Chapter 7

RARE EARTH (Sm, Ndr Pr) MIXED BARIUM MOLYBDATE

SINGLE CRYSTALS - IN GENERAL

7.1 Introduction

When organic matter derived from many living

specimens is calcined, the ashes contain rare earths at a

concentration of a few parts per million . This

phenomenon is widespread but it is not known whether these

trace elements play an essential role in the living

process. The relative abundance of rare earths, elements

and isotopes is of great interest to the cosmologists in

developing the theories of how the universe, galaxies and

stars formed, how the various celestial bodies obtain

their energy and how they decay in cosmic time.

Applications of rare earths include use in hydrogen

storage(LaNi5), computer memory elements and solid state

devices. A considerable amount of information has been

reported regarding the magnetic behaviour of rare earth

compounds[234-2441. In the field of nuclear engineering,

rare earths are being used as control rods, atomic

batteries, shielding magnets and container materials.

Didymium (a mixture of praseodymium and neodymium) has

found application in the glass and ceramic industry for

decolorizing glasses, getting rid of glare, and for

ceramic coating[234, 2451. Rare earth (mixed) materials,

play a vital role in laser materials, fluorescent screens

such as television screens, intense sources of light such

as street lights, and, perhaps, in the future, they will

help in the invention of intense panel lighting, where an

entire wall will fluoresce brilliantly in any shade

desired by the application of an electric potential. It is

therefore not surprising that rare earths have been

receiving a lot of attention in recent years and attempts

are all the time being made to chemically combine rare

earths with different elements to form new compounds of

varying stochiometry.

Rare earth molybdates are finding immense use as

laser materials[246-2481. Trivalent rare earth molybdates

with the formula Ln2(Mo0 ) whlch show considerable 4

utility in laser studies were comprehensively studied by

Nassau et a1.[19]. The profusion of structural types and

the wide range of physical properties gave rise to several

specific studies resulting in a basic contribution to the

knowledge of these materials[249-2571. At the same time

the czochralski growing process was improved to enable

the production of high optical quality crystals[258].

Petrosyan et a1.[259] using physic0 chemical analysis have

constructed the phase diagram of Na2 Mo 04-Ba Mo O4 for

the growth of Ba Mo O4 single crystals. Packter[260]

could obtain Ba Mo O4 crystals as tetragonal bipyramidal

crystals by the precipitation of alkaline-earth metal

molybdate powders from neutral aqueous solution. Flux

growth of calcium molybdate[261] single crystals, low

energy electronic structure of intermediate valence

'golden' Sms[2621, the growth of Bi2(Mo 04)3 by

czochralski method[263], the synthesis and physic0

chemical properties of Ba Mo 04-C2 (Mo 04)3[2641 have

already been reported. The synthesis and phase +

transitions of double molybdates M4Cu (Mo 04)3 (M = Cs,Pb,

K ) [265], the growth of tetragonal Na Bi (Mo O4I2 12661

were also reported.

In this part of the thesis the author tried to grow

rare earth (Sm, Pr, Nd) mixed barium molybdate single

crystals in gel which might have important scientific and

technological uses, as in the case of the molybdates

mentioned above. In this chapter, the procedure of growth,

the characteristics and formula in general, of all

molybdates of this type are discussed.

7.2. Discussion

7.2.1 Growth and effect of different parameters on the

growth of the crystals

The procedure adopted in the present study to grow

rare earth (Sm I Pr, Nd) mixed barium molybdate crystals

was the same. The outer electrolyte- a mixture of

B ~ ( N o ~ ) ~ (barium chloride) and respective rare earth

nitrate-was poured over the gel. On diffusion, colloidal

precipitate was formed and dissolution of it led to

crystallization. The salient features of crystal growth

in gels when one of the nutrients is a colloidal

precipitate[l53] and another variation in which the

colloidal precipitate of a compound is transformed into

single crystals[267] have been reported.

