Solvent Dependent Phase Transition between Two Polymorphic

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Solvent Dependent Phase Transition Between Two Polymorphic Phases of Manganes#Tungstate: From Rigid to Hollow Microsphere Kaustav Bhattacharjee, Satya Prakash Pati, Gopes C Das, and Kalyan Kumar Chattopadhyay Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01575 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Crystal Growth & Design

Solvent Dependent Phase Transition Between Two Polymorphic Phases of Manganese ‒Tungstate: From Rigid to Hollow Microsphere Kaustav Bhattacharjee†, Satya Prakash Pati§, Gopes C. Das† and Kalyan K. Chattopadhyay*‡ †

Dept. of Metallurgical and Material Engineering, Jadavpur University, Kolkata 700032, India.

§

Department of Electronic Engineering, Tohoku University, Sendai 980-0845, Japan.

‡

Thin Film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India.

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1 ACS Paragon Plus Environment


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Abstract:

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Crystallization of manganese (Mn2+) and tungstate (WO42-) ions in the presence of citric acid

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under different water/ethanol mixtures has been systematically investigated under solvothermal

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conditions. A unique phase transition between two polymorphic phases, formulated as MnWO4,

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manganese tungstate and Mn8W12O42(OH)4·8H2O, manganese heteropolytungstate (Mn-HPT),

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was observed along with a striking morphological alteration from rigid to hollow microsphere.

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The effective coordination of citrate ion to tungstate (tungstate-citrate 1:1 complex) in aqueous

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solution before the hydrothermal treatment drives the system to nucleate the less symmetric,

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monoclinic (Space group: P2/c) MnWO4 phase, which is the thermodynamically preferred

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polymorph. However, formation of the tungstate-citrate complex can be tuned by changing the

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dielectric constant of the solvent or by decreasing the citric acid to tungstate molar ratio. Results

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show that both the conditions assist in the formation of the kinetically stable, more symmetric,

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cubic Mn-HPT (Space group: Im-3) phase at same reaction temperature and time. The formation

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of the Mn-HPT phase is mediated by a kinetically advantageous crystallization process from an

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amorphous precursor while later on it gradually converted into more stable MnWO4 phase

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according to ‘Ostwald rule of successive phase transformation’. Optimum reaction conditions for

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the synthesis and plausible growth mechanisms of both the microspheres were proposed on the

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basis of solvent, reaction time, temperature and the presence of citric acid. Magnetic properties

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of both the samples were investigated in order to illuminate the nature of magnetic interaction

42

within the crystal lattice.

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Keywords: Polymorphism, Solvothermal reaction, Classical Nucleation Theory, Ostwald’s rule

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of phase transition.


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1. INTRODUCTION

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The intriguing search for new building blocks and in some case to investigate the intricate

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relationship and possible transformations between different structural motifs within a family of

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compounds has been a fervent research area in recent time. Of the many in the periodic table, the

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third row transition metal element tungsten (W) grabs appreciable research attention because of

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its complexing ability with diverse coordination number and geometries, and the tendency to

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form clusters and polynuclear complexes with a variety of metal ions. In fact, tungsten (VI)

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oxides (WO3) are known to possess several polymorphic forms and can undergo crystallographic

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phase transition even at room temperature,1 that makes it unique among all the oxides.

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Polymorphism is a phenomenon studied for many decades in the fields of organic and inorganic

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chemistry.1-4 However, in recent times the concept of polymorphism has been extended a lot to

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encompass crystal structures that consist of same primary molecules but with different molecular

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partners. These partners are often solvent molecules,2 or counter ion,3 or some other molecules4

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accommodated within the crystal lattice. Solvents, mostly water, often become an important part

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of the crystal structure to form various polymorphic structures, called hydrates (solvates). The

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structural relationship between the hydrate and the anhydrate forms is often found to be derived

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from the interplay between the thermodynamic and kinetic parameters.1

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Several theoretical and experimental studies have been developed in the past few years

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which demonstrate that the polymorphism in WO3 compound is often resulted from the lattice

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phonon and electronic structure of WO3 moiety.1 Heteropolytungstates (HPT), the class of

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compounds with the general formula [XmWnOp]q− (X = Si, P, B, Co, Fe, Cu, ...), are poly-

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condensed products of tungstate anion (WO42-) formed in the presence of a hetero-atom and is

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one of the most studied area of polyoxometalates (POMs) cluster chemistry.5 Under suitable



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condition they form discrete cluster-like structures such as, Keggin, Anderson−Evans,

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Dexter−Silverton, Well−Dawson, Lindquist etc. of definite sizes and shapes by sharing the

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corners, edges, and/or faces of MO6 (M = V, W, Mo, Nb,...) octahedra.5-7 Among all, the

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Dexter−Silverton type structure of polytungstate is very rare and consists of a highly uncommon

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arrangement of six pair of face shared WO6 dimeric units forming a central 12-fold coordination

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site for metal ion.8 Only a few compounds have been synthesized so far with this type of crystal

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structure having general anionic formula [XM12O42]10- (X = Ce, Gd, Mg, Mn and M = Mo, W).8-

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11

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form is the monoclinic/triclinic (Space Group: Pc/2) type metal tungstates which give rise to a

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series of iso-structural compounds known as scheelite/wolframite.12 Among them, MnWO4

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represents the typical wolframite class of tungstates minerals with multi-functionalities like

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humidity sensor and/or coexistence of ferroelectricity, ferromagnetism and ferroelasticity.13 In

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the crystal lattice the MnO6 octahedra with high spin Mn2+ (d5; S = 5/2) ions and the WO6

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octahedra with diamagnetic W6+ (d0) ions share their alternative cis edges to form the zig-zag

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MO4 (M = Mn or W) chains, all running along the c-axis. As a consequence of the complicated

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magnetic interaction between the Mn2+ ions within the crystal structure, this compound

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undergoes three magnetic phase transitions: at TN = 13.5 K (AF3), T2 = 12.8 K (AF2) and T1 =

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6.5-8 K (AF1).14

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A large quantity of research efforts has been dedicated in the last few years for the synthesis of

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MnWO4 hierarchical structures under hydrothermal conditions with tailored properties,15-17 but a

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rather comprehensive understanding of solvent dependent crystallization behavior of different

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polymorphic phases of WO3 with manganese (II) ion is rarely discussed in the literature. In this

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paper we report a unique solvent dependent phase transition from a thermodynamically stable

On the other hand, at higher pH depending on the size of the cation, the most favored crystal


