Pressure-Induced Hexagonal to Monoclinic Phase Transition of

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

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Pressure-Induced Hexagonal to Monoclinic Phase Transition of Partially Hydrated CePO4 Enrico Bandiello,*,† Daniel Errandonea,† Sergio Ferrari,‡ Julio Pellicer-Porres,† Domingo Martínez-García,† S. Nagabhusan Achary,§ Avesh K. Tyagi,§ and Catalin Popescu∥

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Departamento de Física Aplicada-ICMUV, MALTA Consolider Team, Universidad de Valencia, Edificio de Investigación, C/Dr. Moliner 50, Burjassot 46100 Valencia, Spain ‡ Universidad de Buenos Aires, Consejo Nacional de lnvestigaciones Científicas y Técnicas. Instituto de Tecnología y Ciencias de la Ingeniería ’Ing. Hilario Fernández Long’ (INTECIN). Av. Paseo Colón 850, C1063ACV Ciudad Autónoma de Buenos Aires, Argentina § Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India ∥ CELLS-ALBA Synchrotron Light Facility, Cerdanyola del Valles, 08290 Barcelona, Spain ABSTRACT: We present a study of the pressure dependence of the structure of partially hydrated hexagonal CePO4 up to 21 GPa using synchrotron powder X-ray diffraction. At a pressure of 10 GPa, a second-order structural phase transition is observed, associated with a novel polymorph. The previously unknown high-pressure phase has a monoclinic structure with a similar atomic arrangement as the low-pressure phase, but with reduced symmetry, belonging to space group C2. Group− subgroup relations hold for the space symmetry groups of both structures. There is no detectable volume discontinuity at the phase transition. Here we provide structural information on the new phase and determine the axial compressibility and bulk modulus for both phases. They are found to have an anisotropic behavior and to be much more compressible than the denser monazite-like polymorph of CePO4. In addition, the isothermal compressibility tensor for the high-pressure structure is reported at 10 GPa and the direction of maximum compressibility described. Finally, the possible role of water and the pressure medium in the high-pressure behavior is discussed. The results are compared with those from other rare-earth orthophosphates. structure is retained at least up to 20 GPa.16 An apparent pressure-induced structural distortion of monazite CePO4 at 11.5 GPa16 has been later attributed to experimental issues (namely, nonhydrostatic conditions).17,18 However, to the best of our knowledge, information on the structural stability of dehydrated and partially or totally hydrated rhabdophane CePO4 (R-CePO4 in the following) remains scarce. Previous studies have been mainly focused on the effect of temperature on hexagonal CePO4 nanostructures, showing a hexagonal-tomonazite phase transition at high temperature.14,19 There are also studies on the high-temperature structural behavior of several hydrated rare-earth orthophosphates.20 However, there is a total lack of high-pressure studies which, combined with high-temperature experiments, can provide relevant information for the development of long-term storage of nuclear waste.20 In order to improve the knowledge of CePO4 and rare-earth phosphates, here we explore the effects of high-pressure on partially hydrated CePO4 through a detailed study based on synchrotron X-ray diffraction data (XRD). We have found that

1. INTRODUCTION Cerium orthophosphate CePO4 is a material with notable physicochemical properties that has received a fair amount of attention because of its multiple potential real-world applications. Due to mixed electrical conduction properties, CePO4 has been proposed as a promising material for intermediate temperature fuel-cell and oxygen sensors applications.1−3 Photoluminescent terbium-doped CePO4 is used in the fabrication of sensors for the detection of O2, Fe2+, Co2+, and glucose.4−8 Moreover, being a chemically stable material, CePO4 is also a promising alternative to ZnO and TiO2 in sunscreen filters for the protection of human skin from UV rays9,10 and as a better additive for improved corrosion resistance of Ni−P coatings.11 CePO4 is a mineral with a monoclinic crystal structure (space group P21/n), which is the prototype of monazite-type oxides. 12 When hydrated, the material is known as rhabdophane (CePO4·0.667H2O) and has been described in the literature either with a hexagonal or a trigonal crystal structure.13−15 CePO4 can be also obtained in this structure in the anhydrous form but only as nanocrystals.13,14 Previous studies under high-pressure (HP) conditions have confirmed the mechanical stability of monazite CePO4, as the initial © XXXX American Chemical Society

Received: December 31, 2018

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DOI: 10.1021/acs.inorgchem.8b03648 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

P6222.12,28 A similar crystallographic behavior has been observed in the low-temperature−high-temperature phase transition of AlPO4 berlinite, where the loss of a 2-fold symmetry axis parallel to c with decreasing temperature results in a change from the space group P6222 to space group P3121.30,31 With respect to our sample, a careful inspection of the XRD patterns (Figure 1) and the validation of the

it transforms to a previously unreported monoclinic phase at a pressure around 10 GPa, whose structural parameters are reported in the following. The transition is reversible and does not involve a detectable volume collapse. Moreover, we study the axial and bulk compressibility of both phases and their equation of state (EOS). We performed experiments up to a pressure of 21 GPa and found the novel polymorph to be stable up to this pressure. The structural relationships between the low- and high-pressure phases are discussed and the isothermal compressibility tensor for the high-pressure phase is reported. The hexagonal and new monoclinic polymorphs of R-CePO4 are found to be the most compressible materials among rare-earth orthophosphates. In the following sections, the experimental results are presented and subsequently they are discussed in comparison to other rare-earth orthophosphates.