Colloidal solutions, sols, hold an intermediate place

between true solutions and suspensions. Sols can be

obtained either by the combination of molecules or ions of

a solute into aggregates, or by dispersing large

particles. Svedberg[268] has made a systematic and

exhaustive study of the various methods of preparation of

colloids and their properties. The stability and

coagulation of colloidal systems are of great practical

importance in geology, agriculture and biology. The

colloidal systems are thermodynamically and aggregatively

unstable owing to an interface between particles and the

dispersion medium. Coagulation is the most prominent

mechanism through which a sol transforms into a more

stable state. The nature of various factors on

coagulation was studied by Schulze[269], Hardyr270-2711,

Trauber2721 Mukopadhyayal2731, Burton et dl. [274] etc.

Freundlich and Nathansohn[275,276] showed that mixing of

pairs of certain like-charged colloids may result in

mutual precipitation. The importance of pH in the mutual

precipitation of proteins was demonstrated by Michaelis

and David shon[277].

When sodium meta silicate is acidified, it tends to

polymerize. The role of silica gel on the growth of the

crystals from precipitation is very significant. Silica

gel is a polymerised form of silicic acid and it is a

fine-pored adsorbent. Adsorption is a surface phenomena

and it is very important in colloidal systems which have

large surfaces. The adsorbent property of silica gel is

on account of its wider capillaries and its large surface

area of the order of 400-500 mL/~[278]. The substance or

ion adsorbed on the surface of the adsorbent is directly

proportional to the total surface of the adsorbent. The

particles are strongly adsorbed and the aggregation of a

large number of primary particles have an enormous

surface area which therefore adsorb ions, impurities and

various extra ions, substances from the solution, leading

to a rapid growth of these crystallites in spherical form.

The growth kinetics of the crystal in this case are

controlled by the diffusion of the solute from dissolving

particles to the growing particles, rather than by the

concentration gradient of diffusing outer electrolyte. It

is inherent in all systems where there are dispersed

particles of varying size having some solubility in the

surrounding medium[279]. The smaller particles tend to

dissolve and precipitate in large particles and, in the

final state, the system tends to form a single crystal.

This explains the growth of large crystals on the

dissolution of the fine crystals in the precipitate.

Directly or indirectly acidity has a role in the

dissolution of the precipitate. The acidity present in

the gel is free for reaction in the lower portions of the

gel, ie., below the precipitate. So the partial

dissolution in this case is started from the bottom of the

precipitate towards the top region. This process of

partial dissolution influence nucleation, leading to

crystallization. A detailed discussion of

crystallization from colloidal precipitate is given in

chapter 11.

It was observed that, as in the case of the two

component system,the depth of the precipitation and the

partial dissolution [Figs. (4. 3 , 4 ) , (5. 3 , 4 ) & ( 6 . 1 1 1

was directly proportional to the concentration of the

outer electrolyte and inversely proportional to the inner

electrolyte in this multicomponent system.

As the concentration of the outer electrolyte

increased the number of ions diffused also increased.

This increased the depth of precipitation. As the ions of

the inner electrolyte increased,the rate of diffusion of

the outer ions slackened. Therefore the depth of

precipitation decreased. Lower pH values are favourable

for thicker precipitate and dissolution regions, as in

figs. (4.5,6), (5.6) & (6.2). As the pH of the gel

decreased, the pore size increased and paved the way for

greater diffusion.

7.2.2 Morphology

For describing properly the morphology, one must know

the basic appearance of a crystal in the ideal situation

in which external factors are absent or do not play a

role. The majority of the crystals were bipyramidal,

octachedral in shape. The growth layers found in some

crystals may have been due to the local variation of

concentration gradient in the system. The single crystals

were observed at the bottom of the precipitate region and

just below it where the diffusion rate is less. At higher

pH values near to the interface multiple crystals were

observed. The curved nature of the protruding crystals

(Fig 4.7) are evidence of the higher concentration of the

nutrients. In these region the diffusion rate is greater.

