Solid State Chemistry: A Contemporary Overview - American

17 Some Structural and Magnetic Properties of

Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch017

AxVIIxVIII1-xF3Compounds Y . F . LEE, R . F .

WILLIAMSON,

and W . O . J .

BOO

1

Department of Chemistry, T h e University of Mississippi, University, M S 38677

Tungsten

bronze-like

studied changes

on

hexagonal

structure

KxVF

dimensions

with

observed (x =

to be

have lattice

dimensions

or ferrimagnetic

A single

spontaneous

moment

with the

x.

lattice was

Hexagonal

KxVF

(x =

3

that change

ordering

Pseudo-

change

a superstructure

small

compounds

spontaneous

dependent.

for

were

composition

properties.

does not

but

and tetragonal

All of the

systems,

parallel

composition

0.19-0.31)

sition.

magnetic

0.19-0.27)

optimum

compounds

3

the effects of small

and

(x =

3

xF

AxVIIxVIII1

in order to determine

smoothly

reported

Cs VF x

with

displayed

compoantiferro

above 4.2 K. In each of the

magnetic

moments

crystal

of

K .20VF3

only

when

its

0

were

three

composition

revealed

c axis

3

0.450-4.558)

was

a

small aligned

field.

T ) h a s e d i a g r a m s of m i x e d - v a l e n c e systems o f t e n are r e p o r t e d to h a v e single p h a s e d regions that e x t e n d o v e r b r o a d c o m p o s i t i o n ranges. W i t h i n these r e g i o n s , the ratios of m i x e d - v a l e n c e ions, as w e l l as c o m positions of other c o m p o n e n t s , m a y c h a n g e . T h e h e x a g o n a l K WF W

0.19-0.27) a n d C s ^ V F g (x = K a ? V F (x = 3

0.19-0.31) systems (1,2)

0.450-0.558) system (3,4)

S

(x

=

a n d the tetragonal

are r e p o r t e d l y r e l a t e d s t r u c t u r a l l y

to the t u n g s t e n bronzes. A l t h o u g h t h e structures are s i m i l a r , differences exist i n t h e A V F a ?

3

c o m p o u n d s s u c h as superlattices, l a t t i c e d i s t o r t i o n s ,

a n d e v e n space groups. 1

Author to whom correspondence should be addressed. 0-8412-0472-l/80/33-186-315$05.00/l © 1980 American Chemical Society Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

316

SOLID S T A T E

T h e purpose

CHEMISTRY: A CONTEMPORARY OVERVIEW

o f this c h a p t e r i s t o r e p o r t s u b t l e , b u t i n t e r e s t i n g ,

structural a n d magnetic properties of the A . V a

I I

a

.V

I I I

i- .F a

3

p h a s e systems.

I t e m p h a s i z e s t h e fact t h a t p h y s i c a l changes w h i c h often

accompany

c o m p o s i t i o n changes w i t h i n a n a p p a r e n t single p h a s e p r o v i d e a n interest i n g area o f i n v e s t i g a t i o n f o r the s o l i d state c h e m i s t . Experimental Stoichiometric mixtures of K F or C s F , V F , a n d V F were m i x e d t h o r o u g h l y i n s i d e a n i n e r t a t m o s p h e r e g l o v e box. S a m p l e s o f these mixtures were v a c u u m encapsulated i n m o l y b d e n u m b y electron-beam w e l d i n g t e c h n i q u e s . T h e capsules w e r e fired a t 8 0 0 ° C f o r 30 days i n a n e v a c u a t e d stainless steel t u b e i n s i d e a H e v i D u t y f u r n a c e . V F a n d V F w e r e p r e p a r e d i n this l a b o r a t o r y b u t o p t i c a l - g r a d e K F a n d C s F w e r e o b t a i n e d c o m m e r c i a l l y . C h e m i c a l analyses of p r o d u c t s w e r e i n a g r e e m e n t w i t h c a l c u l a t e d v a l u e s w i t h i n e x p e r i m e n t a l error. S m a l l q u a n t i t i e s o f i m p u r i t y phases w e r e s e p a r a t e d f r o m b u l k samples b y Pasteur's m e t h o d u n d e r a n A m e r i c a n O p t i c s stereoscopic m i c r o s c o p e . F i n a l p r o d u c t s w e r e i n s p e c t e d u n d e r a L e i t z D i a l u x p o l a r i z i n g m i c r o s c o p e . S i n g l e crystals o f c o m p o s i t i o n K . 2 o V F , s u i t a b l e for m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s , w e r e o b t a i n e d f r o m a n i n d e p e n d e n t e x p e r i m e n t b y the f o l l o w i n g p r o cedure: a mixture of K F , V F , a n d V F was heated to 1000°C inside a g r a p h i t e - f i n e d n i c k e l f u r n a c e t u b e u n d e r a c o n t r o l l e d gas f l o w o f H F , H , a n d A r . S e v e r a l l a r g e single crystals, w h i c h h a d g r o w n f r o m t h e v a p o r phase, w e r e d e p o s i t e d d o w n s t r e a m i n the cooler p o r t i o n o f t h e f u r n a c e . F r o m these, one s i n g l e c r y s t a l e l o n g a t e d a l o n g its c axis a n d w e i g h i n g 5.0 m g w a s chosen f o r m a g n e t i c a n i s o t r o p y measurements.


