Chapter 1

GENERAL INTRODUCTION AND EXISTING INFORMATION OF

FWRE EARTH MIXED CRYSTALS OF BARIUM MOLYBDATES

AND .COPPER OXAtATES

1.1 Introduction

"Diamonds grow in India, some as big as a bean, some like a hazel nut. They are male and female and from the falling dew they multiply and bring forth small children".

(Sir John blandeville, 1360)

This charming sentence demonstrates vividly how

deeply medieval people were fascinated by the beauty of

natural crystals and how they tried to unravel the mystery

of the formation of crystals in nature. Just as

Descartes. Lord Doi, Bentley and Bhatta Nakaya were

fascinated by snowflakes, many have been attracted by the

beauty and mystery of natural crystals. Nature' s

storehouse of knowledge also reveals methods which can be

utilised by men for crystallization in laboratories.

At one time, natural specimens were the only source

of large, good crystals. Since these were mostly gems and

museun pieces, the workers ilere forced to find some other

means for artificial synthesis of ~rystals~especially, by

the development of solid state devices. Besides,

artificial synthesis enabled men to yet crystalline

materials that were purer and more closely packed. The

development of methods for growing large crystals

artificially also provided a source of material for

intensive scientific study which, in turn, led to major

developments in inorganic chemistry and solid state

physics. In one way or another, it has been possible to

develop the single crystals of a very large number of

materials in recent years, some with relative ease, and

others, after long and painstaking research. The

invention and use of frequency-controlled oscillators,

polarisers, infrared optical devices, transistors,

magnetic devices, masers, lasers etc. is stepping up the

demand for artificial crystals day by day.

The latest developments in the field like high-

temperature optical spectroscopy, high-voltage electron

microscopy, laser annealing techniques, new laser

techniques for fast and accurate measurements of growth

rate, computer solutions of problems in liquid flow, and

of heat and mass transfer etc. have caused immense

increase in our knowledge as to how crystals grow and have

transformed 'crystal growth' into a pure branch of

science. Now, technological progress in this branch of

science is such that it is possible to prepare pure,

defect-free, economically viable and shapely crystals [l]

which are preferred by scientists.

In the past few years there has been considerable

interest in growing various mixed crystals, the important

ones amongst these being, mixed cation garnets[2-41, rare

earth Na-Mo scheelites[51, alkali halides[6-91, certain

ferrites[lO,ll] and I1 to IV compounds[l2]. One thing

common to all of them is that they have been grown by some

high temperature method. The purpose here is to report

the growth of rare earth mixed crystals of molybdates and

oxalates in silica gels at room temperature.

1.2 Mixed crystals of rare earth barium molybdates

1.2.1 Existing information on molybdates

Certain existing information about the molybdates

will prove to be enlightening here. Single crystals of

barium molybdate have found increasing use as laser

material matrices, gamma radiation detectors in

scintillation counters and in other fields[13,14]. It is

used in electronic and optical equipment[l5] and also in

protective coatings[l6]. A transition metal molybdate

like nickel molybdate has interesting magnetic

properties[l7]. The magnetic properties of a number of

divalent transition metal molybdates like nickel

molybdate, cobalt molybdate, manganese molybdate have been

studied by Van Uitert et a1[18]. Trivalent rare earth

molybdates like lanthanum molybdate show cosiderable

utility in laser studies[l8]. It is also used in some

optical equipment on account of its double refraction

property[l9].

A single crystal of lead molybdate (PbMo04) is a

superior medium for acousto-optic devices because it has a

high acousto-optic figure of merit, low acoustic loss, low

optical loss in the region from 0.42 to 3.9 m and

favourable mechanical impedance for acoustic

matching[20-221, One of the devices is an acousto-optic

light deflector to be used as a random-access page

selector for holographic memory systems[23]. Recently,

lead molybdate single crystals[24] (PbMo04) were found to

be a promising medium of acousto-optic light

deflectors[25].

Bismuth molybdate Bi2Mo06, is a polar material, with

potential applications in pyroelectric, ferroelectric,

piezoelectric, electro optic and acousto-optic

devices[26].

Molybdates in general are used as pigments, chemical

reagents, corrosion inhibitors, laser materials,

ferroelectrics etc. Molybdates can be easily doped with

rare earth ions and are available in the form of single

crystals[27,28].

