In Situ Neutron Diffraction Studies of the Flux Crystal Growth of the

Abeysinghe , D.; Cortese , A. J.; Yeon , J.; zur Loye , H.-C. The Two-Step Hydrothermal Method: An Effective Approach for the Crystal Growth of New Ma...
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Article Cite This: Chem. Mater. 2018, 30, 1187−1197

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In Situ Neutron Diffraction Studies of the Flux Crystal Growth of the Reduced Molybdates La4Mo2O11 and Ce4Mo2O11: Revealing Unexpected Mixed-Valent Transient Intermediates and Determining the Sequence of Events during Crystal Growth Dileka Abeysinghe,† Ashfia Huq,‡ Jeongho Yeon,† Mark D. Smith,† and Hans-Conrad zur Loye*,† †

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Chemical and Engineering Materials Division, Spallation Neutron Source, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: Exploratory flux-mediated crystal growth is one route to discovering novel inorganic compounds, including metastable ones; however, the method does not reveal the existence of intermediates or transient species, as only the final products are isolated at the end of the reaction. Uncovering the presence of any potential intermediates or transient species is, thus, of interest and can be achieved via in situ X-ray or neutron diffraction crystal growth studies to observe, in real time, the processes occurring, including phase formation and crystal growth. In this study we investigated the processes involved in the crystal growth of La4Mo2O11 and Ce4Mo2O11, two reduced Mo5+-containing oxides synthesized in a NaCl/CsCl eutectic flux, and studied via in situ diffraction experiments at the POWGEN beamline at the Spallation Neutron Source. These in situ studies reveal the appearance and disappearance of staring materials, intermediates, and final products. Several transient phases that formed as either single crystals or polycrystalline powders were identified and the reaction conditions (temperature and time) for their formation in single-crystals form established, enabling the use of quenching experiments to isolate these singlecrystal transient species and to carry out their structural characterization. Additionally, by changing the reactants-to-flux ratio, different reaction pathways were observed to ultimately result in the same target phase. A higher flux-to-reagent ratio was shown to favor the formation of reduced molybdenum transient phases.

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INTRODUCTION The directed development of novel materials with new or improved properties is challenging but essential for the continued improvement of materials needed for energy storage (lithium batteries), energy-efficient lighting (LEDs), energy conversion (thermoelectrics), energy harvesting (solar energy and H2 production from water), and data recording, processing, and storage (DVDs). To achieve these goals, a directed and rational synthesis is needed for the discovery of materials with desired properties. Unlike in molecular reactions where it is sometimes possible to identify, isolate, and characterize intermediates, solid-state reactions are more challenging, as there rarely are discrete intermediates that can be isolated and identified. More often than not, the reaction proceeds via noncrystalline stages where little structural insight can be gained, preventing the identification and structural characterization of such intermediate materials. Interestingly, while a vast number of materials have been synthesized via solid-state reactions, we know very little about what happens between the time when the reaction starts and the time we isolate the final product(s). For these reasons, predicting, designing, or targeting a complex composition is extremely challenging, and sometimes this process is helped by serendipity.1−6 © 2018 American Chemical Society

Of the many synthetic methods available to the researcher, flux-mediated exploratory crystal growth has been consistently successful at enabling the discovery of new phases, be they oxides,4,7,8 chalcogenides,9,10 halides,11 or intermetallics.12 One reason why materials discovery via crystal growth is so successful is that the process results in new materials in single-crystal form whose structures and physical properties can be determined. Learning more about which conditions are conducive to the formation of which phases, in addition to measuring their physical properties, can lead to a greater understanding of structure−property relationships. Traditional high-temperature solid-state syntheses, which greatly favor thermodynamically stable phases, typically prevent the formation of metastable phases (defined herein as phases that are only stable at lower temperatures) and, as mentioned earlier, tend to proceed via intermediate, poorly crystalline states of matter that are difficult to characterize structurally. Similarly, the molten flux crystal growth method, while popular for exploratory synthesis and more favorable for the formation of metastable phases, also does not reveal the existence of Received: January 6, 2018 Published: January 10, 2018 1187

DOI: 10.1021/acs.chemmater.8b00072 Chem. Mater. 2018, 30, 1187−1197

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Figure 1. Optical and SEM images of representative crystals of Ln4Mo2O11 (Ln = La and Ce). The maximum crystal size is ∼0.75 mm.

of a specific composition and structure. To obtain a more representative real-time understanding of the flux crystal growth process of reduced complex oxides, we investigated the La4Mo2O1132 and Ce4Mo2O11 compositions via in situ diffraction experiments at the POWGEN beamline at the Spallation Neutron Source. Utilizing experimental conditions identical to what was used in the laboratory settingsame size reaction tubes, same sample loading, same temperature profileallowed us not only to reproduce the formation of the designated reduced molybdates but also to monitor the processes taking place inside a sealed fused silica tube by taking advantage of the ability of the neutron beam to penetrate the sample tube and to diffract off the crystalline contents. By analyzing the diffraction data collected continuously during the flux reaction, we were able to observe the appearance and disappearance of transient intermediates, and the nucleation and growth of the product crystals. As the reaction conditions were identical, we were able to reproduce the conditions leading up to the formation of the transient intermediates, quench the reaction, isolate the transient intermediates, and determine their structures and compositions. Herein, we report on the in situ neutron diffraction studies of the flux crystal growth reaction leading to the formation of La4Mo2O11 and Ce 4 Mo 2 O 11 as the final products, the isolation, and identification of the transient intermediates La3Mo4SiO14, La5Mo3O16, La0.5Na0.5MoO4, La2Mo2O7, LaOCl, La2(MoO4)3, and La2MoO6, and the investigation of the magnetic behavior of La4Mo2O11 and Ce4Mo2O11.