2

3

2

0

3

3

2

3

2

T h e samples w e r e c h a r a c t e r i z e d b y X - r a y d i f f r a c t i o n m e t h o d s . B o t h D e b y e - S c h e r r e r and G u i n i e r - H a g g techniques were used o n powder samples. S i n g l e crystals, w h e r e a v a i l a b l e , w e r e s t u d i e d b y W e i s s e n b e r g m e t h o d s f o r s u p e r s t r u c t u r e s , d o m a i n structures, a n d o t h e r s t r u c t u r a l anomalies. A P A R v i b r a t i n g sample magnetometer equipped w i t h a l i q u i d h e l i u m d e w a r a n d g a l l i u m a r s e n i d e t h e r m o m e t e r w a s u s e d for m a g n e t i c s u s c e p t i b i l i t i e s f r o m 2 . 8 - 3 0 0 K a t a constant field o f 10,000 G o n r a n d o m l y o r i e n t e d p o w d e r samples, as w e l l as o n a s i n g l e c r y s t a l o f h e x a g o n a l K . o V F o r i e n t e d w i t h its c axis p a r a l l e l a n d p e r p e n d i c u l a r t o t h e field. Spontaneous m o m e n t s w e r e d e t e r m i n e d a t constant t e m p e r a t u r e s i n fields r a n g i n g f r o m 10,000-1,000 G a n d e x t r a p o l a t e d t o zero field. I n e v e r y e x p e r i m e n t , samples w e r e c o o l e d i n a field o f 10,000 G . Slopes o n plots o f X ^ " versus T w e r e o b t a i n e d b y l i n e a r regression. A D e c - 1 0 c o m p u t e r w a s u s e d t o r e d u c e the X - r a y d a t a . 0

2

3

1

Results and

Discussion A hexagonal

lattice c a n b e described

easily b y a set o f o r t h o r h o m b i c base vectors.

Pseudohexagonal Ka.VF .

I f these vectors are c h o s e n

3

s u c h t h a t ao — 1 / 2 a + b , b = 1 / 2 a*, a n d c = c ; t h e n \a \ = h

h

0

|a | — 2|b | a n d |a |/V3|b | = 0

0

0

0

G

h

h

2/\/3

1.00Q. T h e r e are t w o s i m p l e p o s s i b i l i t i e s

Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

17.

A V

LEE E T AL.

x

7 /

x

V

317

i - ^3

7 / /

for d i s t o r t i o n f r o m h e x a g o n a l s y m m e t r y b y w h i c h the l a t t i c e r e m a i n s orthorhombic.

F o r the first k i n d , | a | / \ / 3 |b | is greater t h a n u n i t y , a n d G

0

for t h e s e c o n d k i n d it is less t h a n u n i t y . F o r distortions of either k i n d , the h e x a g o n a l M i l l e r i n d i c i e s are t r a n s f o r m e d to o r t h o r h o m b i c i n d i c i e s b y the m a t r i c i e s


V4 VS 0'

1 0 0

0 0 1

V2 V4 V i VT.

0

T h e h e x a g o n a l (hOhl) r h o m b i c reflections (h/2,

0

0 0 1

T 0 0

, and

Vi V4 0

0 0 1

reflections are t r a n s f o r m e d i n t o t h r e e o r t h o h/2, I), (71/2, h/2, I), and (TiOZ). T h e

reflections are t r a n s f o r m e d i n t o ( 3 / 2 7 i , h/2, I) (Ohl),

(hh2hl)

a n d (3/2h, h/2,

I n b o t h cases, t w o of t h e three o r t h o r h o m b i c reflections are

I).

degenerate

a n d , i n p o w d e r d i f f r a c t i o n e x p e r i m e n t s , one observes a d o u b l e t of w h i c h one r e f l e c t i o n is t w i c e t h e i n t e n s i t y of t h e other. F o r a d i s t o r t i o n of t h e first k i n d , the (hOhl)

reflections s p l i t i n t o d o u b l e t s of w h i c h t h e first reflec

t i o n (larger d) is h a l f as intense as t h e second, a n d t h e (hh2hl)

reflections

s p l i t i n t o d o u b l e t s of w h i c h the first reflection is t w i c e as intense as t h e second.