2. EXPERIMENTAL DETAILS Hexagonal CePO4 was prepared by precipitation method using aqueous solutions of Ce(NO3)3·6H2O and (NH4)2HPO4. Concentrated solutions were prepared by dissolving 8.68 g of Ce(NO3)3· 6H2O and 2.70 g of (NH4)2HPO4 in water in two separate beakers. The clear solution of (NH4)2HPO4 was added to the solution of Ce3+ ions under constant stirring. A colloidal precipitate formed almost immediately. The liquor with the precipitate was heated at 80 °C for about 4 h under constant stirring. The precipitate was separated out by centrifuging and then repeatedly washed with water and ethanol. The agglomerated precipitate was fragmented and dried in air for 2 days at ambient temperature. The dried pieces were then powdered and heated in an oven at 200 °C for 2 h in order to dehydrate the sample. The powder was finally characterized by XRD. Thermogravimetric (TG) analysis was performed in N2 atmosphere to check for the water content in the sample. The temperature was first stabilized at 30 °C and then increased from 30 to 600 °C at a rate of 10 °C min−1. The Raman spectrum of a powder sample of R-CePO4 was measured with a custom setup, using a 638.2 nm helium−neon laser as the excitation source. A backscattering geometry was used with a Jobin−Yvon spectrometer, in combination with a thermoelectriccooled multichannel CCD detector and an edge filter. The spectral resolution of the setup is below 2 cm−1. We selected the polarization of the dispersed light parallel or perpendicular to the incident one. HP angle-dispersive powder X-ray diffraction (XRD) experiments were carried out at room temperature (RT) employing an Almax− Boehler diamond-anvil cell (DAC) with diamond culets of 280 μm. The sample was loaded in a 90 μm hole drilled on a tungsten gasket preindented to a 30 μm thickness. Before loading it into the cell, the powder has been annealed again for 20 h at 200 °C in an oven. The ruby fluorescence method was used for pressure determination.21 A 16:3:1 methanol−ethanol−H2O mixture was used as the pressuretransmitting medium. Experiments were performed at the BL04MSPD beamline of ALBA Synchrotron.22 We used a monochromatic X-ray beam (λ = 0.4246 Å) focused to a beam size down to 20 μm × 20 μm (full width at half-maximum). The two-dimensional XRD patterns were collected with a Rayonix SX165 CCD detector using a sample-to-detector distance of 240 mm. All the experiments were performed at room temperature. Structural analyses were performed using MAUD and POWDER CELL.23−26 VESTA has been used to draw the crystal structures and to measure the interatomic distances and polyhedral volumes.27

Figure 1. XRD pattern (circles) and Rietveld refinement for hexCePO4 at ambient pressure (red line). Peak positions (vertical bars) and residuals (black line) are also shown. Fitting parameters: RP = 3.46%, RWP = 10.02%, χ2 = 1.82.

structures with CheckCif, led us to assign to R-CePO4 the structure described by space group P6222 (Figure 2). The structure is isomorphic to that of rhabdophane LaPO4/NdPO4, in agreement with Mooney’s latest report.12 The unit-cell parameters and atomic positions resulting from the Rietveld refinement are shown in Table 1 (RP = 3.46%, RWP = 10.02%, χ2 = 1.82). The values compare well with the literature.12,28,29 Notice that the hexagonal structure has Z = 3 but monazite has Z = 4. Consequently, the unit-cell volume per formula unit in R-CePO4 (279.97 Å3/3 = 93.932 Å3) is 25% larger than in monazite (298.78 Å3/4 = 74.69 Å3). From the refined structure, we have determined the bond distances. The results are shown in Table 2. R-CePO4 can be explained as constituted by irregular CeO8 polyhedra and PO4 tetrahedral units.32,33 The CeO8 units have four short and four long Ce−O distances. The long bonds are 12% larger than the short ones, making the effective coordination 6.5. In contrast, the PO4 tetrahedron is regular (P−O distance = 1.501(6) Å). The volume of PO4 tetrahedron is 15 times smaller than that of CeO8 polyhedron. Consequently, the change with pressure of CeO8 will dominate the compressibility of R-CePO4.32,33 When comparing the polyhedral unit with those of monazitetype CePO4,16 it can be seen that the PO4 tetrahedron is slightly smaller in R-CePO4 than in monazite CePO4 (average P−O distance 1.6128 Å). In addition, in monazite this unit is highly irregular while in R-CePO4 it is symmetric.34 The differences are more important for the coordination polyhedron of Ce, which is 8-fold coordinated in R-CePO4 but 9fold coordinated in monazite, with an average distance of 2.540 Å (being 2.510 Å in R-CePO4). Regarding the structural framework, R-CePO4 consists of edged-linked chains of CeO8 polyhedra, which develop along