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low symmetry monoclinic tungstate (MnWO4) form to a kinetically stable high symmetry cubic

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heteropolytungstate, Mn-HPT [Mn8W12O42(OH)4·8H2O] phase and also the Ostwald rule driven

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successive phase transformation to the most stable phase. Unlike the crystallization of colloids

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and proteins which have slow crystallization kinetics, Chung et al. have recently demonstrated a

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direct evidence of the Ostwald’s rule of stages for an inorganic compound in very lesser time.18

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Our study also shows the direct manifestation of the same rule within the comparable

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experimental time frame, where, the formation of the thermodynamically stable MnWO4 crystals

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starts from an aqueous solution containing dissolved ions and the kinetically stable Mn-HPT

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crystals are formed starting from an amorphous intermediate. The structural, thermodynamic and

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kinetic relationships between the two polymorphs are determined by several parameters like,

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dielectric constant, concentration, pH, reaction temperature and time etc. Another important

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issue of this study comes from the extensive screening (by X-ray diffraction and field emission

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scanning electron microscopy) of the samples, is the distinct morphological evolution of the two

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polymorphs. Solid microspheres of MnWO4 are formed under the hydrothermal condition while

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hollow microspheres of Mn-HPT are found after the solvothermal treatment at the same

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temperature and time. Magnetic analyses reveal the nature of magnetic interactions in the

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respective compounds.

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2. EXPERIMENTAL SECTION

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2.1. Materials

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In the present case, we have synthesized two different polymorphs of manganese-

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tungstate by varying different parameters like solvent, time, temperature, pH etc. Sodium

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tungstate dihydrate (NaWO4, 2H2O, 99%) and manganese chloride tetrahydrate (MnCl2, 4H2O,

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99.99%) were purchased from Sigma Aldrich. Citric acid monohydrate and ethylenediamine 5 ACS Paragon Plus Environment


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were procured from Merck, India. All the chemicals used in the present study were of analytical

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grade (AR) and used without further purification. A mixture of water and ethanol in different

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volume ratio were used as the solvents for the synthesis processes. Ultrapure water (specific

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conductivity = 17.4 μS cm-1) was collected from an EASY pure®-II, UV-ultrapure water system

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while 99% ethanol was obtained from Merck, India.

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2.2. Synthesis of the Polymorphs

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In the present study, we have synthesized different sets of samples with varying

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hydrothermal and solvothermal reaction conditions (details described in the Supporting

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Information, section S1). In a typical hydrothermal process, 0.05 mole sodium tungstate Na2WO4

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was first dissolved in 100 mL water. The pH of the solution was measured to be ~ 7. Then 0.05

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mole of citric acid (CA) was added to the mixture and stirred until dissolved. The solution

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remains transparent while the pH is reduced to ~ 3. After 30 min of stirring, 0.05 mole of

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manganese chloride (MnCl2, 4H2O) was added into the above mixture and the stirring was

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continued for another 30 min. The pH was measured to be ~ 3. Calculated amount of

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ethylenediamine (EDA) was added to the reaction mixture to increase the pH to ~ 5. This

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transparent solution was then transferred in a Teflon-lined autoclave (80% filled) of 100 ml of

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capacity and treated at fixed temperature of 180 °C for different time or different temperature at

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fixed time of 6 h. While in a typical solvothermal process, we used the same synthesis protocol

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except that the solvent was a mixture of water and ethanol (50:50 vol%) instead of water alone.

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2.3. Characterization of the Samples

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Phase identification of all the synthesized samples was done by powder X-ray diffraction

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(PXRD) analysis using a Rigaku ultima-III X-ray diffractometer (Bragg−Brentano geometry)

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with Cu-Kα radiation (λ = 0.154 nm). The representations of crystal structures were drawn using


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‘Diamond Crystal and Molecular Structure Visualization program’19,20 (Version 3.2f for

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Windows) on the basis of their PXRD refinement analysis. The surface morphology of all the

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samples was characterized using a field emission scanning electron microscope (FESEM, Hitachi

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S4800, Japan) operating at 5 keV. Transmission electron microscopy (TEM) analysis of the two

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polymorphs was performed with a JEOL JEM 2100 microscope operated at 200 kV. Analyses of

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different vibrational modes of the two different polymorphs were done at room temperature

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using Fourier Transformed Infrared Spectrometer (FTIR, IR-Prestige, Schimatdzu). Room

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temperature Raman spectrum was recorded using Labram Raman instrument with a laser source

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of λ = 532 nm and 50 mW laser power. Elemental analyses of the samples were done by Energy

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Dispersive X-ray Spectroscopy (EDS) attached to the FESEM instrument. X-ray Photoelectron

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Spectroscopy (XPS, SPECS HSA-3500 hemispherical analyzer) analysis was performed using a

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monochromatic X-ray source of Al Kα radiation. The magnetic property of both the samples was

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investigated by using a superconducting quantum interference device (SQUID) magnetometer

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(Quantum Design, MPMS).

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3. RESULTS AND DISCUSSION

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3.1. Crystal Nucleation in the Light of Classical Nucleation Theory

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The classical nucleation theory (CNT) was first derived by Becker, 21 Döring and

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Frenkel22 in the year of 1930 which based on the work of Volmer and Weber,23 Gibbs24,25 and

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Farkas26. According to the theory, nucleation starts with the formation of a small cluster (called

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nucleus) of a new crystalline phase in a supersaturated solution by the stochastic clustering of the

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solute molecule.27 The degree of super-saturation (S) is defined as the ratio of bulk solution

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concentration (C) to the equilibrium concentration (solubility;

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free energy cost is associated with creation of the newly formed solid surface and thus the


) of the solute.23,28 However, a


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nucleation strongly depends on both the degree of super-saturation (S) and the interfacial energy

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(γ) by the relation:28,29

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( )

( ), and

⁄

(1)

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Where, W(n) is the work to form an n-sized cluster, kB is the Bolzmann constant (1.38 × 10−23 J

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K−1) and A(n) (m2) is the surface area of the cluster and n is the number of primary molecules

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involved in the clustering. During the initial periods, these nuclei are unstable and tend to

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dissolve until a critical size is reached. The critical radius (r*) for a stable nuclei is defined as the

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radius at which the free energy is maximum, and can be determined according to the Kelvin

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equation:29

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(2)

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It is known that the logarithm of equilibrium concentration (

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proportional to the dielectric constant (ε) of the solution24 and the dielectric constant of the

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solvent drastically fall at the solvothermal temperature.29

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[

(

]

) of the solute is inversely

(3)