intermediates or the appearance and disappearance of transient species, as we isolate only the final products. Thus, while chemists have the ability to predict new structures and compositions based on their knowledge of radius ratio rules, oxidation states, and the preferred coordination environments of elements,13 these rationales are often not sufficient, and our understanding of the reaction sequences occurring during flux crystal growth remain shrouded. An enhanced knowledge of the underlying chemistry of flux growth synthesis would enable us to pursue a more targeted preparation of new compositions. For this reason, there is a strong desire to perform in situ studies to observe, in real time, the processes occurring during crystal growth and phase formation.14−16 With the advent of better optics in laboratory X-ray machines and synchrotron and neutron user facilities, in situ scattering techniques have gained wider usage and have been applied to a large number of investigations on solid catalysts, biomolecules, mesoporous materials, pharmaceuticals, and other materials of interest.17−20 Although there are several techniques available to follow reactions occurring in real time, X-ray and neutron scattering techniques are generally the most effective techniques, especially for inorganic crystal growth and materials discovery.21−26 In situ studies enable the measurement and structural identification of intermediates, as well as provide insight into the sequence of events that take place during the crystal growth process. Determining when reactants begin to dissolve, when new phases crystallize, whether or not they persist and for how long, are all crucial pieces of information that are necessary for the directed development of new phases. In situ studies can provide unique insights into the syntheses of novel crystal forms and begin the transformation of flux crystal growth from an empirical method to a more deliberate process. We are currently investigating the processes involved in the crystal growth of reduced early transition-metal oxides synthesized in a flux environment. Reduced complex oxides, and their often intriguing magnetic properties, are a relatively unexplored area due to the synthetic challenges involved in achieving and maintaining low oxidation states of the metal (often transition-metal) cations. While our group has been successful in the development of new strategies for the synthesis of novel reduced systems,7,8,27−31 our knowledge of the in situ reduction and crystal formation process is still at the early stages of development. Until recently, monitoring flux crystal growth required quenching and ex situ analysis of the species present. Thus, for each temperature point, one reaction had to be run and the analysis had to be performed under the assumption that the quenching process did not alter the reaction or crystal growth

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EXPERIMENTAL SECTION

Reagents. MoO3 (99.95%, Alfa Aesar) and Zn metal (99.8%, −140 + 325 mesh, Alfa Aesar) were used as received. La2O3 (99.99%, Alfa Aesar) and CeO2 (99.99%, Alfa Aesar) were activated in air at 1000 °C for 12 h and stored in a vacuum desiccator. CsCl (99%, Alfa Aesar) and NaCl (Certified A.C.S., Fisher) were dried and stored in an oven at 260 °C. Synthesis. Single crystals of the title compounds, Ln4Mo2O11 (Ln = La and Ce), were grown via a molten flux method. A mixture of 2 mmol of La2O3 (0.6516 g) or 2 mmol of CeO2 (0.3442 g), 2 mmol of MoO3 (0.2879 g), 2 mmol of Zn (0.1308 g), and 2 g of a CsCl/NaCl eutectic mixture (molar ratio 13:7) were placed into a fused silica tube. The tube was evacuated to a pressure of ∼10−4 Torr and sealed using an oxygen/methane torch. The tube was placed inside a programmable furnace and heated to 1000 °C at a rate of 10 °C/min, held for 24 h, cooled to 450 °C at a rate of 6 °C/h, and then cooled to room temperature by turning off the furnace. Single crystals were isolated from the flux matrix by dissolving the halide salts in water, assisted by sonication. La4Mo2O11 was obtained in a yield of approximately 65%, while Ce4Mo2O11 was obtained in a yield of approximately 25% based on MoO3; however, in the latter case the simultaneous formation of byproduct, Ce5Mo3O16,30 was also noted. Optical and SEM images of 1188