3. RESULTS AND DISCUSSION R-CePO4 has been assigned in the past to two different space groups, namely, trigonal P3121 or hexagonal P6222.12,28,29 Both of them are able to successfully index the experiments,28 making it difficult to unambiguously discern the correct space group. Mooney first assigned R-CePO4 to P3121 and then to B


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Inorganic Chemistry

Figure 2. Structure of hex-CePO4, projected on the different crystallographic planes: (a) (100), (b) (010), and (c) (001) plane. A general view of the crystal structure is presented in (d). Green spheres represent cerium atoms, while purple and red spheres represent phosphorus and oxygen, respectively.

Table 1. Experimental Unit-Cell Parameters, Volume, and Atomic Positions for Hexagonal CePO4 and Comparison with the Literaturea

Table 2. Typical Bond Lengths and Polyhedral Parameters in Hexagonal CePO4 R-CePO4 at ambient pressure

unit-cell parameters a (Å)

R-CePO4 this work literature12,28 literature29 AlPO465 atom site Ce P O Al P O

7.0725(9) 7.055(3) 7.05 5.0402(9) x/a

c (Å)

volume (Å3)

6.4621(9) 6.439(5) 6.43 11.063(2) y/b

279.93(11) 277.6(5) 277.55 243.39 z/c

Atomic Positions (R-CePO4) 1/2 0 1/2 0 0.4602 0.1515 Atomic Positions (AlPO4) 3c 1/2 0 3d 1/2 0 12k 0.4201 0.1915

3c 3d 12k

Ce−O Ce−O ⟨Ce−O⟩ polyhedral volume distortion index effective coordination P−O polyhedral volume distortion index effective coordination

0 1/2 0.3668

2.673 (5) Å (×4) 2.347(5) Å (×4) 2.510 Å 26.14(7) Å3 6.504 × 10−2 6.5577 1.507(2) Å (×4) 1.741(8) Å3 0 4

0 1/2 0.0776

a

Experimental atomic positions are also reported.

the [100] and [010] directions, and are separated by isolated PO4 units. This arrangement leaves void hexagonal channels along the [001] direction (see Figure 4).35 Due to them, up to 0.667 H2O molecules per unit formula of CePO4 can be accommodated in the hexagonal framework.12,36 More importantly, the presence of zeolitic water is deemed necessary to stabilize R-CePO4. On the other hand, high-temperature studies show that up to 400 °C the hexagonal structure is retained, but heating to 800 °C leads to a phase transition to the monazite phase.12,28,37 Notably, XRD patterns for asprepared and dehydrated hexagonal CePO4 (400 °C) show little difference from XRD patterns of samples obtained by a

Figure 3. Structure of hex-CePO4 with emphasis on the quasi-planar structure of the first nearest-neighbors shell of O atoms in the CeO polyhedra.

C


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Inorganic Chemistry

230 °C for CePO4) lead to anhydrous CePO4.38 Our sample lost around 5% water when reaching 200 °C at a heating rate of 10 °C min−1 (17 min of gradual heating). This leads to a CePO4·0.18H2O composition. However, before the DAC loading we kept the sample at 200 °C during 20 h. Thus, it is reasonable to assume that there was in the sample studied under HP a maximum of 0.18 H2O molecules per CePO4 unit formula.36 Considering the rapid experimental sequence between the preheating phase and DAC loading, we believe that complete rehydration of R-CePO4 was unlikely. Notice that after annealing, no traces of water could be detected in our samples by Raman spectroscopy (in the following section). As a part of the characterization of our partially dehydrated R-CePO4, we report the Raman spectrum, shown in Figure 6.

Figure 4. Schematic representation of hexagonal CePO4, where the hexagonal empty channels can be seen.

hydrothermal process similar to ours.14,37 An alternative description of the structure can be obtained from comparison with the high-temperature form of AlPO4 (HT-AlPO4). We provide in Table 1 the atomic positions of it. The atomic coordinates of Al and P in HT-AlPO4 correspond to those of Ce and P in R-CePO4. The oxygen coordinates only appreciably differ in the z coordinate. HT-AlPO4 consists in helixes of alternating corner-linked AlO4 and PO4 tetrahedra. Similar helixes are present in R-CePO4. The only difference is that the four oxygens defining the first Ce neighbor shell do not define a tetrahedron, but are disposed in a planar configuration (see Figure 3). In this way, R-CePO4 can be considered a bridge structure between the monazite structure of orthophosphates and that of berlinite. Anhydrous R-CePO4 is known to rehydrate promptly in ambient conditions.38 Therefore, we took care during sample preparation to minimize the transfer time of the sample from the oven to the DAC (less than 10 min). To assess the water content in our sample, thermogravimetric analysis has been performed in samples before the heating treatment. The results reported in Figure 5 show multiple steps of dehydration.

