2O4 postspinel phases were synthesized at 21 GPa and 1400 °C for

Figure 1. Comparison of (a) CaFe2O4 (CF)-type (space group and setting: Pnma (abc)) and (b) CaTi2O4 (CT)-type structures (space group and setting: Cmcm (abc)). Smallest spheres (red) represent oxygen. Solid lines express unit cells. (c) Double chain structure formed by edge-shared FeO6 or TiO6 octahedra parallel to b or a̅ axes in CF- and CT-type structures, respectively. CaMn2O4-type structure (space group and setting: Pbcm (abc)) has the same sequence of octahedra as the CT-type structure. (d) CaO8 bicapped prism consists of a trigonal prism and two longer oxygen−Ca bonds.

magnetic and electric properties of postspinel phases have been remarked upon because of their characteristic structures, namely, a pseudo-one-dimensional structure of double chains, in which arrangements of magnetic cations cannot satisfy competing spin interactions making a magnetically frustrated spin system. High-pressure syntheses and physical property measurements for CF-type phases such as NaRh2O4,10 CdRh2O4,11 and CdCr2O412 have been made, while those of CT-type phases have not yet been investigated, because studies of AB2O4 compounds at high pressure have been limited in the viewpoint of materials science. From the issues discussed above, it is valuable to find new postspinel phases at high pressure, especially new CT-type phases, for both of geoscience and materials science. Phase relations of postspinel transitions in several AB2O4 compounds have been determined at high P−T conditions.5−9,13−20 These studies indicate that AB2O4 Sp first decomposes to two different types of decomposition phases. One is rock-salt-type AO + corundum (Cor)-type B2O3, and the other is modified ludwigite (mLd)-type (or CaFe3O5-type) A2B2O5 + Cor-type B2O3. At higher pressure, the decomposition phases recombine into a single phase with composition AB2O4. Generally, the CT-type phase is a high-pressure phase of CF-type.7,15,21 In addition, an attempt to categorize types of AB2O4 postspinel phases with available parameters (e.g., ionic radius) would be valuable as a basis for design of postspinel compounds so that one can predict a crystal structure of an unknown compound with AB2O4 stoichiometry. In this study, high-pressure, high-temperature phase transitions in MgV2O4, FeV2O4, and MnCr2O4 were investigated. Previous studies reported high-pressure stability of CTtype MgCr2O4 and FeCr2O4.15,16 Therefore, MgV2O4 and FeV2O4, in which VIV3+ has nearly the same ionic radius as VI 3+ Cr , are expected to crystallize in the CT structure type. MnCr2O4 is a good candidate to examine which type of AB2O4 postspinel phase is stable because of a larger ionic radius of B

DOI: 10.1021/acs.inorgchem.8b00810 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 30 min. The MgV2O4 postspinel phase was made by holding at 25 GPa and 1200 °C for 60 min. A major part of each synthesized sample was pulverized and used for synchrotron powder XRD measurements, as described later. Parts of the samples were mounted on a glass-slide plate with epoxy resin and polished for phase identification and compositional analysis by XRD and electron microscopy. Analyses of Recovered Samples. Phases present in the products were identified using a microfocus X-ray diffractometer (Rigaku RINT 2500 V, MDG) with an X-ray beam collimated to 50 μm and a powder X-ray diffractometer (Rigaku RINT 2500 V), both of which were operated with Cr Kα radiation from a rotating anode operated at 45 kV and 250 mA. The powder X-ray diffractometer was used in stepscan mode (step size of 0.02°) in a 2θ range of 10−140° to determine initial lattice parameters used for the Rietveld refinements. Composition analysis was conducted using a scanning electron microscope (SEM, JEOL JMS-6360) with an energy dispersive X-ray spectrometer (EDS, SGX Sensortech Sirius SD-10133), operating with an acceleration voltage of 15 kV and a probe current of 0.55 nA. Natural fayalite and forsterite and synthetic oxides of Cr2O3, MnO, and V2O5 were used as standard materials for Fe, Mg, Cr, Mn, and V, respectively. For structural analyses of the recovered phases, angle-dispersive synchrotron powder XRD measurements were carried out in the BL15XU (NIMS beamline) for the MgV2O4 phase and in the BL02B2 for the FeV2O4 and MnCr2O4 phases at SPring-8. The diffraction data of the samples at ambient conditions were collected using a Debye− Scherrer camera with an imaging plate in 2θ ranges 0−60° and 0−75° with angle resolutions of 0.003° and 0.01° in BL15XU and BL02B2, respectively. The incident X-ray beam in BL15XU was monochromatized at the K absorption edge of niobium (λ = 0.65297 Å). The wavelength (λ = 0.42044 Å) of X-ray in BL02B2 was determined using diffraction data of fluorite-type CeO2. The synthesized polycrystalline samples were ground to fine grain powders using an agate mortar, and each fine powder was packed in a Lindemann glass capillary. The samples were rotated during diffraction pattern measurements for ∼5 min to suppress preferred orientation effects. Rietveld analyses of the diffraction data were carried out using the RIETAN-FP/VENUS package.25 The initial structure models of MgV2O4, FeV2O4, and MnCr2O4 high-pressure phases were CT-type MgCr2O4,16 CT-type FeCr2O4,15 and CF-type MgAl2O4,26 respectively. The initial lattice parameters for the Rietveld refinements of each sample were determined from the laboratory powder XRD patterns (Cr Kα) using DICVOL06 software27 as explained above. A Legendre polynomial function with 12 parameters was used for fitting of the XRD backgrounds. In the final stage of the analysis, lattice parameters, a scale factor, atomic coordinates, isotropic atomic displacement parameters, and a split-type pseudo-Voigt profile fitting function formulated as given in ref 28 were refined simultaneously. Fe metal and corundum-type Cr2O3 in the FeV2O4 and MnCr2O4 samples, respectively, were included in the Rietveld analyses as small amount impurities, and their lattice parameters and scale factors were refined as well as atomic coordinates for corundum-type Cr2O3 (see the Results section).