F o r a d i s t o r t i o n of the s e c o n d k i n d , the o r d e r of intensities

o b v i o u s l y w o u l d be r e v e r s e d . G u i n i e r - H a g g photographs

s h o w that t h e lattices of K . 2 o V F 0

K0.25VF3 are d i s t o r t e d to a n o r t h o r h o m b i c s t r u c t u r e of the T a b l e I shows X - r a y p o w d e r

d a t a of K0.25VF3.

The hexagonal

i n d i c i e s i n the first c o l u m n w e r e those assigned to l o w - a n g l e

and

3

first

kind. Miller

Debye-

S c h e r r e r d a t a . O n e s u p e r l a t t i c e reflection i n d e x e d as ( 2 1 3 3 ) makes t h e h e x a g o n a l a v a l u e a p p e a r to d o u b l e . T h e s e c o n d c o l u m n i n T a b l e I gives M i l l e r i n d i c i e s of the m u l t i p l e o r t h o r h o m b i c reflections w h i c h are g e n erated f r o m Guinier-Hagg agreement

t h e h e x a g o n a l reflections. films

w i t h the

The

observed

d values

from

are s h o w n i n the t h i r d c o l u m n a n d are i n g o o d calculated values.

A l l of

reflections that w e r e r e s o l v a b l e are o b s e r v e d i n t e n s i t y 1:2, w h e r e a s , a l l (hh2hl)

the hexagonal

(hOhl)

to s p l i t i n t o d o u b l e t s

of

reflections are split i n t o d o u b l e t s

of

i n t e n s i t y 2 : 1 . A d e t a i l e d analysis of this system is p u b l i s h e d elsewhere (2). F i g u r e 1 d i s p l a y s the inverse s u s c e p t i b i l i t y versus t e m p e r a t u r e of a single c r y s t a l of K 0 . 2 0 V F 3 o r i e n t e d p a r a l l e l a n d p e r p e n d i c u l a r to a m a g n e t i c field of 10,000 G over the 4.2-100 K t e m p e r a t u r e r a n g e . T h e insert of F i g u r e 1 is a p l o t of spontaneous m o m e n t s e x t r a p o l a t e d to zero field at fixed

temperatures.

T h e s e m e a s u r e m e n t s r e v e a l a spontaneous

o n l y w h e n t h e c r y s t a l is o r i e n t e d w i t h its C axis p a r a l l e l to t h e

moment

field.

T h e i n v e r s e s u s c e p t i b i l i t y a n d spontaneous m a g n e t i c m o m e n t versus t e m p e r a t u r e of a r a n d o m l y o r i e n t e d p o w d e r s a m p l e of K . 2 o V F is s h o w n 0

i n F i g u r e 2, a n d t h a t of K

0

5VF

2

3

i n F i g u r e 3.

3

S m a l l corrections


for

318

SOLID S T A T E C H E M I S T R Y :

Table I.

Hexagonal (Wi\) (2020)

OVERVIEW

G u i n i e r - H a g g D a t a f o r K0.25VF3: a = 12.88 ± b = 7.39 ± 0.01; c = 7.52 ± 0.01

0.01;

Orthorhombic ^•observed

((200

((no

6.448 6.419

l-observed

w w

+

^calculated

6.438 6.409

(002

3.763

M

{(310 { (020;

3.715 3.698

w

3.246

w

1(220

3.224 3.205

w

((312 {(022

2.643 2.638

W"

1

(4260)

f(510; \ (420

2.438

W"

2

T2.432 •j 2.427 [2.419

(3361)

(031

2.339

W"

3

2.341

((203

2.335

w

-s

(2.336 {2.334

(0002)


A CONTEMPORARY

(2240) (2022) (4040) (2242)

(2023)

((202

1(112 ((400

1(130

1(113

3.760 1

W"

2

3.711 3.694 (3.247 {3.243 3.219 3.204

+

M

2.641 2.635

(4261)

f(511 \ (421

2.314

W"

3

T2.314 •(2.310 (2.303

(2133)

(213

2.229

W"

4

2.227

((600

2.146 2.136

1(131

W

- 3

2.146 2.136

(6060)

1 (330

(2243)

{(313 {(023

2.075

W

(0004)

(004

1.880

w

1.865

W"

1.856

M

1.847

W"

2

(2024)

1.805

W"

3

(1.805 {1.804

(6280)

1.787

W"

4

fl.785 -U.780 [1.776

(6062) (4480)

W"

2

(2.077 {2.074

- 3

1.880 4

1.864 (1.857 {1.855 1.847


17.