DOI: 10.1021/acs.chemmater.8b00072 Chem. Mater. 2018, 30, 1187−1197

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Single-Crystal X-ray Diffraction. X-ray diffraction intensity data from a black colored rod crystal were measured at room temperature on a Bruker SMART APEX diffractometer (Mo Kα radiation, λ = 0.71073 Å). The raw area detector data frames were processed with SAINT+.37 An absorption correction based on the redundancy of equivalent reflections was applied to the data with SADABS.37 The reported unit cell parameters were determined by least-squares refinement of a large array of reflections taken from each data set. Difference Fourier calculations and full-matrix least-squares refinement against F2 were performed with SHELXTL.38 Ce4Mo2O11 crystallizes in the centrosymmetric space group P42/n, as determined by the successful solution and refinement of the structure. The asymmetric unit contains two cerium, one molybdenum, and seven oxygen atoms. Ce(1), Ce(2), Mo(1), and O(1)−O(5) lie on a general position (Wyckoff site 8g). Both O(6) (Wyckoff site 2a) and O(7) (Wyckoff site 2b) reside on the fourfold axis. All atoms were refined with anisotropic displacement parameters. Crystallographic data, atomic positions, and selected interatomic distances of Ce4Mo2O11 are listed in Tables 1, 2, and 3, respectively. The

representative black crystals are shown in Figure 1. La4Mo2O11 was previously reported by Gall and Gougeon as single crystals;32 however, no physical property characterizations were performed. In Situ Neutron Diffraction of Ln4Mo2O11 (Ln = La and Ce). Time-of-flight (TOF) powder neutron diffraction data were collected using the POWGEN (BL-11A) diffractometer at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory, Oak Ridge, Tennessee. Experiment I. A mixture of 0.5 mmol of La2O3 (0.1629 g), 0.5 mmol of MoO3 (0.0720 g), 0.25 mmol of Zn (0.0164 g), and 0.5 g of a CsCl/NaCl eutectic mixture (molar ratio 13:7, reactants:flux = 1:2) were placed into a fused silica tube. The tube was evacuated to a pressure of ∼10−4 Torr and sealed using an oxygen/methane torch. The fused silica tube was placed inside a vanadium sample can and sealed using a boron nitride lid. Finally, the fused silica tube containing vanadium can was loaded in a vacuum furnace at POWGEN and heated to 1000 °C at a rate of 5 °C/min, held for 7 h, and cooled to room temperature at a rate of 5 °C/h. Neutron diffraction data were collected throughout the reaction using the time event mode data collection at SNS. This allowed postprocessing of data as described in a previous report by Peterson et al.33 Experiment II. A mixture of 2.67 mmol of La2O3 (0.8688 g), 2.67 mmol of MoO3 (0.3839 g), 1.34 mmol of Zn (0.0872 g), and 0.665 g of CsCl/NaCl eutectic mixture (molar ratio 13:7, reactants:flux = 2:1) were placed into a fused silica tube. The tube was evacuated to a pressure of ∼10−4 Torr and sealed using an oxygen/methane torch. The fused silica tube was placed inside a vanadium sample can and sealed using a boron nitride lid. The furnace at POWGEN was gradually heated up to 400 °C at a rate of 5 °C/min, stopping at the following temperatures to collect diffraction data: 200, 300, and 400 °C, heated to 500 at a rate of 1 °C/min, and finally to the target temperature 1000 °C at a rate of 5 °C/min. After dwelling for 7 h, the sample was cooled to 500 °C at a rate of 5 °C/min, 500−400 °C at a rate of 1 °C/min, and then to room tempterature (RT) at a rate of 5 °C/min, stopping at the following temperatures to collect diffraction data: 400, 300, and 200 °C. As before, neutron diffraction data were collected throughout the reaction. Experiment III. A mixture of 0.5 mmol of CeO2 (0.0861 g), 0.5 mmol of MoO3 (0.0720 g), 0.25 mmol of Zn (0.0164 g), and 0.5 g of a CsCl/NaCl eutectic mixture (molar ratio 13:7, reactants:flux = 1:2) were placed into a fused silica tube. The tube was evacuated to a pressure of ∼10−4 Torr and sealed using an oxygen/methane torch. The fused silica tube was placed inside a vanadium sample can and sealed using a boron nitride lid. Finally, the fused silica tube containing vanadium can was loaded in a vacuum furnace at POWGEN and heated to 1000 °C at a rate of 5 °C/min, held for 7 h, and cooled to RT at a rate of 5 °C/h. Experiment IV. A mixture of 3.20 mmol of CeO2 (0.5507g), 3.20 mmol of MoO3 (0.4605 g), 3.20 mmol of Zn (0.2093 g), and 1.12 g of KCl flux (reactants:flux = 1:1) were placed into a fused silica tube. The tube was evacuated to a pressure of ∼10−4 Torr and sealed using an oxygen/methane torch. The fused silica tube was placed inside a vanadium sample can and sealed using a boron nitride lid. The furnace at POWGEN was gradually heated up to 400 °C at a rate of 5 °C/min, stopping at the following temperatures to collect diffraction data: 200, 300, and 400 °C, heated to 500 °C at a rate of 1 °C/min, and finally to the target temperature 1000 °C at a rate of 5 °C/min. After dwelling for 7 h, the sample was cooled to 500 °C at a rate of 5 °C/min, 500− 400 °C at a rate of 1 °C/min, and then to RT at a rate of 5 °C/min, stopping at the following temperatures to collect diffraction data: 400, 300, and 200 °C. As before, neutron diffraction data were collected throughout the reaction. The same neutron diffraction experiment was carried out for an empty fused silica tube to obtain neutron diffraction data of the reaction vessel under the identical temperature and time conditions. The neutron diffraction patterns related to the sample were generated after subtracting the scans of the empty fused silica tube at each temperature using MantidPlot software.34 Rietveld refinements of diffraction data were performed using the GSAS software package and the EXPGUI interface.35,36