)

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At the hydrothermal condition, the degree of super-saturation easily becomes greater than unity

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and a large number of tiny nuclei are formed suddenly by the collision of the reacting species

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(mostly WO42- and Mn2+ ions in the present case). This is known as primary homogeneous

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nucleation. On the other hand, as the nucleation energy barrier height is strongly dependent on

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the interfacial energy term, phase with smaller surface energy leads to a lower barrier for the

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formation of the meta-stable product. Thus if any solute is already present (in the crystalline or

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amorphous form) or deliberately added in the solution, then under any solvothermal condition at

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low super-saturation, nucleation can occur at the vicinity of the heterogeneous phase. Nuclei thus


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formed, get sufficient time to grow larger in size. This is called secondary or heterogeneous

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nucleation. According to Ostwald’s rule, the first crystallizing phase should always be the one

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which is closest in free energy to the mother phase, may successively thereafter changes into

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phase with higher thermodynamic stability.30,31 An amorphous phase has significant lower

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surface energy than their dense crystalline counterpart30 and can act as a steady source of

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reacting species under the solvothermal condition. Surfactant induced crystallization of a

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particular polymorph for glycine was also reported by Chen et al.32 For ceramic oxide, in

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aqueous solution crystallization, the nucleation occurs at a much faster rate and thus can be

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adequately explained with the help of classical nucleation theory and Ostwald’s step rule.30

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3.2. Effect of Solvent on the Formation of Two Polymorphs: XRD and FESEM Studies

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When an aqueous solution of sodium tungstate, citric acid and manganese chloride (1:1:1

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molar ratio) at pH 5 (maintained by ethylenediamine) was hydrothermally heated at 180 oC for 6

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h, rigid microspheres of the monoclinic wolframite MnWO4 phase was obtained. Interestingly, if

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we use ethanol as a co-solvent and varying its proportion in the synthesis process, then under the

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same experimental condition, the morphology of the product changes noticeably from solid to

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hollow microspheres. At a certain ethanol volume fraction (35%), this is added with the sudden

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change in phase into cubic hydroxide hydrate of manganese tungstate (Mn4W6O21(OH)2, 4H2O),

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the Mn-HPT phase. Figure 1 shows the FESEM images along with the XRD patterns of the

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powder samples synthesized at 180 oC for 6 h under varying water-ethanol ratio. The details of

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the synthesis process and the experimental conditions for all the samples are described in the

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Supporting Information (Section S1). From the FESEM images of the samples reveal that when

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only water is used, densely packed rigid microspheres of MnWO4 with diameter of 2 − 4 µm are

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formed (Figure 1a). Small volume of ethanol (15% - 25%) does not change the shape much, but



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leads to the formation of disks/plate like building subunits (Figure 1b and c) at the surface. From

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35 % of ethanol volume, the morphology of the surface particle starts to change into polyhedron

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(Figure 1d). These polyhedrons are actually of the HPT phase as confirmed by the XRD data.

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Typically at 50:50 condition, hollow spheres are formed which are made of polyhedron particles

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having one or multiple thick shells (Figure 1e). The powder XRD patterns of all the samples

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(Figure 1f) clearly show the phase change from MnWO4 to HPT at 35% ethanol volume fraction

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and onwards. It has been found that alcohol plays a crucial role on the selective phase

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crystallization and the morphology of CaCO3 polymorphs in water-alcohol binary mixture.33,34,35

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3.3. Effect of Reaction Temperature and Time on the Formation of Two Polymorphs: XRD

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study

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Figure 2a and b show the XRD patterns of samples prepared under various solvothermal

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time (at constant temperature of 180 oC) and temperature (at constant time of 6 h), respectively.

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From the results it is clear that the stability of the HPT phase strongly depends on the

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temperature and time of the reaction. If we increase the temperature to 210 oC and keep it for 6 h

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or maintaining at 180 oC if we increase the time to 12 h, the cubic HPT phase changes to

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monoclinic wolframite phase. Thus we can conclude that the formation of the manganese based

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HPT phase is kinetically controlled under solvothermal conditions and successively changes into

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the more stable monoclinic wolframite phase, directed by the thermodynamic pathways. On the

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other hand, the XRD patterns of samples prepared under different hydrothermal temperature and

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time are shown in Figure 2c and d, respectively. From the figure it is clear that under

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hydrothermal condition MnWO4 was formed over a wide range of temperature (150 oC − 210 oC)

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and once formed, it is stable for long time (3 h − 24 h). However, at considerably lower reaction


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temperature (~ 110 oC), the HPT phase was formed (Figure 2c). This observation again indicates

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the kinetic stability of the HPT phase even in hydrothermal situation.

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3.4. Effect of Citric Acid Concentration, pH and Precursor Concentration on the

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Formation of Two Polymorphs: XRD study

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Interesting observations were found under both the solvothermal and hydrothermal

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conditions when other reaction parameters were varied. In the hydrothermal process, upon

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reducing the tungstate to citric acid ratio to 1:0.5 at the same temperature (180 oC) and time (6

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h), a mixture of the HPT and MnWO4 phases was formed. Fairly, the higher citric acid

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concentrations would result in the formation of MnWO4 only. The observations were shown in

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the left panel of Figure 3. This strongly suggests that the formation of soluble tungsten-citrate

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complexes actually reduces any slim chances of the poly-anion formation in aqueous solution

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when citric acid is present in equimolar ratio with respect to tungstate ion. But if present at lower

239

concentration (half the concentration of tungstate), both the soluble complex and the poly-anion

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can form in the solution and eventually leads to the mixture of phases. Another important role of

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citric acid in this synthesis process was revealed from the FESEM study of the samples. As

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shown in the respective insets of Figure 3, the microspheres were loosely bound in low citric

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acid concentration while the binding efficiency and compactness of the spheres were greatly

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improved at higher concentrations. This signifies the adhesive-like nature of citric acid in the

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microsphere formation.