A^V"^/ " .

LEE E T AL.

1

319

F,

t

X

Table I. Hexagonal fhkiU

Orthorhombic (hkl)

(2224)

^•observed

-l -2

1.664 1.658

1.622

w

((800)

1.610 1.602

w w

(1.623 {1.622

{(802)

1(442)

1.479 1.474

f(822) \(732)

1.374

{(622)

1.664 1.658

1(440)

(8082) _ (64152)

1(152)

W

W W

1(044)

1.610 1.602

2 2

-3 w

1.480 1.474

W

1

W"

1.321 1.319

{(624)

(4484)

W

h e x a g o n a l samples. T h e v a l u e 6 X on V F

magnetic

region

3

fl.374 \ 1.372 U-368

4

1.321 1.318

-3

t e m p e r a t u r e - i n d e p e n d e n t o r b i t a l p a r a m a g n e t i s m of V et a l . ( 5 )

dca'cwlcrted

(1.677 {1.676

1.677

1(042)

(8080)

^•observed -4

{(314)

1(024)

(448*2)


Continued

10" e m u / ( m o l V 4

were made on a l l

+ 3

+ 3

) used by Gossard

w h e n a p p l i e d to o u r d a t a g a v e l i n e a r i t y to the p a r a of

X M ' V S . T p l o t s of 1

a l l of

the h e x a g o n a l

Aa.VF

3

compounds. F o r m i x e d - v a l e n c e c o m p o u n d s of the t y p e A . V f l

C u r i e constant c a n b e expressed and V

+ 3

M

+ 2

V

I I I

i . . F , the molar a

3

as the s u m of c o m p o n e n t s

= xC )

+

i+2

from V

+ 2

(1 - x )

C

( + 3

)

, w h i c h is a s p i n o n l y s y s t e m , A ^

n C ( + 2 )

a n d for V

+ 3

(

+

2

)

W

(3/2)

(3/2 +

1)

3fc

=

i n w h i c h the o r b i t a l m o m e n t is q u e n c h e d ,

W(i)

r

^ f+3)

———

=

w h e r e N is A v o g a d r o s n u m b e r , g Lande

a ;

b y the e q u a t i o n C

For V

I I

s p l i t i n g factors

for V

+ 2

(+2

)

(i + i)

3/c and g

and V

+ 3

( + 3 )

are average v a l u e s of the

, r e s p e c t i v e l y , / A is the b


Bohr

320

CHEMISTRY:

A

CONTEMPORARY

OVERVIEW


SOLID S T A T E

Figure 1. Inverse molar susceptibilities and spontaneous moments versus temperature of a single crystal of K VFg oriented parallel and perpendicular to the field 0t20

m a g n e t o n , a n d k is the B o l t z m a n n constant.

I t has b e e n e s t a b l i s h e d that

g 2) is i s o t r o p i c a n d has a v a l u e of 1.97 i n a n o c t a h e d r a l (+

m e n t (6,7).

F o r V F , the a v e r a g e v a l u e of g ) 3

(+3

is 1.80

s i n g l e - c r y s t a l d a t a of K . 2 o V F , a n average v a l u e of g 0

3

( + 3 )

fluoride

environ

(5).

F r o m the

was

calculated


17.

AJ/ ^/ .

LEE E T AL.

to b e 1.75.

n

321

F

m

t

X

S

C u r i e constants f o r p o w d e r e d samples of K0.20VF3, K0.25VF3,

CS0.25VF3, a n d C s . 3 i V F , c a l c u l a t e d w i t h g > == 1.97 a n d g ) 0

3

(+2

(+3

=

1.75,

are i n excellent a g r e e m e n t w i t h m e a s u r e d v a l u e s as seen i n T a b l e I I . T h i s e v i d e n c e s t r o n g l y supports the c o n c l u s i o n t h a t the o r b i t a l m o m e n t of V

+ 3

is q u e n c h e d

largely i n the hexagonal A V F X

3

compounds

as is

t y p i c a l of t r a n s i t i o n m e t a l ions i n solids. H o w e v e r , some o r b i t a l c o n t r i b u t i o n reduces the effective g v a l u e b e l o w 2.0 o w i n g to s p i n - o r b i t c o u p l i n g . Hexagonal Cs*VF .