Table 1. Crystallographic Data for Ce4Mo2O11 formula weight (g/mol) temperature (K) crystal system space group a (Å) c (Å) volume (Å3) Z density (g/cm3) abs coeff (mm−1) cryst size (mm3) θ range (deg) completeness to θmax (%) R(int) GOF (F2) R(F)a Rw(Fo2)b largest diff. peak and hole

R(F) = Σ∥Fo| − |Fc∥/Σ|Fo|. Σw(Fo2)2]1/2.

a

928.36 294(2) tetragonal P42/n 12.9065(6) 5.6194(5) 2267.74(12) 4 6.587 21.69 0.10 × 0.02 × 0.02 2.23−28.34 100 0.0365 1.106 0.0445 0.0924 4.718 and −1.245 e × A−3 b

Rw(Fo2) = [Σw(Fo2 − Fc2)2/

Table 2. Selected Interatomic Distances (Å) for Ce4Mo2O11 Ce(1)−O(1) Ce(1)−O(2) Ce(1)−O(1) Ce(1)−O(3) Ce(1)−O(1) Ce(1)−O(4) Ce(1)−O(4) Ce(1)−O(5) Mo(1)−O(4) Mo(1)−O(3) Mo(1)−O(2) Mo(1)−Mo(1)

2.354(6) 2.405(6) 2.418(7) 2.481(6) 2.492(6) 2.559(6) 2.689(6) 2.861(6) 1.847(6) 1.909(6) 1.958(6) 2.589(1)

Ce(2)−O(6) Ce(2)−O(7) Ce(2)−O(2) Ce(2)−O(3) Ce(2)−O(4) Ce(2)−O(2) Ce(2)−O(3) Ce(2)−O(5) Mo(1)−O(5) Mo(1)−O(5) Mo(1)−O(1)

2.366(1) 2.392(2) 2.450(6) 2.470(6) 2.554(6) 2.590(6) 2.617(6) 2.618(6) 1.967(6) 2.056(6) 2.200(7)

Cambridge Crystallographic Data Centre (CCDC) number for Ce4Mo2O11 is 1561631. The unit cell parameters of La4Mo2O11 are close to the reported values as shown in Table S1 of the Supporting Information. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction data of Ln4Mo2O11 (Ln = La and Ce) were collected on a Rigaku Ultima IV powder X-ray diffractometer using Cu Kα (λ = 1.5418 Å) radiation. The step scan covered the angular range 10−65° 2θ in steps of 0.02°. 1189

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Chemistry of Materials Table 3. Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for Ce4Mo2O11a Ce(1) Ce(2) Mo(1) O(1) O(2) O(3) O(4) O(5) O(6) O(7)

are held together by the lanthanide ions to form the threedimensional framework as shown in Figure 3.

x

y

z

U(eq)

8030(1) 8820(1) 5249(1) 6485(5) 4707(5) 4564(5) 6268(5) 4139(5) 7500 7500

3874(1) 6815(1) 4090(1) 2965(5) 3062(5) 3393(5) 4535(5) 5013(5) 7500 7500

794(1) −39(1) 780(1) 15(12) 3021(10) −1771(11) 2849(12) 1896(11) −2500 2500

22(1) 22(1) 21(1) 24(1) 22(1) 23(1) 26(1) 25(1) 21(2) 20(2)

Figure 2. Isolated Mo2O10 cluster units formed by edge-sharing two distorted MoO6 octahedra. The cyan and red represent Mo and O, respectively.

a

U(eq) is defined as one-third of the trace of the orthogonalized Uji tensor.

No impurities were observed in the title compounds, and the calculated and experimental PXRD patterns are in excellent agreement (Figure S1). Energy-Dispersive Spectroscopy (EDS). Elemental analysis was performed on all the reported single crystals using a TESCAN Vega-3 SBU scanning electron microscope (SEM) with EDS capabilities. The crystals were mounted on carbon tape and analyzed using a 20 kV accelerating voltage and an accumulation time of 20 s. As a qualitative measure, EDS confirmed the presence of each reported element in the title compounds. Magnetic Susceptibility. The magnetic susceptibility as a function of temperature of Ln4Mo2O11 (Ln = La and Ce) was measured using a Quantum Design MPMS3 SQUID magnetometer. Zero-field-cooled (zfc) magnetic susceptibility of the ground crystals were measured as a function of temperature between 2 and 300 K in an applied field of 1000 Oe. The measured magnetic moment was corrected for shape and radial offset effects using the methods reported by Morrison and co-worker,39 which determines the correction factors by comparing the moments observed from DC and VSM scans at a single temperature, 30 K.

Figure 3. Extended structure of Ln4Mo2O11 (Ln = La and Ce) along the c-axis. The blue, cyan, and red represent Ln, Mo, and O, respectively.

