Pressure and Temperature Dependent Structural Studies on

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Pressure and Temperature Dependent Structural Studies on Hollandite Type Ferrotitanate and Crystal Structure of a High Pressure Phase Samatha Bevara,†,‡ S. Nagabhusan Achary,*,†,‡ Nandini Garg,‡,§ Abhishek Chitnis,†,‡ P. U. Sastry,‡,∥ A. B. Shinde,∥ P. Siva Ram Krishna,∥ and Avesh Kumar Tyagi†,‡ †

Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India § High pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ∥ Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ‡

S Supporting Information *

ABSTRACT: The structural stability and phase transition behavior of tetragonal (I4/m) hollandite type K2Fe2Ti6O16 have been investigated by in situ high pressure X-ray diffraction using synchrotron radiation and a diamond anvil cell as well as by variable temperature powder neutron and X-ray diffraction. The tetragonal phase is found to be stable in a wider range of temperatures, while it reversibly transforms to a monoclinic (I2/m) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced phase transition occurs with only a marginal change in structural arrangements. The unit cell parameters of ambient (t) and high pressure (m) phases can be related as am ∼ at, bm ∼ ct, and cm ∼ bt. The pressure evolution of the unit cell parameters indicates anisotropic compression with βa = βb ≥ βc in the tetragonal phase and becomes more anisotropic with βa ≪ βb < βc in the monoclinic phase. The pressure−volume equations of state of both phases have been obtained by second order Birch− Murnaghan equations of state, and the bulk moduli are 122 and 127 GPa for tetragonal and monoclinic phases, respectively. The temperature dependent unit cell parameters show nearly isotropic expansion, with marginally higher expansion along the c-axis compared to the a- and b-axes. The tetragonal to monoclinic phase transition occurs with a reduction of unit cell volume of about 1.1% while the reduction of unit cell volume up to 6 K is only about 0.6%. The fitting of temperature dependent unit cell volume by using the Einstein model of phonons indicates the Einstein temperature is about 266(18) K.

I. INTRODUCTION

Ideally, hollandite type structures are formed by rutile like edge and corner sharing of octahedral BO6 units. The typical connections of BO6 octahedra lead to 1-D tunnel like empty spaces, and the larger A cations are filled in these empty spaces.1,2,8−14 In the hollandite structure, the walls of the channels are formed by two octahedra (2 × 2) while other closely related structures are formed with a higher number of octahedra in the walls.1,8,14,16,20 In particular, the titanate hollandites are observed with a wider range of compositions, Ax(Ti,B)8O16, where A = Na+, K+, Rb+, Cs+, Sr2+, Ba2+ and B = Mg2+, Al3+, Ga3+, etc. and transition metal ions.10,11,13,21−25 The thermophysical properties and structural transitions of some of the hollandite and related materials have been reported in the literature.21−25 It has been observed that the structure of hollandite is sensitive to the concentration as well as ionic radii of the A+ ions as well as oxygen stoichiometry.11,13,14,21,22,25 Most commonly, for A = K+, Rb+, and Cs+, the tetragonal (I4/ m) structure with 2 × 2 channels is observed while for smaller

Hollandite type materials, in particular manganite or titanates, are observed in nature, and they possess significant structural stabilities under geological conditions.1−3 Silicates with hollandite type structure are also known to occur in the lower mantle region of the earth or under similar conditions.4−7 Weathering of hollandite type minerals under geological conditions often results in wide varieties of compositions with diversified monovalent cations, such as alkali, and divalent cations, such as alkaline earth and Pb2+ cations.1,2,8,9 Thus, hollandite (AxB8O16, where B are cations suitable for octahedral coordination and A are larger cations, such as Cs, Ba, etc.) type materials are considered as a component in the Synroc ensemble to immobilize radioactive Cs and Ba produced in a nuclear reactor.10−14 Besides, hollandite type materials are also of interest for various physical and chemical properties. Spin transitions and Mott−Peierls transitions have been reported in several transition metal containing hollandites.15−17 Hollandite type materials with smaller cations exhibit fast cation conduction due to the free motion of ions in the channels.18,19 © XXXX American Chemical Society

Received: December 1, 2017

A

DOI: 10.1021/acs.inorgchem.7b03028 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ions, such as Na+ and Li+, other low symmetric structures, such as tetragonal superstructures, orthorhombic, monoclinic, etc., are observed.8,11,13,21,22,25 As mentioned, the ionic radii of the interstitial cations play a significant role in governing the structure, and smaller size cations form low symmetric structures. Incorporation of smaller cations in the channel of tetragonal hollandites displaces the octahedral units and then transforms them to lower symmetric structures by destroying the 4-fold symmetry of the original lattice. Such structural differences are clearly observed in Na+ and K+ containing hollandite type ferrotitanates and aluminotitanate compositions.21−24 Ba2+ substituted Cs hollandites also show a transition from tetragonal to monoclinic structure beyond certain concentration of Cs+ ions, and also BaxTi8O16+x shows a structural transition with the variation of Ba2+ ion concentrations.16,25 The distorted structures formed due to the chemical pressure induced displacement of cations of the octahedral units are relaxed with increasing temperature.21−24 Low temperature studies on Fe containing titanates, such as A2Fe2Ti6O16, A = K+, show a specific heat anomaly at around 50 K, which the authors have attributed to a possible structural transition.26 However, due to the lack of structural information at lower temperature, this transition remained unexplained. Electronic or magnetic transitions at lower temperature have been reported in transition metal ions containing tetragonal as well as orthorhombic hollandites.16,27−29 However, no low temperature structural transition or specific heat anomaly has been observed in hollandite type KAlSi3O8.7,30−33 It can be mentioned here that the external thermodynamic parameters, such as pressure and temperature, play opposite roles in the structure, and studies under the influence of pressure or temperature on such materials are useful to understand their crystal chemistry compared to studies by chemical substitutions. It can be expected that pressure can introduce strain and distortion similar to those caused by the chemical substitution. Besides, pressure can lead to structural transition, decomposition, or amorphization in materials depending on the crystal structure and composition of the system. Several studies indicating pressure induced structural transition due to the anisotropic elastic properties7,32−39 or collapsing of loosely packed structure to denser or amorphous phases have been reported in the literature.39−42 Thus, loosely packed and framework type materials are expected to show structural transition or amorphization under compression. As the unit cell of tetragonal hollandite type K2Fe2Ti6O16 has large fractions of empty spaces (about 46%), diversified crystal chemistry can be expected under pressure or temperature. The information on pressure or temperature induced structural variations can shed light on the behavior of hollandite type materials to be used for disposal of nuclear waste in deep geological repositories. Also, they can guide the synthesis of denser transition metal ion containing hollandite or related phases with enhanced magnetic and multiferroic interactions. In this study, the structural stability of hollandite type K2Fe2Ti6O16 with temperature and pressure as well as possible existences of new denser phases has been explored. In order to understand the structural stabilities of K2Fe2Ti6O16 with pressure and temperature, in situ high pressure XRD studies in DAC and variable temperature X-ray and neutron diffraction studies were carried out. It is observed that, at moderate pressure (∼3 GPa), the tetragonal structure reversibly transforms to a monoclinic structure with a large increase in structural distortions. Evolutions of crystal

structures and compressibility behaviors have also been investigated. Temperature dependent studies indicated no structural transition in a wider range of temperatures (6− 1273 K) and nearly isotropic expansion in the tetragonal K2Fe2Ti6O16. The evolutions of structure and pressure−volume equation of states are explained in the following sections.