Figure 6. Raman spectrum of hex-CePO4 in ambient conditions. The sample is in powder form. The incident laser is polarized. We employed nonpolarized, parallel, or crossed detection. Symmetry labels indicated with an asterisk correspond to tentative identifications.

The Raman spectra of rhabdophane phosphates was previously discussed by Assaoudi and Clavier.20,41 Here we will extend this discussion to highlight several relevant issues. First, we underline the similitude of the spectrum with that of GaPO4, which adopts the closely related berlinite structure.42,43 Afterward, we can discuss the identification of the phonons by selection rules. Group theory classifies the lattice vibrations as follows: Γ = 3A1 + 5A 2 + 5B1 + 5B2 + 10E1 + 8E2

(1)

Acoustic modes have A2 and E1 symmetry. B1 and B2 are silent modes. A1 and E2 are Raman active, whereas A2 is infrared active. E1 modes are both infrared and Raman active. There are then 21 Raman active modes, whose Raman tensors are given by jij a 0 0 zyz j z A1 = jjj 0 a 0 zzz jj z j 0 0 b zz k {

Figure 5. Thermogravimetric analysis of CePO4 powder after annealing at 200 °C for 20 h.

Around 1.8% weight is lost up to 150 °C, and 5% weight is lost up to 200 °C. The sample is completed dehydrated at temperatures higher than 400 °C (see Figure 5). This behavior has already been reported for hydrated rare-earth phosphates, attributing the first dehydration phase to the evaporation of water adsorbed and retained between CePO 4 crystal grains.39,40 Temperatures slightly higher than 200 °C (around

ij 0 0 0 yz ij 0 0 −c yz jj zz jj zz E1 = jjj 0 0 c zzz, jjj 0 0 0 zzz jjj zzz jjj zzz k 0 c 0 { k− c 0 0 { D

(2)

(3) DOI: 10.1021/acs.inorgchem.8b03648 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

jij d 0 0 zyz jij 0 −d 0 zyz j z j z E2 = jjjj 0 −d 0 zzzz, jjjj−d 0 0 zzzz jj zz jj zz (4) k0 0 0{ k 0 0 0{ Our Raman measurements are performed with the sample in powder form. If no polarization is employed, then all modes are allowed. E1 modes disappear when the incident and exit polarizations are chosen to be parallel. When polarization is crossed, A1 modes are not observed. The symmetry assignments based on these criteria are compiled in Table 3. The

Once described the symmetry analysis and the relationship with the modes of the PO−3 4 tetrahedron, we will discuss the experimental spectra. The modes with the largest wavenumbers correspond to v3 modes. However, the mode at 1086(1) cm−1 display selection rules associated with A1 symmetry, whereas it can be appreciated from (7) that the v3 does not originate any A1 mode. Given the proximity of the v1 mode, we expect the hybridization of modes derived from v3 and v4. From this perspective, modes observed in the spectral range between 850 and 1150 cm−1 correspond to the stretching Raman active modes associated with v1 + v3, i.e., A1 + 2E1 + 2E2. The E2 modes are clearly observed. The signal/ noise ratio for the Raman emission of R-CePO4 is rather poor, thus complicating the detection of some the weakest modes, as the E1 modes at 1004(5) and 1031(5) cm−1, whose identification should be considered as tentative. According to the discussion presented above, modes appearing in the 400−750 cm−1 range are mainly related to v2 + v4 bending modes, resulting in 2A1 + 2E1 + 3E2 Raman vibrations. In the 425−600 cm−1 interval, two A1 modes and one E2 phonon overlap, and their wavenumbers have been extracted by deconvolution. From the two expected E1 modes, only one is observed. We suggest that the extra E2 mode is associated with Ce atoms. We observe only three modes, 2E1 + E2, in the lowest part of the spectrum, which should correspond to contributions from Ce atoms as well as translations and rotations of the tetrahedron. Even if the experimental setup is conceived to characterize modes as close to the laser as 20 cm−1, the low signal-to-noise ratio did not allow clear identification of any mode under 200 cm−1. The most intense Raman peaks have been previously characterized.14,37,41 Finally, in rhabdophane, stretching Raman modes for zeolitic H2O are expected around 3600 cm−1.20 Unfortunately, our multiple attempts to explore the 3000−4000 cm−1 range have delivered Raman spectra with extremely poor signal-to-noise ratio, indicating the presence of only a residual amount of water molecules, consistent with the TG measurements. Figure 7 shows selected XRD patterns of R-CePO4 upon increasing high-pressure conditions. Up to 9 GPa, the original phase is retained (low-pressure phase, or LP). No significant changes are visible in the diffractograms, apart from the expected shift toward higher values of 2θ due to the shrinking of the unit-cell parameters. We tried to determine variations in the microstructure of R-CePO4 in the low-pressure range. In particular, we refined the atomic positions and found that within the pressure range of stability of the LP phase the pressure change of the atomic coordinates is smaller comparable to the experimental uncertainty. Thus, we concluded that as a first approximation the pressure effect on the atomic positions can be neglected.47 Let us discuss now the HP results. At pressures above 9 GPa, subtle changes appear in the XRD patterns (see Figure 7). In particular, at 11.4 GPa a new peak appears prominently around 2θ = 11° and a small peak around 15° begins to split. We consider this as an evidence that a phase transition takes place between 9 and 11.4 GPa. Since we have not measured any pattern between these two pressures, we have assigned the transition pressure to 11.4 GPa. At higher pressures, we observed that the new peaks become more prominent and there is a broadening of XRD peaks. The peak broadening can be caused by a partial loss of hydrostaticity,48,49 but it can also indicate a gradual distortion of the crystal structure.50