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On the other hand, in solvothermal process, if no EDA was added and the pH was

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maintained at ~ 3 by citric acid only (detail sample preparation is in Table S5, Supporting

248

Information), then Mn-HPT was formed in the reaction (see Figure 3b). However, the

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microspheres were rigid in this case (see Figure S1, Supporting Information) with very tiny



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particles at the surface. Again in the absence of citric acid if pH was maintained at ~ 10 (detail

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sample preparation is in Table S5, Supporting Information) with the same amount of EDA, then

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under the same reaction temperature and time (180 oC/6 h) MnWO4 is formed (see Figure 3b)

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and in this case nanobars were formed with roughly 50 − 60 nm in length and 10 - 15 nm in

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width (see Figure S1, Supporting Information). This proves that citric acid also helps maintain

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the pH of the system suitable for the formation of the polyanion-aggregates and hence the Mn-

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HPT phase. If we reduce the concentrations of the precursor in the solvothermal reaction to 0.01

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M then at same reaction conditions MnWO4 was formed (see Figure 3c). However, at 0.05 M

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and 0.1 M precursor concentrations, Mn-HPT phase was observed (see Figure 3c). This suggests

259

that the dissolution of the precursor molecules into the solvent would control the formation of the

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actual reactive species that can take part in the solvothermal reaction. Due to the lower

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concentrations, precursors were dissolved completely which would lead to the formation of the

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soluble tungstate-citrate complexes even in the mixed solvent and consequently MnWO4 was

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formed in the reaction. From the above discussion we can conclude that the solvation of the

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precursor molecules and the formation of soluble complexes solely depend on the dielectric

265

constant of the solvent, which in turn governs the formation of the kinetically controlled or

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thermodynamically controlled products under the desirable solvothermal conditions.

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3.5. Thermodynamic and Kinetic Relationships for the Formation of the Polymorphs

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The hydrothermal chemistry of potassium tungstate in solutions in the presence of citric

269

acid has recently been studied by Gu et al.36 When citric acid is added to the aqueous solution of

270

equimolar amount of sodium tungstate, the pH of the final mixture is retained at 3. Citric acid

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(CitH3) mostly remains in the dibasic form (CitH2-) at this pH and coordinates tungstate anion to

272

form various soluble tungstate citrate complexes.37,38 The favorable change in enthalpy for the


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expansion of the coordination sphere from four (W4+) to six (W6+) drives the complex formation

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reaction with citric acid.38 Manganese chloride was then added to this solution followed by the

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increase of pH to 5 with small amount of ethylenediamine. The consecutive reactions can be

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expressed by the following equations:

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pWO42- + qCitH3- + rH+= [(WO4)p(CitH)qHr][2p+3q-r]-

(4)

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[(WO4)p(CitH)qHr][2p+3q-r]-+pMn2+= pMnWO4 + qCitH3- + rH+

(5)

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Under hydrothermal condition the nucleation of MnWO4 starts to occur with the release of citric

280

acid according to equation 5. Soon the high rate of aggregation results in the formation of

281

relatively larger particles of wolframite MnWO4. The schematic representation of the process is

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shown in Figure 4. The as-formed MnWO4 belongs to monoclinic crystal system with a space

283

group P2/c as shown in the upper right panel of Figure 4. On the other hand, with the gradual

284

increase in ethanol volume, polarity of the mixed solvent decreases which in turns decreases the

285

solubility of the precursors (sodium tungstate and citric acid), and typically at 50/50 condition, a

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rapid white precipitation appears just after the addition of citric acid, which gradually becomes a

287

faint sol upon prolonged stirring. The precipitation appears due to the formation of paratungstate

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poly-anion according to the equation 6.38,39,40

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6[WO4]2- + 7H+ = [HW6O21]5- + 3H2O

(6)

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[HW6O21]5- + 4Mn2+ + 6H2O = Mn3[MnW6O21](OH)2· 4H2O + 6H+

(7)

291

The local pH of the sodium tungstate solution falls rapidly just after the addition of citric acid

292

which favors the poly-anion formation39 and at the same time the solubility of the polymeric

293

species is decreased due to the presence of ethanol. Addition of Mn2+ into the solution does not

294

make any noticeable change. Finally the solution was adjusted at pH 5 by adding

295

ethylenediamine as in the previous case. The poly-anion aggregates serve as a necessary source



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296

for the nucleation of the HPT phase according to a kinetically advantageous process, called

297

secondary nucleation that passes through a relatively smaller nucleation energy barrier. Thus

298

under solvothermal condition a large number of tungstate poly-anion, in association with the

299

metal ions (Mn2+) and water molecules, are reassembled together to form relatively larger nuclei

300

of the HPT phase according to equation 7 (step-I in the schematic). A very uncommon yet highly

301

ordered arrangement of six pair of face shared WO6 dimeric units forming a 12-fold coordination

302

site for the Mn2+ ion was noticed in the present compound. Generally, more rigid the pre-

303

organization of the solvent (host) molecules with the cation, slower would be the kinetics of the

304

formation of solvates4. As the number of water molecules in a given proportion of the solvent is

305

less in the solvothermal condition, the activity of water and hence the pre-organization, decreases

306

sufficiently so that it favors the formation of the hydrated manganese poly-tungstate (HPT)

307

phase. A representative image of the HPT unit cell is shown in the lower right panel of Figure 4.

308

Interestingly, if reaction time or temperature is increased, the phase changes into

309

thermodynamically stable MnWO4 phase (Figure 2a and 2b). According to Ostwald’s rule of

310

successive phase transformation, the thermodynamically least stable phase will crystallize first

311

and will be successively replaced in time by the more stable phase.41,42 Based on non-equilibrium

312

thermodynamics where a distribution in size exist in a polymorphic system, due to the varying

313

curvature of different particulate interfaces, heterogeneous preceded growth of new phase results

314

followed by grain coarsening.42 This would lead to the crystallization of more dense products

315

(which in the present case is MnWO4, step-II in the schematic), as also the fraction of water

316

molecule in liquid phase, which otherwise stabilizes the HPT lattice, will drop upon

317

temperature.43 Thus it can be logically assumed that the successive phase transformation process

318

in the present case is accompanied with the gain in the change in entropy at the cost of the


Page 15 of 35

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319

change in enthalpy associated with the water molecules. Hence, from the standard

320

thermodynamic relationship we can say that the overall driving force for the crystallization of

321

both the phases, is denoted by, ΔG, is negative, while the activation energy for the first transition

322

is less than that of the second,

323

diagram for the formation and phase transition behavior of the HPT phases under solvothermal

324

condition is illustrated in the Figure 4.