II. EXPERIMENTAL METHODS A polycrystalline sample of K2Fe2Ti6O16 was prepared by solid state reaction of K2CO3, Fe2O3, and TiO2. In order to remove any adsorbed moisture or OH groups, Fe2O3 and TiO2 were preheated overnight at around 300 and 800 °C, respectively. K2CO3 was heated at around 200 °C just before weighing. Weighed quantities of K2CO3, Fe2O3, and TiO2 (molar ratio 1.1:1:6) were thoroughly mixed in an acetone medium and then dried in air. The dry mixture of reactants was pelletized and heated at progressively increasing temperature, viz. 500 °C/12 h, 800/12 h, and 1250 °C/12 h, with intermittent grindings. The products obtained after heating at 1250 °C for 12 were characterized by powder XRD and used in this study. Regular and nearly spherical grain of the powder along with a few particles of gold (pressure marker) were loaded into a 100 μm hole of a preindented tungsten gasket of thickness ∼40 μm in a modified Mao-Bell kind of diamond anvil cell. The tungsten gasket transmits the scattered X-ray only through the sample hole and hence effectively reduces the background and significantly improves the signal-to-noise ratio. A methanol:ethanol (4:1) mixture was used as pressure transmitting medium (PTM), and the pressure was determined from the equation of state of gold.43 To ensure that the sample experiences hydrostatic or quasi hydrostatic pressure conditions at higher pressure, very little amount of sample was filled into the gasket hole. These experiments were carried out at the BL11 beamline of the INDUS2 synchrotron source44 using X-rays of wavelength 0.5026 Å up to pressures 31 GPa while increasing and decreasing pressure. The data was collected with the help of a two-dimensional imaging plate detector MAR345. CeO2 was used to determine the sample to detector distance (230 mm) and also to calibrate the imaging plate detector. The two-dimensional diffraction images were converted to one-dimensional diffraction patterns using the FIT2D software.45 About 4 g of powder sample was filled in a thin platinum tube and sealed for high temperature neutron diffraction studies. The neutron diffraction data were collected with a linear 5-PSD based Debye− Scherrer type powder diffractometer (PD-2) at the 100 MW Dhruva Research Reactor, BARC, Mumbai, using beams of monochromatic (λ = 1.2443 Å) neutrons. The sealed platinum tube was placed inside a vanadium tube under high vacuum. This vanadium container was placed inside an ILL type high-temperature furnace with a dynamic vacuum better than 10−6 mbar. Eight cylindrical vanadium foils were used as heating element and heat shield to maintain uniformity of the temperature across the sample. The sample temperature is raised to a desired temperature and equilibrated for 1 h, and then the diffraction data were collected. The diffraction data at each temperature were accumulated over a period of 8 h. The scattered signals were corrected for the contribution of platinum and empty furnace background. About 6 g of finely ground sample of K2Fe2Ti6O16 was filled in a vanadium can and capped for low temperature neutron diffraction studies. For low temperature study, the vanadium can filled with sample was placed inside a CCR based cryostat. The sample was cooled to lowest temperature, i.e. 6 K, and allowed to equilibrate for 6 h, before data collection. The sample was heated inside the cryostat to a desired temperature and again equilibrated for 1 h prior to the collection of diffraction data. High temperature XRD data were collected by using Cu Kα radiation on a rotating anode based X-ray diffractometer (Rigaku, Japan). Finely powdered sample was filled in the groove of a platinum strip and placed inside a cylindrical heater. The heater had an opening allowing the incident and outgoing X-rays. The sample was heated at a rate of 20 K/min to a particular temperature and equilibrated for about 5 min prior to data collection. The diffraction data were collected in the angle range of 10 to 90° with a step width of 0.02° and step time 2 B


Article

Inorganic Chemistry s. The structural analyses and refinements of all the temperature and pressure dependent diffraction data were performed by using the Fullprof-2000 package.46

III. RESULTS AND DISCUSSION A. Structural studies at ambient pressure and temperature. The powder XRD pattern of the dark brown color product shows intense reflections attributable to tetragonal hollandite type K2Fe2Ti6O16 reported earlier in the literature.22 However, a few weak additional reflections at 2θ around 11.3, 24.0, 29.0, 34.2, 38.9° etc. observed in the XRD pattern could not be accounted for the tetragonal hollandite type phase. The analyses of these peak positions suggest that they belong to a layered lepidocrocite type phase.47−49 The formation of such a layered phase has been investigated in several studies, and they indicate that the composition as well as temperature of the reaction have a significant role in their formation. The K rich composition is primarily formed at lower temperature, and on decomposition they transform to piredite or hollandite type Kx(Fe/Ti)8O16 compositions at higher temperature.47 A slight K-excess lepidocrocite type phase observed as secondary phase in the sample is found to be stable up to 1250 °C, the maximum temperature of preparation. The composition of such lepidocrocite (FeOOH type) or α or α′ phases has needle or plate type habits and crystallizes in either body-centered or C-centered lattices.47,49 All the extra reflections could be assigned to the reported C-centered orthorhombic phase of KxFe2+yTi6−xO16. The ambient temperature XRD pattern was analyzed by Rietveld refinement using the structural data of tetragonal (I4/m) hollandite type phase and Cmcm (α-phase, KxFe2Ti6O1647,49) phase as model parameters. The complete powder XRD pattern could be successfully refined by considering these two phases, and the refined unit cell parameters are a = 10.1258(1) Å, c = 2.9778(1) Å, V = 305.31(1) Å3, for the tetragonal K2Fe2Ti6O16 phase, and a = 3.8051(8) Å, b = 15.771(3) Å, c = 2.9688(8) Å, V = 178.16(7) Å3 for orthorhombic (Cmcm) KxFe2Ti6O16 with an x ∼ 1.2 phase. The typical Rietveld refinement plots for the ambient condition powder XRD data and the refined structural data are given as Supporting Information (Table S1 and Figure S1). Further characterization of the structure has been carried out from the powder neutron diffraction data recorded at 300 K. The powder neutron diffraction data was refined by considering the identified phase in a similar manner as in the case of XRD data. The refined unit cell parameters of the hollandite type phase are a = 10.1241(5) Å, c = 2.9791(2) Å, and V = 305.35(3) Å3, and they are in good agreement with those observed from XRD studies. The final Rietveld refinement plot of the powder neutron diffraction data recorded at 300 K is shown in Figure 1. The refined structural parameters of the tetragonal K2Fe2Ti6O16 phase as observed from powder neutron diffraction data are given in Table 1. The quantitative estimation of the hollandite type phase as observed from the Rietveld refinement is about 85(1) wt %. Both Ti and Fe are statically occupied in 8c sites, and the K+ ions are considered in 2a sites. The typical interatomic distances in the octahedral BO6 (where B = 0.75Ti + 0.25Fe) units are B−O1 = 1.948(5) Å, B−O1 = 1.927(3) Å × 2, B−O2 = 1.993(4) Å, B− O2 = 2.019(3) Å × 2. From the dispersion of bond lengths, the ⎛ ∑i=1(d − d ̅) ⎞2 1 ⎟ , where n = 6, d = length of distortion (Δ = n × ⎜ n d ̅i i ⎝ ⎠ th i bond, d̅ = mean bond length) in the octahedral BO6 unit is

Figure 1. Rietveld refinement plot of powder neutron diffraction data of K2Fe2Ti6O16 collected at 300 K. (Upper vertical ticks: tetragonal hollandite phase; and lower vertical ticks: orthorhombic α-phase (a = 3.804(4) Å, b = 15.845(14) Å, c = 2.969(3) Å, V = 178.9(3) Å3, wt % = 15(1) %, Rp = 6.72%, Rwp = 8.57%, χ2:8.06).

Table 1. Refined Structural Parameters of K2Fe2Ti6O16 at 300 K (from powder neutron diffraction data)a Atom

wyc

x

y

z

Occ.