Table 1. Results of High-Pressure, High-Temperature Experimentsa run no.

pressure (GPa)

S161024 S160927 S161108 S161119 S161111 I121009 S170204 S161213 S161202 S160808 S160803 S160727 S160720 S160709 S161126

5 8 11 13 15 25 3.5 4.5 6 8 11 15 19 20 8

A170730 A171010 A170801 A170720 A170921 A170622 S170116 A170705 I140515

10 12 17 10 12 15 15 17 21

S170111 S170124 S161215 S161221 S161203 S161026 S161028 S170120 S170126 I140514

10 12 7 9 11 15 19 9 11 21

temperature (°C) MgV2O4 1200 1200 1200 1200 1200 1200 1600 1600 1600 1600 1600 1600 1600 1600 1800 FeV2O4 1000 1000 1000 1200 1200 1200 1200 1200 1400 MnCr2O4 1000 1000 1200 1200 1200 1200 1200 1400 1400 1400

time (min)

phase

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

Sp Pc + Cor Pc + Cor Pc + Cor CT ≫ Pc + Cor CTb Sp Sp + Pc + Cor Pc + Cor Pc + Cor CT ≫ Pc + Cor CT CT CT Pc + Cor

120 120 120 120 120 120 60 120 30

Wu + Cor CT CT Wu + Cor CT CT CT CT CTb

120 120 60 60 60 60 60 60 60 30

Sp CF Sp Sp CF CF CF Sp CF CFb

a

Abbreviations: Sp, spinel-type phase; Pc, MgO periclase; Wu, FeO wüstite; Cor, corundum-type V2O3; CF, CaFe2O4-type phase; CT, CaTi2O4-type phase. bThese recovered samples were used for synchrotron X-ray diffraction measurements to conduct the Rietveld refinements.

a new FeV2O4 phase at ∼12 GPa which was identified as CTtype FeV2O4, as shown later. In the MnCr2O4 system (Figure 2c), Sp-type MnCr2O4 directly transforms to a new CF-type phase at ∼10 GPa and 1000−1400 °C. Detailed characterization of the newly synthesized postspinel phases of MgV2O4, FeV2O4 and MnCr2O4 and their crystal structure refinements are described later. In the present experiments on MgV2O4, FeV2O4, and MnCr2O4, we did not observe the decomposition phase assemblage of A2B2O5 + B2O3. In the system MgAl2O4, the decomposition phase assemblage of mLd-type Mg2Al2O5 + Cor-type Al2O3 is stable above 2000 °C below the pressure at which CF-type phase becomes stable. The temperature is higher than the temperature ranges (1200−1800 °C for MgV2O4 and 1000−1400 °C for FeV2O4 and MnCr2O4) in this study. 8 Therefore, it may be possible that the

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RESULTS Phase Relations. Table 1 summarizes experimental conditions and the products of the individual runs for phaserelation experiments on MgV2O4, FeV2O4, and MnCr2O4. Figure 2 shows the results of the high-pressure phase-relation experiments on MgV2O4, FeV2O4, and MnCr2O4. In the system MgV2O4 (Figure 2a), at 1200−1600 °C, Sp-type MgV2O4 first decomposes to MgO periclase + Cor-type V2O3 at 4−7 GPa, and subsequently they reconstitute to a new MgV2O4 phase at 11−15 GPa. Structural and compositional analyses indicated that the new phase was CT-type MgV2O4, as shown below. Phase relations in the system FeV2O4 (Figure 2b) are similar to those in MgV2O4. Sp-type FeV2O4 first dissociates into FeO wüstite + Cor-type V2O3 below ∼10 GPa, and they reconstitute C

DOI: 10.1021/acs.inorgchem.8b00810 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. High-pressure high-temperature phase relations in (a) MgV2O4, (b) FeV2O4, and (c) MnCr2O4. Sp, spinel-type phase; Pc, MgO periclase; Wu, FeO wüstite; Cor, corundum-type V2O3; CF, CaFe2O4-type phase; CT, CaTi2O4-type phase. Solid lines are phase boundaries between two stability fields.

reliability indexes converged to sufficiently small values (