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J O U R N A L O F M A T E R I A L S S C I E N C E 2 ( 1 9 6 7 ) 9 L E T T E R S

etters

The Growth of Large Single rystals of

Lithium Ferrite

The g rowth is repor ted o f s ing le crys ta ls o f

l i th ium ferri te with dimen sions sufficient for the

produc t ion o f magnetoelas t ic delay-l ine speci -

mens. P re l iminary measurements have been

made o f some magnet ic and microwave p roper-

ties of the crystal.

I t i s wel l know n th at the magnet ic an d e las t ic

propert ies of l i th ium ferri te lead one to expect

that th is mater ia l would f ind ready app l ica t ion

in the f ie ld o f magnetoacoust ic delay l ines and

magnetoelas t ic parametr ic ampl i f ica t ion . How-

ever, such application has been prevented to-

date by the lack of l i th ium ferri te single crystals

o f the requ ired d imensions . The p resen t com-

municat ion repor ts an a t tempt to p roduce a

single crystal suitable for th is purpose.

The c ompos i t ion o f the mel t was as fo l lows :

Li2CO3 Fe20~ B2Oa PbO

mole ~ wt) 12.3 1 23.09 20.28 44.32

The mel t was heated to 1060~ in a 500 ml

p la t inum crucib le fo r a per iod o f 12 h , and

a negat ive temperatu re g rad ien t o f 0 .7 ~ C/cm

was main ta ined over the heigh t o f the crucib le

dur ing th is per iod . The temperatu re was then

reduc ed at 0 .8 ~ C/h to 600 ~ C, w hen the furna ce

was switched off . The result ing l i th ium ferri te

crystals consisted of six specimens of approxi-

ma tely 1 cm side, and one specimen, sho wn in

fig. 1 , with a side of 2 cm and of weight 36 g .

Gam ma- rad iog raphs o f th is la t ter crys ta l showed

regions, clear of f lux inclusions, which are large

enough to cu t ou t rods su i tab le fo r acoust ic

resonance exper iments . However , the gamma-

rad iographs , together wi th X-rad iographs ,

showed the existence of str iae which are parallel

to the g rowth in ter face and which may be

associa ted wi th e i ther the occurrence o f con-

s t i tu t ional supercoo l ing o r f luctuat ions in the

temperatu re con tro l dur ing the coo l ing cycle .

Also , the appearance o f the larger crys ta l i s

consistent with the existence of parallel growth.

The smaller crystals showe d crystal faces which

were more near ly perfect bu t wi th the bas ic

s t ructu re seen in the pho tograph .

Atomic absorp t ion spect rum analys is has

shown that the i ron- to - l i th ium ra t io o f the

crysta ls co rresponds to a chemical fo rmula:

Lio.45Fe2.5204.

igure I

1.0 in. = 25.4 ram)

The in i t ia l measurements car r ied ou t on the

crystal y ielded the following results:

Res istivity 25000 ~Q cm

Curie tem pera ture 635.5 ~ C

Satu ra t ion magnet isa t ion 4052 G

0 ~ K)

Mag net ic mo me nt per molecu le 2 .519 FB

0 ~ K)

g-fa cto r 300 ~ K) 2.071

2K M 300 ~ K) --558 Oe

AH 300 ~ K) as pre pare d 15 Oe

from mel t )

AH 300 ~ K) quenched af ter 7 0 e

4 h ann eal ing at 750 ~ C)

The increase in l inewid th over the value o f

1 . 8 0 e , r e p o r t e d b y R e m e i k a a n d C o m s t o c k

[1], and the values o f the resistiv ity and the

magnet ic moment may be due to the excess o f

iron in th e cryst al giving rise to Fe ~+ ions.

cknowledgements

The au thors wish to thank Messrs C . Campkin

and D. J . R ippon fo r under tak ing the gamma-

293

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JOURNAL OF MATERIALS S CIENCE 2 (1 96 7 ) 9 LETTERS

radiographs. One of us (J.M.R.) wishes to

acknowledge the award of a Research Fellow-

ship by the Governors of the Portsmouth College

of Technology.

e f e r e n c e

1. S. P. REMEI KA

and g. J.