In the last few years a new group of laser active

materials, namely stoichiometric or "pure" active

crystals, have enlarged the class of known and already

applied doped laser materials[29,30]. At present the

stoichiometric laser crystals used are those of the rare

earth pentaphosphates or tetraphosphates (Eg. NdP5014,

P 0 Ndx Lal-x 5 14' NdxYl-x P5°14f LiNdP4012), all emitting

light in the infrared ( A = 1.05 m). The small size

crystals of (Pr P5 014) praseodymium pentaphosphate

obtained are of laser quality; thus superradiance as well

as laser action in these crystals are obtained in the

visible ( > = 637.4 nm) region.

Part of this thesis deals with rare earth mixed

barium molybdates viz. samarium barium molybdate,

neodymium barium molybdate and praseodymium barium

molybdate.

1.2.2 Chemistry of molybdates[31]

Molybdenum and tungsten have a wide variety of

stereochemistry in addition to the variety of oxidation

states and their chemistry is among the most complex of

the transition elements. Molybdenum and tungsten can give

a variety of complexes whose constitution is not

especially wsll known, with organic hydro 0x0 compounds

such as sugars, tartaric acid etc. Molybdenum occurs

chiefly as molybdenite,MoS2 and molybdates. Molybdenum

and tungsten are found in the 5th and 6th periods of the

periodic system. They play materially different parts in

the geochemical process of mineral formation and yield a

wide series of isomorphous mixtures in the course of

formation of minerals.

In general, normal molybdates contain discrete

tetrahedral ions of Moo4 2- and are insoluble in water

with the exception of ammonium, sodium, potassium,

rubidium, lithium, magnesium, beryllium and thallium

salts. But these salts dissolve readily in acids and form

polymolybdates in solution. An outstanding

characteristic of molybdatesis their strong tendency to

form condensed complex or polymeric anions of iso-

polymolybdates in acid solution. The normal molybdates

resemble, in genera1,the sulphates and,even more closely,

the normal chromates and tungstates.

1.2.3 Structure and symmetry

Anisodesmic structures contain some cations ( B )

usually of high charge and small size linked to its co-

ordinating anion by bonds of strength greater than one

half the charge of these anions. Common structures of A B 0 4

molybdates and tungstates[32] are scheelite and

wolframite. They are BX4 ionic compounds of anisodesmic

structures[33]. The most common BX4 groups with distorted

- - 2 - tetrahedral co-ordination are Moo4 , W04 , 104 etc.

Scheelite structured molybdates are distorted Moo4

tetrahedral where the oxygen co-ordination round the

cation is 8 fold and each 02- ion is linked to one Mo 6 +

and two cations. The wolframite structured crystals

consist of flattened Moo4 2 - tetrahetra and a deformed 2 -

octahedra of 0 ions co-ordinating cations. ie., a 6-fold

oxygen co-ordination is round the cation.

The scheelite structure of AB04 molybdate crystal, is

a cubic close-packed array of A ~ + and B04 2 - units which

are ordered. Crystals of this class belong to the space

group C4h6(141,a)[34,35]. They possess a tetragonal axis

z perpendicular to an equatorial plane of symmetry[36].

This structure is adopted by a number of oxy-salts XY04

including NaI04, KI04, CaMo04, CaW04 and the analogous

compounds of Ba, Pb and KRe04. A study of Sille'n and

Nylander[37] on the molybdates and tungstates of calcium,

strontium, barium and lead leads to very similar

parameters for all compounds, these being appreciably

different from earlier values.

The wolframite structure may be described as made up

of hexagonally close packed oxygens with certain

octahedral sites filled by A and B cations in an ordered

way. The oxygens are three co-ordinated to cations in

both scheelite and wolframite structure. In worlframite,

half is co-ordinated to two B cations and half is co-

ordinated to two A cations. Many ?+Moo4 molybdates

normally have the scheelite structure but pressure is

required to form molybdates with the wolframite

structure[38,39]. All known ?+Moo4 can have either the

wolframite or scheelite structure with the exception of

Hg Mo04[40]. The crystals referred to in this thesis,

samarium barium molybdate, neodymium barium molybdate and

praseodymium barium molybdate,are of the form ABMo04. No

reference has been obtained about the structural form of

these upto this time.

Dem'yanetes et a1.[41] and Nicol et a1.[42] described

a relation between the scheelite and wolframite

structures. Some scheelites do transform to wolframite

structure at high pressures, but they apparently revert to

the scheelite structure when the pressure is released.