The structure consists of two crystallographically unique Ln centers, namely, Ln(1) and Ln(2). Both Ln(1) and Ln(2) are located in an irregular eight-coordinate polyhedron with Ln−O distances ranging from 2.354(6) to 2.903(2) and 2.366(10) to 2.637(2) Å, respectively. There is only one crystallographically unique Mo center present in the structure forming irregular MoO6 octahedra with Mo−O distances ranging from 1.847(2) to 2.207(2) Å.32 The distance between the two Mo atoms in the Mo2O10 unit of La4Mo2O11 and Ce4Mo2O11 are short 2.5905(5) and 2.5892(15) Å, respectively, because of the presence of direct Mo−Mo bonding, and shorter than the distances between nonbonding Mo−Mo neighbors (>3 Å).41 By comparison, in the isostructural Nd4Re2O11, the Re atoms in the Re2O10 dimer units are only 2.421(1) Å apart, reflecting the presence of a ReRe metal bond.40 On the basis of the chemical composition of the title compounds, La4Mo2O11 and Ce4Mo2O11, the average oxidation state of Mo is +5, as further confirmed by bond valence sum (BVS)42,43 calculations for La4Mo2O11 and Ce4Mo2O11, yielding 5.03 and 5.01, respectively. The presence of Mo−Mo bonds in reduced molybdates often leads to the formation of isolated dimers, infinite chains, or quasidiscrete clusters. Both La4Mo2O11 (Mo5+), which contain Mo2O10 octahedral units, and La2MoO5 (Mo4+), which contain Mo2O8 square planar units, are examples of structures with isolated dimers.41 Additionally, fused Mo2O10 units are present in both La2Mo2O7 (Mo4+)44 and La5Mo4O16 (Mo4+/5+);45 however, they are not isolated, and are either corner-shared to each other

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RESULTS AND DISCUSSION Synthesis. We have successfully demonstrated that Zn metal can be used as a reducing agent to prepare reduced oxides.7,8,28,31 The low melting point of zinc (419.6 °C) is advantageous, as having Zn in a liquid state helps to speed up the kinetics of the reduction reaction and improve the overall yield. At the reaction temperature of 1000 °C, Zn is above the boiling point (907 °C), which is further helpful as the tube will contain a Zn vapor atmosphere. Zn metal readily reduces Mo6+ to Mo5+ as well as Ce4+ to Ce3+ in situ. The use of redox-neutral alkali halides is suitable as a flux as it allows efficient dissolution of oxides while not interfering with the reducing agent. Adopting our synthetic approach, we were able to synthesize high-quality single crystals of Ln4Mo2O11 (Ln = La and Ce). In 1992, Gall and Gougeon reported the synthesis of single crystals of La4Mo2O11 using Mo metal as a reducing agent in a sealed Mo crucible at 1700 °C; however, no further characterization was reported.32 Structure. Ln4Mo2O11 (Ln = La and Ce) is isostructural with Nd4Re2O11.40 The structure of Ln4Mo2O11 (Ln = La and Ce) crystallizes in the tetragonal space group P42/n and exhibits a three-dimensional crystal structure consisting of distorted MoO6 octahedra and irregular LnO8 polyhedra. Two distorted MoO6 octahedra are edge-shared with each other to form isolated Mo2O10 dimer units, Figure 2. These dimer units 1190

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Chemistry of Materials to form infinite chains or corner-shared to other MoO6 octahedra, respectively. Isolated infinite chains of MoO6 octahedra can be observed in MoO2 (Mo4+) as well as Ln5Mo2O12 (Mo4+/5+, Ln = Y, Eu−Er)29,46 whereas Mo3, Mo6, and Mo10 clusters can be observed in LiZn2Mo3O8 (Mo4+/3+),47 Ln4Mo4O11 (Mo2+/3+, Ln = Y, Nd−Lu),48,49 and LnMo5O8 (Ln = La−Gd)50 or Ln16Mo21O56 (Ln = La−Nd),51 respectively. Magnetic Susceptibility. The temperature dependence of the magnetic susceptibilities of Ln4Mo2O11 (Ln = La and Ce), measured in an applied field of 1000 Oe, is shown in Figure 4.

Figure 4. Temperature dependence of the molar magnetic susceptibility, χ, and inverse susceptibility, 1/χ, of Ln4Mo2O11 (Ln = La and Ce) in an applied field of 1000 Oe.