325

3.6. Vibrational Mode Analysis of the Two Polymorphs

>

.18 Based on these arguments the energy profile

326

Vibrational mode analyses of all the samples were done at room temperature by FTIR

327

and Raman spectroscopy. A comparative analysis of the FTIR and Raman spectra for the two

328

polymorphs were shown in Figure 5a and b, respectively. A more clear and progressive change

329

in the FTIR spectra of the samples prepared under different solvothermal (50/50 water/ethanol

330

mixed solvent) and hydrothermal conditions for different time and temperature are shown in the

331

Supporting Information, Figure S2. All the bands were properly assigned corresponding to each

332

of the polymorphic lattices and were listed in the Supporting Information, Table S6 and S7. The

333

stretching and bending modes of vibrations for the characteristic structural water molecule

334

present in the Mn-HPT lattice were detected in the range of 3530 − 3230 cm-1 and 1600 cm-1,

335

respectively. While for MnWO4, a broad band appears in the range of 3500 − 3200 cm-1

336

corresponding to the surface adsorbed water molecules. For MnWO4, we found three weak bands

337

at 1640, 1574 and 1384 cm-1 which correspond to the symmetric (first two) and asymmetric

338

stretching mode (third) of coordinated –COO- group of citric acid, respectively.44 Citric acid

339

often plays the role of structure directing agent under hydrothermal condition by selectively

340

coordinating in different lattice planes and thus remains incorporated in the compound.45 On the

341

other hand, strong signals for –OH and –CH bending vibration were observed in this



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Page 16 of 35

342

wavenumber region for the HPT phase.11 The bands appeared in the Raman spectra of the two

343

polymorphs were properly assigned and were listed in the Supporting Information, Table S8. The

344

most intense band for the MnWO4 sample appears at 878 cm-1 which corresponds to the

345

symmetric stretching vibration (Ag) of a short terminal W-O bond.13 While for Mn-HPT, the

346

most intense band appears at 960 cm-1 which is due to the symmetric stretching of W=O

347

(terminal) bond.46

348

3.7. Compositional Analysis of the Two Polymorphs

349

Compositional analyses of the individual samples were done by EDS and XPS studies.

350

Figure 6a and b shows the EDS spectrum of the Mn-HPT and MnWO4 samples, respectively,

351

which supports the presence of C, W, Mn, and O in both the samples. The atomic ratio of Mn

352

and

353

Mn7(MnW12O42(OH)2)·8H2O for Mn-HPT and 1:1, as in the formula MnWO4. The XPS survey

354

scans and high resolution scans for the W 4f and O 1s levels were performed for both the Mn-

355

HPT and MnWO4 samples and the observations are shown in Figure 6c and d, respectively. The

356

elemental ratio calculated from the spectrum was put in the inset of each spectrum. Doublets

357

correspond to W 4f7/2 and W 4f5/2 photoelectrons with spin-orbit coupling 2.1 eV and 2.3 eV,

358

respectively for Mn-HPT and MnWO4 measured from the spectrum which strongly suggests W6+

359

oxidation state in both the compounds.47 While O 1s spectrum with maximum at 529.5 eV and

360

531 eV for Mn-HPT and MnWO4 indicate the presence of W6+=O2− in both the compounds.11,47

361

A small hump appeared at 532.2 eV for O 1s of Mn-HPT sample due to the lattice water

362

molecules.11 These observations are consistent with the Raman analysis where a strong signal for

363

W−O bond at 959 cm−1 (Figure 2c) and relatively weak signal for W−H2O bond at 360 cm−1

364

have been detected. No indication for reduced tungsten cations (W5+ or W4+) has been found in

W

was

found

to

be

almost

stoichiometric,


8:12,

as

in

the

formula

Page 17 of 35

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365

any of the compounds, which is indicative of fully oxidized state of tungsten atom and the

366

yellow coloration of the samples.

367

3.8. Growth Mechanism of the Rigid and Hollow Microsphere

368

To understand the growth mechanism of the MnWO4 rigid microspheres we studied

369

different hydrothermal samples by FESEM. The detail temperature and time dependent analyses

370

were given in the Supporting Information, Figure S3. From the analysis it is clear that the

371

microspheres are formed at 3 h of hydrothermal treatment at 180 oC and are mainly made of

372

many thin flakes which are attached themselves from core to surface, however, the packing

373

efficiency is poor. The microspheres formed after 6 h are relatively tight and additional

374

deposition of MnWO4 nanoparticle is observed at the surface. Nevertheless, size does not change

375

significantly. After 10 h, microspheres of MnWO4 with diameter 5 − 8 µm are fully formed and

376

also the irregularity in the shape and size decreases. Again at lower temperature, 150 oC after 10

377

h the microspheres are formed but loosely compact and irregular in shape (Figure S3, Supporting

378

Information). However, at further lower temperature, 120-110 oC, the phase changes into the

379

kinetically stable HPT under the hydrothermal condition as can be seen in Figure 2d. The TEM

380

images of 3 h and 10 h hydrothermally treated samples were shown in the Supporting

381

Information (Figure S4) which confirms that the packing density is weak at lesser reaction time.

382

From this analysis we conclude that hydrothermal temperature and time have profound effect on

383

the formation of densely packed rigid microsphere of MnWO4. Additionally, by varying the

384

molar ratio of citric acid to tungstate we have already shown that citric acid has an adhesive role

385

in the formation of the MnWO4 rigid microspheres. Thus from the above analysis we proposed a

386

plausible formation mechanism of the rigid microsphere which is schematically shown in Figure

387

7. Formation of citrate-tungstate complex in aqueous medium reduces the chance of the



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Page 18 of 35

388

formation of MnWO4 before hydrothermal treatment but provide the necessary source of

389

tungstate ion at the nucleation stage. Thus, first the tiny nuclei of MnWO4 are formed in the

390

solution, which are then agglomerated further in the presence of citric acid to form flakes or

391

disks or plates of MnWO4 building blocks and set one upon one from center-to-surface to form

392

the rigid microsphere structure. The packing efficiency of the microsphere under the

393

hydrothermal condition strongly depends on the temperature, time and concentration of citric

394

acid. To investigate the mechanism of the formation of the hollow microsphere, we also made an

395

intensive FESEM study by varying the temperature and time of the solvothermal condition. The

396

FESEM and TEM images of the samples were shown in the Supporting Information (Figure S5

397

and S6). A complete mechanistic description of the formation of this unique core-shell like

398

hollow microsphere was reported in our previous study,11 from where we proposed the formation

399

mechanism of the hollow microsphere under the solvothermal condition as shown in Figure 7.

400

Which interesting here, is that after 8 h, no significant growth of the HPT hollow spheres was

401

noticed instead some deposition of particles was noticed at the surface of the hollow spheres.

402

XRD analysis of this sample (Figure 1b) shows that the phase starts to change into MnWO4 from

403

this stage and thus the newly deposited materials are considered to occur by dissolution of the

404

HPT phase and re-crystallization into MnWO4 phase. After 12 h, formation and deposition of

405

MnWO4 continues in expense of the HPT phase, and after 24 h, the phase completely changes

406

into wolframite MnWO4 and the hollow morphology was completely lost.