Table 3. Detected Raman Phonons of hex-CePO4a mode

notes

281(1) 339(5) 377(3) 465(1) 499(5) 543(5) 574(2) 626(2) 713(1) 726(3) 979(1) 1004(5) 1031(5) 1086(1) 1088(1)

E1 E1* E2* E2 A1 A1 E2 E2 E2 E1 E2 E*1 E*1 A1 E2

a

Tentative assignments are indicated by an asterisk.

interpretation of the Raman spectra of monazites has taken profit of the strong character of the P−O bond in PO4 tetrahedra.44 Raman modes are then classified as external [rotations F1T(R) and translations F2T(T)] or internal [AT1 (v1), ET(v2), F2T(v3), F2T(v4)] modes of the tetrahedra. The wavenumbers associated with internal modes in isolated PO4−3 ions are v1 = 938, v2= 420, v3 = 1017, and v4= 567 cm−1.45 In R-CePO4, this approach is not straightforward. However, the correlation schemes provided by Assaaoudi et al.41 present inconsistencies between the number of modes according to the factor group and site symmetry. We thus follow the method of the equivalent representation as described by Dresselhaus et al.46 PO4−3 tetrahedra are located at P sites. The equivalent representation associated with both the Ce and P sites is Γequiv = A1 + E2. The representation associated with the vibration pattern of most tetrahedral modes is readily obtained: ΛAT1 = A1, ΛFT1 = ΛFT2 = A2 + E1. However, the ET representation in Td has basis functions which do not define a representation in D6. It is necessary to consider F1T. In isolated PO4−3 ions the closest F2T mode to the ET(v2) vibration at 420 cm−1 is the v4 mode at 567 cm−1. We expect then the interaction between ET(v2) and F2T(v4) vibrations, whose combined representation is reduced as ΛET+FT2 = A1 + E1 + E2. Finally, lattice modes are given by Γ = Γequiv ⊗ ΓT: Γν1 = A1 + E2

(5)

Γν2 + ν4 = 2A1 + A 2 + B1 + B2 + 2E1 + 3E2

(6)

Γν3, ΓT , ΓRΓCe = A 2 + B1 + B2 + 2E1 + E2

(7) E


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Inorganic Chemistry

is shown in Figure 8. This new structure can be still described in terms of Ce atoms that are 8-fold coordinated with oxygen

Figure 8. XRD pattern (circles) and Rietveld refinement for HPCePO4 at 16 GPa (red line) using C2 space group. Peak positions (vertical bars) and residuals (black line) are also shown. Fitting parameters: RP = 5.76%, RWP = 12.33%, χ2= 2.97.

Figure 7. Evolution of XRD patterns of hex-CePO4 under pressure. Changes in the diffractograms appear beyond 9 GPa. Asterisks denote peaks of the new phase and arrows indicate splitting of peaks.

atoms, and tetrahedral PO4 units (see Figure 9). In a similar fashion than in the LP phase, in HP-CePO4, CePO8 polyhedra describe “s-shaped” chains along [100] and [010] and are separated by PO4 tetrahedra along [001]. Again, void hexagonal channels are present along the c-axis. As confirmed by the subtle variations in the XRD patterns, apart from the unit-cell geometry, the transition from R-CePO4 to HP-CePO4 does not imply a drastic atomic rearrangement. The cell and microstructure parameters for HP-CePO4 at 16.1 GPa are shown in Table 4 (the goodness of fit parameters are RP = 5.76%, RWP = 12.33%, χ2 = 2.97). As we will discuss next, at the phase transition there is no detectable volume change. This fact, the reversibility of the transition, and the group−subgroup relationships between both polymorphs suggest that the reported phase transition is a second-order transformation.55,56 From the experiments, we have obtained the pressure dependence of unit-cell parameters of both polymorphs. The results are shown in Figure 10. In Figure 11 we show the pressure dependence of the volume. For both phases, the pressure behavior of unit-cell parameters is linear. The functions describing their behavior are given in Table 5. In the LP phase, the linear compressibilities, defined as 1 ∂a ka = − a ∂P are ka = 3.97 × 10−3 GPa−1 and kc = 2.03 × 10−3 GPa−1, which correspond to a relative shrinking of 3.66 and 1.83% in the 0−9 GPa range. Thus, the response of RCePO4 to pressure is nonisotropic, being more compressible along the [100] and [010] directions than along [001]. This phenomenon, often observed in APO4 orthophosphates, is due to the lower compressibility of P−O bonds with respect to A− O bonds.52,57 In R-CePO4, the tetrahedral PO4 units are preferentially aligned along [001], alternated to CeO8 polyhedra (see Figure 1), being this the reason for the anisotropic compressibility. In HP-CePO4 we also found that the compressibility is nonlinear. In particular, the linear compressibilities of the different axes (at 11.4 GPa) are ka = 4.48 × 10−3 GPa−1, kb = 1.24 × 10−3 GPa−1, and kc = 1.23 × 10−3 GPa−1, and the relative variations of the parameters in the range 11.4−21 GPa are 1.2% for a and c and 4.6% for b. The coordination of Ce and P atoms with oxygen atoms for the high-pressure phase is the same as in R-CePO4. Moreover, the volumes of the