Unpaired electrons present in the Mo(V) d1 molybdenum as well as Ce(III) f1 rare-earth site contribute to the magnetism. The sole contributor for the magnetic susceptibility in the La analogue is molybdenum; however, in this case the La analogue is observed to be diamagnetic, as can be seen in Figure 4. The unpaired electrons present in the Mo sites are localized in the Mo−Mo single bond; hence, no magnetic coupling is observed. The Ce analogue, on the other hand, has a Ce(III) f1 contribution, which can be observed in the magnetic susceptibility. The inverse susceptibility follows Curie−Weiss behavior in the temperature range of 175−300 K. The effective magnetic moment, 4.95 μB/formula unit, is in good agreement with the theoretical value of 5.08 μB, indicating that the Mo(V) electrons again are localized and only the Ce(III) f1 electrons contribute to the moment. The negative Weiss constant, θ = −85.2 K, suggests the presence of antiferromagnetic interactions; however, no long-range order is evident in the susceptibility data. In Situ Neutron Diffraction of Ln4Mo2O11 (Ln = La and Ce). Experiment I. The initial and final Rietveld refinements of the starting mixture (La2O3, MoO3, Zn, NaCl, and CsCl) as well as the final products (La4Mo2O11, NaCl, and CsCl) of experiment I (reagents:flux = 1:2) were carried out using neutron diffraction data collected at 25 °C, which are shown in Figure 5. The 2D maps of the heating and cooling cycles as a function of d-spacing (Å) were generated using the neutron diffraction data obtained at respective temperatures, Figure 6A,C. Figure 6B shows the 1D neutron diffraction plot as a function of d-spacing (Å) during the dwelling at 1000 °C. According to the heating 2D map, there are no diffraction peaks present after ∼450 °C, which indicates that the flux and the starting reagents are molten/dissolved around this temperature. However, new diffraction peaks appear during the dwelling period at 1000 °C that, surprisingly, do not correspond to the

Figure 5. Rietveld refinements of (A) starting materials, La2O3, MoO3, Zn, NaCl, and CsCl, and the (B) final products (La4Mo2O11, NaCl, and CsCl) of experiment I.

target phase, La4Mo2O11. Interestingly, these peaks disappeared on cooling and peaks belonging to the target phase, La4Mo2O11, and to the recrystallized flux, NaCl/CsCl, start to appear at ∼450 °C. (Figure 6C) The Rietveld refinement of the final products (Figure 5B) confirms the presence of the expected species (La4Mo2O11, CsCl, and NaCl) and contains a weak peak at 3.15 Å, which could not be identified. The cooling 2D map shows that this unidentified peak starts to appear at ∼450 °C along with the La4Mo2O11 and the solidified flux. Because of the high background in the data (low signal-tonoise) at elevated temperatures, it was difficult to assign peaks and identify phase(s) during the dwelling period. Hence, subsequent laboratory quenching experiments at 1000 °C were carried out to isolate the phases formed. This quenching experiment, interestingly, revealed four different types of transient intermediate lanthanum molybdate phases: namely, La3Mo4SiO14, La5Mo3O16, La0.5Na0.5MoO4, and La2Mo2O7, which were subsequently characterized via powder and singlecrystal X-ray diffraction methods. Prior to the in situ neutron diffraction investigation, the crystal growth reactions of La4Mo2O11 were run multiple times with a variety of heating and cooling rates and the only phase ever observed was the target phase, La3Mo2O11, and at times, unreacted starting materials. It was therefore quite unexpected to observe the formation of four transient molybdate species and, furthermore, that none of the transients transformed into the target phases, but were instead simply reabsorbed by the flux. 1191

DOI: 10.1021/acs.chemmater.8b00072 Chem. Mater. 2018, 30, 1187−1197

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fused silica tube itself.7 The structure (Figure 7A) consists of Mo3O13 cluster units, edge-shared infinite MoO6 octahedra, SiO4 groups, and La polyhedra. The structure of La5Mo3O16 (black cubes) contains isolated MoO4 tetrahedra that are corner-shared to La polyhedra. In this phase, Mo is in +5 and +6 oxidation states (Figure 7B). Adopting the scheelite structure with isolated MoO4 tetrahedra connected via La/Na polyhedra, La0.5Na0.5MoO4 formed as gray bars (Figure 7C). Unlike the other intermediates, Mo in this structure is not reduced. The structure of La2Mo2O7 (Mo4+), as discussed earlier, consists of Mo2O10 units that are corner-shared to each other (Figure 7D). The resulting oxidation states found in these structures, Mo6+, Mo5+, and Mo4+, suggest that the structure/ cation size fit criterion is important to stabilize certain structure type. A unique aspect of the in situ work described herein is that it uses, as mentioned earlier, the identical experimental conditions to what was used in the laboratory settingsame size reaction tubes, same sample loading, and same temperature profile enabling us to identify from the neutron data the conditions (time and temperature) at which the transient species formed and to readily reproduce their formation in the laboratory and to isolated them. This is an important consideration since, as described next, even the reactant-to-flux ratio can influence whether or not these transients form; therefore, only by using the identical conditions for the in situ and laboratory work can their formation and isolation be predictably achieved. Analysis of the neutron diffraction data at elevated temperatures was very difficult because of the large amount of flux present relative to the small amount of the reagents in the first experiment. Increasing the amount of starting materials relative to the flux would increase the absolute amount of products as well as generate data with a better signal-to-noise ratio; hence, a second in situ experiment that utilized more reagents (reagent:flux ratio = 2:1) was carried out at POWGEN (BL-11A). Experiment II. The initial and final Rietveld refinements of the starting mixture (La2O3, MoO3, Zn, NaCl, and CsCl) and final products (La4Mo2O11, ZnO, NaCl, and CsCl) of experiment II were performed using neutron diffraction data collected at 25 °C, shown in Figure 8. The Rietveld refinement of the final products has the same impurity peak (3.15 Å) that is observed in the last experiment. All attempts to identify this unknown peak using subsequent laboratory experiments and databases such as the ICSD and PDF-4+ were unsuccessful. Although the last neutron experiment showed the reaction path of La and Mo cations, it failed to reveal what happened to the Zn metal after dissolution at ∼450 °C. As evident in Figure 8B, Zn metal has become ZnO. We believe that the reason why ZnO was not observed in the previous experiment is the relatively small amount of ZnO present in the system, which was not sufficient to show up in the neutron diffraction data. On the basis of the neutron diffraction data obtained throughout the reaction, 2D maps of heating and cooling cycles as a function of d-spacing were generated as shown in Figure 9A,C. Figure 9B shows the Rietveld refinement at 1000 °C. Although both experiments yielded the expected target phase La4Mo2O11 upon completion of the heating/cooling cycle, we obtained different insights into the reaction and information concerning the crystal growth from the two different in situ experiments, which we attribute to the change in the reagentsto-flux ratio. As can be seen in the heating 2D map (Figure 9A), at roughly 400 °C starting materials begin to disappear;