Anion packing is nearer to close packing in the wolframite

structure than in the Scheelite structure.

The rare earth mixed crystal prepared according to

Zambonini have the formula Wa La (M00~)~[43]. An extensive

study on the molybdates and tungstates of rare earth

metals was made by Zambonini[44] who claims to have

prepared crystals, isomorphous with scheelite of a large

number of M2(XO4I3. Shungal et a1.[451 produced

Na La MOO^)^. H20 crystals by the reaction between

lanthanum nitrate and sodium molybdate in an aqueous

solution at room temperature.

Rare earth barium molybdate crystals are orthorhombic

crystals. These crystals do not occur naturally.

Naturally occurring orthorhombic form of molybdate is

Koechlinite structure [(Bi02)2 (Moo4)]. This is found as

an alternate product of wolframite at the Germania mine,

Washington.

1.2.4 Properties

Samarium barium molybdate is a transparent light

yellowish white crystal. It consists of 37.9hf barium

0.6% of samarium, 38.00% ofmolybdemm and 23.5% of

oxygen. It is sparingly soluble in acids and insoluble in

water. It is found to be highly volatile around 4 0 0 ~ ~ . The

atomic radius of Ba2+ is 1.34O~[46] with an

electronegativity of 0.85. It is too large to be replaced

by other elements. To a certain extent, a trivalent rare

earth can substitute for Ba2+ without charge compensation.

Npodymium barium molybdate crystals are light violet

in colour, sparingly soluble in acids and insoluble in

water. These crystals are volatile around 400° C. They

consist of 33.29% barium, 10.49% neodymium, 33.72%

molybdenum and 22.50% oxygen.

Praseodymium barium molybdate consists of 38% barium,

4% praseodymium, 35% molybdenum and 23% oxygen. It is

sparingly soluble in acids and insoluble in water. It is

a light greenish crystalline of varying dimensions and

found to be volatile around a temperature of 4 2 0 ~ ~ .

These crystals assume the habits and morphology of

the orthorhombic system. Discussion of this system

introduces several open forms, namely prisms and

pinacoids. Closed forms are double pyramids known as

dipyramids. Koechlinite commonly exhibits thin, square to

rectangular plates and laths flatened parallel, striated

parallel and pseudo-tetragonal.

1.3 Mixed crystals of rare earth copper oxalates

1.3.1 Existing information on oxalates

Rare earths combine with a large variety of metals

and non metals to produce rare earth inter-metallics.

These compounds exhibit interesting physical and chemical

properties and find application in industry[48-501.

Single crystals of sodium hydrogen oxalate monohydrate,

manganese oxalate dihydrate, cobalt oxalate dihydrate,

copper oxalate monohydrate[511, cadmium oxalate

trihydrate[52] and bismuth oxalatel531 have been

successfully grown by the gel technique.

Transition metal oxalates are generally synthesized

in aqueous media by double decomposition of soluble salts,

with alkali metal oxalates or oxalic acid. This procedure

results in the immediate precipitation of the usually very

insoluble transition metal oxalates in the form of

powdered aggregates. Different attempts were made

to grow single crystals of oxalates but without

success[54,55]. Gel growth has proved very successful for

materials with low solubility including many

oxalates[51,56-591.

Part of this thesis deals with the simpler and better

conditions of crystal growth, the performance and

potential in producing more perfect single crystals in the

case of samarium copper oxalate, neodymium copper oxalate

and praseodymium copper oxalate, by gel technique.

1.3.2 Chemistry of oxalates

Oxalic acid has been reported to be one of the most

abundant low-molecular-weight organic acids present in

solutions of a 'mor humus layert[60,61]. Oxalates are also

a common component of soil solution and a concentration

of lo-' mol dm-3 has been reported[62]. Furthermore, the

minerals whewellite [Ca(C204)H20] and weddellite

[Ca(C204)2H20] have been found in the 'little layer' of

several different soils[62], indicating that oxalates are

a major metabolic product of fungi in natural

environment. Many species of fungi also produce oxalates

in laboratory culture[63]. It is, however, important to

realize that the biodegradability of oxalates is high and

that their abundance in natural systems is governedby a

dynamic equilibrium between formation and destruction

rates.