Debye-Scherrer and G u i n i e r - H a g g X-ray pow

3

d e r d a t a r e v e a l that C s . 5 V F Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch017

0

2

3

and C s . 3 i V F 0

3

retain hexagonal symmetry

b u t h a v e l a t t i c e constants that increase s l i g h t l y w i t h x.

These are: a

T,°K

Figure 2.

Inverse molar susceptibility and spontaneous moments versus temperature of a powder sample of K VF 0

20

S


=

322

A

CONTEMPORARY OVERVIEW


SOLID S T A T E C H E M I S T R Y :

7.50 ±

0.01 A , c — 7.67 ±

dz 0.01 A , respectively.

0.01 A a n d a =

7.51 ±

p e r a t u r e of CS0.25VF3, s h o w n i n F i g u r e 4, i n d i c a t e s a n t r a n s i t i o n at 22 material.

Like

K.

No

0.01 A , c —

7.69

T h e inverse m a g n e t i c s u s c e p t i b i l i t y versus t e m spontaneous m o m e n t was

K0.20VF3 a n d

K0.25VF3, t h e

inverse

antiferromagnetic o b s e r v e d for susceptibility

this of

C s . 3 i V F , s h o w n i n F i g u r e 5, indicates no m a g n e t i c o r d e r i n g t r a n s i t i o n . 0

3

( M a t e r i a l s that o r d e r f e r r i m a g n e t i c a l l y u s u a l l y do not d i s p l a y m a x i m a


LEE ET AL. Table II.

X

X

X

Magnetic Constants of Aa.VF

CM

^served)

Compound

(cm

3

K0.20VF3 K0.25VF3 CS0.25VF3

Cs .3iVF Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch017

0

3

K0.450VF3 K0.498VF3 K0.558VF3

323

A V" V"V F,

0.99 1.02 1.02 1.09 1.39 1.49 1.51

deg

(

CM

calculated)

3

Compounds a

© K

TV K

MB

-10 - 3 -20 -11 -65 -75 -78

8 10 22 12 48 47 45

0.05 0.13 0.00 0.06 0.28 0.010 0.004

mol' ) 1

0.98 1.03 1.03 1.10

—

300 T, K

Figure 4.

Inverse susceptibility versus temperature of Cs . VF 0 25

of a powder

3


sample

324

CHEMISTRY: A

CONTEMPORARY OVERVIEW


SOLID S T A T E

in

)

H o w e v e r , a r e m n a n t m a g n e t i c m o m e n t w h i c h appears b e l o w T

N

,

a n d d i s a p p e a r s a b o v e T , gives e v i d e n c e o f m a g n e t i c o r d e r i n g a n d defines N

t h e o r d e r i n g t e m p e r a t u r e v e r y w e l l . A s u m m a r y of t h e m a g n e t i c constants is g i v e n i n T a b l e I I . Tetragonal Ka.VF . 3

T h e l a t t i c e constants of t h e t e t r a g o n a l K a . V F

3

p h a s e r e p o r t e d l y c h a n g e s m o o t h l y w i t h x (4)

a n d a superstructure w i t h

d i m e n s i o n s \a

was observed for the com

\ = 2\/2Ja

suvteT

position x = Ka.VF

3

0.558.

B U b

|; c ' s

upe

r = 2 c

s u b

T h e i n v e r s e m a g n e t i c s u s c e p t i b i l i t y of t e t r a g o n a l

indicated long-range magnetic ordering near 45 K

(4).


The

17.

LEEETAL.

A V" V x

x

I 7 /

325

i -J?3

d e p e n d e n c e of spontaneous m o m e n t o n c o m p o s i t i o n , d i s p l a y e d i n F i g u r e 6, is a n i m p o r t a n t effect t h a t has not b e e n r e p o r t e d p r e v i o u s l y . T h e d i m e n s i o n s |a er| = 2 2|a |; c = 2 c was observed for the c o m r e m n a n t m o m e n t s o n r a n d o m l y o r i e n t e d p o w d e r samples of c o m p o s i t i o n x — 0.558 a n d 0.498 are s m a l l , b u t for x — 0.450 the m a g n i t u d e of t h e m o m e n t p e r v a n a d i u m a t o m is 0.28 PB- A s u m m a r y of t h e m a g n e t i c constants of t e t r a g o n a l K z V F is i n c l u d e d i n T a b l e I I . sub

BllP

s u p e r

s u b

3


Conclusions T h e b r o n z e - l i k e A ^ V F a structures are p e r o v s k i t e - l i k e , as V - F - V b o n d angles are a l l close to 180°.

T h e y are u n l i k e perovskites, h o w e v e r , i n

0.30 _

0.20 _

Solid State Chemistry: A Contemporary Overview - American

Recommend Documents