K1 Ti1:Fe1 O1 O2

2a 8h 8h 8h

0.0000 0.3376(4) 0.0424(2) 0.2968(3)

0.0000 0.1500(3) 0.3371(4) 0.3426(3)

0.0000 0.0000 0.0000 0.0000

1 0.75:25 1 1

a

Tetragonal (I4/m): a = 10.1241(5) Å, c = 2.9791(2) Å, V = 305.35(3) Å3, Bov = 0.60(2) Å2.

found to be 4.090 × 10−4. This suggests a nearly regular octahedron is formed around the Bn+ ions. Similarly the distortions in angle were calculated from the octahedral bond angles using the relations for bond angle variance (σ2) and distortion index (DI).50−52 For the BO6 octahedral units, the calculated value of bond angle variance (σ2), 1 n (σ 2 = 1 − n × ∑i = 1 (αi − 90)2 ), where αi = bond angle and n = 12 for octahedra and 14 for cube, is 52.24, and DI (DI(Y )(O − M − O) =

1 n

n

× ∑i = 1

(

(αi − αm) αm

2

) ), where α

m

=

mean bond angle, is 0.00592. The smaller distortion parameters also support only feeble distortion in them. The temperature and pressure dependent structural studies were carried out by using the refined structural parameters from powder neutron diffraction data as model structure. As mentioned earlier, the habits of the hollandite and α-phase are different; the hollandite phase can be discriminated easily under a microscope, and tiny grains of hollandite phase were separated and used for studying under high pressure while the bulk sample containing both phases was used for temperature dependent studies, and they are explained below. B. Pressure dependent structural studies and phase transition. Typical XRD patterns of the sample recorded in DAC at different pressures in both up and down strokes are shown in Figure 2. The data recorded at the lowest pressure in DAC show only reflections due to the tetragonal hollandite type phases. However, no reflections due to the layered α-phase indicate that the selected grains belong to tetragonal hollandite type phase. Additional reflections due to the Au, used as pressure marker, and W, used as gaskets, are observed in the diffractions data. The analyses of the XRD data were carried out C


Article

Inorganic Chemistry

Figure 2. Typical powder XRD pattern of K2Fe2Ti6O16 recorded at different pressures (λ = 0.5026 Å). Expanded portion shows the characteristic splitting of 110 and 020 peaks with pressure.

by considering all these three phases. The Rietveld refinements were carried out by considering the unit cell and structural parameters observed under ambient conditions. It may be noted here that only one thermal parameter (overall thermal parameters) was used in the refinement. The background was modeled by linear interpolation of selected points to create a smoothly varying profile for background. The peak profile was generated by pseudo-Voigt profile function. The refined unit cell and position coordinates observed at 0.4 GPa are summarized in Table 2. The final Rietveld refinement plot for the corresponding XRD data is shown in Figure 3a. Table 2. Structural Parameters of Tetragonal K2Fe2Ti6O16 at 0.4 GPa (upper row) and Pressure Released (lower row) Sample inside a Diamond Anvil Cella Atom

wyc

K1

2a

Ti1/Fe1

8h

O1

8h

O2

8h

x 0.0000 0.0000 0.341(7) 0.329(8) 0.038(11) 0.033(20) 0.296(9) 0.319(17)

y 0.0000 0.0000 0.158(7) 0.143(7) 0.343(5) 0.337(11) 0.334(10) 0.314(19)

z 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Occ. 1 0.75:0.25 1 1

a

Tetragonal (I4/m): a = 10.123(5) Å, b = 10.123(5) Å, c = 2.976(2) Å, V = 305.0(3) Å3 (at 0.4 GPa). a = 10.13 (1) Å, b = 10.13(1) Å, c = 2.977(4) Å, V = 305.7(7) Å3 (Pressure release). Figure 3. Rietveld refinement plots of powder XRD data of K2Fe2Ti6O16 recorded at (a, top) 0.4 GPa (Rp: 0.72%, Rwp: 1.06%, χ2:0.02) and (b, bottom) 8.7 GPa (Rp: 0.90%, Rwp: 1.29%, χ2:0.02). In both panels, vertical ticks indicate positions of Bragg reflections for K2Fe2Ti6O16 (top row), gold pressure marker (middle row), and tungsten gasket (lower row).

A comparison of the XRD patterns recorded at different pressures (Figure 2) shows a distinct difference in the pattern recorded at pressure 3.8 GPa compared to that recorded at 1.6 GPa. The typical features such as splitting of peaks, broadening of some peaks, as well as appearance of new peaks are in accordance with a possible structural transition around this pressure. Further, it can be seen that all the intense peaks are observed at almost similar positions as those observed in the D


Article

Inorganic Chemistry ambient pressure tetragonal phase. This indicates that the transition is displacive type and occurs without much alteration in the original structure. Well-defined peaks of the transformed phase could be observed at higher pressure (∼8.7 GPa). Further comparison of the evolution of peaks shows the separation of splitting of peaks increases with pressure, and the positions of some of the peaks shift faster to higher angles compared to others, which suggests appreciable anisotropy in the compression behavior, and this aspect is further discussed later in the paper. In order to deduce the structure of the transformed phases, analysis of the XRD pattern recorded at 8.7 GPa pressure was carried out. A comparison of the splitting and relative intensity of the peaks indicates a monoclinic hollandite type structure for the high pressure phases. Further, all the observed peaks of the transformed phase could be indexed on a monoclinic (I2/m) lattice similar to those reported for Ba2−xMn8O16 type phases.21,53 The unit cell parameters of the monoclinic (I2/ m) structure are closely related to those of the original tetragonal (I4/m) structure, and they can be related as am ∼ at, bm ∼ ct, and cm ∼ bt and β ∼ 90°, where the subscripts t and m represent tetragonal and monoclinic phases, respectively. The refinement of the observed XRD data was carried out in a similar manner as that of the lower pressure. The position coordinates of Ba2‑xMn8O1653 were used as initial model parameters. The position for K atoms in the hollandite structure is sensitive to the external thermodynamic conditions, such as pressure as well as temperature, and is often displaced from the special position. The structural analysis of the monoclinic hollandite structure suggests anisotropic displacement of K atoms and displaces predominantly along the c-axis of the tetragonal lattice. The complete XRD pattern could be refined considering this monoclinic phase, Au, and W. The final Rietveld refinement plot for the XRD data recorded at 8.7 GPa is shown in Figure 3b. The refined position coordinates for the high pressure monoclinic structure are given in Table 3.

Figure 4. Crystal structures of (a, left) tetragonal and (b, right) monoclinic K2Fe2Ti6O16. BO6 polyhedra are shown in both. In the tetragonal phase all the octahedra are symmetrically equivalent. In the monoclinic structure two sets of symmetrically equivalent octahedra are shown as green: B1 = (Fe/Ti)1 and brown: B2 = (Fe/Ti)2. Larger spheres are for K+ ion and smaller spheres are for oxygen. In the monoclinic phase the K+ ions are placed at 2a sites for clarity.

hollandite tunnel as well as 1 × 1 rutile tunnels are also closely similar. The shift in the positions of Bn+ cations and the distortion in the octahedral units deform the regular tunnels and thus lower the symmetry. In general the deformation of the hollandite structures can be observed from the diameter of the tunnels or by comparing the shortest distance of the oxygen atoms in the tunnels. Typical surroundings around K+, bond distances and bond angles of ambient and high pressure phases are given in the Supporting Information (Figure S2, Table S2 and Table S3). It is observed that the deformation distorts the tunnels, which can be noticed from the original bond distances of cubical KO8 (O1−O1 (7.01 Å) and O2−O2 (5.32 Å)) of the ambient pressure phase and those of the high pressure phase (viz. O1−O1 (5.03 Å), O2−O2 (5.21 Å), O3−O3 (7.51 Å), and O4−O4 (5.60 Å)) (Figure 5a and b). This dispersion is originated from the rotations and deformations of octahedral

Table 3. Structural Parameters of High Pressure Monoclinic (I2/m) Phase of K2Fe2Ti6O16 at 8.7 GPaa Atoms

Wyc

x

y

z

Occ

K Fe1:Ti1 Fe2:Ti2 O1 O2 O3 O4

4g 4i 4i 4i 4i 4i 4i

0.00000 0.161(6) 0.357(8) 0.214(16) 0.164(17) 0.135(14) 0.530(14)

0.43(7) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0000 0.341(8) −0.167(6) 0.142(16) −0.203(17) 0.524(16) −0.204(17)

0.5 0.25:0.75 0.25:0.75 1 1 1 1

a a = 10.241(16) Å, b = 2.934(5) Å, c = 9.448(16) Å, β = 92.61(15)°, V = 283.6(8) Å3. Bov: 0.25 Å2.