407

3.10. Optimization of the Reaction Parameters for the Formation of Two Polymorphs

408

In order to determine the optimum reaction conditions to obtain either of the two

409

crystallographic phases in hydrothermal and solvothermal condition, the following parameters,

410

e.g. temperature, time, pH and concentration of the precursors, were optimized by a standard


Page 19 of 35

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411

Response Surface Methodology (RSM) design,48,49,50,51 named Box-Behnken Design (BBD).51

412

The detail analysis process was given in the Supporting Information (Figure S7); the optimum

413

values extracted from the analysis are presented in Table 1.

414

3.11. Magnetic Property analyses

415

Field and temperature dependent magnetization analyses of both the samples were carried

416

out in the temperature range 3 - 300 K and magnetic field of ± 5 T. As shown in Figure 8a, the

417

M-H curve for MnWO4 sample shows typical paramagnetic behavior at temperature range 100 -

418

300 K, while small amount of hysteresis was observed at sufficiently low temperature (as shown

419

in the inset). MnWO4 is well-known for its multiple antiferromagnetic phase transitions below 13

420

K.14,52,53,54 and Mn2+ is known to be moderately frustrated antiferromagnetic system.14 Thus the

421

high temperature paramagnetic behavior is expected for the independent Mn2+ (high-spin; S =

422

5/2) ions present in the lattice, while the low temperature hysteresis behavior is the manifestation

423

of weak ferromagnetic ordering in the probably arising due to the small degree of spin-canting

424

behavior associated with the antiferromagnetic material and/or some uncompensated surface

425

spins in the sample.16 The temperature dependent magnetization measurement for the MnWO4

426

sample carried out under zero field cooled (ZFC) and field cooled (FC) conditions in the applied

427

field of 10 kOe was shown in Figure 8b. It can be seen that the magnetization initially increases

428

much slowly with decreasing temperature but shows noticeable increase below 50 K. The FC

429

and ZFC curve run almost parallel up to a maximum temperature of 12.6 K, below which there is

430

a small divergence between FC and ZFC curve was observed. The sharp maxima at 12.6 K is the

431

onset of antiferromagnetic phase transition in the material,54,55 which in contrast to bulk MnWO4,

432

does not show multiple characteristic, although there are evidences for stabilization of different

433

magnetic phases at different applied fields. The invers susceptibility vs. temperature plot as



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Page 20 of 35

434

shown in the inset of Figure 8b is well described by the Curie-Weiss type paramagnetic

435

behavior;54 1/χ(T) = (T-θ)/C, for temperature range 300 - 50 K. The Curie−Weiss law is

436

observed down to temperatures lower than the absolute Weiss temperature (θ(0)), obtained from

437

the fitting analysis ∣θ(0)∣ = 60(±2) K, indicating the presence of spin frustration in sample.54 The

438

so-called frustration factor54,56 (∣θ(0)∣/TN) = 4.75, is found to be nearly same as that of the bulk

439

MnWO414 implying the absence of finite size effect in the sample. From the value of the fitting

440

parameter C = 0.0084 (emu-K/g-Oe) we calculate the effective magnetic moment (

441

individual Mn2+ magnetic ion (= 4.6

442

is susceptibility per gram,

,57 where,

) according to the formula,

is number of magnetic ions/gram,

) for

is the Bohr magneton,

443

is Boltzmann’s constant,

444

weight percent and atomic weight of the magnetic ion. This value is slightly lower than the

445

single Mn2+ (d5, high spin) ion, 5.2-5.9

446

results from the presence of small extent of spin frustrations in the magnetic site.58

temperature in K,

,

and

are the effective magnetic moment,

. The lowering of magnetic moment must be therefore

447

On the other hand, the magnetization data of the Mn-HPT sample reveals relatively

448

simpler nature of the magnetic interaction in the compound. As shown in Figure 8c and the inset,

449

the magnetization curves (M-H) at different temperatures (300 - 5 K) show typical paramagnetic

450

nature of the sample. Because of the paramagnetic property, the Curie law, 1/χ(T) = T/C is used

451

to fit the experimental 1/χ vs. T data of the compound.59,60 It can be found from Figure 8d that

452

the theoretical fittings are in excellent agreement with the experimental results for C = 0.042

453

(emu-K/g-Oe). From this fitting value we calculate the effective magnetic moment (

454

individual Mn2+ magnetic ion (= 5.4

455

agreement with single Mn2+ (d5, high spin) ions, implying no significant magnetic interaction

456

between the magnetic centers.

) for

) according to the above formula which is in closes


Page 21 of 35

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457


4. CONCLUSIONS

458

In conclusion, we report a unique solvothermal approach to control both the phase and

459

the morphology of two different manganese tungstate based polymorphic phases namely,

460

MnWO4 and Mn-HPT. Different parameters such as, (i) ethanol content in the mixed solvent, (ii)

461

pH, (iii) citric acid concentration, (iv) temperature and (v) time, that control the formation of the

462

two crystallographic phases were identified and optimized in order to obtained the best

463

experimental condition for the synthesis of the two polymorphic phases. Two different growth

464

mechanisms, both under the light of classical nucleation theory, have been proposed for the two

465

types of microsphere which further help us to understand the stability of the samples under the

466

said solvothermal conditions. Magnetic study of the powder MnWO4 sample shows

467

characteristic antiferromagnetic phase transition (T2) at 12.6 K with the indication of spin

468

frustration, suggesting significant interaction between the magnetic centers, while the powder

469

Mn-HPT sample show typical paramagnetic behavior throughout the measuring temperature.

470

■ ASSOCIATED CONTENT

471

Supporting Information

472

The Supporting Information is available free of charge on the ACS Publications website

473 474 475 476 477

■ AUTHORS INFORMATION *Corresponding Author: Email: [email protected] (KKC); Fax: +91-332414-6007. Tel: +919433389445.

■ ACKNOWLEDGMENTS The authors would like to thank the University Grants Commission (UGC), the Government of

478

India for financial assistance under the ‘University with potential for excellence (UPE II)’

479

scheme. K. B acknowledges the cooperation received from Dr. Suman K. Mishra, NML,

480

Jamshedpur, India for the Raman spectra measurements.