At lower pH values (ie between 5-31 no

crystallization was observed, only Liesegang rings were

found. These rings were clearly spaced disc patterns. It

was found that greater acidity in the gel is not suitable

for the nucleation of these crystals.

At higher pH values, near to the interface along with

the multiple crystals spherulites were observed (Fig.

6.11). Spherulites were not observed at bottom region

where the diffusion rate is less. Spherulites were formed

due to the fast diffusion of the nutrients.

According to Buckley[280] impurity concentration in a

crystalline phase may be one of the reasons for

spherulitic crystallization. The presence of small

quantities of free acid or alkali which should make a very

material addition to the 'H' or 'OH' ion concentration,

does not appear to make any difference to the large

majority of crystals[280]. A very small percentage of

other impurities in H N 0 3 in the range 0.1 x to 0.1 x

10 -* % can accentuate spherulitic crystallization on

account of their impurity action[280].

A considerable number of minerals frequently exhibit

a spherulitic habit. 'Spherulite', spherical in form,

radial or concentric in structure and closely related

aggregates are called, reniform, botryoidal, pisolytic,

mammillary and globular[281]. It is generally observed

that the presence of a gel appears to be highly favourable

for the growth of artificial spherulites. Spherulites of

volcanic rocks as well as those found in slowly cooled

artificial glasses must have grown in media of high

viscosity. This suggests the possibility that viscosity

has some effect somewhat similar to that of the gels in

producing spherulitic habit.

Spherulitic morphology was observed by many authors

in phenyl benzoate[282], praseodymium tartrate[283,284]

etc.

7.2.3. Identification and characterization

The X-ray data of certain molybdates like R2(Mo O4I3

1285, 2861 [where R2 = LA2, Sm2, Dy2, yB2, EE2, Eu2, Ho2,

Lu2, PR2, GD2, ER2, DYTB, DYGD, Np2, TB2TPl2], B A Mo

04[287], CA Mo 04[288], NDK (Mo 04)2[289J have al-ready

been reported. X-ray spectroscopy provides much useful

information about the structure of matter[290,291]. The

X-ray data obtained prove the crystallinity of the

crystals. The lattice parameters obtained are shown

below.

Lattice parameters

Samarium Neodymium barium barium molybdate rnolybdate

Praseodymium barium molybdate

These crystals belong to the orthorhombic system.

The Id' values obtained were characteristic of each

crystal.

The infrared absorption spectroscopy is based on the

transition between the two vibrational levels of the

molecules in the electronic ground state and are usually

observed as absorption spectra. From a quantum mechanical

point of view, a vibration is active in the infrared

spectrum if the dipole moment of the molecule is changed

during the vibration. The IR obtained as in figures

(4.12), (5.14) & (6.13) are indicative of molybdate

skeleton. An alternate and simpler method than band

spectra for obtaining vibrational and rotational

frequencies of molecules is through observation of the

Raman effect. From IR and Raman spectra there was no

indication of water molecules in these crystals.

The assignments of IR and Raman are based on the

analyses of other molybdates[292-2951 on comparative

basis. The frequencies of these modes compared favourably

to those described by Barraclough et a1.[2961 in their

studies of molybdenum-oxygen bonds. The interactions

between the molybdate ions led to the formation of

dimetric Mo2 O8 systems containing oxygen bridge bonds.

Spectra data and band assignments of the rare earth

barium molybdate mixed crystals are given in the table.

0.1).