The analyses of the structures of ambient and high pressure phases indicate that both structures are closely similar and occur through a feeble distortion in the octahedral walls. The Bn+ cation sites split into two sets of sites, and thus, the 4-fold rotation axis of the I4/m structure is transformed to a 2-fold rotation axis with the monoclinic angle (β) ∼ 90°. Such structural transition in hollandite type structures is of ferroelastic nature and observed earlier with temperature, pressure, as well as compositions.7,31−33 Typical crystal structures showing the octahedral frame of BO6 in the ambient and high pressure phases are shown in Figure 4a and b. The basic arrangements of the octahedral units in the 2 × 2

Figure 5. Typical 2 × 2 and 1 × 1 tunnels formed by BO6 octahedra in the tetragonal and monoclinic structures of K2Fe2Ti6O16 are shown. Typical inert−anion distances in the tunnels are marked for both structures. E


Article

Inorganic Chemistry BO6 units. The typical distortion (Δ) in the octahedral units at 0.4 GPa is 12.53 × 10−4, and average B−O bond lengths are 1.96 Å, while those in the monoclinic phase at 8.7 GPa are ⟨B1−O⟩ 1.95 Å and ⟨B2−O⟩ 1.94 Å and Δ = 20.28 × 10−4 and 10.29 × 10−4. The decreases in octahedral bond lengths are only noticeable in both the octahedra. The deformation is also reflected in increasing coordination around the K+ ions; that is, the regular cubes (KO8) of tetragonal phase transform to ten coordinated (bicapped cube, KO 10 ) polyhedra in the monoclinic phase. In the tetragonal phase the K atoms are surrounded by O2 atoms of BO6 octahedra forming a regular cube. The typical K−O2 distances in the KO8 of tetragonal phase are 3.08(8) Å (at 0.4 GPa). As the structural distortion occurs, the KO8 becomes distorted and two additional oxygen atoms from the octahedral MO6 become closer to K and thus form KO10 polyhedra in the monoclinic phase. Typical K−O bonds in the monoclinic phase are K−O1 = 2.8(2) Å × 2, K− O1 = 3.0(2) Å × 2, K−O2 = 2.9(2) Å × 2, K−O4 = 2.8(2) Å × 2, and K−O4 = 3.1(2) Å × 2. The typical surrounding of K+ ions in KOn (n = 8 and 10, within the limit of 3.1 Å) polyhedra in the 2 × 2 tunnel of tetragonal and monoclinic phases is shown in the Supporting Information (Table S2). From the bond length dispersions, the distortions (Δ) around the K in KO8 of the tetragonal (0.4 GPa) and monoclinic (8.7 GPa) phases are 0.002 × 10−4 and 8.96 × 10−4, respectively. The average K−O bond lengths in KO8 decrease from 3.05 to 2.89 Å on increasing pressure from 0.4 to 8.7 GPa. Similarly the distortions (σ2 and DI) calculated from bond angles (Supporting Information Table S3) indicate only a small change in octahedral units. However, the two distinct octahedra of the monoclinic phase have different bond angle distortions, viz. σ2 = 27.5, DI(Y) = 0.0032 in B1O6 and σ2 = 61.8. DI(Y) = 0.0070 in B2O6 (at 8.7 GPa) (Supporting Information Table S3). Considering the additional two K−O4 bonds, the distortion in the KO10 (average bond length 2.93(5) Å) of the high pressure phase is 15.33 × 10−4. This suggests that the tunnel formed in the 2 × 2 hollandite distorts appreciably with increasing pressure. Further insights on the tunnel could be obtained by comparing the bond angle distortions (σ2 and DI) in the KO8 (n = 8) polyhedra of tetragonal and monoclinic phases. Both σ2 and DI for KO8 increase in the phase transition (σ2 = 474.5, DI(Y) = 0.0563 at 0.4 GPa and σ2 = 609.6, DI(Y) = 0.0726 at 8.7 GPa, Supporting Information Table S3). This also indicates that the phase transition affect significantly the tunnel compared to the wall in 2 × 2 hollandite. Further, to understand the pressure evolution of structure, the unit cell parameters of both ambient and high pressure phases observed at different pressures are compared. The evolutions of unit cell parameters with pressure are shown in Figure 6. It can be seen that the unit cell parameters decrease monotonically in the tetragonal phase. However, the unit cell parameters disperse at the phase transition pressure, viz. expansion along one axis (am) in the transformed monoclinic phases. The variation of the a-axis of the monoclinic phase is observed to be almost invariant up to about 25 GPa and then shows only a marginal decrease with increasing pressure. The other two axes (bm and cm) of the unit cell decrease continuously with increasing pressure. The anisotropic variation is related to the monoclinic distortions (β), which show a continuous but nonlinear increasing trend with pressure. From the linear fits, the axial compressibilities of the ambient and high pressure phases are obtained, and they are given in Table

Figure 6. Pressure evolution of unit cell parameters of K2Fe2Ti6O16.

Table 4. Compressibility, EOS Parameters (a),a and Thermal Expansion (b) Data of K2Fe2Ti6O16a (a) Tetragonal

Monoclinic

βa (GPa−1)

3.33 × 10−3

βc (GPa−1) βV (GPa−1) Vo (Å)3 Bo (GPa) Bo′

1.54 × 10−3 8.16 × 10−3 306.1(0.3) 122(12) 4

βa (GPa−1) βb (GPa−1) βc (GPa−1) βV (GPa−1) Vo (Å)3 Bo (GPa) Bo′

0.27 × 10−3 1.11 × 10−3 3.8 × 10−3 5.15 × 10−3 303.4(1.8) 127(8) 4

(b) α (6K−1273 K) K a (Å) c (Å) V (Å)3

10.11 × 10−6 10.79 × 10−6 31.42 × 10−6

−1

x0

ϵ

ϑE (K)

10.1018(8) 2.9731(2) 303.39(5)

0.028(4) 0.010(1) 2.82(21)

244(29) 286(15) 266(19)

a

EOS parameters are obtained from the second order BM-II equation of states. ∂ ln x

4. A comparison of axial compressibilities ( βx = − ∂P , where x = unit cell parameters) indicates that the ratio βa:βb:βc is 2.17:2.17:1.00 for the tetragonal phase, while the same ratio for the monoclinic phase is 1.00:3.99:13.97. Thus, the compression behaviors of the ambient and HP phase are different, which can be accounted by the variation of the coordination of K+ ions in the 2 × 2 tunnel. It may be suggested that the strain introduced in compression of KO8 is relieved by distorting the lattice and deviating the angle from 90°. The pressure evolution of unit cell volume of K2Fe2Ti6O16 (shown in Figure 7) indicates an almost smooth variation in unit cell volume with pressure in the complete range of pressure. It can be noticed that the decrease in unit cell volume of tetragonal phases is relatively faster compared to the monoclinic phase. However, no noticeable volume discontinuity is observed at the transition, which is in accordance with a displacive type phase transition which occurs F