However, no evidence of any additional phase transition is found up to 21 GPa, the highest pressure reached in the experiments. On top of that, the phase transition appears to be completely reversible, as the initial pattern is recovered upon reverting pressure from 21 GPa to ambient conditions. The pressure stability of partially dehydrated R-CePO4 is contrasting the HP behavior of many dehydrated compounds. For instance, anhydrous AgClO4 undergoes a phase transition at 5 GPa51 and dehydrated rhabdophane-type BiPO4 at pressures lower than 0.5 GPa.52 In the last case, the presence of empty porous in the crystal structure favor the collapse of it and the transformation into the monazite-type polymorph. The stability of our compound up to 9 GPa suggests a possible rehydration due to the presence of water in the pressure medium in the DAC (5 vol %). Indeed, this phenomenon and even the absorption of alcohol in the porous of R-CePO4 due ̈ hypothesis and has been to compression is not a naive observed in other oxides with large empty cavities.53 The exploration of this hypothesis will require the performance of experiments with dehydrated R-CePO4 immersed under a selection of pressure media with molecules of different size,54 which is beyond the scope of the present work. To the best of our knowledge, the HP phase of R-CePO4 seen above 9 GPa (HP-CePO4, in the following) has never been reported so far. The gradual changes in the XRD patterns above suggest that the crystal structure of the new polymorph could be a distorted version of R-CePO4. An accurate analysis of the XRD patterns shows that HP-CePO4 has a monoclinic structure, with a β angle of 90.3473° at P = 11.4 GPa (see Table 5). More specifically, following the group-subgroup relationships available in the International Tables of Crystallography, it has been concluded that HP-CePO4 belongs to the space group C2. Using this monoclinic structure as the starting model, after background subtraction, we performed Rietveld refinements and successfully explained the XRD patterns measured from 11.4 to 21 GPa. In the refinements, only the overall scale-factor, unit-cell, and lineshape parameters were refined. The atomic positions were fixed to those obtained from the transformation of space group P6222 to C2. A Rietveld fit of the XRD pattern at P = 16.1 GPa F


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Figure 9. Structure of HP-CePO4 at 11.4 GPa, projected on the different crystallographic planes: (a) (100), (b) (010), and (c) (001) plane. A general view of the crystal structure is presented in (d). Green spheres represent cerium atoms, while purple and red spheres represent phosphorus and oxygen, respectively.

Table 4. Experimental Unit-Cell Parameters, Volume Data, and Atomic Positions Monoclinic CePO4 (HP Phase) Obtained at P = 16.1 GPa unit-cell parameters a (Å) HP-CePO4

b (Å)

6.6037(2)

c (Å)

11.6503(9) 6.2795(9) atomic positions

β (deg)

volume (Å3)

90.79(7)

483.07(2)

atom

site

x/a

y/b

z/c

Ce Ce P1 P2 O1 O2 O3 O4 O5 O6

2b 4c 4c 2a 4c 4c 4c 4c 4c 4c

0 0.2648 0 0.2648 0.3576 0.6424 0.7179 0.3069 0.4280 0.5608

0.5185 0.2524 0.5085 0.2585 0.0742 0.9258 0.1409 0.8592 0.2788 0.7146

1/2 0.1701 0 0.6818 0.1559 0.1559 0.4892 0.4892 0.8101 0.8226

occur by rotation of the PO4 tetrahedral units to allow for sharing edges as well as corners. Thus, the structures of both CePO4 and AlPO4 form hexagonal tunnel-like empty channels. The incorporation of water molecules in the structure during the preparation stage stabilizes the hexagonal lattice, which is also retained upon dehydration. It can be also mentioned here that the structure of hydrated LnPO4, Ln = Sm, Gd, and so on, has been explained in as a monoclinic (C2) lattice, which is closely related to the hexagonal (P6222 and P3121) lattice and the HP phase we are proposing here.36,38 On dehydration a lattice expansion is observed, while retaining the monoclinic structure (sublattice of the hydrated phase). Thus, it can be pointed out that the degree of hydration plays a crucial role governing the symmetry of the hydrated LnPO4.36,38 Similar

coordination polyhedra are essentially conserved in the phase transition, so the anisotropic compressibility of the HP phase has to be ascribed, again, to the uncompressible bonds in the phosphate groups, which in very rough terms are preferentially arranged along [010] (see Figure 9). The β angle, however, increases around 1% in the considered pressure range, pointing toward a slightly less symmetric structure. As mentioned earlier, the hexagonal phase of CePO4 bears close resemblance with the HT-AlPO4, which has been explained in the same space group as that of CePO4. The LT and HT polymorphs of AlPO4 are formed by tetrahedral AlO4 and PO4 groups. However, the displacement of oxygen positions to accommodate larger cations like Ce3+ increases the coordination number of Ce3+ to 8. These rearrangements G