Absorption measurements based upon ultraviolet or

visible radiation find widespread application in the

qualitative and quantitative determination of molecular

species. The ions of most lanthanide and actinide

elements are absrbed in the ultraviolet and visible

regions. In distinct contrast to the behaviour of most

inorganic and organic absorbers, their spectra consist of

narrow, well defined and characteristic absorption peaks,

which are little affected by the type of ligand associated

with the metal ion. The transitions responsible for

Table 7.1 -1

SPECTRA DATA ( ~ n ) AND BAND ASSIGNMENTS OF MIXED CRYSTALS OF RARE EARTB (Sm , Nd 6 Pr) BARIUM MOLYBDATE

Samarium barium Neodymium Praseodymium Assign- molybdate barium molybdate barium molybdate ments -------------- ---------------- ----------------

IR Raman IR Raman IR Raman

890vs ILMo04 (Sym. )

830s $3 MOO 4 (asystr . )

810s MOO 785s

660w >Mo-0-Mo

460w 373m MOO+

MOO 4 d4(asy .bend) MOO 4 Rot. Trans. Trans. Trans. Trans. Trans.

vs = very strong; s = strong; m = medium; w = weak; vw = very weak; v w = very very weak.

absorption by elements of the lanthanide series appear to

involve the various energy levels of 4f electrons, while

it is the 5f electrons, of the actinide series that

interact with radiation. These inner orbitals are largely

screened from external influences by electrons occupying

orbitals with higher principal quantum numbers. AS a

consequence the bands are narrow and relatively unaffected

by the nature of the solvent or the species bonded by the

outer electrons. The peaks obtained at 401.5, 521.60 and

443.5 X AO in the uv-visible spectra correspond to the

samarium, neodymium and praseodymium, respectively present

in the crystals. The peaks corresponding to barium also

were observed.

To establish the elemental incorporation in the

crystals, EDAX work was taken up. The data relating to

the integrated counts of x-ray photo electrons taken for a

definte time interval was helpful in getting the

quantitative analysis. The presence of different

constituents related to La , L y , Lr, Koc , K p lines

obtained, which agree with the EDAX international chart

are shown below.

Samarium Neodymium Praseodymium Barium Molybdenum

Kev Kev Kev Kev Kev

According to Lange the solubility of Ba Mo O4 is

58mg/litre (58 ppm). The solubility of barium is

0.00212g in lOOml at loo0 C. Due to high insolubility of

barium, the major part is played by barium and then by

molybdenum in the crystal. Rare earth incorporation is

less compared with other components.

XRF analysis was utilised as a qualitative method,

the crystals were pressed at 15000 psi and made into 10 mm

diameter discs and applied with mylor film support. LiF

220 X-ray crystal was used. The peaks at different 2 8

values were noted and in each case presence of the

constituent elements was confirmed with the following

Values.

Elements

From the XRF plots obtained the dominance of Mo and

Ba over rare earth can be observed. These data support

the EDAX results.

Thermogravimetric curves are characteristic for a

given compound or system because of the unique sequence of

physic0 chemical reactions which occur over definite

temperature ranges. These crystals are found to be

volatile around a temperature of 400° C. In the DSC

analysis no particular peak was obtained. There is no

evidence of water of crystallization in the crystals and

is supported by IR and Raman analyses.

7.3. Conclusion

From the above discussions it is evident that rare

earth (Sm, Pr, Nd) nixed barium molybdates have a common

procedure of growth and a similarity in their

characteristic properties. The percentage composition can

be slightly varied by changing the concentration of

nutrients[329-3311.

The property that one rare earth ion can readily be

substituted for another in the lattice of almost any rare

earth crystal with very little strain resulted in -the

development of substitutional element formation. In the

present study the rare earth formation in the crystals may

be by substitutional basis or occupying in the

interstices.

It may also be concluded that by substituting other

lanthanides in the place of r.are earth, similar crystals

discussed in this thesis can be grown which are assumed

to be useful in optical, acousto optical studies. The

growth of certain molybdates like K2 Bi (Mo 04)4 ~ d ~ +

single crystals[297], potassium gadolinium molybdate

x-, Gd (Mo 04)21[29 I , do~ble molybdates K 2 Nd (Mo 04)4, Kg Bi (Mo 04)4, Rb5 Gd (Mo 04)4[2991 and Cesium lanthanum

nitrate CS2 [La NO^)^. (H20)21[300] have already been

reported. From all these data a general formula for rare

earth mixed barium molybdates may be derived as

Y1-x Bax (Mo O4In.