Article

Inorganic Chemistry

B0 = 83 GPa to monoclinic, B0 = 85 GPa34) and BaTe2O6 (orthorhombic, B0 = 123.2 GPa to monoclinic, B0 = 129.3 GPa35) have been reported in the literature. Also the changes in unit cell volume at the transition in such materials are only marginal. The change in volume as well as bulk modulus can be attributed to the variation of free volume in the unit cell. The increase in packing results from the displacements of the octahedral units and the increase of the coordination number around the K+ ions. Since the atoms are only feebly displaced, the atomic arrangement in the high pressure phase remains closely similar to that in the ambient pressure phase of K2Fe2Ti6O16, and hence the transition occurs with a small volume discontinuity as well as small increase in bulk modulus. The original tetragonal phase also reverts back without any hysteresis. The structural relations between the ambient and high pressure phases and complete reversal of structure without any hysteresis suggest a second order nature of the transition. High pressure studies on lead alumina silicate (Pb0.8Al1.6Si2.4O8) prepared at HP-HT conditions (16.5 GPa and 1450 °C) show a tetragonal (I4) structure while no monoclinic phase has been observed, which may be attributed to the simultaneous effects of pressure and temperature.55 Ballaran et al. have reported a ferroelectric I4/m to I2/m phase transition at around 19 GPa in hollandite type K0.8Na0.2AlSi3O8, prepared under similar HPHT conditions (20 GPa and 1700 °C).33 The equation of state of the tetragonal and high pressure monoclinic phase obtained by similar BM-II EOS indicates lower bulk modulus for the tetragonal phase (174 GPa) compared to the monoclinic phase (198 GPa). It can be mentioned here that the bulk modulus of hollandite type KAlSi3O8 is about 208 GPa and the tetragonal to monoclinic phase transition occurs at significantly higher pressure, such as ∼23 GPa.7 It can be mentioned here that the crystal chemistry of hollandite type materials is related to the ionic radii (r) of the octahedral (B) and tunnel cation (A). Post et al. has proposed the tetragonal to monoclinic structural transition occurs at the limit rB/rA ∼ 0.48.20 The rB/rA (∼0.41) for K2Fe2Ti6O16 (for octahedral coordination, rFe3+ = 0.645 Å,

Figure 7. Pressure evolutions of the unit cell volume of K2Fe2Ti6O16. Solid lines are calculated from second order EOS parameters of tetragonal and monoclinic phases.

without much variation in the overall arrangements of ions. In addition, on decreasing pressure the monoclinic phase completely reverts back to tetragonal phase near approximately the same pressure as in upstroke data. Complete reversal of tetragonal phase is observed in the pressure released sample (Figure 2). The structural data of the pressure released phase is included in Table 2. The pressure−volume data was fitted with the second order Birch−Murnaghan (BM) equation of state (EOS).54 The obtained EOS parameters for the high pressure monoclinic phase are V0 = 303.4(1.5) Å3, B0 = 127(8) GPa, and B0′ = 4.0. Similarly, from the second order BM EOS, the obtained V0 and B0 for the ambient pressure phase are found to be 306.1(0.3) Å3 and 122(12) GPa, respectively. The increase in bulk modulus is marginal (∼4%) and can be attributed to almost similar structural arrangements in both ambient and high pressure phases. Similar marginal increases in bulk modulus in the pressure induced phase transitions in Sr2ZnGe2O7 (tetragonal,

Figure 8. Evolution of powder neutron diffraction patterns (λ: 1.2443 Å) and XRD patterns (λ = Cu Kα) with temperature. G


Article

Inorganic Chemistry rTi4+ = 0.605 Å; for cubic coordination, rK+ = 1.51 Å56) is close to the limiting value compared to the silicates (∼0.35). For Na2Fe2Ti6O16 this ratio is ∼0.52 (for cubic coordination, rNa+ = 1.18 Å56) and thus forms only a stable monoclinic lattice. The lower tetragonal to monoclinic transition pressure reported by Ballaran et al.33 for K0.8Na0.2AlSi3O8 may be related to the incorporation of smaller cations such as Na+ in the sites of K+ ions. In a similar analogy, Na2Fe2Ti6O16 should reversibly transform to tetragonal K2Fe2Ti6O16 type structure at higher temperature.21,22,25 Since the octahedral units of such ferrotitanates are relatively rigid, the increase or decrease in the ionic radius of the A-site should govern the transition pressure. Since the rB/rA of K2Fe2Ti6O16 is close to the limiting value, thus a feeble compression in the KO8 polyhedra reduces the symmetry. Thus, it is expected that with increasing pressure the cations become less labile inside the tunnels and become more incompressible. This aspect is further explained while discussing the temperature dependent studies. C. Temperature dependent structural studies. The stability of K2Fe2Ti6O16 with temperature has been investigated by both variable temperature powder neutron diffraction (in between 6K to 975 K) and XRD (in between 298 to 1273 K). Typical diffraction patterns recorded at different temperatures are shown in Figure 8. It can be seen that the PND or XRD patterns recorded at different temperatures are closely similar to their respective ambient temperature data, which suggests the absence of any structural transition in the complete range of studies (6−1273 K). The analyses of the PND and XRD data recorded at different temperatures are carried out in a closely similar manner as mentioned earlier. Refined position coordinates and unit cell parameters at 6 K, 975 K, and 1273 K are given in Table 5. Typical Rietveld refinement plots for these temperatures are shown in Figure 9. Table 5. Structural Parameters of Tetragonal (I4/m) K2Fe2Ti6O16 at Different Temperatures, 6 K (PND), 975 K (PND), and 1273 K (XRD)a Atom

Wyc

x

y

z

K1

2a

Ti1

8h

O1

8h

O2

8h

0.0000 0.0000 0.00000 0.3338(4) 0.3375(4) 0.3315(4) 0.0464(2) 0.0435(2) 0.0492(9) 0.3002(3) 0.2954(3) 0.3075(9)

0.0000 0.0000 0.0000 0.1466(3) 0.1488(3) 0.1493(3) 0.3412(5) 0.3372(5) 0.3401(8) 0.3416(3) 0.3408(2) 0.3417(8)

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Occ. 1

Figure 9. Rietveld refinement plots of tetragonal K2Fe2Ti6O16 at some representative temperatures: (a) at 6 K (neutron diffraction data, Rp: 2.85%, Rwp: 3.68%, χ2:1.99); (b) at 975 K (neutron diffraction data, Rp: 4.79%, Rwp: 6.18%, χ2:4.23); (c) at 1273 K (XRD data, Rp: 18.0%, Rwp: 23.8%, χ2:2.48). In all the figures, the upper rows of ticks are Bragg positions of tetragonal K2Fe2Ti6O16 phase and lower rows of ticks are for orthorhombic Cmcm, α-phase.

0.75:0.25

1

1

BO6 octahedra also does not show appreciable variation (4.15 × 10−4 at 6 K, 3.81 × 10−4 at 975 K (from PND data), and they are significantly lower compared to the distortion introduced by pressure. The variations of unit cell parameters with temperature are shown in Figure 10. It is observed that unit cell parameters systematically decrease with decreasing temperature and become almost invariant at lower temperature, below 150 K. The coefficients of average axial and volume expansion are calculated from the observed unit cell parameters at 6 and 1273 K, and they are αa = 10.11 × 10−6 K−1, αc = 10.79 × 10−6 K−1, and αV = 31.42 × 10−6 K−1. The variation of unit cell parameters and volume could be fitted by considering the Einstein model, XT = X 0 + ε(e ϑε / T − 1)−1, where X is the unit cell parameter and ϵ and ϑϵ are the Einstein constant and

a 6 K: a = 10.1021(5) Å, c = 2.9728(2) Å, V = 303.38(3) Å3, Overall tem. factor = 0.91(6) Å2. 975 K: a = 10.1976(6) Å, c = 3.0030(2) Å, V = 312.29(4) Å3, Overall tem. factor = 1.75(3) Å2. 1273 K: a = 10.2315(2) Å, c = 3.0134(1) Å, V = 315.46(1) Å3, Overall tem. factor = 1.79(8) Å2.