Figure 6. (A) 2D plot of the heating, (B) 1D plot of the dwelling, and (C) 2D plot of the cooling as a function of d-spacing of La4Mo2O11.

Out of the four phases obtained, La3Mo4SiO14, La5Mo3O16, and La0.5Na0.5MoO4 formed as single crystals whereas La2Mo2O744 formed as a black polycrystalline powder. Both La3Mo4SiO1452 and La0.5Na0.5MoO453 were previously reported as single crystals in the literature; however, La5Mo3O16 was first identified based on powder X-ray diffraction by Hubert et al.54 Later McCarroll et al.55 were able to prepare small single crystals of La5Mo3O16+x by fused salt electrolysis. The crystallographic data of La3Mo 4 SiO14 , La 5 Mo3 O 16, and La0.5Na0.5MoO4 are listed in Table S2. The Cambridge Crystallographic Data Centre (CCDC) number for La5Mo3O16 is 1562641. La3Mo4SiO14 formed as black-colored rods where Mo is in +4 and +5 oxidation states. The source of the silicate is the 1192

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Figure 7. Extended structures of intermediate phases: (A) La3Mo4SiO14, (B) La5Mo3O16, (C) La0.5Na0.5MoO4, and (D) La2Mo2O7 formed at 1000 °C. The yellow, green, blue, and red represent La(Na), Mo, Si, and O, respectively.

however, unexpectedly, new phase(s) begin to form almost immediately. Diffraction peaks persist throughout the dwelling period and were identified as final products (La4Mo2O11and ZnO) as demonstrated by the Rietveld refinement shown in Figure 9B. Laboratory quenching experiments further confirm this result. The final products, La4Mo2O11 and ZnO, observed at 1000 °C persist throughout the cooling period and throughout the flux recrystallization period starting at 400 °C, (Figure 9C) Every neutron diffraction pattern was refined sequentially using the EXPGUI interface to GSAS. To identify the intermediate phases formed in the reaction, ex situ laboratory quenching experiments were carried out. Weight percentages of the phases obtained from the refined patterns were used to record the reaction progression as a function of temperature and a “reaction progress map” was generated, shown in Figure 10. This reaction map depicts the dissolution of reagents, appearance and disappearance of transient intermediate phases, and final product formation during the in situ reactions of La2O3, MoO3, and Zn in NaCl/CsCl flux. Experiments III and IV. In situ neutron and ex situ laboratory synthesis and diffraction studies of Ce4Mo2O11 were carried out using the same procedure and quantities used to study La4Mo2O11 using CeO2, instead of La2O3, as the rare-earth reagent. The fact that the CeO2 has to be reduced by Zn to Ce2O3 prior to being incorporated into the product phase results in different kinetics for this process and the flux crystal growth results in a smaller yield of the target phase, Ce4Mo2O11, and the formation of a large amount of Ce5Mo3O16 as a secondary phase. While this poses little problem in the laboratory setting where the flux can be fully removed, it makes the product analysis significantly more challenging in the in situ neutron environment, as the signal-tonoise is a function of concentration. The formation of only a small amount of Ce4Mo2O11 prevented us from refining this phase in the product mixture by Rietveld refinement. On the other hand, the major product of the reaction, Ce5Mo3O16,

Figure 8. Rietveld refinements of (A) starting materials, La2O3, MoO3, Zn, NaCl, and CsCl and the (B) final products (La4Mo2O11, NaCl, and CsCl) of experiment II.