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481

■ NOTES AND REFERENCES:

482

(1) Cazzanelli, E.; Vinegoni, C.; Mariotto, G.; Kuzmin,à, A.; Puransà, J. J. Solid State

483 484 485 486 487 488 489

Chem.1999, 143, 24−32. (2) Griesser, U. J. Hilfiker R (ed). In Polymorphism: in the Pharmaceutical Industry, Wiley, Weinheim, 2006, Chapter 8, 211. (3) Braga, D.; Maini, L.; de Sanctis, G.; Rubini, K.; Grepioni, F.; Chierotti, MR.; Gobetto, R. Chem. Eur. J. 2003, 9, 5538−5548. (4)Vishweshwar, P.; McMahon, J. A.; Peterson, M. L.; Hickey, M. B.; Shattock, T. R.; Zaworotko, M. J. Chem. Commun. 2005, 4601−4603.

490

(5) Jeannin, Y. P. Chem. Rev. 1998, 98, 51−76.

491

(6) Miras, H. N.; Yan, J.; Long, D.-L.; Cronin, L. Chem. Soc. Rev. 2012, 41, 7403−7430.

492

(7) Long, D. L.; Tsunashima, R.; Cronin, L. Angew. Chem. Int. Ed. 2010, 49, 1736−1758.

493

(8) Dexter, D. D.; Silverton, J. V. J. Am. Chem. Soc. 1968, 90, 3589−3590.

494

(9) Wu, C. D.; Lu, C. Z.; Zhuang, H. H.; Huang, J. S. J. Am. Chem. Soc. 2002, 124, 3836−3837

495

(10) G nter, J. . Schmalle, H. W. Dubler, E. Solid State Ionics. 1990, 43, 85−92.

496

(11) Bhattacharjee, K.; Chattopadhyay, K. K.; Das, G. C. J. Phys. Chem. C. 2015, 119,

497

1536−1547.

498

(12) Hyde, B. G.; Andersson S. In Inorganic Crystal Structure; John Wiley & Sons, New York,

499

1989.

500

(13) Heyer, O.; Hollmann, N.; Klassen, I.; Jodlauk, S.; Bohat´y, L.; Becker, P.; Mydosh, J. A.;

501

Lorenz, T.; Khomskii, D. J. Phys.: Condens. Matter. 2006, 18, L471–L475.

502

(14) Arkenbout, A. H.; Palstra, T. T. M.; Siegrist, T.; Kimura, T. Phys. Rev. B. 2006, 74,

503

184431(1-7).


Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


504

(15) Zhang, L.; Lu, C.; Wang, Y.; Cheng, Y. Mater. Chem. Phys. 2007, 103, 433–436.

505

(16) Zhou,Y-X.; Zhang, Q.; Gong, J-Y.; Yu, S-H. J. Phys. Chem. C. 2008, 112, 13383–13389.

506 507

(17) Nguyen, T-D.; Mrabet, D.; Vu, T-T-D.; Dinh, C-T.; Do, T-O. Cryst. Eng. Comm. 2011, 13, 1450–1460.

508

(18) Chung, S-Y.; Kim, Y-M.; Kim, J-G.; Kim, Y-J. Nat. Phys. 2008, 5, 68−73.

509 510

(19) Bergerhoff, G.; Berndt, M.; Brandenburg, K. J. Res. Natl. Inst. Stand. Technol. 1996, 101, 221−225.

511

(20) Brandenburg, K.; Berndt, M. J. Appl. Crystallogr. 1999, 32, 1028–1029.

512

(21) Becker, R.; Döring, W. Ann. Phys. 1935, 24, 719−752.

513

(22) Frenkel, J. J. Chem. Phys. 1939, 7, 200−201.

514

(23) Volmer, M.; Weber, A. Z. Phys. Chem., Stochiom. Verw. 1926, 119, 277−301

515

(24) Gibbs, J. W. Trans. Connect. Acad. Sci. 1876, 3, 108−248.

516

(25) Gibbs, J. W. Trans. Connect. Acad. Sci. 1877, 3, 343−524.

517

(26) Farkas, L. A. Z. Phys. Chem. Stochiom. Verw. 1927, 125, 236−242.

518

(27) Denis, G.; Antje, V.; Helmut, C. Science. 2008, 322, 1819−1822.

519

(28) Hailu, F.; Baohong, G.; Guangming, J.; Matthew, Z. Y.; Zhongbiao, W. Cryst. Growth Des.

520

2012, 12, 1388−1394.

521

(29) Chen, H. I.; Chang, H. Y. Colloids and Surfaces A. 2004, 242, 61−69.

522

(30) Alexandra N. Proc. Natl. Acad. Sci. 2004, 101, 12096−12101.

523

(31) Tian, H. Z.; Xiang, Y. L. J. Am. Chem. Soc. 2007, 129, 13520−13526.

524

(32) Chen, C.; Cook, O.; Nicholson, C. E.; Cooper, S. J. Cryst. Growth Des. 2011, 11, 2228–

525

2237.

526

(33) Sand, K. K.; Rodriguez-Blanco, J. D.; Makovicky, E.; Benning, L. G.; Stipp, S. L. S. Cryst.

527

Growth Des. 2012, 12, 842−853. 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

528

(34) Manoli, F.; Dalas, E. J. Cryst. Growth. 2000, 218, 359−364.

529

(35) Seo, K-S.; Han, C.; Wee, J-H.; Park, J-K.; Ahn, J-W. J. Cryst. Growth. 2005, 276, 680−687.

530

(36) Gu, Z.; Ma, Y.; Zhai, T.; Gao, B.; Yang, W.; Yao, J. Chem. Eur. J. 2006, 12, 7717−7723.

531

(37) Cruywagen, J. J.; Krüger, L.; Rohwer, E. A. J. Chem. Soc. Dalton Trans. 1997, 1925−1930.

532

(38) Cruywagen, J. J.; Krüger, L.; Rohwer, E. A. J. Chem. Soc. Dalton Trans., 1991, 1727−1731.

533

(39) Simon, N. G. K. Y.; Gulari, E. Polyhedron. 1984, 3, 8, 1001−1011.

534

(40) Aveston, J. Inorganic Chemistry. 1964, 3, 7, 981−986.

535

(41) Noorduin, W. L.; Vlieg, E.; Kellogg, R. M.; Kaptein, B. Angew. Chem. Int. Ed. 2009, 48,

536

9600−9606.

537

(42) Madras, G.; J. McCoy, B. Cryst. Growth Des. 2003, 3, 981−990.

538

(43) Yang, J.; Li, W.; Li, J.; Sun, D.; Chen, Q. J. Mater. Chem. 2012, 22, 17744−17752.