The analysis of the refined structural parameters observed at low and high temperature indicates that the octahedral BO6 remains almost similar and average B−O bonds increase only marginally (∼1.5%) with temperature. The average M−O bonds of the MO6 octahedra at 6 and 1273 K are 1.970(2) and 2.008(3) Å, respectively. Similarly, the distortion around the H


Article

Inorganic Chemistry

octahedra in 2 × ∞ sheets. Since the intercation distance (Bn+− Bn+) is lower along the c-axis compared to the a-axis of the tetragonal phase (Figure-5 and Figure S2), the later shows larger compression compared to the former. The channels containing the K+ ions can easily shrink with pressure. However, the expansion of the channel with increasing temperature is restricted by the octahedral units. D. Pressure and temperature dependent crystal chemistry. Further analyses of the variation of unit cell volume with pressure or temperature indicates that the unit cell volume decreases only by 0.6% by decreasing temperature down to 6 K while the volume drops to about 1.1% by increasing pressure of 1.6 GPa. The reduction of unit cell volume to about 2.2% (at around 3.6 GPa) leads to structural instability and hence leads to a phase transition from tetragonal to monoclinic structure. Thus, it can be anticipated that the tetragonal structure remains unaffected by lowering temperature. Similar comparison of temperature dependent unit cell parameters indicates that the unit cell volume expands to about 3.3% up to 1273 K Earlier thermal studies on tetragonal hollandite type A2Al2Ti6O16 (A = monovalent alkali ions) indicated that the anisotropy in axial expansion is sensitive to the nature of the cation in the 2 × 2 tunnel, viz. αa < αc for K2Al2Ti6O16 while αa > αc for A = Rb and Cs.22 Also the coefficients of volume expansion vary nonmonotonously with increasing radii of A+ ions, viz. αV (K−1) are 30.4 × 10−6 (for K2Al2Ti6O16), 32.8 × 10−6 (for Rb2Al2Ti6O16), and 31.6 × 10−6 (for Cs2Al2Ti6O16) (for coordination number 8, ionic radii of K+, Rb+, and Cs+ are 1.51, 1.61, and 1.74 Å, respectively56). The anomalous behavior can be due to the larger expansion of A−O bonds in Cs and Rb hollandite. Knyazev et al. have summarized the system of thermal expansion with ionic radii of A+ and suggested lower expansion with Na+ hollandite compared to others (Na2Al2Ti6O16: 24.8 × 10−6 K−1; Na2Fe2Ti6O16: 27.01 × 10−6 K−1.22,24 However, the results indicate no systematics in αV for tetragonal A2Fe2Ti6O16, A = K+ (32.85 × 10−6 K−1), Rb+ (29.37 × 10−6 K−1), and Cs+ (34.06 × 10−6 K−1), hollandites, but it can be suggested that compounds with A2B2Ti6O16 type hollandites have larger expansion and they are in the range between 30 and 34 × 10−6 K−1. Xu et al. have prepared series of alkali and alkaline earth containing hollandite type materials and indicated that the lowering of unit cell volume to about 1.3% retains the tetragonal structure.60 However, the change in symmetry from tetragonal to monoclinic due to lowering of the average ionic radius of the A site cation has been reported in the literature.13,25 Carter has pointed out the stoichiometry of BaxM8O16 is the governing factor of tetragonal to monoclinic transitions, where the lower concentration of Ba2+ favors tetragonal while at higher concentration the monoclinic structure is favored.25 Thus, the decrease in volume is not the sole criteria for the transition; rather, it can be explained by the internal strain in the lattice and in particular in 2 × 2 hollandite type tunnels. Larger distortion favors the monoclinic structure while lower distortion is in favor of the tetragonal structure. From the temperature and pressure dependent studies it can be suggested that the transition from tetragonal to monoclinic phase is related to distortion around the K+ ions. The spherical surrounding or regular cubical polyhedra around K+ stabilizes the tetragonal phase. However, the distorted polyhedra around the tunnel cation, either due to displacement of octahedral units or local strain arising from the smaller ionic radius of the cation, transform the tetragonal structure to the monoclinic structure. Thus, the structure transforms to the

Figure 10. Evolutions of unit cell parameters of tetragonal K2Fe2Ti6O16 with temperature. Solid lines are fits with the Einstein model.

Einstein temperature, respectively.57,58 The typical fit parameters are given in Table 4. The slightly larger expansion along the c-axis compared to the a-axis can be attributed to the packed configuration octahedral units along the c-direction. A comparison of the pressure and temperature dependent studies indicates the a-axis is more compressible compared to the c-axis in tetragonal hollandite (βa/βc = 2.17) while the aaxis has smaller expansion compared to the c-axis (αa/αc = 0.94). This fact corroborates with the close packing of octahedral MO6 units along the c-axis, where the expansion and compression are mainly controlled by the expansion of chemical bonds and intercation repulsions. In addition, it can be mentioned here that the variation of K−O bond lengths with temperature is appreciably higher compared to the (Fe/ Ti)−O bond lengths. The average K−O bond length at 6 and 975 K are 2.974 and 3.040 Å, respectively. The (Fe/Ti)−O bond lengths at 6 and 975 K are 1.970 and 1.981 Å, respectively. Typical bond lengths and bond angles at some representative temperatures are given in the Supporting Information (Table S4 and Table S5). Thus, the coefficients of average thermal expansion of the K−O and (Fe/Ti)−O bonds are 22.90 × 10−6 K−1 and 5.76 × 10−6 K−1, respectively. This is consistent with the weak nature of the K−O bonds. This fact can also be explained from the ionicity of the bonds. In general, compressions or expansions of chemical bonds are higher for bonds with larger ionic character. Thus, the bonds with large covalent characteristics are stiff and show lower expansion with temperature or lower compression with pressure.59 Since Fe−O and Ti−O bonds are more covalent than K−O bonds, temperature or pressure affects significantly to K−O bonds than Fe/Ti−O bonds. Thus, the larger expansion of the K−O bond reduces the ratio of ionic radii of octahedral and tunnel cations and, hence, favors the tetragonal structure at higher temperature. It can be noted here that the differences in expansion of bonds are accommodated by the deformation of octahedra. It can also be suggested that the expansion or compressibility of K2Fe2Ti6O16 is essentially controlled by the bonding in the I


Article

Inorganic Chemistry Author Contributions

monoclinic structure at appreciably lower pressure. The distorted monoclinic phase is retained up to very high pressure. However, the post hollandite phase cannot be ruled out at higher pressure, but no other transition is expected with temperature. The observed specific heat anomaly at lower temperature observed earlier might be related to the electronic transition, which may be attributed to a change of electronic structure as observed in hollandite type manganese and titanate.16,27−29 Further detailed studies on electronic properties may be able to unravel the electronic structure aspects in hollandite type ferrotitanates.

The manuscript was written through contributions of all authors. All the authors have equal contribution to this manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. K. I. Priyadarsini, Head, Chemistry Division, Bhabha Atomic Research Centre, for the support and encouragement of this work. The authors also thank Dr. V. Srihari, High pressure and Synchrotron Radiation Physics Division, BARC, for his help in data collection at BL-11, Indus2 Synchrotron Radiation Source.

IV. CONCLUSION In summary, the detailed structural stability of tetragonal hollandite type K2Fe2Ti6O16 has been investigated in a wide range of temperature and pressure. These studies indicated high thermal stability of the lattice yet an instability of the tetragonal lattice at a moderate pressure (∼3.6 GPa). Under the influence of pressure, the tetragonal lattice distorts and transforms to a monoclinic lattice with marginal alteration in the structural arrangements. The compressibility and equation of states of both ambient and high pressure phases indicate only about 4% increase in bulk modulus by the phase transition. The tetragonal hollandite structure of K2Fe2Ti6O16 shows almost isotropic expansion at higher temperature and almost no change in the unit cell parameters below 150 K. No structural transition is observed in the low temperature diffraction data which suggests the earlier reported specific anomaly is related to possible electronic phenomena in K2Fe2Ti6O16. This study supports the high stability of the hollandite type ferrotitanates at higher pressure and temperature and can account for their existence in the lower mantle. Also their stabilities support them as promising materials for usage as geological repositories of nuclear wastes.