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Inorganic Chemistry

distortion in the lattice can be attributed to the hexagonal-tomonoclinic transition due to pressure. Broadly speaking, the crystal chemistry of hydrated CaSO4 and LnPO4 are related by the distortions in the MO8 polyhedra, which could be caused by temperature, pressure, or the incorporation of water molecules in the lattice. Regarding the fate of the absorbed water in our sample after the phase transition, we cannot make a definitive statement. By considering that in the both the HP phase and in the lowpressure phase the hexagonal zeolitic cavities are present, we can speculate that water will remain absorbed in the sample. It is even possible that pressure could favor the penetration of other molecules of the pressure medium, favoring the structural distortion we have observed. The study of these ideas will require the performance of HP neutron diffraction experiments which can be carried out in the pressure range of our study.60 We hope the present results will open the possibility for them. In a monoclinic structure, the compressibility tensor can be calculated analytically, given the pressure dependence of the unit-cell parameters. In this case, the tensor is symmetric and has only four non-null independent components β11, β22, β33, and β13. Their analytical expressions have been given by Knight.61 We have then access to the eigenvalues and eigenvectors of the compressibility tensor, which we report in Table 6. One eigenvector is parallel to the unique

Figure 10. Evolution of the cell parameters for the LP hexagonal phase (hexagons) and the HP monoclinic phase (diamonds). To improve the readability of the plot, for the monoclinic phase the value of a/2 is shown. The lines are linear fits.

Table 6. Isothermal Compressibility Tensor Coefficients, βij, Their Eigenvalues, λi, and Eigenvectors, evi, and Angle of Maximum Direction of Compression, ψ, for HP-CePO4 at 11.4 GPa

Figure 11. Pressure−volume data for hexagonal and HP-CePO4 (symbols) and corresponding Birch−Murnaghan third-order EOS fit (lines). For the HP phase we plot V/2 because the size of the unit-cell is the double than in the LP phase. The inset shows the pressure dependence of the β angle in the HP phase. The symbols are the experimental results, and the solid line represents a linear fit.

β11 (GPa−1) β22(GPa−1) β33 (GPa−1) β13 (GPa−1) λ1 (GPa−1) ev1 λ2 (GPa−1) ev2 λ3 (GPa−1) ev3 ψ (deg)

behavior of symmetry transformation due to degree of dehydration has been explained in CaSO4·xH2O.58,59 The trigonal (P3121) lattice of CaSO4·0.8 H2O transforms into the monoclinic (C2) lattice by a feeble decrease in water contents. In all these structures, the empty hexagonal tunnels accommodate the water molecules, and the removal of the water molecules distorts the CaO8 polyhedra and hence lowers the symmetry. In the present study, the samples used were preheated to dehydrate them completely and to remove any adsorbed water molecules. A comparison of the present pressure-induced hexagonal-to-monoclinic transition to the temperature-induced hexagonal and monoclinic phases reported by Mesbah et al.38 indicates that the distortion rendered in the LnO8 polyhedra governs the symmetry changes. A stiff raise in the unit cell volume occurs upon dehydration, which might be due to the effect of temperature on the lattice. In the present case, though, there is no abrupt volume collapse, so the

4.715 × 10−3 1.254 × 10−3 1.251 × 10−3 −7.958 × 10−4 1.0769 × 10−3 (−0.21369, 0, −0.97690) 1.2543 × 10−3 (0, 1, 0) 4.8891 × 10−3 (−0.97690, 1, 0.21369) 77.66

crystallographic b-axis, and the other two lie in the plane perpendicular to it. According to the eigenvalues, the minimum, intermediate, and maximum compressibilities are 1.08 × 10−3, 1.25 × 10−3, and 4.89 × 10−3 GPa−1, respectively. These values shows that nearly 60% of the total compression of HP-CePO4 takes places along the c-axis, which is the direction of maximum compressibility. However, the two directions of compression are perpendicular to the b-axis forming between them an angle of approximately 77.7°. To conclude the

Table 5. Linear Functions Describing the Pressure Dependence of Unit-Cell Parameters for Both Phases hexagonal (LP) intercept a b c β