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disappeared at around 380 °C, which is before the melting point of Zn. The possible reasons for this observation is likely the initiation of the reaction between Zn and molybdenum, leading to a decrease in the amount of Zn present after 380 °C and becoming insufficient to detect via neutron diffraction. The NaCl/CsCl eutectic melt (mp 486 °C) acts to dissolve the reagents early in the reaction and accounts for the disappearance of the diffraction lines belonging to La2O3 and MoO3. Intermediates. Figure 10 clearly displays the formation of two different intermediates, LaOCl56 and La2(MoO4)3,57 both appearing at 400 °C and disappearing at 520 and 540 °C, respectively. In the tetragonal unit cell of LaOCl, LaO layers alternate with double Cl layers. Each Cl ion is connected to three La cations in the opposite LaO layers. Adopting the scheelite structure, La2(MoO4)3 consists of isolated MoO4 tetrahedra that are connected to La cations. The next intermediate phase, La2MoO6,58 starts to appear at 480 °C. This compound also has isolated MoO4 tetrahedra; however, unlike La2(MoO4)3 these tetrahedra are present in alternating layers that are connected via 2D sheets of edge-sharing La2O12 dimers. The La2MoO6, which first appeared at 480 °C, disappears by 940 °C, before reaching the final reaction temperature 1000 °C. In both intermediates, La2(MoO4)3 and La2MoO6, Mo is present in the +6 oxidation state, suggesting that the reduction process of molybdenum is not finished at the melting point of Zn, but continues all the way up to 1000 °C. Final Products. Before La2MoO6 disappears from the system, surprisingly, the target phase, La4Mo2O11, along with ZnO already appears during the heating phase at 720 °C. We did not observe the appearance of the target phase this early in the reaction or the behavior of the Zn metal becoming ZnO in the previous experimentlikely due to the smaller amount of reagents in the system. Once formed, La4Mo2O11 and ZnO remain in the system throughout the dwelling and cooling periods until the reaction reaches room temperature. No other intermediate phases appeared or disappeared afterward; however, flux recrystallization, as expected, is observed at 400 °C. We were able to observe the appearance and the disappearance of phases during the in situ experiment that would not be observed in a normal flux crystal growth experiment. Although we observed several reduced molybdenum-containing intermediate phases (Mo4+, Mo5+) in the second in situ experiment, it was very different from the first, as we observed La4Mo2O11 (Mo5+) as the only reduced molybdate. Even though in both experiments the target phase was achieved, the reaction pathways were changed due to the change in reactants-to-flux ratio. With more flux in the system, we were able to isolate reduced intermediate phases as single crystals. In contrast, with less flux present in the system we observed oxidized intermediate phases as polycrystalline powders. A potential cause is the large amount of molten flux present in the system, which favors and accelerates dissolution of starting materials, hence, creating an efficient interaction of cations and the reducing agent to form more metastable reduced phases.

Figure 9. (A) 2D plot of the heating, (B) 1D plot of the dwelling, and (C) 2D plot of the cooling as a function of d-spacing of La4Mo2O11.

could be identified. As in the case of La4Mo2O11, the presence of the large amount of flux, relative to products, interfered with the phases analysis. Unlike for La4Mo2O11, however, in the case of Ce4Mo2O11 changing the flux or the amount of flux to a 1:1 ratio further decreased the relative yield of Ce4Mo2O11. Repeating this process in the future starting with Ce2O3 instead of CeO2 may help the relative yield and may lead to the formation of analogous transient species, Ce3Mo4SiO14, Ce5Mo3O16, Ce0.5Na0.5MoO4, and Ce2Mo2O7. This will now be explored in the laboratory setting using the information garnered from the La4Mo2O11 in situ experiments. Reaction Map: La4Mo2O11. Starting Materials. The low melting point of Zn metal (419.6 °C) is very advantageous as it will be molten at the crystal growth temperature, thereby providing a close interaction with the cations dissolved in the flux, which enhance the reduction by speeding up the reaction. According to the reaction progress map, the Zn in the reaction

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CONCLUSION Flux-mediated in situ crystal growth provides valuable insight into the reaction pathway, especially the identification or isolation of intermediates, which would not otherwise be observed in a normal flux growth experiment. To better 1194

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Figure 10. Reaction progress map constructed from the in situ neutron diffraction data of the reaction La2O3 + MoO3 + Zn and NaCl/CsCl flux.

understand the flux crystal growth process of reduced oxides, we investigated the progress of the reactions leading to the formation of Mo5+-containing La4Mo2O11 via in situ neutron diffraction at the POWGEN beamline. In the first experiment with more flux in the system, we isolated four different transient phases by carrying out laboratory quenching experiments at 1000 °C, three of them as single crystals and the fourth one as a polycrystalline powder. Molybdenum in these transient phases was found in different oxidation states, Mo6+, Mo5+, and Mo4+, suggesting that the structure and the cation size fit criteria are important for stabilizing specific structure types. To obtain data with better signal-to-noise ratio, a second experiment containing a higher reagent-to-flux ratio was carried out. With use of the results of the Rietveld refinements, a reaction progress map was assembled that clearly shows the appearances and disappearances of staring materials, intermediates, and final products. Because of the change of the reaction-to-flux ratio, different reaction pathways were observed although both reactions yielded the same target phase La4Mo2O11. In the second experiment, three different intermediate phases were identified as polycrystalline powders; however, none of them contained molybdenum in oxidation states lower than +6. It was found that increased amounts of flux in the system leads to better dissolution of starting materials and consequently to more efficient interaction of cations and the reducing agent to form the metastable reduced phases.

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ORCID

Hans-Conrad zur Loye: 0000-0001-7351-9098 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

Financial support for this work was provided by the National Science Foundation under DMR-1301757 and is gratefully acknowledged. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00072.

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