2 + Besides forming sparingly soluble salts with Ca ,

oxalate forms strong complexes with aluminium. This

complexation tends to increase the concentrations of the

nutrients K, Mg and Ca due to increased weathering of

silicate minerals. According to Boyle et dl-[641, oxalic

acid, like HC1, has been found to weather biotite more

rapidly than other low-molecular-weight organic acids.

Oxalato complexes are also common, the main linkage

types[65] being

All the 4f (lanthanide or rare-earth) metals except

Pm are known to be polymorphic, some with as many as four

different structures at various temperatures and

pressures. The abnormal chemical properties of the

elements immediately proceeding gel and Lu have already

been noted in the account of the 4f elements. In the

elementary state also Eu and Yb are abnormal. The

majority of the 4f metals crystallize at ordinary

temperatures with the h or ha type of closest packing,

though the 9-layer sequence chh is the normal structure of

Sm[66].

1.3.3 Structure and symmetry

The common forms of oxalates occurring in nature are

whewellite, weddellite, humboldtine and oxammite[67].

Whewellite ordinarily is of organic origin and occurs

in association with vegetal remains, but it has been found

as a primary hydrothermal deposit in ore veins. The

substance also occurs as microscopic crystals in living

plant cells and as calculi or as a sediment in the human

urinary tract. Natural occurrences are found at

Burgk near Dresden, Saxony, where crystals up to several

inches in size occur with calcite.in the footwall of a

coal seam. Also,from veins at Recsk and Kapnik in Hungary

and from the Maikop district in the northern Caucasus,

USSR, in calcite veins in shale on the Yareg River,

Southern Timan, USSR[68]. Ca (C2 04) H20 of whewellite

crystallize in monoclinic system (prismatic-2/m) with

a : b : c = 0.8696 : 1 : 1.3695. These types have a habit

of short prismatic (001) and the faces are usually

irregularly developed, twins with twin planes e(101) are

common. Calcium oxalate monohydrate crystals are

transparent, colourless, sometimes yellowish or brownish.

Calcium oxalate dihydrate (Ca C2 04, 2H20) is another

naturally occurring crystal coming under weddellite

structure. These crystallize in tetragonal, tetragonal - dipyramidal (4/m) systems with a : c = 1 : 0.591. These

exhibit pyramidal (101) formsand sometimes are aggregated

into groups. These are insoluble in water and decompose

into CaCo3 at about 2 7 0 ~ ~ . These are found as tiny isolated

crystals in the bottom muds of the central weddell sea,

Antartica. Weddellite and whewellite are the principal

constituents of the oxalate type of urinary calculi in

humans[69].

Ferrous oxalate dihydrate (Fe (C204. 2H20) is

categorised under'humboldtine structure which crystallizes

in orthorhombic form having a:b:c = 0.7730:1:1.1039. In

this case distinct crystals are rare, they exhibit as

prisms elongated (001) or as plates, usually capillary,

botryoidal or incrusting forms with. a fibrous structure.

These naturally occur in Czechoslovakia with gypsum in

brown coal at Kollosoruk south west of Bilin.

Oxammite has a naturally occurring crystal, ammonium

oxalate monohydrate[70] ( N H ~ ) '2'4. H2° which

crystallizes in orthorhombic (disphenoidal -222) system.

They are colourless in transmitted light. They are

naturally occurring at Mascagnite in the guano deposits of

the Guanape Islands, Peru.

1.3.4 Properties

Samarium copper oxalate crystals grown by gel method

are transparent yellowish in colour. The spherulites of

these crystals consist of 2.29 % samarium, 32.37 % copper,

12.79% carbon, 34.07% oxygen and 18.48% water, since it is

hydrated. These crystals are readily soluble in acids and

insoluble in water.

Hydrated neodymium copper oxalate crystals consist

of 37.32% neodyminum, 2.06% copper, 10.11% carbon, 26.92%

oxygen and 23.59% water. These are transparent pinkish

violet shining crystals easily soluble in acids and

insoluble in water.

Praseodymium copper oxalate grown in the laboratory

is a light greenish transparent crystal soluble in acids

and insoluble in water. It consists of 36.51%

praseodymium, 2.36 % copper, 10.27% carbon,27.40% oxygen

and 23.46% water.

These crystals become polycrystalline powder even on

slight heating.

1.3.5 Habits

Crystals coming under the category of Humboldtine

and oxammite assume the habits and morphology of the

orthorhombic system. They exhibit different habits like,

elongated prisms, plates, capillary, botryoidal or

incrusting forms with a fibrous structure.