■

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03028. Results of Rietveld refinement of XRD data of K2Fe2Ti6O16 at 298 K and corresponding Rietveld refinement plot, typical surroundings around K+ ions in ambient and high pressure phases of K2Fe2Ti6O16, typical interatomic distances at some representative temperature and pressure (PDF) Accession Codes

CCDC 1589367−1589373 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

REFERENCES

(1) Miura, H.; Iura, H. The crystal structure of hollandite. Mineral. J. 1986, 13, 119−129. (2) Reguir, E. P.; Chakhmouradian, A. R.; Mitchell, R. H. Pb-bearing hollandite-type titanates: A first natural occurrence and reconnaissance synthesis study. Mineral. Mag. 2003, 67, 957−965. (3) Ching, S.; Driscoll, P.; Kieltyka, K.; Marvel, M.; Suib, S. Synthesis of a new hollandite-type manganese oxide with framework and interstitial Cr(III). Chem. Commun. 2001, 23, 2486−2487. (4) Schmidt, M. W. Experimental constraints on recycling of potassium from subducted oceanic crust. Science 1996, 272, 1927− 1930. (5) Irifune, T.; Ringwood, A. E.; Hibberson, W. O. Subduction of continental crust and terrigenous and pelagic sediments: An experimental study. Earth Planet. Sci. Lett. 1994, 126, 351−368. (6) Ishii, T.; Kojitani, H.; Akaogi, M. High-pressure phase transitions and subduction behavior of continental crust at pressure−temperature conditions up to the upper part of the lower mantle. Earth Planet. Sci. Lett. 2012, 357−358, 31−41. (7) Sueda, Y.; Irifune, T.; Nishiyama, N.; Rapp, R. P.; Ferroir, T.; Onozawa, T.; Yagi, T.; Merkel, S.; Miyajima, N.; Funakoshi, K. A new high-pressure form of KAlSi3O8 under lower mantle conditions. Geophys. Res. Lett. 2004, 31, L23612. (8) Cheary, R. W. An analysis of the structural characteristics of hollandite compounds. Acta Crystallogr., Sect. B: Struct. Sci. 1986, 42, 229−236. (9) Vasconcelos, P. M.; Renne, P. R.; Brimhall, G. H.; Becker, T. A. Direct dating of weathering phenomena by 40Ar/39Ar and K-Ar analysis of supergene K-Mn oxides. Geochim. Cosmochim. Acta 1994, 58, 1635−1665. (10) Ringwood, A. E.; Kesson, S. E.; Ware, N. G.; Hibberson, W. O.; Major, A. Immobilisation of high level nuclear reactor wastes in SYNROC. Nature 1979, 278, 219−223. (11) Aubin-Chevaldonnet, V.; Caurant, D.; Dannoux, A.; Gourier, D.; Charpentier, T.; Mazerolles, L.; Advocat, T. Preparation and characterization of (Ba,Cs)(M,Ti)8O16 (M = Al3+, Fe3+, Ga3+, Cr3+, Sc3+, Mg2+) hollandite ceramics developed for radioactive cesium immobilization. J. Nucl. Mater. 2007, 366, 137−160. (12) Tumurugoti, P.; Clark, B. M.; Edwards, D. J.; Amoroso, J.; Sundaram, S. K. Cesium incorporation in hollandite-rich multiphasic ceramic waste forms. J. Solid State Chem. 2017, 246, 107−112. (13) Xu, Y.; Wen, Y.; Grote, R.; Amoroso, J.; Nickles, L. S.; Brinkman, K. S. A-site compositional effects in Ga-doped hollandite materials of the form BaxCsyGa2x+yTi8−2x−yO16: implications for Cs immobilization in crystalline ceramic waste forms. Sci. Rep. 2016, 6, 27412. (14) Cheary, R. W. A structural analysis of potassium, rubidium and caesium substitution in barium hollandite. Acta Crystallogr., Sect. B: Struct. Sci. 1987, 43, 28−34. (15) Moetakef, P.; Wang, L.; Maughan, A. E.; Gaskell, K. J.; Larson, A. M.; Hodges, B. C.; Rodriguez, E. E. Tuning the electronic band

AUTHOR INFORMATION

Corresponding Author

*Phone: 0091-22-25592328. Fax: 0091-22-25505151. E-mail [email protected]; acharysn@rediffmail.com. ORCID

S. Nagabhusan Achary: 0000-0002-2103-1063 J


Article

Inorganic Chemistry structure of microporous titanates with the hollandite structure. J. Mater. Chem. A 2015, 3, 20330−20337. (16) Murata, R.; Sato, T.; Okuda, T.; Horibe, Y.; Tsukasaki, H.; Mori, S.; Yamaguchi, N.; Sugimoto, K.; Kawaguchi, S.; Takata, M.; Katsufuji, T. Electronic phase transition in hollandite titanates BaxTi8O16+δ. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 20408. (17) Lium, S.; Akbashev, A. R.; Yang, X.; Liu, X.; Li, W.; Zhao, L.; Li, X.; Couzis, A.; Han, M.-G.; Zhu, Y.; Krusin-Elbaum, L.; Li, J.; Huang, L.; Billinge, S. J. L.; Spanier, J. E.; O’Brien, S. Hollandites as a new class of multiferroics. Sci. Rep. 2015, 4, 6203. (18) Kadoma, Y.; Oshitari, S.; Ui, K.; Kumagai, N. Synthesis of Hollandite-type LixMnO2 by Li+ Ion-exchange in Molten Salt and Lithium Insertion Characteristics. Electrochim. Acta 2007, 53, 1697− 1702. (19) Wang, E.; Tarascon, J. M.; Colson, S.; Tsai, M. Li Insertion and Ion Exchange Reactions in the Ionic Conducting Tl2(M,Ti)8O16 Phases with Hollandite-Type Structure. J. Electrochem. Soc. 1991, 138, 166−172. (20) Post, J. E.; Von Dreele, R.B.; Buseck, P. R. Symmetry and cation displacements in hollandites: structure refinements of hollandite, cryptomelane and priderite. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, 38, 1056−1065. (21) Michiue, Y. Phase relations in NaxCrxTi8−xO16 at 1350 °C and crystal structure of hollandite-like Na2Cr2Ti6O16. J. Solid State Chem. 2006, 179, 2578−2583. (22) Knyazev, A. V.; Maczka, M.; Ladenkov, I. V.; Bulanov, E. N.; Ptak, M. Crystal structure, spectroscopy, and thermal expansion of compounds in MI2O-Al2O3-TiO2 system. J. Solid State Chem. 2012, 196, 110−118. (23) Xu, H.; Costa, G. C. C.; Stanek, C. R.; Navrotsky, A. Structural Behavior of Ba1.24Al2.48Ti5.52O16 Hollandite at High Temperature: An In Situ Neutron Diffraction Study. J. Am. Ceram. Soc. 2015, 98, 255− 262. (24) Knyazev, A. V.; Chernorukov, N. G.; Ladenkov, I. V.; Belopolskaya, S. S. Synthesis, structure, and thermal expansion of M2Fe2Ti6O16 and MFeTiO4 compounds. Inorg. Mater. 2011, 47, 999− 1005. (25) Carter, M. L. Tetragonal to monoclinic phase transformation at room temperature in BaxFe2xTi8−2xO16 hollandite due to increased Ba occupancy. Mater. Res. Bull. 2004, 39, 1075−1081. (26) Knyazev, A. V.; Chernorukov, N. G.; Letyanina, I. A.; Zakharova, Y. A.; Ladenkov, I. V. Crystal structure and thermodynamic properties of dipotassium diiron(III) hexatitanium oxide. J. Therm. Anal. Calorim. 2013, 112, 991−996. (27) Ishiwata, S.; Bos, J. W. G.; Huang, Q.; Cava, R. J. Structure and magnetic properties of hollandite Ba1.2Mn8O16. J. Phys.: Condens. Matter 2006, 18, 3745−3752. (28) Ishige, Y.; Sudayama, T.; Wakisaka, Y.; Mizokawa, T.; Wadati, H.; Sawatzky, G. A.; Regier, T. Z.; Isobe, M.; Ueda, Y. Interplay between Mott physics and Peierls physics in hollandite-type vanadates with a metal-insulator transition. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 125112. (29) Larson, A. M.; Moetakef, P.; Gaskell, K.; Brown, C. M.; King, G.; Rodriguez, E. E. Inducing Ferrimagnetism in Insulating Hollandite Ba1.2Mn8O16. Chem. Mater. 2015, 27, 515−525. (30) Akaogi, M.; Kamii, N.; Kishi, A.; Kojitani, H. Calorimetric study on high-pressure transitions in KAlSi3O8. Phys. Chem. Miner. 2004, 31, 85−91. (31) Kawai, K. K.; Tsuchiya, T. First-principles study on the highpressure phase transition and elasticity of KAlSi3O8 hollandite. Am. Mineral. 2013, 98, 207−218. (32) He, Y.; Sun, Y.; Lu, X.; Gao, J.; Li, H.; Li, H. First-principles prediction of fast migration channels of potassium ions in KAlSi3O8 hollandite: Implications for high conductivity anomalies in subduction zones. Geophys. Res. Lett. 2016, 43, 6228−6233. (33) Ballaran, T. B.; Liu, J.; Dubrovinsky, L. S.; Caracas, R.; Crichton, W. High-pressure ferroelastic phase transition in aluminosilicate hollandite. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 214104.