7.065(4) Å 6.465(2) Å 6.465(2) Å

monoclinic (HP) slope −1

−0.0281(9) Å GPa −0.0131(5) Å GPa−1 −0.0131(5) Å GPa−1

H

intercept

slope

7.116(9) Å 11.889(5) Å 6.4068(2) Å 89.31(5)°

−0.0319(5) Å GPa−1 −0.0147(3) Å GPa−1 −0.0079(1) Å GPa−1 0.090(3)° GPa−1 DOI: 10.1021/acs.inorgchem.8b03648 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

calculated and the main directions of compressibility found. In addition, a room-temperature P−V equation state has been determined for each polymorph. The determined bulk moduli shows that hexagonal and HP-CePO4 are two of the most compressible APO4 phosphates. This is a consequence of their low density and of the large voids in the structures, which are present in both phases as hexagonal channels running along the c axis.

discussion on the compressibility tensor, we would like to mention that the bulk modulus can be estimated as 1 B = β + β + β , βii being the diagonal elements of the 11

22

33

compressibility tensor. In our case, we got B (at 11.4 GPa) = 138(5) GPa. Finally, we have determined a RT P−V equation of state (EOS) for both polymorphs. In particular, we found that the experimental results can be properly described by a third order Birch−Murnaghan EOS.62 The results of the fits can be seen in the Figure 11. For the low-pressure phase, we have determined the following EOS parameters: unit-cell volume at ambient pressure, V0 = 279.9(1) Å3, bulk modulus at ambient pressure, B0 = 83.9(7) GPa, and its pressure derivative B′0 = 3.7(3). For the HP phase, we obtained V0 = 560(1) Å3, B0 = 81(4) GPa, and B′0 = 3.7(3). The parameters for both phases agree within the uncertainties (considering the volume of the HP phase is twice that of the LP phase). This is consistent with the idea that a second-order transition is taking place at 11.4 GPa. Considering B0 and B′0, the value of the bulk modulus can be estimated to be 130(10) GPa at 11.4 GPa, which is consistent with the value obtained from the compressibility tensor. The obtained bulk modulus can be compared with other APO4 compounds, in particular monazite-type CePO4, which has B0 = 120 GPa.16 This means that our two polymorphs are more compressible than is monazite. This is reasonable given the lower density and presence of large void spaces in our two polymorphs. In other words, the larger B0 of monazite is due to the more efficient packing of this structure. In addition, RCePO4 and HP-CePO4 are also much more compressible than the rest of rare-earth APO4 phosphates (A = La, Nd, Eu, Gd, Er, Pr, Sc, and Y) which have bulk moduli larger than 100 GPa.18,63−65 The only compound which has a similar bulk modulus is SbPO4-type BiPO4, with B0 = 78(4) GPa.52 This polymorph has also a less dense structure than monazite, and has a similar polyhedral packing than our polymorphs. Therefore, it is reasonable that it would be as compressible as the CePO4 polymorphs here studied. Finally, R-CePO4 and HP-CePO4 apparently have a larger bulk modulus than BiPO4−I, which is isomorphic to R-CePO4. The theoretically computed bulk modulus for BiPO4−I is 64.4 GPa.52 However, a large pressure derivative is associated with it, B′0 = 5.53. Since B0 and B′0 are correlated parameters, to properly compare BiPO4−I with our polymorphs we fit the BiPO4−I results, fixing B′0 = 3.7 GPa and obtaining B0 = 77(3) GPa which, as expected, is comparable with the bulk moduli here reported.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1879403 and 1879404 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Enrico Bandiello: 0000-0003-0956-3195 Daniel Errandonea: 0000-0003-0189-4221 S. Nagabhusan Achary: 0000-0002-2103-1063 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support of the Spanish Ministerio de Ciencia, Innovación y Universidades, the Spanish Research Agency (AEI), and the European Fund for Regional Development (FEDER) under Grant No. MAT2016-75586-C4-1/2-P and by Generalitat Valenciana under Grant Prometeo/2018/ 123 (EFIMAT). E.B. thanks the Generalitat Valenciana for his postdoctoral contract (ValI+D, APOSTD2017). He also ́ thanks Dr. David Santamariá (Departamento de Fisica Aplicada, Universidad de Valencia) for the helpful discussions. HP XRD experiments were performed at MSPD beamline at ALBA Synchrotron Light Facility with the collaboration of ALBA staff.

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REFERENCES

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4. CONCLUSIONS By means of high-pressure experiments on partially hydrated hexagonal cerium orthophosphate, we discovered a hitherto unreported monoclinic polymorph of CePO4, which belongs to the C2 space group. Apart from the space group change, the transition does not involve a large atomic rearrangement and the microstructures of the low- and high-pressure phases are very similar. Indeed, the transition can be characterized as second order. The sluggish transition may be due to the presence of water in the large void channels present in the hexagonal framework. Water molecules are likely to remain absorbed in the crystal even after the phase transitions. We have also determined the axial compressibility of both phases, being the response to pressure anisotropic. In the case of the HP monoclinic phase, the compressibility tensor has been I


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Pressure-Induced Hexagonal to Monoclinic Phase Transition of

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