539

(44) Matzapetakis, M.; Raptopoulou, C. P.; Tsohos, A.; Papaefthymiou, V.; Moon, N.;

540

Salifoglou, A. J. Am. Chem. Soc. 1998, 120, 13266−13267.

541

(45) Zhang, K.; Liang, J.; Wang, S.; Liu, J.; Ren, K.; Zheng, X.; Luo, H.; Peng, Y.; Zou, X.; Bo,

542

X.; Li, J.; Yu, X. Cryst. Growth Des. 2012, 12, 793−803.

543

(46) Ross-Medgaarden, E. I.; Wachs, I. E. J. Phys. Chem. C. 2007, 111, 15089−15099.

544

(47) Nguyen, T. D.; Mrabet, D.; Vu, T. T. D.; Dinh, C. T.; Do, T. O. Cryst. Eng. Comm. 2011,

545

13, 1450−1460.

546

(48) Edginton, A.; Sheridan, P.; Boermans, H.; Thompson, D.; Holt, J.; Stephenson, G. Arch.

547

Environ. Contam. Toxicol. 2004, 46, 216−223.

548

(49) Shieh, C. J.; Lai, Y. F. J. Agric. Food Chem. 2000, 48, 1124−1128.

549

(50) McCarron, P. A.; Woolfson, A. D.; Keating, S. M. Int. J. Pharm. 1999, 193, 37−47.

550

(51) Tang, H-Y.; Xiao, Q-G.; Xu, H-B.; Zhang, Y. Org. Process Res. Dev. 2013, 17, 632−640.


Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

551 552 553 554 555 556 557 558


(52) Chaudhury, R. P.; Lorenz, B.; Wang, Y. Q.; Y. Sun, Y.; Chu, C. W. Phys. Rev. B. 2008, 77, 104406(1−6). (53) Ye, F.; Fishman, R. S.; Fernandez-Baca, J. A.; Podlesnyak, A. A.; Ehlers, G.; Mook, H. A.; Wang, Y.; Lorenz, B.; Chu, C. W. Phys. Rev. B. 2011, 83, 140401(R)(1−4). (54) Meddar, L.; Josse, M.; Maglione, M.; Guiet, A.; La, C.; Deniard, P.; Decourt, R. Lee, C.; Tian, C.; Jobic, S.; Whangbo, M-H.; Payen. C. Chem. Mater. 2012, 24, 353−360. (55) Yang, J.; Chen, J.; Fang, Y.; Han, Z. D.; Yan, S. M.; Qian, B.; Jiang, X. F.; Wang, D. H.; Du, Y. W. RSC Adv. 2016, 6, 3219–3223.

559

(56) Dachs, H. Solid State Commun. 1969, 7, 1015−1017.

560

(57) Parks, G. A.; Akhtar, S. American Minerologist. 1968, 53, 406−415.

561

(58) Clemens, O.; Rohrer, J.; Nénert, G. Dalton Trans. 2016, 45,156–171.

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(59) Zhang, X.; Dou, J.; Wang, D.; Zhang, Y.; Zhou, Y.; Li, R.; Yan, S.; Ni, Z.; Jiang, J. Cryst.

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Growth Des. 2007, 7, 1699−1705. (60) Zhang, X.; Wang, D.; Dou, J.; Yan, S.; Yao, X.; Jiang, J. Inorg. Chem. 2006, 45, 10629−10635.

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Figure 1: FESEM images of samples synthesized under different solvothermal conditions at 180

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o

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65:35, (e) 50:50. (f) XRD patterns of the same samples.

C for 6 h at pH 5, with varying water:ethanol percentage (a) Pure water (b) 85:15, (c) 75:25, (d)

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Figure 2: XRD patterns of samples prepared under 50:50 water:ethanol solvothermal condition

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at 180 oC for (a) different times and for 6 h at (b) different temperature as mentioned inside the

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graphs. XRD patterns of samples prepared under hydrothermal condition for 6 h at (c) different

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temperature, and at 180 oC for (d) different times as mentioned inside the graphs.

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Figure 3: (a) XRD patterns of different hydrothermal samples prepared in presence of different

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citric acid to tungstate ratio. Inset of each pattern shows the corresponding FESEM images of the

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samples. XRD patterns of samples prepared under 50/50 water/ethanol solvent at different (b) pH

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and different (c) precursor concentration.

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Figure 4: Schematic representation of energetic of two different polymorphs as a function of

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reaction time (not to scale). Differences in activation energy and time dependent phase stability

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of the two polymorphs are shown.

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Figure 5: (a) FTIR and (b) Raman spectra of two polymorphic phases.

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Figure 6: EDS spectrum of (a) Mn-HPT and (b) MnWO4 samples. XPS analyses of (c) Mn-HPT

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and (d) MnWO4 samples.

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Figure 7: Growth mechanism of the rigid and hollow microspheres.

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Figure 7: Magnetic properties of two samples. For MnWO4: (a) Comparison of M(H) plots at

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three different temperature. Inset shows the M(H) plot at lowest temperature. (b) Chi (χ = M/H)

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plot under ZFC and FC condition at 10 kOe field. Inset shows the Curie-Weiss fitting of the

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inverse chi plot.; For Mn-HPT: (c) Comparison of M(H) plots at three different temperature.

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Inset shows the M(H) plot at lowest temperature. (d) Chi (χ = M/H) plot under ZFC and FC

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condition at 10 kOe field. Inset shows the Curie fitting of the inverse chi plot.

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Table 1: Optimized values for the reaction parameters predicted by RSM.

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Phase*

Temperature (oC) Time (h)

pH

Conc. of precursors (mole)

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MnWO4

150

8

3

0.01

Mn-HPT

150

4

3

0.0358

709 710

*each phase was synthesized in optimal solvent condition. MnWO4 in hydrothermal and Mn-

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HPT in 50/50 water-ethanol mixture.

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For Table of Contents Use only

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Solvent Dependent Phase Transition Between Two Polymorphic Phases of

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Manganese‒Tungstate: From Rigid to Hollow Microsphere.

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Kaustav Bhattacharjee†, Satya Prakash Pati§, Gopes C. Das† and Kalyan K. Chattopadhyay*‡

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A unique solvent dependent crystallization behavior and polymorphic relationship between two manganese-tungstate based polycrystalline phases and their distinct morphological evolutions under different solvothermal conditions was exemplified in detail. Both the phases, has remarkable applications in various field of science and technology.

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Solvent Dependent Phase Transition between Two Polymorphic

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