(34) Achary, S. N.; Errandonea, D.; Santamaria-Perez, D.; Gomis, O.; Patwe, S. J.; Manjón, F. J.; Hernandez; Placida, R.; Muñoz, A.; Tyagi, A. K. Experimental and Theoretical Investigations on Structural and Vibrational Properties of Melilite-Type Sr2ZnGe2O7 at High Pressure and Delineation of a High-Pressure Monoclinic Phase. Inorg. Chem. 2015, 54, 6594−6605. (35) Mishra, K. K.; Achary, S. N.; Chandra, S.; Ravindran, T. R.; Pandey, K. K.; Tyagi, A. K.; Sharma, S. M. Study of Phase Transformation in BaTe2O6 by in Situ High-Pressure X-ray Diffraction, Raman Spectroscopy, and First-Principles Calculations. Inorg. Chem. 2016, 55, 227−238. (36) Errandonea, D.; Gomis, O.; García-Domene, B.; Pellicer-Porres, J.; Katari, V.; Achary, S. N.; Tyagi, A. K.; Popescu, C. New Polymorph of InVO4: A High-Pressure Structure with Six-coordinated Vanadium. Inorg. Chem. 2013, 52, 12790−12798. (37) Mishra, K. K.; Achary, S. N.; Chandra, S.; Ravindran, T. R.; Sinha, A. K.; Singh, M. N.; Tyagi, A. K. Structural and Thermal Properties of BaTe2O6: Combined Variable-Temperature Synchrotron X-ray Diffraction, Raman Spectroscopy, and ab Initio Calculations. Inorg. Chem. 2016, 55, 8994−9005. (38) Errandonea, D.; Kumar, R. S.; Gracia, L.; Beltrán, A.; Achary, S. N.; Tyagi, A. K. Experimental and theoretical investigation of ThGeO4 at high pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 094101. (39) Errandonea, D.; Manjón, F. J. Pressure effects on the structural and electronic properties of ABX4 scintillating crystals. Prog. Mater. Sci. 2008, 53, 711−773. (40) Achary, S. N.; Mukherjee, G. D.; Tyagi, A. K.; Godwal, B. K. New polymorph of HfMo2O8 synthesized using a high-pressure and high-temperature toroid anvil setup. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 1841061−1841065. (41) Mukherjee, G. D.; Vijaykumar, V.; Karandikar, A. S.; Godwal, B. K.; Achary, S. N.; Tyagi, A. K.; Lausi, A.; Busetto, E. Compressibility anomaly and amorphization in the anisotropic negative thermal expansion material NbOPO4 under pressure. J. Solid State Chem. 2005, 178, 8−14. (42) Krogh Andersen, A. M.; Carlson, S. High-pressure structures of α- and δ-ZrMo2O8. Acta Crystallogr., Sect. B: Struct. Sci. 2001, 57, 20− 26. (43) Dewaele, A.; Loubeyre, L. P.; Mezouar, M. Equations of state of six metals above 94 GPa. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 094112. (44) Pandey, K. K.; Poswal, H. K.; Mishra, A. K.; Dwivedi, A.; Vasanthi, R.; Garg, N.; Sharma, S. M. Energy-dispersive X-ray diffraction beamline at Indus-2 synchrotron source. Pramana 2013, 80, 607−619. (45) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. Two-dimensional detector software: From real detector to idealised image or two-theta scan. High Pressure Res. 1996, 14, 235−248. (46) Rodriguez-Carvajal, J. Fullprof 2000: A Program for Rietveld, Profile Matching and Integrated Intensity Refinements for X-ray and Neutron Data, Version 1.6; Laboratoire Leon Brillouin: Gif sur Yvette, France, 2000. (47) Endo, T.; Nagayama, H.; Sato, T. Crystal growth of potassium titanates in the system K2O-Fe2O3-TiO2. J. Cryst. Growth 1986, 78, 423−430. (48) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. The Stability and Structure of Csx[Ti2‑x/4□x/4]O4, 0.61 < x < 0.65. J. Solid State Chem. 1987, 66, 7−19. (49) Gao, T.; Fjellvåga, H.; Norby, P. Protonic titanate derived from CsxTi2−x/2Mgx/2O4 (x = 0.7) with lepidocrocite-type layered structure. J. Mater. Chem. 2009, 19, 787−794. (50) Robinson, K.; Gibbs, G. V.; Ribbe, P. H. Quadratic elongation: a quantitative measure of distortion in coordination polyhedra. Science 1971, 172, 567−570. (51) Wildner, M. On the geometry of Co(II)O6 polyhedra in inorganic compounds. Z. Kristallogr. 1992, 202, 51−70. K


Article

Inorganic Chemistry (52) Ertl, A.; Hughes, J. M.; Pertlik, F.; Foit, F. F.; Wright, S. E.; Brandstätter, F.; Marler, B. Polyhedron distortions in tourmaline. Can. Mineral. 2002, 40, 153−162. (53) Ishiwata, S.; Bos, J. W. G.; Huang, Q.; Cava, R. J. Structure and magnetic properties of hollandite Ba1.2Mn8O16. J. Phys.: Condens. Matter 2006, 18, 3745−3752. (54) Birch, F. Finite strain isotherm and velocities for single-crystal and polycrystalline NaCl at high pressures and 300 K. J. Geophys. Res. 1978, 83, 1257−1268. (55) Downs, R. T.; Hazen, R. M.; Finger, L. W.; Gasparik, T. Crystal chemistry of lead aluminosilicate hollandite: A new high-pressure synthetic phase with octahedral Si. Am. Mineral. 1995, 80, 937−940. (56) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (57) Bevara, S.; Achary, S. N.; Mishra, K. K.; Ravindran, T. R.; Sinha, A. K.; Sastry, P. U.; Tyagi, A. K. Temperature dependent structural, vibrational and magnetic properties of K3Gd5(PO4)6. Phys. Chem. Chem. Phys. 2017, 19, 6030−6041. (58) Chatterji, T.; Hansen, T. C. Magnetoelastic effects in Jahn− Teller distorted CrF2 and CuF2 studied by neutron powder diffraction. J. Phys.: Condens. Matter 2011, 23 (276007), 1−9. (59) Achary, S. N.; Tyagi, A. K. Diffraction and thermal expansion of solids. In Thermodynamic Properties of Solids; WILEY-VCH Verlag GmbH & Co: Weinheim, 2010; Chapter 6. (60) Xu, H.; Wu, L.; Zhu, J.; Navrotsky, A. Synthesis, characterization and thermochemistry of Cs-, Rb- and Sr-substituted barium aluminium titanate hollandites. J. Nucl. Mater. 2015, 459, 70−76.

L


Pressure and Temperature Dependent Structural Studies on

Recommend Documents