COMSTOCK, .L

Appl. Phys.

35 (1964) 3320.

ydrothermal Synthesis of Iron Molybdates

The crystal structures of the transition metal

molybdates and tungstates are of interest

because the structures of these complex oxides

reveal a surprisingly large number of changes

going along the series increasing the number of

3d or 4f electrons. The marked changes in

volume per formula weight found in going from

one member of the series to another also

suggests that there might be interesting and cor-

related changes in the structure of a single

species at different pressures. The crystal

structures of some of the transition metal

molybdates have been studied by Abrahams [1 ]

and others [2]. A study of trivalent molybdates

[3] has shown that F%(MoO4)3 is isomorphous

with scandium molybdate, which has the same

orthorhombic structure as scandium tungstate

[4]. However, Pliasova

et al

[5] have reported

tha t Fe2(MoO~)3, when crystallised hydro-

thermally [6], has a monoclinic structure. It was

therefore of interest to investigate whether

F%(MoQ)3 can exist in different, pressure-

dependent polymorphs.

The dry oxides Fe203 and MoO3 were mixed

together in the molar ratios of 1 : 3 and sealed

into platinum capsules with a solution of FeC13.

Two sizes of platinum capsule were used, with

internal capacities of ~ and 89 m3. The charges of

solid oxides were 50 and 30 mg respectively.

Solutions of FeCla which were 5 and 12 wt

both gave the same results. The platinum

capsules were pressurised and heated in a

standard "Tem-Pres" hydrothermal apparatus

for periods between 4 to 12 days, and then

quenched and opened.

It was found that, at pressures below about

400 bars (1 bar = 750 torr), the product was

F%(MoO4)a. After 4 days at 460 ~ C, the crystals

were only a few tenths of a millimetre in size,

and the product contained unreacted F%O~ and

MoOs. After 12 days at 475 to 485 ~ C, the re-

action was complete. Single crystals of

294

17 January 1967

h.J. POINTON

J M

ROBERTSON

Physics Department

Portsmouth College o f Technology

Portsmouth UK

Fe2(MoO4)a grew as greenish-yellow, thick

plates up to 1 mm in size. These monoclinic

crystals were examined by X-ray diffraction

using a precession camera, and the lattice

constants were found to be [7]

a = 15.55 i 0 . 0 5 A

b = 9.27 ~0 .0 4 A

c = 18.08 ~: 0.07 A

fl = 124.8 :~ 0.2 ~

These are in good agreement with those deter-

mined by Pliasova, which were

a = 15.52 A

b = 9.21 A

c = 18.10 A

= 2 5 ~

In the absence of good single-crystal measure-

ments for orthorhombic F%(MoO~)3, it is

not possible to determine with certainty which

form has the smaller volume per unit formula

weight. Approximate calculations from powder

patterns suggest that the X-ray density of the

orthorhombic form is slightly the smaller, and

so it might be expected to form at the lower

pressures. The lowest pressure investigated was

170 bars and, at this pressure, the product was

still monoclinic F%(MoO4)a. The transition

point between the monoclinic and orthorhombic

polymorphs must therefore lie between 170 bars

and ambient pressure.

At pressures greater than about 650 bars, the

same reactants , at 480 ~ C, produced deep-blue

acicular crystals up to a few millimetres in

length, but very thin. The crystals commonly

grew as rosettes of radiat ing clusters, and the

reaction appeared to be complete within 4 days.

This product was identified as the high-pressure

form of FeMoO~, having the wolframite struc-

ture [7]. It is interesting to note that Young and

Schwartz [8] synthesised this compound in

powder form by heating FeO and MoOz

together at 900 ~ C and 60 kbars; whereas, in this

work, it has been produced hydrothermally as